Accelerator neutrino beams are currently the object of intense discussion and development. They provide a necessary tool for the detailed study of neutrino oscillations and in particular the observation of potential CP-violating effects that are born from the interference of transitions among the three known species of neutrino. Neutrino interaction cross-sections are tiny, so the challenge in studying their properties has been to produce ever increasing beam intensities. The next challenge in neutrino physics will be to establish precisely the parameters of the oscillations and then compare the oscillations of neutrinos with anti-neutrinos (or the oscillation probability as a function of neutrino energy) to search for CP-violation. This will require precise measurements of the transitions of neutrinos into each other, which will in turn demand a much better knowledge of the neutrino beams.
At present – and probably for the next decade – neutrino beams are generated by the conventional technique: a beam of multi-giga-electron-volt protons, as powerful as possible (up to around 500 kW beam power), is directed at a target to produce a large number (1012 or more) of hadrons, mainly pions with a small admixture (5–10%) of kaons. These are then focused in the direction desired for the neutrino beam and they decay – producing neutrinos – in a decay tunnel.
In the absence of a good theory of hadronic interactions, a precise prediction of the properties of such neutrino beams requires measurements of particle production at an unprecedented level of precision. The role of the NA61/SHINE experiment at CERN’s Super Proton Synchrotron (SPS) is to perform these hadron production measurements. More specifically, it has taken data for the T2K experiment in Japan, both with a thin carbon target and a full replica of the target used in T2K. These data have already proved important for the extraction of the first results on electron-neutrino appearance and muon-neutrino disappearance in T2K. As statistics increase in T2K, they will become more and more essential.
The collaboration behind the SPS Heavy Ion and Neutrino Experiment (SHINE, approved at CERN as NA61) is an unlikely marriage between aficionados of the heaviest and lightest beams on offer. Ions as heavy as lead nuclei have been accelerated in the SPS, while neutrinos have the lightest mass (now famously non-zero) of all particles apart from photons. So what is the unifying concept between these communities that are a priori so different?
The NA49 detector in CERN’s North Area offers excellent tracking with its immense set of time-projection chambers (TPCs), time-of-flight (TOF) detectors and flexible beamline. To perform systematic measurements at energies at the onset of quark–gluon plasma creation, the heavy-ion physicists were interested in upgrading the detector to allow higher event statistics and lower systematic uncertainties. At the same time, neutrino physicists, attracted by the extensive coverage of the detector, were interested in running it in a simple configuration, but also with high statistics, so as to have the first data ready in time for the start of T2K.
The main upgrades relevant for all of the NA61/SHINE physics programmes concerned the TPC read-out, an extension of the TOF detectors and an upgrade of the trigger and data-acquisition system. Figure 1 shows the upgraded detector. Its acceptance fully covers the kinematic region of interest for T2K.
The NA61/SHINE experiment was approved in April 2007 and took data in a pilot run the following September, with 600,000 triggers on the thin carbon target and 200,000 triggers on the replica (long) T2K target. More extensive data-taking for the T2K physics programme took place in 2009 and 2010, both with thin (6 million triggers in 2009) and long targets (10 million triggers in 2010). In parallel, data were recorded for the NA61/SHINE heavy-ion and cosmic-ray programmes.
As a first priority, the cross-sections for producing charged pions from 30 GeV protons on carbon were measured with the thin-target data taken in 2007 (Abgrall et al. 2011). The systematic errors are typically in the range of 5–10% and smaller than the statistical errors. These data have already been used for an improved prediction of the neutrino flux in T2K (Abe et al. 2011). Furthermore, they also provide important input to improve the hadron-production models needed for the interpretation of air showers initiated by ultra-high-energy cosmic rays.
However, these first NA61/SHINE measurements provide only a part of what is needed to predict the neutrino flux in T2K. A substantial fraction of the high-energy flux, and in particular the electron-neutrino contamination, originates from the decay of kaons. Charged kaons are readily identified in NA61/SHINE from the suite of particle-identification techniques – dE/dx in the TPC and the TOF in the upgraded detector (see figure 2) – and a first set of cross-sections has been produced already. Neutral kaons can be reconstructed using the V0-like topology of K0S→ π+π– decays.
A large fraction (up to 40%) of the neutrinos originates from particles produced by re-interactions of secondary particles in the target, which for T2K is 90 cm long. This is difficult to calculate precisely and it motivates a careful analysis of the data taken with the long target. Long-target data are notoriously more difficult to reconstruct and analyse but they provide much more directly the information needed for extracting the neutrino flux. The NA61/SHINE collaboration presented a pilot analysis at the NUFACT meeting at CERN in early August (Abgrall 2011). The ultimate precision will come from the full analysis of the long-target data taken in 2010. The collaboration is working hard to complete these analyses in time for the high-statistics measurements that will become possible in T2K when the experiment resumes data-taking after recovering from damage in the massive earthquake in north-eastern Japan that occurred in March this year.
It all started innocently enough with a review article I wrote in 2004 about the nuclear driplines, which described the exploration of the most neutron- and proton-rich isotopes (Thoennessen 2004). The article included tables listing the first observation of each isotope along the proton- and neutron-dripline. The idea to expand this list to cover all isotopes lingered for a few years until in 2007 I mentioned it to an undergraduate student as a possible research project. At the beginning we did not appreciate the magnitude of the project; after all, there are more than 3000 isotopes presently known. However, with the help of many undergraduate students performing elaborate literature searches and carefully judging the merits of the individual papers we continued, even though we extrapolated that the project would take about 10 years to reach completion.
We have described details of the discovery of each isotope in short paragraphs, arranged by elements, which are published in a series of articles in Atomic Data and Nuclear Data Tables. In a summary table, the first author, year, journal, laboratory, country and method of discovery are presented. Now, only four years after we started, the project is almost completed. We finished the initial discovery assignment for all isotopes and are currently finalizing the paragraphs for the last four elements: actinium, thorium, protactinium and uranium.
The master table of all elements is a rich source of interesting information. Along the way it has been fascinating to see how not only the physics and technology changed over time, but also the style of the papers. For example, the average number of authors per article increased from 1.1 in 1930 to 16.4 in 2000.
One piece of information – the number of isotopes discovered by the different laboratories around the world as a function of time – was recently highlighted by a Nature News article and has drawn a lot of attention over the past few weeks (Samuel Reich 2011). The article reveals the labs and individuals that have discovered the largest number of new isotopes. The results show that while Lawrence Berkeley National Laboratory leads by almost a factor of two, other laboratories in Japan and Europe – most notably GSI in Germany – have made most of the new discoveries in the past couple of decades. A graph displaying the number of isotopes discovered per laboratory as a function of time was featured as the “Trendwatch” in a recent issue of Nature (Trendwatch 2011). The graph seems to indicate that the top five laboratories are Berkeley, Cavendish, GSI, RIKEN, and JINR in Dubna; however, RIKEN was included only because of the large number of recent discoveries. But in reality, CERN’s ISOLDE has played a pioneering role in the discovery of isotopes, especially with the “isotope-online” technique, and ranks number five on the list.
Now why is the information contained in the database significant? The discovery of isotopes has a long history beginning with the discovery of radioactivity of uranium (later identified as 238U) by Becquerel in 1896. The discovery of new isotopes is closely linked to developments of new techniques and new accelerators (Thoennessen and Sherrill 2011). Creating and detecting new isotopes is the first prerequisite to being able to study them, automatically putting the laboratories that produce the most exotic isotopes in the best position for doing the most exciting science with these isotopes. The techniques to produce, separate and identify these isotopes are also critical to make and deliver clean beams of less exotic isotopes at higher intensities, which can then be used to explore the properties of these nuclei. The recent conference on Advances in Radioactive Isotope Science, ARIS 2011, highlighted not only the tremendous interest in the field and the most recent advances in physics but also the technical developments making these experiments with exotic isotopes possible (ARIS 2011 charts the nuclear landscape).
The data presented in the Trendwatch indicate that the balance of power pushing the field forward has shifted away from the US. The article did not stress that 2010 was the most productive year for the discovery of isotopes. For the first time more than 100 isotopes were discovered in a single year. This points to a renaissance of the field, which is driven by the start of a new accelerator system in RIKEN, Japan, and new technical developments at GSI. During the past 20 years, most new isotopes were discovered at projectile fragmentation facilities, thus the next major step will be the new accelerators currently being designed at the Facility for Antiprotons and Ion Research (FAIR) at GSI and the Facility for Rare Isotope Beams (FRIB) at Michigan State University in the US. FRIB is absolutely critical for the US to play a leading role in nuclear physics in the future.
As the 1970s turned into the 1980s, two projects at the technology frontier were battling it out in the US accelerator community: the Energy Doubler, based on Robert Wilson’s vision to double the energy of the Main Ring collider at Fermilab; and Isabelle (later the Colliding Beam Accelerator) in Brookhaven. The latter was put in question by the difficulty in increasing the magnetic field from 4 T to 5 T – which turned out to be much harder than originally thought – and eventually gave way to Carlo Rubbia’s idea to transform CERN’s Super Proton Synchrotron into a p–p collider, allowing the first detection of W and Z particles. Fermilab’s project, however, became a reality. Based on 800 superconducting dipole magnets with a field in excess of 4 T, it involved the first ever mass-production of superconductor and represented a real breakthrough in accelerator technology. For the first time, a circular accelerator had been built to work at a higher energy without increasing its radius.
When the Tevatron began operation at 540 GeV in 1983, Europe was just starting to build HERA at DESY. This electron–proton collider included a 6 km ring of superconducting magnets for the 820 GeV protons and it came into operation in 1989. The 5 T dipoles for HERA were the first to feature cold iron and – unlike the Tevatron magnets, which were built in house – they were produced by external companies, thus marking the industrialization of superconductivity.
Meanwhile the USSR was striving to build a 3 TeV superconducting proton synchrotron (UNK), which was later halted by the collapse of the Soviet Union, while at CERN the idea was emerging to build a Large Hadron Collider in the tunnel constructed for the Large Electron–Positron (LEP) collider (CERN Courier October 2008 p9). However, the US raised the bid with a study for the “definitive machine”. The Superconducting Super Collider (SSC), which was strongly supported by the US Department of Energy and by President Reagan, would accelerate two proton beams to 20 TeV in a ring of 87 km circumference with 6.6 T superconducting dipoles. With this size and magnetic field, the SSC would require decisive advances in superconductors as well as in other technologies. When the then director-general of CERN, Herwig Schopper, attended a high-level official meeting in the US and asked what influence on the scientific and technical goals the Europeans could have by joining the project, he was told “none, either you join the project as it is or you are out”. This ended the possibility of collaboration and the competition began.
To compete with the SSC, the LHC had to fight on two fronts: increase the magnetic field as much as possible so as to reduce the handicap of the relatively small circumference of the LEP tunnel; and increase the luminosity as much as possible to compensate for the inevitable lower energy. In addition, CERN had to cope with a tunnel with a cross-section that was tiny for a hadron collider, which many considered a “poisoned gift” from LEP. However, the interest for young physicists and engineers lay in the “impossible challenges” that the LHC presented.
To begin with, there was the 8–10 T field in a dipole magnet. Such a large step with respect to the Tevatron would require both the use of large superconducting cable to carry 13 kA in operating conditions of 10 T – almost double the capability of existing technology – and cooling by superfluid helium at 1.8–1.9 K. Never previously used in accelerators, superfluid helium cooling had been developed for TORE Supra, the tokamak project led by Robert Aymar but on a smaller scale. Then, to fit the exiting LEP tunnel, the magnets would have to be of an innovative “two-in-one” design – first proposed by Brookhaven but discarded by US colleagues for the SSC – where two magnetic channels are hosted in the iron yoke within a single cold mass and cryostat. In this way, a 1 m diameter cryostat could house two magnets, while the geometry of the SSC (with separate magnets but with 30% lower field than the LHC) simply could not fit in the LHC tunnel. Figure 1 shows the various main-dipole cross-sections for the various hadron machines.
A critical milestone
In 1986 R&D on the LHC started under the leadership of Giorgio Brianti, quietly addressing the three issues specific to the LHC (high field, superfluid helium and two-in-one), while relying on the development done for HERA and especially for the SSC for almost all of the other items that needed to be improved. The high field was the critical issue and had to be tested immediately. Led by Romeo Perin and Daniel Leroy, CERN produced the first LHC coil layout and provided the first large superconducting cable to Ansaldo Componenti. This company then manufactured on its own a 1-m long dipole model – single bore, without a cryostat – that was tested at CERN in 1987. Reaching a field of 9 T at 1.8 K, it proved the possibility of reaching the region of 8–10 T (CERN Courier October 2008 p19). This was arguably the most critical milestone of the project because it gave credibility to the whole plan and began to lay doubt on the strategy for the SSC.
These results were obtained with niobium-titanium alloy (Nb-Ti), the workhorse of superconductivity. CERN had also a parallel development with niobium-tin (Nb3Sn) that could have produced a slightly higher field at 4.5 K, with standard helium cooling. This development, pursued with the Austrian company Elin and led by CERN’s Fred Asner, produced a 1-m long 9.8 T magnet and also a 10.1 T coil in mirror configuration, the first accelerator coil to break the 10 T wall. However, in 1990 the development work on Nb3Sn was stopped in favour of the much more advanced and practical Nb-Ti operating at 1.9 K. This was a difficult decision, as Nb3Sn had a greater potential than Nb-Ti and would avoid the difficulty of using superfluid helium, but it was vitally important to concentrate resources and to have a viable project in a short time. The decision was similar that taken by John Adams in the mid-1970s to abandon the emerging superconducting technology in favour of more robust resistive magnets for CERN’s Super Proton Synchrotron.
For the development of the superconducting cable there were three main issues. First, it should reach a sufficient critical current density with a uniformity of 5–10% over the whole production, which also had to be guaranteed in the ratio between the superconductor and the stabilizing copper matrix, illustrated in figure 2. The critical current was to be optimized at 11 T at 1.9 K, maximizing the gain when passing from 4.2 to 1.9 K. The second issue was to reduce the size of the superconducting filaments to 5 μm without compromising the critical current. This required, among other features, the development of a niobium barrier around Nb-Ti ingots. Third was to control the dynamic (ramping up) effect in a large cable, as some effects vary as the cube of the width.
Again, the strategy was to concentrate on specific LHC issues – the large cable, the critical current optimization at 1.9 K – and rely on the SSC’s more advanced development for the other issues. There is, indeed, a large debt that CERN owes to the SSC project for the superconductor development. However, when the SSC project was cancelled in 1993, the problem of eliminating the dynamic effect arising from the resistance between strands composing the cable was still unresolved – but it became urgent in view of the results on the first long magnets in 1994 and after. Later, CERN carried out intense R&D to find a solution suitable for mass production relatively late, at the end of the 1990s. This involved controlled oxidation, after cable formation, of the thin layer of tin-silver alloy with which all the copper/Nb-Ti strands were coated – a technology that was a step beyond the SSC development.
Returning to the magnet development, after the success of the 1987 model magnet, which was replicated by another single-bore magnet that CERN ordered from Ansaldo, the R&D route split into two branches. One concerned the manufacture of 1-m long full-field LHC dipole magnets to prove the concept of high fields in the two-in-one design, with superconducting cable and coil geometry close to the final ones. A few bare magnets, called Magnet Twin Aperture (MTA), were commissioned from European Industry (Ansaldo, Jeumont-Scheinder consortium, Elin, Holec) under the supervision of Leroy at CERN.
The second line of development lay in proving the two-in-one concept in long magnets and a superfluid-helium cryostat. This involved assembling superconducting coils from the HERA dipole production, which had ended in 1988, in a single cold mass and cryostat, the Twin Aperture Prototype (TAP). The magnet, under the supervision of Jos Vlogaert with the cryostat and cold-mass, under Philippe Lebrun, tested successfully in 1990, reaching 5.7 T at 4.2 K and 7.3–8.2 T at 1.8 K – thus supporting the choices of the two-in-one magnet design, of the superfluid helium cooling and the new cryostat design.
At the same time, the LHC dipole was designed in the years 1987–1990, featuring an extreme variation: the “twin” concept, where the two coil apertures are fully coupled, i.e. with no iron between the two magnetic channels (figure 3). We now take this design for granted, but at the time there was scepticism within the community (especially across the Atlantic); it was supposed to be much more vulnerable to perturbations because of the coupling and to present an irresolvable issue with field quality. It is to the great credit of the CERN management and especially Perin, who for a long time was head of the magnet group, that they defended this design with great resilience – because among many advantages it also made an important 15% saving in the cost.
The result of the first sets of twin 1-m long magnets came in 1991–1992. Some people were disappointed because they felt that the results fell short of the 10 T field “promised” in the LHC “pink book” of 1990. However, anyone who knows superconductivity greatly appreciated that the first generation of twins went well over 9 T. This was already a high field and only 5–10% less than expected from the so-called “short sample” (the theoretical maximum inferred by measuring the properties of a short 50–70 cm length of the superconducting cable); accelerator magnets normally work at 80%, or less, of the short-sample value. The results of the 1–m LHC models also made it clear that the cable’s mechanical and electrical characteristics and the field quality of the magnet (both during ramp and at the flat top) were not far from the quality required for the LHC.
A final step would be to combine the two branches of the development work and put together magnets of the twin design with a 10 m cold mass in a 1.8 K cryostat to demonstrate that full-size, LHC dipoles of the final design were feasible. However, the strict deadline imposed by the then director-general, Rubbia, dictated that the LHC should have the same time-scale as the SSC and be ready at the end of 1999. This meant that CERN was forced to launch the programme for the first full-size LHC prototypes in 1988, i.e. well before the end of the previous step, the construction in parallel of 1 m LHC MTA models and the 10-m long TAP.
At this point, CERN was just finishing construction of LEP and beginning work on industrialization of the components for LEP2; it was a period of shortage of personnel and financial resources (not a new situation). So Brianti and collaborators devised a new strategy: for the first time CERN would seek strong participation from national institutes in member states in the accelerator R&D and construction. In 1987–1988 the president of INFN, Nicola Cabibbo, and CERN’s director-general, Schopper, agreed – with a simple exchange of letters (everything was easier in those days) – that INFN would given an exceptional contribution to the LHC R&D phase. The total value was about SwFr12 million (1990 values) to be spread over eight years.
Towards real prototypes
In 1988 and 1989, INFN and CERN ordered LHC-type superconducting cables for long magnets and in 1989 INFN ordered two 10-m long twin dipoles from Ansaldo Componenti in Italy, some nine months before CERN had the budget to order three long dipoles, one from Ansaldo and two from Noell, a German company that had been involved in the construction of HERA quadrupoles. A fourth CERN long magnet, without the twin design, was ordered from the newly formed Alstom-Jeumont consortium (even at CERN some people still doubted the effectiveness of the twin design). The effort was decisive in being able by 1993 to have the magnets qualified by individual tests and then put into a string, consisting of dipoles and quadrupoles connected in series to simulate the behaviour of a basic LHC cell.
Parallel to the INFN effort, the French CEA-Saclay institute established collaboration with CERN and took over the construction of the first two full-size superconducting quadrupoles for the LHC. While CERN provided specifications and all of the magnet components (including superconducting cable), CEA did the full design and assembly of these quadrupoles, for a value a few million Swiss francs over the eight years of R&D (CERN Courier January/February 2007 p25). This was the start of a long collaboration; the French also continued to support the project after the initial R&D, throughout industrialization and construction phases, with an in-kind contribution on quadrupoles, cryostats and cryogenics for about SwFr50 million (split between CEA and CNRS-IN2P3).
The challenge of the prototyping was hard and covered many aspects. In particular for the dipoles, CERN first had to convince industry to pay enough attention and to invest resources in the LHC; the allure of the SSC, a much larger project (6000 main dipoles of 15 m length, 2000 quadrupoles, etc), was difficult to ignore. CERN’s project was much more challenging technically, with the required accuracy of the tooling a factor of five or so higher than for the HERA magnets. There was also the usual fight in a prototyping phase: good results required building expensive tooling for one or two magnets, with insufficient budget and no certainty that the project would be approved and the tooling cost thus paid for.
A delay of one year was the price to pay for the many developments and adjustments. Meanwhile, in 1993 the project had to pass a tough review devoted to the cryo-magnet system led by Robert Aymar, who as CERN’s director-general 10 years later would collect the fruit of the review. With the review over and completion of the long magnet prototypes approaching, the credibility of the LHC project increased. In autumn 1993, the SSC came to a halt – certainly because of high and increasing cost (more than $12 billion) and the low economic cycle in the US, but also because the LHC now seemed a credible alternative to reach similar goals at a much lower cost ($2 billion in CERN accounting). Rubbia, near the end of his mandate as director-general, which was the most critical for the R&D phase, led the project without rival. In a symbolic coincidence, the demise of the SSC occurred at the same as leadership of the LHC project passed from Giorgio Brianti, who had led the project firmly from its birth through the years of uncertainty, to Lyn Evans, who was to be in charge until completion 15 years later. The end of the SSC and the green light for the LHC was marked by the delivery to CERN of the first INFN dipole magnet in December 1993, just in time to be shown to the Council. This was followed four months later by the second INFN magnet and then by the CERN magnets, as well as by the two CEA quadrupoles designed and built by the team of Jacques Perot and later Jean-Michel Rifflet (figure 4).
Returning to the first dipole, which had been delivered from INFN at the end of 1993, a crash programme was necessary to overcome an unexpected problem (a short circuit in the busbar system – a problem that in a different form would later plague the project), so as to test it by in time for a special April session of the Council in 1994. The magnet passed with flying colours, going above the operational field of 8.4 T at the first quench, beyond 9 T in two quenches, and a first quench above 9 T after a thermal cycle i.e. full memory (figure 5). Its better-than-expected performance was actually misleading, giving the idea that construction of the LHC might be easy; in fact, it took a long six years before another equally good magnet was again on the CERN test bench. However, the other 10-m long magnets performed reasonably well and with the two very good CEA quadrupoles (3.5 m long), CERN set up the first LHC magnet string, to test it thoroughly and finally receive the approval of the project in December 1994.
Many other formidable challenges were still to be resolved on the technical, managerial and financial sides. These included: the nonuniformity of quench results and the problem of retraining that plagued the second generation of LHC prototypes; the unresolved question of the inter-strand resistance; the change of aluminium to austenitic steel as the material for the collars, implemented by Carlo Wyss; and the lengthening of the magnets from 10 m to 15 m with the consequent curvature of the cold mass, etc.
Looking back at the period 1985–1994, when the base for the LHC was established, it is clear that a big leap forward was accomplished during those years. The vision initiated by Robert Wilson for the Tevatron was brought to a peak, pushing the limit of Nb-Ti to its extreme on a large scale. New superconducting cables, new superconducting magnet architectures and new cooling schemes were put to the test, in the constant search for economic solutions that would be applicable later to large scale production. This last point is an important heritage that the LHC leaves to the world of superconductivity: the best performing solution is not always going to be really the best. Economics and large-scale production are very important when a magnet is part of a large system and integration is critical. “The best is the enemy of the good” has been the guiding maxim of the LHC project – a lesson from the LHC for the world of superconductivity in this 100th anniversary year.
The OPERA experiment in Italy’s INFN Gran Sasso Laboratory has sent ripples round the world with its findings that neutrinos created 730 km away at CERN arrive at the detector slightly earlier than if they were travelling at the speed of light.
The result is based on the observation of more than 15,000 neutrino events measured by the experiment, which observes the beam produced by the CERN Neutrinos to Gran Sasso (CNGS) project. Using high-statistics data taken in 2009, 2010 and 2011, the collaboration has measured the velocity of the muon-neutrinos reaching the detector with much higher accuracy than previous studies conducted using accelerator neutrinos. Upgrades to the CNGS timing system and to the OPERA detector, as well as the use of high-precision geodesy to measure the neutrino baseline, allowed the collaboration to achieve comparable systematic and statistical accuracies.
To perform the study, the OPERA collaboration teamed up with experts in metrology from CERN and other institutions to make a series of high-precision measurements of the distance between the source and the detector, and of the neutrinos’ time of flight. The distance between the origin of the neutrino beam and OPERA was measured with an uncertainty of 20 cm over the 730 km travel path. The neutrinos’ flight time was determined with an accuracy of less than 10 ns by using sophisticated instruments, including advanced GPS systems and atomic clocks. The time responses of all of the elements of the CNGS beamline and of the OPERA detector have also been measured with great precision.
The results indicate that neutrinos from CERN arrive early at Gran Sasso by 60.7 ± 6.9 (stat.) ± 7.4 (sys.) ns compared with the time that would be taken assuming the speed of light in vacuum. This anomaly corresponds to a relative difference of the muon-neutrino velocity, v, with respect to the speed of light, c, (v-c)/c = (2.48 ± 0.28 (stat.) ± 0.30 (sys.) × 10–5.
Given the potential far-reaching consequences of such a result, independent measurements are certainly needed before the effect can either be refuted or firmly established. While OPERA continues to gather more data, the MINOS collaboration in the US is planning to improve its measurement of the neutrino time of flight with the beam from Fermilab to the Soudan Underground Laboratory, about 730 km away.
For several years the European gravitational-wave detectors GEO600 (a collaboration between Germany and the UK), close to Hanover, and Virgo (a collaboration between Italy, France, the Netherlands, Poland and Hungary), close to Pisa, have been performing data-taking runs together with the LIGO detectors in the US. About a year ago the LIGO collaboration turned off its detectors to start an important upgrade, so this summer the European detectors joined forces to step up their search for gravitational waves in a last three-month data-taking run before Virgo also shuts down for its own upgrade.
GEO600 and Virgo had the good fortune to be on with an impressive 82% duty cycle at the time of the recent nearby supernova explosion. Unfortunately, the event on 24 August was too far away and of Type 1a, so releasing only a small amount of energy as gravitational waves. Analysis is nevertheless continuing at full speed.
These detectors are kilometre-scale Michelson laser-interferometers that work by measuring tiny changes caused by a passing gravitational wave in the lengths of their orthogonal arms. Laser beams sent down the arms are reflected from mirrors, suspended under vacuum at the ends of the arms, to a central photodetector. The periodic stretching and shrinking of the arms is then recorded as varying interference patterns.
The worldwide detector upgrades that are just starting will be a fundamental step forward. With current sensitivities, the probability of detecting a gravitational-wave burst in one full year of data-taking is estimated to be of the order of a few per cent. The upgrades aim to improve the sensitivities by a factor of 10 with respect to the present values, which should then extend the “listening” distance by a factor of 10. This will increase the volume of universe explored and the detection probability by a factor of 1000, offering the “certainty” of catching several gravitational-wave events a year.
The non-detection of gravitational waves so far has nevertheless allowed the derivation of several important scientific results. For example, important limits have been established on the production of gravitational waves of cosmological origin and by known pulsars. Improving the spin-down limit of the Crab and Vela pulsars should put limits on the ellipticity of the stellar mass-distributions, which are expected to be related to the magnetic asymmetries in these systems.
“Multimessenger” astrophysics has meanwhile begun, looking for coincidences of candidate gravitational-wave signals with gamma-ray bursts and signals from space-borne cosmic-ray detectors as well as neutrino and optical telescopes. Such clues will have paramount importance in studying the sources as soon as genuine gravitational-wave detection becomes routine after 2015, when detector upgrades are expected to be completed.
Hadronic bound systems with strange quarks, such as kaonic hydrogen, are well suited for testing chiral dynamics, especially in view of the interplay between spontaneous and explicit symmetry breaking. Effective field theories with coupled channels based on chiral meson–baryon Lagrangians have become well established as a framework for describing K–nucleon interactions at threshold, including much disputed Λ(1405) resonances and deeply bound antikaonic nuclear clusters lying just below the respective thresholds.
A recent precision measurement at the Laboratori Nazionali di Frascati of the strong-interaction-induced shift and width of the 1s level in kaonic hydrogen sheds new light on these basic problems in strong-interaction binding and dynamics. Kaonic hydrogen, in which a K replaces the electron, is produced by the capture of stopped K from the decay of φ mesons in hydrogen gas. The φ mesons are generated nearly at rest at the DAΦNE e+e– collider, operating in a new, high-luminosity collision mode.
The shift and width of the kaonic 1s state is deduced from precision X-ray spectroscopy of the K-series transitions in the kaonic hydrogen. The emitted K-series X-rays, with energies of 6–9 keV, were detected by the recently developed Silicon Drift Detector for Hadronic Atom Research by Timing Application (SIDDHARTA) experiment, which performs X-ray–kaon coincidence spectroscopy using microsecond timing and the excellent energy resolution of about 180 eV FWHM at 6 keV of 144 large-area (1 cm²) silicon drift detectors that surround the hydrogen target cell. This method reduces the large X-ray background from beam losses by orders of magnitude. It has led to the most precise values for the 1s level shift, ε1s= –283 ± 36 (stat.) ± 6 (syst.) eV, and width Γ1s = 541 ± 89 (stat.) ± 22 (syst.) eV for kaonic hydrogen (Bazzi et al. 2011).
A recent study using next-to-leading-order chiral dynamics calculations of the shift and the width has shown excellent agreement with these measurements (Ikeda et al. 2011). Further measurements with similar accuracy are planned for the K-series X-rays from kaonic deuterium, using an improved SIDDHARTA-2 set-up to disentangle the isoscalar and isovector scattering lengths.
While searches with 2011 LHC data for the Higgs and new physics caught the headlines over the summer, detailed studies of 2010 data continue to yield high-precision physics. For example, the ATLAS collaboration has published a number of results on the production of vector bosons (W and Z) based on the full 2010 dataset of 37 pb–1, including measurements that require additional jets in the final state.
The challenge of precision measurements of Standard Model vector-boson production is to understand and control the systematic uncertainties; this contrasts with many analyses that are still dominated by statistical uncertainties and can thus “simply” wait for more data. This challenge will increase in analyses of the larger 2011 data set, where ATLAS will probe higher jet-multiplicities and higher jet transverse-momenta. In addition to precise measurements of electroweak parameters, the study of W and Z bosons at the LHC tests perturbative QCD (pQCD) and it constrains the distribution of partons (quarks and gluons) inside the proton. W and Z bosons are also studied as background to other Standard Model signals and to look for new physics.
Two recent ATLAS results have focused on the production of a vector boson together with jets from b-quarks. The Z measurement is still statistically limited, while the W measurement is dominated by systematic uncertainties. The cross-section for inclusive Z + b-jets production agrees with next-to-leading-order pQCD calculations. For the production cross-section of a W with one or two b-jets, the results are again consistent within uncertainties, although the value observed is slightly higher than predicted (Fig. 1). These measurements with b-jets not only test pQCD for heavy quarks, they also assess what is a significant background source in searches, for example for associated Higgs production, where H→bb.
Considering the ratio of cross-sections rather than their absolute value has the advantage that many sources of systematic uncertainty cancel. ATLAS has recently published a measurement of the ratio of W and Z cross-sections with exactly one associated jet, complementing measurements of the individual channels. The ratio is measured as a function of the jet transverse-momentum. The systematic and statistical uncertainties are of comparable size, thereby providing the basis for a precision test of the Standard Model (Fig. 2). The results are in reasonably good agreement with a number of Monte Carlo predictions.
While primarily designed for proton–proton collisions, the ATLAS detector is also an excellent tool to perform measurements in the hot, dense environment of heavy-ion collisions, where temperatures reach tera-kelvin scales. So far, results include detailed measurements of collective properties of the system, such as “elliptic flow”, as well as of “hard probes”, such as jets, quarkonia and vector bosons.
Using the initial 2010 heavy-ion collision data from the LHC, the ATLAS collaboration published the first direct evidence that jets lose energy as they pass through the hot, dense medium, a process called jet “quenching”, leading to event-by-event asymmetries in the energies of the two jets. To characterize the effects of quenching from a different perspective, the next major jet measurement in lead–lead collisions undertaken by ATLAS was to establish the overall reduction in the rate of jets in more “central” collisions, where the two nuclei overlap more completely.
For the Quark Matter 2011 conference, ATLAS compared the rates for central events with those in more peripheral events that consist primarily of a few simultaneous nucleon–nucleon collisions. One surprising result is that, for jets above 100 GeV, the measured jet-suppression factor is independent of the measured jet energy. An even more surprising finding is that this result is the same for jets reconstructed with different “cone” radii, implying that the suppression is not accompanied by a substantial modification of the distribution of energy within a jet. By contrast, an ATLAS measurement of W boson yields using single muons showed no suppression at all.
This comparison, shown in the figure, was quantified using the variable RCP, the ratio of yields measured in central and peripheral collisions, each yield normalized by the relevant number of binary collisions. This quantity is unity if jets are produced in proportion to the number of binary collisions, but falls below one if the yields are suppressed in more central collisions.
The higher luminosities expected in 2011 will provide increased jet statistics, allowing the measurement of jets with even higher energies. At the same time, a more precise understanding of the fluctuations of soft particles, mainly from a rich spectrum of collective modes, will allow the measurement of lower-energy jets, which in preliminary results from the Relativistic Heavy Ion Collider show stronger modification from passage through the medium.
About a year ago, the CMS collaboration released its first publication on studies of the top quark – the measurement of the tt production cross-section at 7 TeV. The measurement was based on a data set of only 3 pb–1 of integrated luminosity and the top quarks were identified through the leptonic decay channels of the W boson. Now, a plethora of results on the top quark based on luminosities of 1–2 fb–1 have been released for the summer conferences, in particular for the TOP2011 workshop, held at the end of September at Sant Feliu de Guixol, Spain.
Top quarks decay almost exclusively into a W boson and a b-flavoured quark jet, leading to different event final states that can be used for selecting tops. Figure 1 gives an overview of the CMS results, which use more or less all of the decay modes. The most precise single measurement is the analysis where one W boson decays into leptons while the second W decays into hadrons and b-quark identification is used, giving a cross-section of 164.4 ± 14.3 pb, i.e. a precision of 8.5%. Precise measurements of the cross-section can also be converted into measurements of the top quark’s mass, within a given theoretical scheme. Currently, the CMS cross-section measurements allow for a precision on the top mass of about 7–8 GeV in such data extractions.
Further new analyses include a measurement of the difference in mass of the t and t, which is an interesting test of CPT invariance. For this study, data are used where one of the W bosons decays into a muon, allowing the event to be classified as t or t decay, depending on the charge of the muon. The difference in mass between the t and t is found to be 1.2±1.3 GeV, i.e. the result is compatible with equal mass within the uncertainty. This is the most precise result on this quantity to date.
Another interesting measurement concerns the charge asymmetry in top production. The experiments at Fermilab’s Tevatron reported asymmetries that are larger than expected. At the LHC, tt production is also slightly asymmetric in rapidity as a result of the different roles that the valence and sea quarks play in the production. CMS has studied this asymmetry by measuring the different widths of the rapidity distribution for t and t. The result gives an asymmetry of 1.6% with an uncertainty of about 3.5%; an asymmetry of about 1.3% is expected from theory. The agreement with the Standard Model is good within the measured uncertainties.
Finally, a challenging new measurement on the electroweak production of single top has been undertaken, namely tW associated production. While single top production in the top-quark channel was reported by the LHC experiments earlier this year, this measurement analyses a different final state; also, this channel is not accessible at the Tevatron. CMS finds an excess over expected background events with a significance of 2.7 σ, and is compatible with the expectation for tW production.
With several tens of thousands of top-quark pairs recorded so far, the detailed study of the properties of the heaviest quark is merely starting. Results based on the full 2011 data sample should be ready in time for the 2012 winter conferences.
The Daya Bay Reactor Neutrino Experiment has begun its quest to answer some of the puzzling questions that still remain about neutrinos. The experiment’s first completed set of twin detectors is now recording interactions of antineutrinos as they travel away from the powerful reactors of the China Guangdong Nuclear Power Group, in southern China.
The start-up of the Daya Bay experiment marks the first step in the international effort of the Daya Bay collaboration to measure a crucial quantity related to the third type of oscillation, in which the electron-neutrinos morph into the other two flavours of neutrino. This transformation occurs through the least known neutrino-mixing angle, θ13, and could reveal clues leading to an understanding of why matter predominates over antimatter in the universe.
The experiment is well positioned for a precise measurement of the poorly known value of θ13 because it is close to some of the world’s most powerful nuclear reactors – the Daya Bay and Ling Ao nuclear power reactors, located 55 km from Hong Kong – and it will take data from a total of eight large, virtually identical detectors in three experimental halls deep under the adjacent mountains. Experimental Hall 1, a third of a kilometre from the twin Daya Bay reactors, is the first to start operating. Hall 2, about a half kilometre from the Ling Ao reactors, will come online in the autumn. Hall 3, the furthest hall, about 2 km from the reactors, will be ready to take data in the summer of 2012.
The Daya Bay experiment is a “disappearance” experiment. The detectors in the two closest halls will measure the flux of electron-antineutrinos from the reactors; the detectors at the far hall will look for a depletion in the expected antineutrino flux. The cylindrical antineutrino detectors are filled with liquid scintillator, while sensitive photomultiplier tubes line the detector walls, ready to amplify and record the telltale flashes of light produced by the rare antineutrino interactions. As a result of the large flux of antineutrinos from the reactors, the twin detectors in each hall will capture more than 1000 interactions a day, while at their greater distance the four detectors in the far hall will measure only a few hundred interactions a day. To measure θ13, the experiment records the precise difference in flux and energy distribution between the near and far detectors.
The experimental halls are deep under the mountain to shield the detectors from cosmic rays and the detectors themselves are submerged in pools of water to shield them from radioactive decays in the surrounding rock. Energetic cosmic rays that make it through the shielding are tracked by photomultiplier tubes in the walls of the water pool and muon trackers in the roof over the pool so that events of this kind can be rejected.
After two to three years of collecting data with all eight detectors, the Daya Bay Reactor Neutrino Experiment should be well positioned to meet its goal of measuring the electron-neutrino oscillation amplitude – and hence sin2 2θ13 – with a sensitivity of 1%.
The start up of the experiment begins after eight years of effort – four years of planning and four years of construction – by hundreds of physicists and engineers from around the globe. China and the US lead the Daya Bay collaboration, which also includes participants from Russia, the Czech Republic, Hong Kong and Taiwan. The Chinese effort is led by project manager Yifang Wang of the Institute of High Energy Physics (IHEP), Beijing, and the US effort is led by project manager Bill Edwards of Lawrence Berkeley National Laboratory and chief scientist Steve Kettell of Brookhaven National Laboratory.
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