Jobs – Page 298 of 524

AMS: the search for exotic matter goes into space

Installation of AMS-02 — Astronauts inspect the installation of AMS-02 on the ISS.
Image credit: NASA.

16 May 2011, 08:56 EDT : The Endeavour space shuttle takes off for its final trip into space. In its payload bay, it is carrying the Alpha Magnetic Spectrometer (AMS-02), a large particle detector. For the 600 physicists and engineers who have been working on the AMS project for more than 15 years, it is a fantastic accomplishment, but the real excitement is only just beginning. Two days after take off, Endeavour will reach the International Space Station (ISS), where AMS-02 is installed. From this vantage point in space, the advanced detector will catch the tracks of cosmic rays for years to come.

Cosmic messengers

Detectors on board AMS-02. — Fig. 1. The various particle detectors on board AMS-02.

The Big Bang that gave rise to the universe about 13.7 billion years ago should have produced particles and antiparticles in equal amounts, as indeed happens in experiments at particle accelerators. So why is there no evidence of antimatter in the universe? This is one of the big questions of modern physics – one that the international AMS collaboration, led by Nobel laureate Sam Ting, is seeking to address. The mission will also join the search for dark matter in the universe and gather information on sources of cosmic rays.

Cosmic rays are energetic particles from outer space. Protons and the nuclei of helium together represent about 99% of the spectrum, while the remaining 1% is composed of heavier nuclei and electrons. The details of their origins remain unknown but they are probably produced in different cosmic objects, especially the most violent, such as supernova remnants. The sources must be powerful natural accelerators – more powerful than any achievable in laboratories on Earth.

AMS-02 is the first large magnetic spectrometer to fly into space for a long period of time, following on from its prototype, AMS-01, which flew aboard the space shuttle Discovery for 10 days in June 1998. It will allow the cosmic-ray spectrum to be measured with high precision. Such precise measurements are possible only from space, because the cosmic rays that bombard the Earth interact with the atmosphere. At the Earth’s surface it is possible to detect only the cosmic-ray showers that they produce, as the Pierre Auger Observatory in Argentina does, for example. The magnetic fields in space mean that it is impossible to deduce the location of any source of the primary charged particles by measuring their direction. However, determining the composition of the cosmic rays precisely is a key to knowing where they come from. All of the natural elements are present in cosmic rays in approximately the same ratio as in the Solar System, so detailed differences could reveal crucial information about the nature of their source – rather like fingerprints – and make it possible to identify the sources indirectly.

Working on AMS-02 in a clean room at CERN in July 2010.

The requirements for operating in space are challenging and AMS-02 has been designed specially for this hostile environment. It has to resist vibrations and large temperature variations; it must operate in the vacuum of space and in the absence of gravity. In addition, the electronics have to be resistant to radiation. Other vital constraints also had to be considered in designing the detector, such as its weight and electrical consumption, both of which were strictly limited.

At the heart of AMS-02 is a powerful, permanent magnet that bends charged particles and antiparticles in opposite directions. This “magic ring” is designed specifically to ensure that the magnet has a negligible net dipole-moment, so avoiding a coupling with the Earth’s magnetic field that would otherwise disturb the orbit of the ISS. Around this magnet, several layers of detectors work together to identify particles passing through (figure 1, p19). The silicon tracker measures the trajectory deflection of charged particles, the ring-imaging Cherenkov (RICH) detector estimates their velocity and the electromagnetic calorimeter (ECAL) measures their energy. In addition, the transition radiation detector (TRD) identifies light particles by the detection of the X-rays that they emit. The time-of-flight (TOF) system acts as a trigger, alerting the subdetectors to an incident cosmic ray. An anti-coincidence counter was also developed to sort the events in real time, rejecting cosmic rays traversing the magnet walls and keeping the significant ones that really cross the overall detector. As a whole, AMS-02 is able to digitize 300,000 channels of data some 2000 times a second.

First events seen by AMS-02 — One of the first events seen by AMS-02, collected only moments after the detector was activated on 19 May. The particle is identified as a 42 GeV carbon nucleus.
Image credit: AMS-02.

AMS-02 can recognize one antiparticle among a billion particles. This represents an increase in sensitivity of three orders of magnitude in comparison with previous experiments. With such precision, the detector will provide the composition of the cosmic-ray spectrum with unprecedented accuracy. This will enable the AMS collaboration to find either an explanation for the disappearance of antimatter or proof of its existence hidden away in a remote corner of the universe. The observation of just one antihelium nucleus would provide evidence for the existence of a large amount of antimatter somewhere in the universe – large enough for antiprotons to have undergone a process of nucleosynthesis. This matter could only have been generated soon after the Big Bang and would represent a real breakthrough in the current view of the universe.

Antiworlds and the dark universe

There are other puzzles about the universe that AMS can try to solve. Only 4% of the universe is accessible to telescopes and detectors through the radiation that it emits. The rest appears to be in the forms known as dark matter and dark energy, which account for about 23% and 73%, respectively, of the total matter and energy in the universe. In this case, AMS-02 is expected to play a key role by tracking possible signals from the annihilation of supersymmetric particles. One of the candidates for a dark-matter particle is the neutralino, a hypothetical particle that is predicted by supersymmetry. If neutralinos do indeed exist they could interact with each other, producing excesses of charged or neutral particles – creating anomalies in the overall cosmic-ray spectrum.

Particle physics, astroparticle physics and cosmology are certainly at a key moment in their history as AMS-02 enters the race

For this reason, AMS-02 has been eagerly awaited because it will probably be the only experiment able to confirm or invalidate results from other experiments that have recently been in the spotlight. In particular, two satellites, PAMELA and FERMI, as well as the ATIC balloon experiment flown above Antarctica, have all reported an excess of electrons and positrons in the cosmic-ray spectrum. Even though these different measurements are inconsistent with each other, they could all possibly fit with the scenario of dark-matter annihilation. Using a completely different approach, underground experiments such as Xenon, CDMS or Edelweiss, are currently providing strong competition in the detection of dark-matter particles. Moreover, in parallel the LHC is producing data that could in the next two years provide the first exciting news on dark matter.

Particle physics, astroparticle physics and cosmology are certainly at a key moment in their history as AMS-02 enters the race. “Better to light a candle than to curse the darkness,” goes a Chinese proverb. All eyes are now focused on the AMS-02 experiment, hoping that it will precisely light the candle on both the dark universe and the antiuniverse.

• AMS was built by an international collaboration involving large European participation from France, Germany, Italy, the Netherlands, Spain and Switzerland,together with China, Taiwan, Russia and the US. It has been supported by the national high-energy institutes INFN, IN2P3, CIEMAT, the US Department of Energy, Academia Sinica (Taipei), Swiss National Fund, and by the space agencies ASI, DLR, NASA, and ESA. The detectors were integrated at CERN by the collaborating groups. Space qualification was at the ESTEC facilities in ESA.

Endeavour takes passion, precision and patience

CCtin1_06_11 — Sam Ting, left, with Saul Gonzalez of the US Department of Energy’s Office of High Energy Physics, at the Kennedy Space Center on launch day.
Image credit: Mike Struik.

“Most of our understanding of our cosmos up to now comes from measuring light. Besides light rays, there are charged particles, which have not been used nearly as much as light to understand the universe.” Samuel Ting, the principal investigator for the Alpha Magnetic Spectrometer (AMS-02) experiment, is talking to the assembled press at the 2.00 p.m. briefing in the crowded auditorium at the Kennedy Space Center, Cape Canaveral. His experiment is soon to fly on board the space shuttle Endeavour, prior to installation on the International Space Station (ISS). “AMS is the first detector to study charged particles from cosmic rays directly in space, thanks to its magnet – the first magnet in space – and it will do so for the next 20 years, for the space station’s entire lifetime,” he explains. “It is the only fundamental science experiment in the space station.”

This was the main argument that Ting, well known as a leading particle physicist and Nobel laureate, used to convince the US Congress in 2008 to request that NASA “shall take all necessary steps to fly one additional space shuttle flight to deliver the Alpha Magnetic Spectrometer and other scientific equipment and payloads to the International Space Station prior to the retirement of the space shuttle” (US government 2008). AMS-02 had been grounded in 2005, in response both to the accident of the space shuttle Columbia on re-entry in 2003 and to the decision to retire the shuttle by late 2010. Were it not for its primary payload, the 2011 launch of Endeavour might never have been scheduled.

Unexpected discoveries

The principal scientific goals of AMS-02 are to search for dark matter and antimatter (AMS: the search for exotic matter goes into space). However, AMS’s biggest discovery might come in a totally unexpected area. Ting points out that the major discoveries in particle physics over the past 50 years were made in areas of physics that had not been anticipated when building the facilities where the discoveries were made. Take, for example, neutral currents, which Ting regards as the first major discovery made at CERN’s Proton Synchrotron. “When the Proton Synchrotron was built nobody thought about neutral currents,” he says. Likewise for the Brookhaven National Laboratory, which built the Alternating Gradient Synchrotron (AGS) during the same period. “The purpose was to study the nuclear force; instead it discovered a second kind of neutrino, CP violation and the J particle,” he explains. “So what you predict and what you discover are often very different things. With AMS, we are going to explore new territory with a precision instrument, and that is the key to discovery. What we will really see, nobody knows. This is how science advances.” This philosophy was also a key to his Nobel-prize-winning experiment at the AGS, which opened up a new world of particle physics based on a fourth kind of quark, charm, in what has become known as the “November revolution” of 1974.

The story of the AMS experiment goes back nearly 20 years, to the cancellation of the Superconducting Supercollider project in 1993. It was then that Ting first had the idea to send a relatively large-scale particle detector into space. “I began to think maybe I should do something different, not necessarily with accelerators, and then I remembered that in early 1964 I did an experiment together with Professor Leon Lederman to show how an antiproton and an antineutron form an antideuterium. A similar experiment was also done by Professor Zichichi’s group at CERN. So I began to think, maybe I should do an experiment in space. In the 1990s, together with a group of colleagues, we saw the ISS as an opportunity to mount an experiment to study cosmic rays. With support from NASA and the US Department of Energy, an international consortium started work on AMS, and we flew a precursor instrument on the STS-91 shuttle mission in 1998.”

The key component of AMS-01 was the magnet, as in its successor, but the detector was much simpler. “It was intended as a proof of principle, proof that a magnet could go to space and it did so by flying 10 days on the space shuttle Discovery,” explains Ting. The test flight in June 1998 not only showed that everything worked, but also made some initial intriguing measurements of cosmic rays in space. This provided the ground work for AMS-02, which is intended to operate for 20 years. “It has greater, more precise subdetectors, with many channels, whose size, scope and precision are totally different from AMS-01,” he says. “We made them as precise as we could manage.”

So how does Ting feel to see AMS-02 finally being about to launch after the long journey that began with AMS-01? “I am actually very calm; I am confident everything will be OK. This detector spent two decades in the workshop: at CERN, we tested the detector twice with a beam from the SPS accelerator, then we tested it in the thermovacuum chamber at ESA-ESTEC. We took it apart and re-assembled it three times, so we’re quite familiar with what’s going on inside. All of the subdetectors measure energy in a repetitive way, so I think everything will work.”

It is not surprising that CERN’s facilities were used in testing AMS-02. Ting’s relationship with the laboratory goes back nearly half a century, his first day at CERN being on 13 March 1963, as a Ford Foundation Fellow. “There, I had the good fortune to work with Giuseppe Cocconi at the Proton Synchrotron, and I learnt a lot of physics from him,” he recalls. Particle physics and CERN have certainly both evolved a great deal since then. “When I first came to CERN, high-energy physics was dominated by the US,” he says. “Most people at CERN were looking at what was done at Brookhaven and tried to do similar experiments. Now the picture has completely changed. Most US particle physicists come to CERN, and CERN now really has become the centre of high-energy physics in the world.”

With all of the current interest in CERN and particle physics, Ting has some serious, practical advice for young people aspiring to become physicists. “If you want to be a scientist, whether it is a physicist, mathematician or biologist, you need to remember that you’re doing this only for interest, not for fame or glory, because only very few people in their lifetime accomplish what they really want,” he explains. “Physics is a very difficult thing; particle physics involves large groups of people working together. Unless you think that physics is the most important thing in your life, you should not do it. It takes passion, precision, patience.”

Patience is a quality that Ting certainly has, waiting for this “last Endeavour” for his AMS project and never giving up hope. So when does he expect the first important results? “We have no competition,” he says. “We are going to do this very slowly, very carefully. We won’t publish any preliminary results; we’ll only publish the data that we’re absolutely sure about.” Whatever AMS discovers, the final answers, like much that Ting has achieved, will be the result of passion, precision and patience.

Eyewitness: the final countdown for Endeavour

CCtin2_06_11 — Lightning threatens the Vehicle Assembly Building at the Kennedy Space Center on 28 April, the night before the scrubbed launch attempt.
Image credit: Mike Struik.

Thursday 28 April, LD–1. It’s launch-minus-one day at the Kennedy Space Center (KSC) in Cape Canaveral and so far it’s “go” for tomorrow’s 10-minute launch window at 3.47 a.m. EDT, the time set for space shuttle Endeavour’s final lift-off. I am one of 1500 members of the international press accredited at the KSC and one of the expected half-a-million viewers to witness the launch.

Much of the attention surrounding this mission has focused on the fact that this will be the final flight of Endeavour and the penultimate mission of the entire space-shuttle programme, as well as that the mission commander, Mark Kelly, is married to Congresswoman Gabrielle Giffords, who is recovering from a shooting more than four months ago. And, according to the latest rumours among the press at KSC, the “first family” is expected to attend tomorrow.

It’s T–11 hours and holding, one of the longest pauses (around 14 hours) built into the countdown procedure. I join the media registered for witnessing the removal of Endeavour’s Rotating Service Structure (RSS). This is one of the important milestones performed in the T–11 hold in the countdown. Around midnight, the metallic gantry around the shuttle starts to move away under the enthralled gaze of the press representatives who were brave enough to stay, revealing Endeavour in all of its splendour. Once the operation has been performed, there are still 11 hours and so many unknowns before lift-off. Moreover, the weather does not seem promising, with lightning threatening NASA’s Vehicle Assembly Building and launch pad 39.

Friday 29 April, T–3 hours and… scrub. After a short night (the RSS removal took place after midnight), we wake up early not to miss another milestone in the countdown schedule – the astronauts’ “walk-out” and departure for launch pad. We have to be early at the media centre for the usual “K-9” controls (dogs checking for explosives). On the way, I stop at AMS’s premises at KSC, which happen to be close to the Operations and Checkout building where all astronauts spend the night before launch, since the time of the Apollo missions. We see three of them jogging – their last chance for a while.

Walk-out takes place at 11.58 a.m. as planned. I barely manage to shout “Forza Roberto” to my compatriot Roberto Vittori, before my voice is drowned in the crowd of media and NASA staff cheering the STS-134 crew, as they proceed to the Airstream van (also used by all crews since Apollo times).

However, in the media bus taking us back to the press centre, we see the Airstream van backing up – a clear sign that something has gone wrong. At a press briefing we learn that, while the astronauts were on their way to the pad, the launch team identified a fault in the heaters of the auxiliary power unit that prevents the shuttle’s fuel from freezing. This is enough to scrub the launch window.

Sunday 15 May, T–11 hours. I’m back at KSC for the second launch attempt and the legendary countdown clock is again on T–11 hours and holding. The faulty box in the shuttle’s aft compartment that resulted in the launch postponement has been replaced and the entire system re-tested. The weather forecast for tomorrow’s slot is “70% go”. Countdown will resume soon.

Monday 16 May,T–9 minutes and counting. The Mission Management team has just given the final “go” for launch. In less than 9 minutes Endeavour will lift off with AMS cradled in its cargo bay. I am on the media-centre lawn, less than 5 km from the launch pad, one of the closest points to watch a launch at KSC. I’m grateful to Prof. Ting for the invitation. No words can convey the emotion; it’s a lifetime experience not to be forgotten.

LHC physics meets philosophy

CCphi1_06_11 — Participants take a break from physics and philosophy for the group photo session, with Nobel laureate Gerardus ’t Hooft, centre front.
Image credit: IZWT-Wuppertal.

At the end of March, the first Spring School on Philosophy and Particle Physics took place in Maria in der Aue, a conference resort of the archbishopric of Cologne in the rolling hills of the area called Bergisches Land, between Cologne and Wuppertal. It was organized by the members of the Deutsche Forschungsgemeinschaft’s interdisciplinary research project, “Epistemology of the Large Hadron Collider”, which is based at the Bergische Universität Wuppertal. Part of the time was reserved for lecture series by distinguished representatives of each field, including: Wilfried Buchmüller, Gerardus ’t Hooft, Peter Jenni and Chris Quigg from physics; Jeremy Butterfield, Doreen Fraser and Paul Hoyningen-Huene from philosophy; and Helge Kragh from the history of science. The afternoons were devoted to five working groups of philosophy and physics students who discussed specific topics such as the reality of quarks and grand unification. The students then presented their results at the end of the school.

The large number of applications – more than 100 for 30 available places – from PhD students and young post-docs from all over the world demonstrated the strong interest in this interdisciplinary dialogue. There was an almost equal share of applicants from physics and philosophy. The pairing of students and lecturers from such different backgrounds made the school a great success. Almost all of the students rated it “very good” or “excellent” in their evaluations.

Theory and reality

The diverse academic backgrounds of the participants stimulated plenty of discussions during the lectures and working groups, as well as late into the night over beer. They centred on the main lecture topics: the reality of physical theories and concepts, experimental and theoretical methods in particle physics, and the history and philosophy of science.

For example, one of the working groups was concerned with the question, “Are quarks real?” Most physicists would, of course, answer “yes”. But then again, the existence of quarks is inferred in a way that is indirect and theory laden – much more than for, say, chairs and tables. Are there different levels of reality? Or are quarks just auxiliary constructs that will be superseded by other concepts in the future, as happened with the ether in the 19th century, for example? A comprehensive picture of philosophical attitudes towards the reality content of physical theories was discussed by the philosopher Hoyningen-Huene of the University of Hannover. His lecture series also presented critically other aspects of the philosophy of science, focusing on the classic ideas of Karl Popper and Thomas Kuhn: What qualifies as a scientific theory? Are physical theories verifiable? Are they falsifiable? How do physical theories evolve over time?

Fraser, of the University of Waterloo, and Butterfield, of the University of Cambridge, discussed the scope and applicability of particle and field concepts in the interpretation of quantum field theory (QFT), an area that is certainly one of the most successful achievements in physics. However, Fraser pointed out that the need for renormalization in QFT, as used in particle physics, reflects a conceptional problem. On the other hand, the more rigorous algebraic QFT does not allow for an interpretation in terms of particles, at least in the traditional sense.

Another topic that has attracted the attention of philosophers in recent years concerns gauge theories and spontaneous symmetry breaking, as Holger Lyre, of Otto-von-Guericke-Universität Magdeburg, discussed in his lecture. He asked whether it is justified to speak of “spontaneous breaking of a gauge symmetry” given that gauge symmetries are unobservable, a theme that was also discussed in a working group. Again, most physicists would take the pragmatic view that it is justified as long as all physical predictions are observed. Philosophers, however, look for the aspects of gauge theories that can count as being “objectively real”.

The contrarian attitudes between physicists and philosophers were put in a nutshell when a renowned physicist was asked whether he considers the electron to be a field or a particle, and the physicist replied: “Well, I usually think of it as a small yellow ball.” Pragmatism – motivated by a remarkably successful theoretical and experimental description of particle physics – clashed with the attempt to find unambiguous definitions for its basic theoretical constructs. It was one of the goals of the school to understand each other’s viewpoints in this context.

The physics lectures covered both experiment and theory. On the experimental side, Jenni, of CERN, and Peter Mättig, of the University of Wuppertal, discussed methods and basic assumptions that allow us to deduce the existence of new particles from electronic detector signals. As also discussed in one of the working groups, the inference from basic (raw) detector signals to claiming evidence for a theory is a long reach. The related philosophical question is on the justification of the various steps and their theory-ladenness; i.e. in which sense do theoretical concepts bias experimentation, and vice versa. Close to this is the additional question addressed in the discussion as to what extent the LHC experiments are fit to find any new particle or interaction that may occur.

The theory lectures of Robert Harlander, of the University of Wuppertal, Michael Krämer, of RWTH Aachen, and Quigg, of Fermilab, focused on the driving forces for new theories beyond the Standard Model. Apart from cosmological indications – comprehensively reviewed by DESY’s Buchmüller in one of the evening sessions – there is no inherent need for such a theory. Yet, almost everyone expects the LHC to open the door to a more encompassing theory. Why are physicists not happy with the Standard Model and what are the aims and criteria of a “better” theory? One of the working groups discussed specifically the quest for unification as one of the driving forces for a more aesthetic theory.

A current, highly valued guiding principle for model building is the concept of “naturalness”. To what extent are small ratios of natural parameters acceptable, such as the size of an atom compared with the size of the universe? As Nobel laureate ’t Hooft discussed in an evening talk, again there is no direct physics contradiction in having arbitrarily small parameters. But the physicists’ attitude is that large hierarchies are crying out for an explanation. Naturalness requires that a small ratio can arise only from a slightly broken symmetry. This is the background for many models that increase the symmetry of the Standard Model to justify the smallness of the weak scale relative to the Planck scale. Another idea that ’t Hooft discussed is to invoke anthropic arguments fuelled, for example, by the discovery of the string landscape consisting of something like 10^⁵⁰⁰ different vacua.

Closely related to the philosophy of science is the history of science. The development of the Standard Model was the subject of one of the working groups and was also comprehensively discussed by Kragh, of the University of Aarhus. Looking at the sometimes controversial emergence of the Standard Model revealed lessons that may well shape the future. Kragh reminded the audience that what is considered “certain” today only emerged after a long struggle against some “certain facts” of former times.

At first glance, philosophical questions may not be directly relevant for our day-to-day work as physicists. Nevertheless, communication between the two fields can be fruitful for both sides. Philosophy reminds us to retain a healthy scepticism towards concepts that appear too successful to be questioned. In return, the developments of new experimental and theoretical methods and ideas may help to sharpen philosophical concepts. Looking into the history of physics may teach us how sudden perspectives can change. Coming at the brink of the possible discovery of new physics at the LHC, the school was a great experience, reflecting about what we as physicists take for granted. The plan is to have another school in two years.

ICARUS takes flight beneath the Gran Sasso

CCica1_06_11 — Fig. 2. Centre: Schematic view of the whole ICARUS T600 plant in Hall-B at the Gran Sasso National Laboratory. Top-right: photo of the actual detector installation. Bottom-right : details of the cryo-coolers plant.

Historically, imaging detectors have played a crucial role in particle physics. In particular, bubble-chamber detectors – such as Gargamelle at CERN – were an incredibly fruitful tool, permitting the visualization and measurement of particle interactions in an unprecedented way and providing fundamental contributions, in particular in neutrino physics. However, in the search for rare phenomena, bubble chambers are limited mainly by the impossibility to scale their size to larger masses and by their duty cycle, which is intrinsically limited by the mechanics of the expansion system.

The concept of the liquid-argon time-projection chamber (LAr-TPC) was conceived more than 30 years ago: it allows the calorimetric measurement of particle energy together with 3D track reconstruction from the electrons drifting in an electric field in sufficiently pure liquid argon (Rubbia 1977). The LAr-TPC successfully reproduces not only the imaging features of the bubble chamber – its medium and spatial resolution being similar to those of heavy-liquid bubble chambers – but it also has the further achievement of being a fully electronic detector, which is potentially scalable to multikilotonne masses. In addition, it provides excellent calorimetric measurements, with the big advantage of being continuously sensitive and self-triggering.

The ICARUS LAr-TPC

CCica2_06_11 — Fig. 1. Illustration of the ICARUS T600 working principle: left to right, from the ionization path of a charged-particle in liquid argon to its geometrical reconstruction.

The ICARUS T600, the largest LAr-TPC ever built, contains 760 tonnes of liquid argon (LAr). It represents the state of the art of this technique and marks a major milestone in the practical realization of large-scale LAr detectors. Installed in Hall B of the underground Gran Sasso National Laboratory (LNGS) of the Instituto Nazionale di Fisica Nucleare (INFN), it is collecting neutrino events from the beam of the CERN Neutrinos to Gran Sasso (CNGS) project. Produced at CERN, the neutrinos reach Gran Sasso after a journey of around 730 km. The detector also acts as an underground observatory for atmospheric, solar and supernovae neutrinos. In addition it will search for proton decay (in particular into exotic channels) in one of its 3 × 10^³² nucleons, with zero background.

The ICARUS T600 detector consists of a large cryostat that is split into two identical, adjacent half-modules (with internal dimensions of 3.6 × 3.9 × 19.6 m^³), which are filled with ultrapure liquid argon (Amoruso et al. 2004). Each half-module houses two TPCs separated by a common cathode, with a drift length of 1.5 m. Ionization electrons, produced by charged particles along their paths, are drifted under a uniform electric field (E_D = 500 V/cm) towards the TPC anode made of three parallel wire planes that face the drift volume (figure 1). A total of approximately 54,000 wires are deployed with 3 mm pitch, orientated on each plane at a different angle (0°, +60° and –60°) with respect to the horizontal direction. By appropriate voltage biasing, the first two planes (the induction-1 and induction-2 planes) provide signals in a non-destructive way; finally, the ionization charge is collected and measured on the last plane (the collection plane).

The relative time of each ionization signal, combined with the electron drift-velocity information (v_D ˜ 1.6 mm/μs), provides the position of the track along the drift coordinate. Combining the wire coordinate on each plane at a given drift time, a 3D image of the ionizing event can be reconstructed with a remarkable resolution of about 1 mm^³. The absolute time of the ionizing event is provided by the prompt UV-scintillation light emitted in the LAr and measured through arrays of photomultiplier tubes (PMTs), installed in the LAr behind the wire planes.

CCica3_06_11 — Fig. 3. (Left) First CC interaction from the CNGS beam as recorded by ICARUS-T600 (collection view, top) together with a ν_μ CC interaction with a 18 m-long muon (middle) and a nice ν_μ CC interaction with a clean π^⁰ signature as recorded in the collection, induction-1 and induction-2 views (bottom).

The electronics for data acquisition allow continuous read-out, digitization and independent waveform recording of signals from each wire of the TPCs. The electronic noise is 1500 electrons r.m.s. to be compared with around 15,000 free electrons produced by a minimum-ionizing particle in 3 mm.

To permit electrons produced by ionizing particles to travel “unperturbed” from the point of production to the wire planes, electronegative impurities (mainly O₂, H₂O and CO₂) in the LAr must be kept at a low concentration level (below 0.1 ppb). Therefore, both gaseous and liquid argon are continuously purified by recirculation through standard Hydrosorb/Oxysorb filters.

Preassembly of the ICARUS T600 detector began in 1999 in Pavia and one of the two 300-tonne half-modules was brought into operation in 2001 and tested with cosmic rays at the Earth’s surface. To meet safety and reliability requirements for underground operation in Hall B at LNGS, the ICARUS T600 module – illustrated in figure 2 – was equipped with dedicated technical infrastructures. Assembly of the complete detector was achieved in the first months of 2010 and it was finally brought into operation with its subsequent commissioning.

Operation at LNGS

In the spring of 2010, the detector was filled with ultrapure LAr and activated immediately. Events from the CNGS neutrino beam and cosmic rays were observed with a trigger system that relied on both the scintillation light signals provided by the internal PMTs and the CNGS proton-extraction time. The “early warning” signal, sent from CERN to LNGS some 80 ms before the first proton spill extraction, allows the opening of two gates of around 50 μs, corresponding to the predicted extraction times. The first observed CNGS neutrino event is shown in figure 3 other beautiful events with a muon crossing both chambers of a module and two neutral pions are shown in the middle and bottom parts of figure 3, respectively.

CCica4_06_11 — Fig. 4. Free-electron lifetime evolution in the west (left) and east (right) cryostats as a function of the elapsed time for the first 330 days of operation of the T600 detector.

LAr purity is monitored continuously by measuring the charge attenuation along the tracks of ionizing cosmic muons that cross the full drift path. With the liquid recirculation turned on, the LAr purity steadily increased, the value of the free-electron lifetime exceeding 6 ms in both half-modules after a few months of operation (figure 4). This corresponds to a maximum free-electron yield attenuation of 16%. Sudden degradations of purity owing to periodic pump stops for maintenance are always recovered promptly within a few days.

The performance of LAr-TPCs has been studied progressively over the past two decades by exposing different detectors to cosmic rays and neutrino beams, culminating in the successful achievement of the T600 operation. The high resolution and granularity of the detector imaging allow the precise reconstruction of event topology, which is completed by a calorimetric measurement.

Particles are identified by studying both the dE/dx versus range and the decay/interaction topology. Electrons are identified by the characteristic electromagnetic showering, being well separated from π^⁰ via γγ reconstruction, dE/dx signal comparison and the π^⁰ invariant mass measurement at the level of 10^{^–3}. This feature guarantees a powerful identification of the charged current (CC) electron-neutrino interactions, while rejecting neutral-current (NC) interactions to a negligible level. The electromagnetic energy resolution σ(E)/E = 0.03/√(E(GeV)) ⊕ 0.01 is estimated in agreement with the π^⁰ → γγ invariant mass measurements in the sub-giga-electron-volt energy range, while σ(E)/E = 0.30/√(E(GeV)) has been inferred for hadronic showers.

CCica5_06_11 — Fig. 5. An example of ν_μ CC interaction from the CNGS beam in collection view (A); together with close-up views of the primary vertex for the collection (B) and induction-2 (C) projections, which allow a complete solution of the event topology.

For long muon tracks that escape the detector, momentum is determined by measuring the displacements arising from multiple scattering along the track. The procedure, implemented through a Kalman filter algorithm and validated on stopping muons, allows a resolution of Δp/p that can be as good as 10%.

During the 2010 CNGS run, the T600 acquired neutrino interaction events with steadily increasing efficiency, a live time of up to 90% and increasing quality. In the last 2010 period, about 100 neutrino CC events were collected and classified, in agreement with expectations.

As an example of the detector capabilities, figure 5 shows a CNGS ν_μ CC event with a 13 m-long muon track, together with zoomed projections on the collection and induction-2 planes. The use of two different views allows the recognition of two distinct electromagnetic showers pointing to – but detached from – the primary vertex. Even though the two photons overlap in the collection view it was possible to determine the associated invariant mass m₁₂* = 125±15 MeV/c^², which is compatible with the π^⁰ mass. The initial ionization of the closer photon amounts to 2.2 minimum ionizing particles. This is a clear signature for pair conversion, thus confirming the expected e/π^⁰ identification capabilities of the detector.

CCica6_06_11 — Fig. 6. The two half-module cryostats of ICARUS T600 just after they arrived in hall B at LNGS in December 2004.
Image credit: Francesco Arnedo LNGS-INFN.

The momentum of the long muon track in figure 5 has been measured to be via the multiple-scattering method p_μ = 10.5±1.1 GeV/c. The other primary long track is identified as a pion that interacts to give a secondary vertex. A short track from the secondary vertex is identified as a kaon, decaying in flight into a muon. From the decay topology and energy deposition, the kaon momentum can be evaluated as 672±44 MeV/c.

The capability for identifying and reconstructing low-energy kaons is a major advantage of the LAr-TPC technique for proton-decay searches. In the event described, the kaon momentum is not far from the average (300 MeV/c), for instance in the p → ν^{^–} K^⁺ channel. Also, the ability to identify π^⁰s, as in this event, is effective for many nucleon-decay channels, as well as for the discrimination of NC events when looking for ν_μ → ν_e oscillations.

The missing transverse-momentum reconstructed is 250 MeV/c. Despite the non-full containment of the event, this value is consistent with the theoretical expectation from the Fermi motion of target nucleons. The reconstructed total energy is 12.6±1.2 GeV, well within the energy range of the CNGS beam (Bailey et al. 1999).

Hildred Blewett: a life with particle accelerators

Hildred Blewett in the 1940s. Her bequest supports women returning to physics.
Image credit: Blewett family.

One of the most generous schemes to support women returning to physics – and possibly the most valuable to result from a personal bequest – is the M Hildred Blewett Fellowship of the American Physical Society (APS). When Hildred died in 2004, she left nearly all that she had to the APS to set up the scholarship, which funds a couple of women a year in the US or Canada to the tune of up to $45,000. So far, nine recipients have benefited from the bequest, including two in nuclear and particle physics – not far removed from Hildred’s own field of work in accelerator physics. Indeed, she played an important role in the design of accelerators on both sides of the Atlantic, as well as in the organization of their exploitation.

Hildred Hunt was born in Ontario on 28 May 1911. Her father, an engineer who became a minister, supported her interests in mathematics and physics, although the family did not have much money and Hildred had to take a time out from college – a factor that appears to have influenced the future bequest. Nevertheless, by 1935 she had graduated from the University of Toronto with a BA in physics and maths. Stints of research followed, first at the University of Rochester, New York, and then at Cambridge’s Cavendish Laboratory – which was still under Ernest Rutherford – together with her husband John Blewett, who had also studied in Toronto. After returning to the US, in 1938 Hildred joined Cornell University as a graduate student, with Hans Bethe as her thesis supervisor. Writing in APS News more than 60 years later, physicist Rosalind Mendell recalled Hildred saying that as John was working on magnetrons at General Electric (GE) “she had gone back for her doctorate because she loved physics and could no longer endure life as a ‘useless’ company wife” (Mendell 2005). Rosalind had arrived at Cornell in 1940, when she was just short of 20 years old, joining 50 men plus Hildred – “the cheerful, confident and breezy Canadian blonde”. Hildred took the younger woman under her wing, a characteristic that was seen later with other junior colleagues and was also reflected in her final bequest.

The entry of the US into the Second World War changed everything and by the summer of 1942, Bethe was working with Robert Oppenheimer in California on some of the first designs for an atomic bomb. In November Hildred joined GE’s engineering department; her thesis work was left behind, never to be fulfilled. While at GE she developed a method of controlling smoke pollution from factory chimneys. However after the war, a bright future opened up for scientific research in the US and in 1947 both Blewetts were hired by the newly established Brookhaven National Laboratory to work on particle accelerators. Hildred’s forte was in theoretical aspects, while John had already worked with betatrons at GE.

The Blewetts were part of the team that worked on the design and construction of a new accelerator that would reach an energy of 3 GeV, an order of magnitude higher than in any previous machine and in the range of cosmic-ray energies, hence the name of “Cosmotron”. The machine came into operation in 1952 and Hildred edited a special issue of Review of Scientific Instruments, which contained articles on many key aspects, some of which she also co-authored (Blewett 1953a).

Birth of the PS

That same year saw the emergence of the alternating gradient or “strong-focusing” technique, which offered the possibility for an accelerator to go up to much higher energies and gave birth to the Alternating Gradient Synchrotron (AGS) at Brookhaven. The idea was also conveyed to a group of physicists from several European countries who visited Brookhaven in the summer of 1952 to learn about the Cosmotron and how they might build a similar but somewhat larger machine for the nascent organization that would become CERN. Following the visit, and a busy period of study, the decision was indeed taken to build a strong-focusing machine of 25–30 GeV, the future Proton Synchrotron (PS). The group invited the two Blewetts and Ernest Courant – one of the inventors of the principle of strong focusing – to Europe to help plan the new laboratory.

Kjell Johnsen, left, shows Willi Jentschke and Hildred Blewett some of the emerging results from the ISR in January 1971.

By the end of March 1953, the provisional Council had agreed to build the strong-focusing machine, but as CERN did not yet officially exist, the work was split among groups in several European institutions. On six months’ leave from Brookhaven, the Blewetts went to Odd Dahl’s institute in Bergen, where they contributed to the initial design of the PS. The arrangement turned out to be more complex than initially thought, and they pushed to have everything moved to Geneva, once the site had been selected and ratified by the cantonal referendum in June 1953. The advance guard of the PS group, including the Blewetts, arrived there at the beginning of October. At the end of the month Geneva hosted a conference on the theory and design of an alternating-gradient proton synchrotron; Hildred edited the proceedings (Blewett 1953b).

Both Blewetts were full members of the PS group, engaged in all aspects, from theoretical research to cost estimates, and their collaboration continued, even after they returned to the US. By January 1954, the decision had been taken to build the 33 GeV AGS at Brookhaven, so the collaboration between the US and Europe was important to both. Hildred commented later that there were even times when “in many ways Brookhaven got more from the co-operation than CERN did” (Krige 1987). She returned to Geneva to attend accelerator conferences in 1956 and 1958, and visited CERN for three months in 1959, when the PS was near completion. Well known photos record her presence in the PS control room on the magical evening of 24 November when the “transition” took place; her written recollections still bring the day vividly to life (CERN Courier November 2009 p19).

Hildred at an interdivisional board to select CERN fellows in November 1975. She joined CERN in 1969.

Back in Brookhaven Hildred made major contributions to the design of the AGS, in particular she “presided over the design of the magnets” (Blewett 1980). Courant also recalls that she devised an elaborate programme to make detailed field measurements of each of the 240 magnets, which enabled the team to assign the positions of the magnets in the ring so as to minimize the effects of deviations from the design fields.

The AGS began operation in 1960, a few months after the PS at CERN. Alan Krisch, then a graduate student at Cornell, worked on a large-angle proton–proton scattering experiment, which was one of the first to be approved. Hildred “sort of adopted” him and he remembers her as a “formidable woman from whom he learnt much”. She was the one, for example, who suggested that the Cornell group acquire a trailer to provide a cleaner environment where they could collect their data near their AGS experiment. “It was a great idea,” he says, “and soon everyone had trailers.”

The Blewetts split up around that time, as professional divergences increased. These included, Krisch recalls, a disagreement about whether the AGS should add a high-intensity linac or colliding beams. After the divorce, Lee Teng, a colleague and friend, invited Hildred to the Argonne National Laboratory, where he had become director of the Particle Accelerator Division. “I remembered that at Brookhaven she got along very well with and was respected by all of the AGS users,” he says, so he suggested that Hildred become the liaison with the users of Argonne’s Zero Gradient Synchrotron (ZGS). She took on the work with characteristic dedication, bringing all of her experience from Brookhaven, taking care of the needs of the users. One of these was Krisch, who at 25 was a newly appointed assistant professor at the University of Michigan and spokesperson for one of the first experiments on the ZGS. Under Hildred, the experimental areas worked well, “probably the best of any place I’ve worked at”, he says. During this time at Argonne, papers by Hildred show that she continued to work on magnet design, as well as on costings for experimental facilities.

By 1967, on leave from Argonne, she was already involved with the 300 GeV project at CERN, for example as co-ordinator of utilization studies across the member states to look into the exploitation of the machine that would become the Super Proton Synchrotron (ECFA 1967). She joined the CERN staff in 1969 and collaborated in the Intersecting Storage Rings (ISR), which started up in 1971. That same year she was heavily involved in the organization of the 8th International Conference on High-energy Accelerators in Geneva, nearly a quarter of a century after the conference (also in Geneva) that had foreshadowed the PS. She ran the finances of the ISR Division, keeping a careful eye on how resources were spent, as well as being secretary of the ISR Committee (ISRC), serving the new community of users at CERN. Again, the users included Krisch, this time as the first US spokesperson on a CERN experiment, together with a trailer flown over from Argonne; and again Hildred’s expertise proved invaluable, advising on how to run the cabling etc. By the time she retired she had been secretary for 60 meetings of the ISRC and left behind her a perfect organization, in the words of her successor.

She retired in August 1976, but remained at CERN until July 1977 as a scientific associate. During this final year, reports were published on the concept for a 100 GeV electron–positron machine and on studies of 400 GeV superconducting proton storage rings – the future Large Electron–Positron collider and Large Hadron Collider, respectively – both of which involved Hildred (Bennet et al. 1977 and Blechschmidt et al. 1977). She also organized the 1st International School of Particle Accelerators “Ettore Majorana” in Erice, which laid the foundations for the CERN Accelerator School.

The recollections of some of the people who knew Hildred not only paint a picture of a strong woman who cared a great deal for others, but also give some insight into her interests beyond physics. Mendell remembers that they walked together on the Physics Department hikes at Cornell and Courant recalls that she was “an avid folk dancer”, organizing weekly classes in which he and his wife participated enthusiastically. Krisch recalls that during his third encounter with Hildred at CERN, she invited him to Geneva’s English Theatre Club to see her star as the Bulgarian heroine in George Bernard Shaw’s Arms and the Man.

After a few years in Oxford, which suited her interests in music, amateur dramatics and fine arts, Hildred returned to Canada to be closer to her brother and his family. She died in Vancouver in June 2004, at the age of 93. Her career was characterized by her concern that others too should be able to make the most of their time in the field she clearly enjoyed – from the young people she mentored to the user communities she served in several major laboratories and to the beneficiaries of her generous bequest.

The ISR’s totally unexpected results

CCisr1_06_11 — Fig. 1. The effective hadronic energy 2E_{_had}/sub> available for particle production as a function of ISR beam energy, E_{_inc} (left) and the total nominal energy √s = 2E_inc(right).

Our understanding of QCD continues to depend on the contributions from totally unexpected results found at the Intersecting Storage Rings (ISR). Some of the most relevant of these also represent an interesting frontier for research at the LHC. In particular, the “effective energy” remains the basic parameter for revealing the features of universality in the multiparticle hadronic systems that are produced, while the “leading” effect is still present at LHC energies, with all of its consequences, such as the total independence of the two hemispheres of the interaction.

Effective energy

The perceived wisdom, up to the moment when the first unexpected results came from the ISR, was that each pair of strongly interacting particles produces its own final state. The properties of these final states were measured to be all different, as shown by a remarkable number of well established quantities, namely: the fractional momentum distribution dσ/dx_F (where x_F is Feynman x); the average number of charged particles < n_ch >; the ratio of the average energy in the charged channel to the total energy < E_ch>/< E_total >; the transverse-momentum distributions dσ/dp_T^²; the normalized transverse-momentum distribution dσ/(dσ/p_T /< p_T >); the event planarity; the two-particle correlations; and the scale-breaking effects.

Out of these eight quantities, the most popular is the average number of charged particles, < n_ch>. It was taken for granted that different pairs of interacting particles – πp, Kp, pp, p^{^–}p, etc. – had to give different values for < n_ch>. That the other seven quantities were different for different pairs of interacting particles was considered a natural consequence of the fact that different initial states have to produce different final states. This perceived wisdom was shown to be wrong when the effective energy was discovered at the ISR.

At the ISR, and at any other collider, the quantity √s = √(q₁^{^inc} + q₂^{^inc})^² = 2E_inc was considered to be the total energy available in the centre-of-mass system (E_inc being the incident energy of each colliding proton). The Bologna-CERN-Frascati (BCF) group proved that this is not true: the quantity √s should be considered as the “nominal”, not the “effective”, value for the total energy available.

The key point is that in a pp collision, such as at the ISR, the total energy available for particle production is not (√s)_pp = 2E_inc. In fact, the incoming proton can carry a large fraction of the primary energy away into the final state. If you examine the final state of a pp interaction, in 90% of the cases you find in each hemisphere a “leading” particle: q₁^{^leading} and q₂^{^leading}. On average, they carry 50% of the nominal energy, 2E_inc. The hadronic system produced in each hemisphere has at its disposal the quadrimomentum q^{^had} = q^{^inc} – q^{^leading}, which gives rise to the quantity 2E_had. This is the effective energy.

CCisr2_06_11 — Fig. 2. Examples of inclusive single-particle fractional-momentum distributions obtained at the ISR in different ranges of 2E_had at different √s_pp show the remarkable agreement with e^⁺e^– data from the low energies of Mark I at SPEAR (left) to TASSO at PETRA (centre). At the right is the sum of the distributions measured at √s_pp = 62 GeV, which Gribov called the “QCD light”.

The BCF group measured the detailed features of 10^⁵ pp collisions, on an event-by-event basis, to identify the effective energy of each collision. Once this quantity is taken as the correct energy for a given process, the eight quantities quoted above are the same, no matter what the nature of the pairs of interacting particles or the type of interaction. The leading effect is a very general phenomenon that is present when a hadron interacts – whether strongly, electromagnetically or weakly (Basile et al. 1981a and 1981b). When a hadron in the final state shares energy with all other particles produced in this highly privileged way, the effect must be accounted for correctly to compare the properties of the multiparticle hadronic system produced in the interaction. This is how we found the first evidence for universality features between pp and e^⁺e^{^–} data using pp interactions at the nominal ISR energy, (√s)_pp = 62 GeV (Basile et al. 1980).

As figure 1 (p39) indicates, this fixed nominal energy corresponds to a set of effective energies available for particle production, 2E_had, in a range of about 5–40 GeV. We collected data at three ISR energies, (√s)_pp = 30, 44, and 62 GeV for the following reason. It was crucial to show that the multiparticle hadronic systems produced in pp interactions with the same values of 2E_had, but with different values of E_inc, had the same properties in terms of the eight quantities mentioned above.

Let us take one example. The BCF group discovered that the fractional momentum distribution, dσ/dx_F, of a pion produced in the reaction pp→π^⁺+ X at the nominal total energy (√s)_pp = 62 GeV is the sum of the fractional momentum distributions at different effective energies (figure 2). This is why Vladimir Gribov liked to call the pion spectrum the “QCD-light”.

CCisr3_06_11 — Fig. 3. (a) Left: The “leading” quantity L for different types of baryons produced in pp collisions at the ISR as a function of the number of propagating quarks. The centre-of-mass energy ranges from 25 to 62 GeV. The dashed curve was obtained by using a parametrization of the single-particle inclusive cross-section (Basile *et al.* 1981a). (b) Right: The “leading” quantity L for the Λ⁰ produced in ν–p and e^–p interactions, where the number of propagating quarks is two.

The effective energy also dismantled another myth, the one that gave a special role to “high” transverse-momentum phenomena. We found at the ISR that the multiparticle systems (jets) produced at high p_T and at low p_T show the same universality features (Basile et al. 1988).

The leading effect

The discovery of effective energy was the driving force to study in detail the leading effect, which is a basic non-perturbative QCD phenomenon. In another impressive set of totally unexpected results, the BCF group at the ISR established the following five properties of the effect, which need to be explained by QCD.

1. The leading effect depends on the quantum number flow. For example, if in the initial state there is a proton (uud) that goes into the final state, the probability of having the same “proton” (uud) with more energy-momentum than all the other particles is very high. The proof is in figure 3a, which shows how the number of quarks that go from the initial to the final state varies with the value of a function, L, which is proportional to the probability of having the particle in the privileged leading status with respect to all other particles in the final state. The leading effect decreases with the number of propagating quarks from the initial to the final state. For example, when the process is p(uud) → p(uud), all three quarks of the initial state go into the final state and L _˜ 3. However, in both p(uud) → Λ^⁰(uds) and p(uud) → Σ^⁺(uus) there are two propagating quarks and L _˜ 1. For p(uud) → Σ^⁰(dds) there is only one propagating quark and L _˜ 0.5. The minimum value of L is when there are no quarks going from the initial to the final state.

2. The leading effect is flavour independent. The neutron, produced in the process p(uud) → n(udd), seems to indicate some deviation from the other cases with two propagating quarks in figure 3a. To study if there is a flavour dependence in the leading effect, we extended our research to all heavy flavours. This is how we discovered the leading production of Λ_c^⁺ (Basile et al. 1981c) and found for the first time the Λ_b^⁰, which at the time was the heaviest particle known (Basile et al. 1981d). The search for the Λ_b^⁰ proved that the leading production mechanism is valid also for the Λ_b^⁰ and that the same leading-baryon effect is present in the Λ_b^⁰, Λ_c^⁺ and Λ_s^⁰ production mechanisms. So, despite the large mass difference between the strange (s), the charm (c), and the beauty (b) quarks, the production of these differently flavoured baryonic states shows the same leading effect (Basile et al. 1981e). The conclusion is that there is no mass-dependence in the leading effect and that it is flavour-invariant.

3. The leading effect is present in deep inelastic scattering (electromagnetic and weak). Figure 3b shows two examples of deep inelastic scattering: one is electromagnetic (e^{^–}p) and the other is weak (ν–p). In both cases the leading effect is present. Taking into account the results previously reported, where the interactions were all strong, the data prove that no matter if the interaction is strong, electromagnetic or weak, the leading effect is there and depends on the flow of quantum numbers from the initial to the final state.

4. The leading effect also exists when there are no hadrons in the initial state, as in e^⁺e^{^–} annihilation. The fractional momentum distribution measured by the TASSO experiment of the particles produced in e^⁺e^{^–} annihilation at √s = 34.4 GeV at the PETRA collider deviated markedly from the expected spectrum when a leading D^{^*} was present in the final state. Using pp data from the ISR at effective energies in the range 10–16 GeV, the TASSO data showed excellent agreement (figure 4). The only correction needed was the subtraction of the D^{^*} leading effect.

5. There are no long-range correlations in the leading effect. For many years experimental data have given evidence for a correlation between the two ISR hemispheres. The same problem needs to be studied at the LHC. The BCF group proved that these correlations disappear when the data are analysed in terms of the effective energy. The best proof came from the study of p₁^{^inc} + p₂^{^inc} → p₁^{^leading} + p₂^{^leading} + anything, where p_1,2^{^inc} indicate the two incident protons, and p_1,2^{^leading} the two leading protons. The data-taking was performed using unbiased events to have a set of genuine inclusive pp interactions. The results, shown in figure 5, prove that the two hemispheres are totally independent and – contrary to what had been believed before – there are no long-range correlation effects.

CCisr4_06_11 — Fig. 4. Data from the BCF analysis at the ISR of proton–proton interactions with effective energy in the range 10–16 GeV show excellent agreement with the e⁺e⁻ data at √s = 14 GeV.

Two final remarks should be made on the leading effect. First, when there is no quantum number flow from the initial to the final state, the leading effect depends on the probability that the fragmentation products recombine themselves into one leading particle. From high-p_T data at the ISR we have: σ(single particle) E_T^{^inclusive}/σ(jet) E_T^{^inclusive} ≈ 10^{^–2} for E_T _˜ 5–10 GeV; from e^⁺e^{^–} annihilation the results are: σ(e^⁺e^{^–} → D* + other particles)/σ(e^⁺e^{^–} → C + other particles) ≈ 10^{^–2}. We can deduce that if a single particle carries a large part of a certain available energy, it must “pay” a factor of around 10^{^–2}. How this recombination into one leading particle can be possible is a problem for non-perturbative QCD theorists.

CCisr5_06_11 — Fig. 5. Left. Scatter plot of the fractional energies of the two leading protons, x_1,2^leading = E_1,2^leading/E_1,2^inc, in the range 0.4 < x_1,2^leading <0.9, at the pp centre-of-mass energy √s = 62 GeV.  Right: The average value < x₂^leading > for fixed x₁^leading against x₁^leading. The value of < x₂^leading > is obtained from the scatter plot.

A second point is that in pp interactions at ISR energies, about 20 mb of cross-section is in the leading effect. At the ISR this effect is dominated by the quantum number flow. In e^⁺e^{^–} there is no quantum number flow from the initial to the final state and only about 1% of the total cross-section is in the leading effect. For an explanation we must wait until non-perturbative QCD can give the correct “predictions”.

In conclusion

The ISR were the source of a series of totally unexpected results. The effective energy with its universality features plays a fundamental role in all QCD processes. The leading effect is flavour independent, has no long-range correlation and exists no matter whether the process originates in a strong, electromagnetic or weak interaction. The next frontier is to find out if these properties are still valid at LHC energies.

CERN and the EPS: a joint endeavour

CCvie1_06_11 — Luisa Cifarelli became president of the EPS in April.
Image credit: EPS.

The European Physical Society (EPS) was founded at CERN in 1968. Today it represents more than 100,000 physicists through its 41 national member societies and it provides a scientific forum for more than 3000 individual members from all fields of physics.

Around 50 universities, research institutes, laboratories and enterprises that are active in physics research are also present as EPS associate members. CERN was the first to join and has supported the EPS since the very beginning. Many leading personalities from CERN have been EPS presidents: Gilberto Bernardini, the founder and first president of the EPS, who at the time was CERN’s research director; Antonino Zichichi; Maurice Jacob and Herwig Schopper.

The EPS is a non-profit association whose purpose is to promote physics in Europe and across the world. In 1968, when European integration was still rather vague, the establishment of the EPS was, to quote Bernardini’s inaugural address in the CERN Council Chamber, “a demonstration of the determination of scientists to make their positive contribution to the strength of European cultural unity.”

Today the EPS continues to play an important role in fostering the scientific excellence of European physicists, through high profile activities, in enhancing communication among physicists in Europe and across the world, and in bringing major issues in physics, and science in general, to the attention of the public and policymakers.

So how is the EPS organized? EPS members decide the priorities of the society, allocate resources for its activities and hold positions of responsibility. The scientific activities of the EPS segment into divisions and groups, which are governed by boards. Such activities include renowned topical conferences, seminars and workshops.

The divisions and groups also develop outreach activities, for students and for the general public, and support measures to help physicists from less-favoured regions of Europe and from scientifically emerging countries worldwide to participate in EPS initiatives.

A number of prestigious prizes are awarded by the EPS divisions and groups in recognition of outstanding achievements in all fields of physics. These often anticipate the Nobel awards.

The EPS has 11 divisions, covering specific fields of physics research: Atomic, Molecular and Optical Physics, Environmental Physics, High Energy and Particle Physics, Nuclear Physics, Physics in Life Sciences, Plasma Physics, Quantum Electronics and Optics, Solar Physics, Statistical and Nonlinear Physics.

In addition there are seven groups that look at questions of common interest to all physicists, such as: Accelerators, Energy, and Technology; but also the History of Physics and Physics for Development. Finally, a number of committees deal with social questions: European Integration, Gender Equality in Physics, Mobility, Physics and Society and Young Minds.

Like all learned societies the EPS publishes a letters journal (Europhysics Letters), a scientific bulletin (Europhysics News) and, more recently, an electronic newsletter (e-EPS). These are produced in partnership with a number of member societies and their respective publishing houses.

As a consequence of its expansion and evolution over the past 40 years, the EPS has undergone several revisions to assess and define its two-fold role of learned society and federation of national societies, so that it can act as an authoritative, scientific opinion-maker.

In 2010 the society sketched out its new strategy plan and identified new guidelines. The EPS needs to gain more visibility, to strengthen and highlight the activities of its divisions and groups and to generate a greater spirit of belonging and cohesion among its members. It also needs to bring added value and provide a louder common voice to its member societies and associate member institutions. It should increase its potential for co-operation and solidarity with less-favoured countries.

The preservation of the quality of European publications, in particular EPS journals and those related to or recognized by the EPS, and their integration into the context of global publishing is another main objective. Finally, establishing and strengthening links with other scientific societies worldwide – physical, astronomical and chemical – is among the new EPS priorities.

In this perspective, further intensifying the good relations and privileged interactions between CERN and the EPS would be highly desirable. Both institutions are on the same wavelength, share the same vision and support excellency in joint fundamental and applied research. They are concerned with technology transfer and industry’s involvement in physics research and they care deeply about education matters, outreach, knowledge dissemination and public awareness.

As CERN’s director-general Rolf Heuer repeatedly emphasizes, “We must bring science closer to society.” A tighter collaboration between the unique European research laboratory that is CERN and the EPS could serve this common goal; moreover CERN could help considerably to boost the future of the EPS.

The Quantum Story

By Jim Baggott

Oxford University Press

Hardback: £16.99 $29.99

The Quantum Story provides a detailed “biography” of the 111-year-old quantum physics, from its birth with Planck’s quantum of action all of the way up to superstrings, loop quantum gravity and the start of the LHC – a machine that is expected to put physics back on the right track, with experimental measurements forcing some “figments of the theoretical mind” to confront reality.

The first chapters are simply delicious and ideally suited for summer reading on a sunny, late afternoon with a fresh drink close by. I was pleased to revisit most of the stories and characters I met as a teenager when reading books by or about Einstein, Bohr, Pauli, Heisenberg, de Broglie, Schrödinger, Dirac and many other universal heroes. Baggott explains the basics and wonders of quantum physics in a surprisingly clear way, despite its intrinsically “unsettling” and “wholly disconcerting” nature. A multitude of advances and a fair share of dead ends are exposed with excitement and suspense, almost as in a detective story, and the pace of the action is such that I was often reminded of Dan Brown’s novels. You begin to wonder if some of the main characters ever slept, such as during the Solvay conference in 1927, when each breakfast time Einstein would attack with a new gedankenexperiment, which Bohr would counter throughout dinnertime in Brussels’ “Hotel Britannique”.

We all know about Einstein’s “year of miracles” when, perhaps inspired by not having a respectable position to lose in the academic world, he revolutionized physics with an incredible succession of amazing papers. It is less known that he also wrote several “unpublished papers”, some of which influenced new and important ideas, such as Born’s probabilistic view of Schrödinger’s wavefunctions, submitted for publication in June 1926. This “hastily written” paper was followed one month later by a second one giving a “more considered” perspective, complemented by a note added to the proofs of the first, mentioning that the probabilities are proportional to the square of the wavefunctions.

Somehow, it had not crossed my mind that even in those days many physicists were in a hurry to get their ideas in print. The “publish or perish” motto has long applied. Pauli submitted a paper deriving Balmer’s formula from matrix quantum mechanics just five days before Dirac did the same; maybe Dirac’s delay was caused by his proverbial perfectionism with clear language. Baggott mentions other notes added by the authors in the proofs of their papers, as when Heisenberg writes that: “Bohr has brought to my attention that I have overlooked essential points in the course of several discussions in this paper [on uncertainties].” Ouch… this must have hurt. It continues: “I owe great thanks to Professor Bohr for sharing with me at an early stage the results of these more recent investigations of his.” The Copenhagen interpretation did not have an easy birth.

The topic of quantum reality strikes back later in the book, in chapters 30 to 35, where the reader needs a higher level of concentration to follow detailed developments regarding the topics of hidden variables, Bell’s and Leggett’s inequalities, entanglement and the surprisingly accurate experimental work recently made in this area. In chapters 18 to 29, the reader learns the crucial steps in the development of quantum field theories, quantum electrodynamics, quantum chromodynamics, quark asymptotic freedom and infrared confinement, the J/Ψ revolution, the discovery of the intermediate vector bosons, etc. This must be the nicest introduction to the Standard Model that I have read so far.

Given the style (and target audience) of the book, the almost complete absence of mathematics is quite understandable and I should say that the author succeeds remarkably well in explaining many leading-edge physics topics without the help of equations. It is true that “modern theoretical physics is filled with dense, impenetrable, complex mathematical structures”, which often obscure the deep meaning of what is being done. Nevertheless, and with the confidence gained after reading the 410 pages of main text plus several end-of-book notes, I dare to express the wish of seeing this book reprinted in a “special illustrated edition” (following the nice examples of Bill Bryson’s A Short History of Nearly Everything and Stephen Hawking’s A Brief History of Time), with more diagrams, pictures and equations.

In summary, this is a truly exceptional book, which I highly recommend. It will be enjoyable reading for many professional physicists as well as for bright high-school students waiting for something to trigger a decision to follow a career in physics.

Present at the Creation: The Story of CERN and the Large Hadron Collider

By Amir D Aczel

Crown

Hardback: £15.73 $25.99

Mathematician and science writer Amir D Aczel is well known for his factually convincing and captivating story of Fermat’s Last Theorem. His recent book on CERN follows a similar recipe for writing a gripping story: impressions from several visits to the laboratory – notably witnessing the LHC restart from the CERN Control Centre on 5 March 2010 and from the CMS Control Centre earlier in the day – as well as interviewing respective experts and leading physicists, including 13 Nobel laureates.

The story develops in 14 chapters that are illustrated with colour photographs, black-and-white line drawings, photographs and tables. An afterword, notes and a bibliography complete the picture, together with three more “technical” appendices: how an LHC detector works; particles, forces and the Standard Model; and the key physics principles used in the book. Aczel covers the LHC and its potentialities and risks, the four big detectors, symmetries of nature and Yang–Mills theory, the Standard Model, the Higgs particle, string theory, dark matter, dark energy and the fate of the universe. The result is a splendid effort to inform a wider public of CERN’s achievements set in an appropriate context.

As would be expected, Aczel is at his best when explaining mathematical theories such as that of Yang and Mills. Given the breadth of the material covered, it is not surprising that there are some lacunae and even errors. What struck me as an accelerator physicist was the erroneous explanation for the PS Booster synchrotron in the accelerator chain that feeds the LHC, which he attributes to the limited increase of particle velocity in a given synchrotron. In fact, the need for the Booster arose from the luminosity requirements of the Intersecting Storage Rings (and successive storage rings) – that is higher beam intensity and (phase space) densities or, in other words, limited transverse and longitudinal beam emittances. It would have been helpful if Aczel had been able to interview the late Nobel laureate Simon van der Meer.

Altogether, however, it is a book that can be highly recommended to anybody who wants to know “everything” about CERN and who likes a narrative style. I would personally be interested to know how much a complete newcomer understood after a first reading.

Crashes, Crises, and Calamities: How We Can Use Science to Read the Early-Warning Signs

By Len Fisher

Basic Books

Hardback: £13.99 $23.99

When I received the book, I was eager to start reading it, particularly because of its subtitle: How We Can Use Science to Read the Early-Warning Signs. How can we? In fact, after reading the book, the conclusion is that we cannot.

Although realizing it caused some disappointment, I can confirm that, even without the million-dollar answer, the book is an interesting read. Len Fisher is an experienced writer, capable of explaining difficult concepts with simplified – but never simplistic – language. The book talks about equilibrium states, physical and mathematical models, negative feedback etc. When you study such topics in textbooks, you can quickly become bored that everything seems so obvious. However, the mathematics that formalizes all of this is far from being obvious; and the million-dollar question has no answer precisely because of this.

Fisher’s writing is engaging because it moves the hard concepts into everyday life, giving them a framework that makes the reader forget about the complex physics and mathematics behind them. Thus, the equilibrium states that remain theoretical in textbooks, are here explained in real and contextual situations, so that the reader learns about the evolution of biological species, the main facts that determine the solidity of a newly constructed bridge (but it could be your house) and even the factors that lead the dynamics between two people who become a couple.

I found this enjoyable reading and the disappointment of the missing conclusion was partly compensated for by the genuine attention that the author pays to the reader’s entertainment. I recommend the book to a non-scientific readership, which, I believe, will greatly profit from Fisher’s explanation of how and why things work, or, conversely, why they don’t work and can break down.

Topics

Reset your password

Registration complete