Work for the LEP electron–positron collider continues to drive ahead, however LEP is far from being the last word in CERN’s long term plans. A clue was already in the LEP Design Study ” …by the adoption of a beam height of only 80 cm, there is enough room left (in the tunnel) for the installation of a second machine at a later stage…”.
A workshop, organized by ECFA and CERN in March 1984, examined the feasibility of a hadron collider in the LEP tunnel (Lausanne LHC workshop). There the idea emerged for a ring of superconducting magnets, installed above the LEP ring, to collide protons together (or protons with antiprotons) at as high an energy as possible. Since this meeting, considerably more work has been done to firm up ideas.
Using 10 Tesla dipole bending magnets, collision energies of 17 TeV (8500 GeV per beam) could be achieved with a respectable collision rate (luminosity 1033 cm–2 s–1). A ‘two-in-one’ aperture solution for the superconducting magnets is recommended for economy and compactness.
It is the relative ease of colliding proton beams (as compared to the difficulties of first making and then handling antiprotons) which promise high collision rates and make the proton–proton option the preferred solution. Despite the need to provide a large number of bunches (a figure of 3564 has been quoted), the two proton rings in the LEP tunnel could be filled using CERN’s existing 450 GeV SPS machine and its proton supply in only a few minutes. Of course new injection lines would have to be built.
• July/August 1986 pp5–4 (abridged).
Elsewhere
In Europe the news of the initial approval for the US Superconducting Supercollider was received enthusiastically as it showed that the future of high-energy physics is regarded as being of paramount importance at the highest levels. While the US plans gather momentum, the possibility of a hadron ring in the LEP tunnel at CERN is still attractive. Although restricted in energy by the ‘modest’ dimensions of the LEP tunnel compared to the SSC (27 km circumference against 84), the LHC scheme scores points for the magnificent beam injection systems already in place at CERN, a complete tunnel, and several collision options.
• March 1987 p2 (abridged).
Superconducting magnet success
Technical preparations for a possible proton–proton collider (LHC) in the LEP tunnel have made substantial progress with the successful testing of the first LHC superconducting high-field 1 m long model magnet. The single aperture niobium-titanium wound dipole was designed by R Perin and his LHC magnet study team, and manufactured by Ansaldo Componenti, Genova.
Operating at 2 K, it reached and passed its 8 Tesla nominal field without any quench, the first three quenches occurring at central fields of 8.55, 8.9 and 9 Tesla respectively. It then attained 9.1 Tesla without quenching and operated at this level for some time.
This is the first time a high field ‘accelerator quality’ superconducting dipole model has been designed and built as a joint venture between a scientific laboratory and industry. CERN provided most of the know-how and the superconductor, while manufacture was taken over by Ansaldo.
The superconducting proton ring being built for the HERA electron–proton collider at DESY has already demonstrated that niobium-titanium technology is mature, even on an industrial scale. The HERA-type design (coils around the beam-pipe, mechanical support collars and cold iron return) has gone on to become widely adopted, but reaches its natural limit for dipole construction using niobium-titanium near 10 Tesla.
This is now well understood and has been demonstrated with several test magnets developed in a collaboration between CERN and Italian supplier Ansaldo. A similar geometry was used with niobium-tin in a collaboration between CERN and Elin (Austria) which reached a record field for this kind of magnet of 9.45 Tesla in September 1989.
CERN’s proposed LHC collider in the LEP tunnel envisages 10 T fields with a double aperture carrying the two beam pipes for the proton beams inside a single cryostat. Four contracts have been placed with European firms for the development of one-metre, double-aperture niobium-titanium magnets with a view to placing further orders for full-scale, 10 m prototype units. Using superfluid helium at 1.8 K instead of conventional 4.2 K cryogenics provides the necessary additional potential.
Two research teams at Michigan State University’s National Superconducting Cyclotron Laboratory (NSCL) have reported fresh findings about neutron-rich nuclei. In separate experiments, groups measured a critical energy gap in oxygen nuclei and achieved their first-ever success using a new technique for finding isomers.
One important area of study with these nuclei focuses on the neutron drip line – the limit in the number of neutrons (N) that can bind to a given number of protons. For oxygen, that line was known to lie at 16 neutrons, and indeed indicated a new shell closure at N=16 in neutron-rich nuclei. However, theoretical calculations disagreed on the difference in binding energy between 24O, with a closed shell of 16 neutrons, and 25O, the first isotope beyond the drip line – in other words, the binding energy of the 17th neutron.
Calem Hoffman from Florida State University and colleagues have now pinpointed this quantity. The group used the NSCL’s coupled cyclotrons to accelerate a beam of 26F onto a fixed target, where they observed 25O for the first time. The 25O decays too quickly for direct detection, but the group was able instead to track its decay products: 24O and a single free neutron, measured with the Modular Neutron Array. The team then used the angles, energies and momenta of the decay products to calculate the mass of the 25O, which in turn allowed them to infer the difference in binding energy from 24O, and ultimately the N=16 shell gap, which they find to be 4.86(13) MeV (Hoffman et al. 2008).
The second experiment, conducted by NSCL’s Georg Bollen and colleagues, focused on nuclear isomers, in which neutrons are excited to a higher-energy arrangement for anywhere from fractions of a second to years. The team has discovered a previously unknown isomer of 65Fe, a nucleus that is intriguing for its proximity in terms of proton and neutron numbers to 68Ni, a particularly enigmatic isotope. 68Ni displays some characteristics of doubly magic nuclei, but nuclei with slightly fewer protons and neutrons than 68Ni reveal pronounced changes in structure – which generally is not the case for isotopes near others that are doubly magic. Researchers have little idea what is happening in this nuclear region, and so are keen to make more measurements.
These nuclei are a target for the Low Energy Beam and Ion Trap (LEBIT), which experimenters at NSCL use to collect high-speed products of cyclotron-spawned collisions. After firing a beam of germanium nuclei into a thin target, Bollen’s team captured the products in LEBIT and directed them into a Penning trap, allowing them to make very precise mass measurements of the particles caught. The team measured two distinct masses for 65Fe, indicating nuclei with different energy states – one the ground state and one a novel isomer at an excitation energy of 402(5) keV (Block et al. 2008) This is the first use of Penning trap mass spectrometry of this kind. Previous isomer studies have instead employed gamma-ray spectroscopy.
In recent years neutrinos have moved onto centre stage in both astrophysics and particle physics, and the latest developments were on show at the XXIII International Conference on Neutrino Physics and Astrophysics on 26–31 May. Supported by the International Union of Pure and Applied Physics, Neutrino 2008 took place in Christchurch, New Zealand, where it was organized by the University of Canterbury and the IceCube collaboration, which uses Christ church as its staging area and gateway to Antarctica. Conference-goers celebrated the 100th anniversary of the award of the Nobel Prize to a former undergraduate of the University of Canterbury, Ernest Rutherford, whose life was the topic of the opening presentation by Cecilia Jarlskog from Lund.
The question “Where are we?” is beloved of neutrino physicists. Alexei Smirnov of the Abdus Salam International Centre for Theoretical Physics in Trieste noted that a quarter of the papers found on the SPIRES high-energy physics database with this title are in neutrino physics. With the discoveries of neutrino masses and lepton-flavour mixing now established, there is a standard neutrino scenario in which neutrinos have masses in the sub-electron-volt range and there are two large mixings and one small or zero mixing between the three neutrino flavours. Neutrino experiments have moved into an era of precision measurements, motivated by the belief that neutrino mass and mixing are manifestations of physics beyond the Standard Model. However, as Smirnov noted, despite many years of effort and many trials, the physics underlying neutrino mass and mixing remains unidentified.
Roadmap of theoretical possibilities
Understanding neutrinos is a two-step process. The first step is to determine the values of the three mixing angles, the masses of the three mass eigenstates, and the value of the CP-violating phase. It is also necessary to find out whether the neutrino is its own antiparticle, that is whether it is as described by the physics of Paul Dirac or of Ettore Majorana. The second step is to try to understand why the neutrino matrix elements and the neutrino masses are what they are and what they tell us about physics well beyond the Standard Model. Stephen King from Southampton presented a roadmap of theoretical possibilities, including extra dimensions and possible grand unified theories, with each theoretical path linked to future experimental results.
Two of the mixing angles are now well determined: one through the solar-neutrino experiments and the other through the atmospheric- and accelerator-neutrino studies. The third angle, θ13, is much less constrained but is no less important because it determines how close the mixing matrix is to the theoretically interesting, highly symmetric “tribimaximal” configuration. The best limits on θ13 are currently from the Double Chooz experiment. If θ13 is large enough, it may be possible to observe CP violation with neutrinos, and Yosef Nir from the Weizmann Institute explained how a large value for the CP-violating parameter, δ, could explain the observed baryon asymmetry in the universe via the process called leptogenesis.
Speakers from solar-neutrino experiments were the first to present their results, beginning with reports from the Borexino detector located at Gran Sasso National Laboratory in Italy, and from the third and final phase of the Sudbury Neutrino Observatory (SNO) in Canada. SNO’s third phase included 3He proportional counters to measure the rate of neutral-current interactions in the detector’s heavy water. The Borexino experiment has results from 192 days of data taking and, as with earlier solar-neutrino measurements, these are best described by neutrino-flavour oscillation. The electron-neutrino flavour eigenstate, to a good approximation, is a linear combination of two mass eigenstates with masses m1 and m2. Neutrinos from the same energy range but at a much shorter baseline are detected by the KamLAND experiment in Japan, which observes antineutrinos from nuclear reactors. A combined analysis of the solar and KamLAND data now gives precise results for the mixing angle, Δ12, and mass difference Δm122, of the two mass eigenstates. The result of analysis with two flavours gives Δ12 = 33.8 + 1.4 –1.3 ° and Δm122 = 7.94 + 0.42 – 0.26 × 10–5 eV2.
The Super-Kamiokande experiment in Japan is now fully recovered from the accident in 2001, which destroyed around half of the original photomultiplier tubes. It has provided a high-precision measurement of neutrino oscillations by detecting atmospheric neutrinos in an energy range of hundreds of millions of electron-volts to a few tera-electron-volts. Jennifer Raaf from Boston gave the results from a combined analysis of the pre-accident and post-accident data taking. These include a mixing angle with sin22θ23 > 0.94 at 90% confidence, which is the best constraint so far obtained for this parameter. The experiment also places limits on non-oscillation physics, such as neutrino decoherence, which is excluded at 5.0 σ, and neutrino decay, which is excluded at 4.1 σ.
Neutrino beams produced at particle accelerators offer the greatest control over the neutrino sources. They have been used to study the same neutrino oscillations that take place in atmospheric neutrino oscillation. The KEK-to-Kamioka (K2K) experiment was the first long-baseline neutrino experiment to operate, using neutrinos sent from the KEK laboratory to the Super-Kamiokande detector 250 km away. The K2K collaboration has previously reported results consistent with the Super-Kamiokande atmospheric neutrino results using data collected between 1999 and 2004. At the conference Hugh Gallagher from Tufts University presented new results from the Main Injector Neutron Oscillation Search (MINOS) experiment. This uses a muon–neutrino beam that is produced at Fermilab and observed at two sites: a near detector at Fermilab and a far detector 734 km away at the Soudan Underground Laboratory in Minnesota. MINOS now has the tightest constraint on the mass difference, finding Δm232 = 2.43 ±0.13 × 10–3 eV–2 and a result for the mixing angle that is consistent with that for Super-Kamiokande.
The conference also heard reports on future experiments that aim to measure θ13. These include the reactor-neutrino experiments Double Chooz in France, Daya Bay in China and the Reactor Experiment for Neutrino Oscillation at Yonggwang in Korea, as well as the accelerator-neutrino experiments T2K, OPERA at the Gran Sasso National Laboratory, and NOvA at Fermilab.
Many efforts are under way to determine directly the absolute neutrino mass scale in laboratory experiments through nuclear beta-decay or neutrinoless double beta-decay, which is possible if the neutrino is Majorana. Beta-decay experiments can be categorized by the detector type and there were reviews of tracking, solid-state, calorimetric and scintillator detectors, with energy resolution being the crucial common ingredient. The neutrino mass scale can also be probed through cosmology; the relic neutrino density influences the evolution of large-scale structure in the universe. Richard Easther from Yale presented the latest results obtained by combining cosmic microwave background and supernova observations. The best fit constrains the mass sum from all neutrino flavours to be less than 1 eV, with better precision obtainable if the Hubble constant is known independently.
Neutrinos also probe a range of physical processes, from the heat source of the Earth to the location of high-energy cosmic accelerators. Bill McDonough of Maryland discussed how the detection of geoneutrinos can put limits on the amount of heat generated by uranium and thorium inside the Earth. KamLAND has already placed limits on this but is restricted by the background from reactor neutrinos. The next step may be the Hawaii Anti-Neutrino Observatory, HANOHANO – a proposed 10 kilotonne liquid scintillation detector designed to be transportable and deployable in the deep ocean. Its goal is to measure the neutrino flux from the Earth’s mantle for the first time.
Cosmic neutrinos may also unveil the very high-energy, cosmic-ray accelerators. Unlike photons or charged particles, neutrinos can emerge from deep inside their sources and travel across the universe uninterrupted. Julia Becker of Gothenberg University discussed some potential sources of cosmic neutrinos, including some of the most energetic objects in the universe, such as supernova remnants, microquasars and active galactic nuclei. To date, no experiment has observed extraterrestrial high-energy neutrinos, but cubic-kilometre telescopes (e.g. KM3Net, which is planned for the Mediterranean, and IceCube, under construction at the South Pole) are expected to be large enough to observe these cosmic neutrinos. Spencer Klein from the Lawrence Berkeley National Laboratory gave an update on the IceCube neutrino observatory, which uses the ice at the South Pole as a Cherenkov medium for the detection of high-energy neutrinos. The observatory comprises an in-ice, three-dimensional array of photomultiplier tubes and a surface air shower array. In February, half of the detector had been deployed, bringing the instrumented volume to roughly 0.5 km3.
Although the field of neutrino physics has moved into a precision era, many puzzles remain and there is still much to be explained. A number of experiments are anticipating new results in the near future, so we can look forward to the next Neutrino conference, to be held in Athens in 2010.
This spring the XVI International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS 2008) took place at University College London (UCL), and was jointly organized by the high-energy particle physics groups of the University of Oxford and UCL. Some 300 participants attended the workshop, which was held on 7–11 April and consisted of approximately 270 talks covering a multitude of subjects.
The provost and president of UCL, Malcolm Grant, opened the first day, which consisted mainly of plenary talks, with speakers detailing recent experimental and theoretical highlights, and looking at future developments in the field of deep-inelastic scattering (DIS), QCD and collider physics. The opening plenary speakers greatly helped to set the tone of the meeting with excellent overviews and positive outlooks. In the late afternoon, the workshop split into working groups with specialized talks, with up to six groups in parallel at any one time.
The parallel sessions covered a range of subjects, including structure functions and low-x; diffraction and vector mesons; electroweak measurements and physics beyond the Standard Model; hadronic final states and QCD; heavy flavours; spin physics; and future facilities. There were many excellent presentations, including high-quality results from both experiment and theory, together with extensive discussions. The parallel sessions continued throughout the next two days, culminating with a packed additional session organised by Hannes Jung from DESY on “What HERA can still provide”. That so many people were prepared to forego an evening meal to participate in an extra session at the end of a busy day demonstrates the unique legacy of HERA, the world’s first and only electron–proton collider, which ceased operation at DESY in June 2007. On the afternoon and morning of the final two days, the convenors of the working groups reported on the highlights of their sessions. Finally, Brian Foster of the University of Oxford beautifully summarized the whole workshop, again highlighting the vitality of both the field and the workshop.
Work on the structure of the proton – the main subject of the DIS workshop series – has seen tremendous advances recently. The H1 and ZEUS collaborations have made the first measurements of the longitudinal structure function, FL, and have combined data on inclusive DIS cross sections from the HERA I run in a preliminary HERA fit of the parton density functions. The quantity FL is an integral part of the description of the proton’s structure and is directly sensitive to the gluon density and the QCD evolution with momentum transfer. Both collaborations have measured FL using two special low-energy proton runs taken at the end of HERA data taking. While the data are consistent with QCD predictions of the parton densities, which are based on fits to the inclusive measurements of F2, they cannot yet distinguish between different predictions, although significant improvements to the measurements are expected.
Taking advantage of the different detectors and their systematics, the combination of the F2 measurements from H1 and ZEUS has produced results that are significantly more precise than the simple effect of doubling statistics. The effective “cross calibration” has led to uncertainties of 1–2% over a wide range in Bjorken-x and in photon virtuality, Q2. The combined HERA data alone have in turn been used in a fit of the parton distributions in the proton and this leads to results that are competitive with global fits that use data from many different sources (see figure). Data from the Tevatron at Fermilab are also placing strong constraints on the structure of the proton. Results on the charge asymmetry of the W particle from the CDF experiment have a precision that is significantly better than the uncertainties on the parton distribution functions. Additionally, inclusive-jet cross sections from the D0 experiment yield constraints at the highest scales, up to 600 GeV. They also provide a wonderful verification of QCD predictions across 10 orders of magnitude in the cross section, differential in jet pT and rapidity.
All of the above results are crucial inputs to our understanding of QCD, and in particular the structure of the proton, which is needed as the starting point for most of the physics at CERN’s LHC. Along with the new measurements, theory is keeping pace with a number of advances that are either already made or planned. With the recent development of next-to-next-to-leading order QCD corrections (NNLO) for F2, groups are working on the implementation of NNLO for general 2 &raar; 2 parton scattering and the extension to the next order for F2. Of course with every order in the perturbation expansion, the number of diagrams increases exponentially, but new approaches using formal mathematics developed for other applications, such as twistors, are helping to reduce the number of diagrams by over an order of magnitude.
Spin physics – fully polarized DIS – attracted many talks. The exquisite experiments of HERMES at HERA, COMPASS at CERN and those at RHIC and Jefferson Lab are matched by exotic new varieties of observables and dreams of reconstructing the proton structure in 3D. Despite all this activity, however, the “spin crisis” remains. The quarks do not carry much of the proton’s spin, and new results show that neither do the gluons. That leaves angular momentum – dubbed “dark angular momentum” by Xiangdong Ji of Maryland during his introductory talk on spin, because it will be so difficult to measure. Much remains to be done to clarify this area at the upgraded Jefferson Lab and/or RHIC.
The workshop programme made room for several social events including a welcome reception, held in the North Cloisters at UCL, and a brilliant concert at the Queen Elizabeth Hall by violinist Jack Liebeck and pianist Katya Apekisheva. The social highlight was the dinner held at Lord’s Cricket Ground – “the home of cricket”. After an excellent dinner, Norman McCubbin from the Science and Technology Facilities Council/Rutherford Appleton Laboratory gave a speech entitled “The scattering of balls: an English obsession”. He explained the delights of this English game, such as its length, the many and complicated options for when tea can be taken and the history of Lord’s. This was all supported by props showing how the game relates to physics and specifically deep-inelastic scattering.
DIS 2008 demonstrated how “DIS and Related Subjects” permeates almost all areas of high-energy physics, from hadron colliders to spin physics, neutrino physics and more. There is still much to be done and learnt in the field. Apart from the immediate excitement of the LHC start-up, another promising development for the future is the LHeC project, discussed on the last day, which would see the introduction of an electron ring in the LHC tunnel, allowing electron–proton collisions.
The European Committee for Future Accelerators has recently approved a conceptual design study and work is rapidly increasing on this project to assess its physics potential and technical realization, with a series of dedicated workshops starting this year. We are now all looking forward to seeing how this flourishing subject will be continued in Madrid at DIS 2009.
• The workshop was generously supported by CERN, DESY, FNAL, Jefferson Lab, STFC, IPPP Durham, UCL Maths and Physical Sciences Faculty, John Adams Institute, Cockcroft Institute, Cambridge University Press and Oxford University Press. As co-chairs we would like to thank all members of the Local Organizing Committee, in particular Christine Johnston, who quietly and efficiently carried most of the administrative burden, and the student helpers who made the conference such a great success.
The BaBar collaboration, working at SLAC has observed the ground state of the bottomonium family, the ηb meson. Bottomonium particles are bound states of a bottom quark and its antiquark. The first such state, the Υ(1S), was discovered 30 years ago and revealed the existence of the bottom quark. Physicists have been searching for the lowest-energy state of the system ever since.
The ηb was observed in the energy distribution of the photons produced in the radiative decay of the Υ(3S). The two-body decay, Υ(3S) → γηb, produces a monochromatic line with an energy that can be used to determine the ηb mass. The crucial point of the analysis was to understand the photon backgrounds, especially those that form peaks in the spectrum. These include photons emitted in radiative processes such as e+e– → γΥ(1S), which produces photons with energies close to the expected ηb signal, and transitions to intermediate bottomonium states, χbJ(2P).
The team used more than 100 million Υ(3S) events produced from e+e– collisions recorded with the BaBar detector at the PEP-II accelerator. These data were recorded in the final data-collection run of the experiment in 2008. After the analysis selection, approximately 19,000 ηb candidates were identified as forming a peak in the photon-energy spectrum at 921.2 MeV. The significance of this peak is 10 σ.
The corresponding mass of the ηb is 9388.9+3.1-2.1±2.7 MeV/c2, giving a hyperfine mass splitting of 71.4+2.3–3.1±2.7 MeV/c2 between the Υ(1S) and the ηb. This measurement represents the first experimental data on hyperfine mass-splittings in the heaviest meson system, and will allow for more precise tests of the role of spin–spin interactions in QCD.
The BaBar collaboration expects to release further results on bottomonium spectroscopy in the near future.
The D0 collaboration at Fermilab has announced the observation of pairs of Z bosons produced in proton–antiproton collisions. This is the final and rarest state in the series of gauge boson pairs observed and studied by D0 and the CDF experiment at the Tevatron: Wγ, Zγ, WW, WZ and ZZ. Earlier this year CDF published evidence for ZZ production, but the D0 results presented on 25 July showed for the first time sufficient significance to rank as an observation.
D0 observed ZZ production in 2.7 fb–1 of data with a combination of two analyses that look for Z decays into different final states. One analysis looked for a Z decaying into two electrons or two muons, the other for a Z decaying into neutrinos. The neutrino signature is challenging experimentally, but worthwhile to pursue because it occurs relatively frequently, although even this decay signature is predicted to occur less than once every 1012 collisions. The process of both Zs decaying to either electrons or muons is an even rarer process. In this analysis, three candidate events were observed with an expected background of less than 0.2 events. The statistical significance of the combined analysis is 5.7 σ, which firmly establishes the discovery of ZZ production at the Tevatron.
D0 measured a cross section for ZZ production of 1.5±0.6 pb, which is in excellent agreement with the prediction of the Standard Model. This is important as Z bosons in the Standard Model do not couple directly to one other. A higher rate would have implied anomalous self-couplings.
The observation of ZZ is connected with the search for the Higgs boson in several ways. The next rarest diboson production processes after ZZ are those involving Higgs bosons; seeing ZZ is an essential step in demonstrating the ability of an experiment to see the Higgs. Pairs of Z bosons also constitute one of the backgrounds to Higgs searches. At small values of the Higgs mass, ZZ can mimic the signature for a Higgs boson produced in association with a Z boson. At large values of the Higgs mass, the Higgs can decay into WW or ZZ. In more ways than one, ZZ observation is an essential prelude to finding, or excluding, the Higgs boson at the Tevatron.
This autumn, commissioning should be in full swing on the LHC at CERN, the world’s largest laboratory for the study of subnuclear physics. So it is entirely appropriate that the 46th Course of the International School of Subnuclear Physics, the oldest of the 123 schools of the Ettore Majorana Foundation and Centre for Scientific Culture (EMFCSC) in Erice, will look at what may come from the LHC – both the expected and the unexpected.
This year’s course, directed by Antonino Zichichi and Gerardus ’t Hooft, is to be held in Erice in September. It will provide the perfect opportunity to focus on the highlights from CERN, and in particular the goals of the LHC. This was also the theme of the 45th in the series, held in 2007, when CERN’s director-general, Robert Aymar, stated that these goals “could determine the future course of high-energy physics and should allow us to go beyond the Standard Model”.
Physics beyond the Standard Model first appeared before the Standard Model itself, when Raymond Davis observed neutrinos from the Sun in the 1960s. At Erice last year, Alessandro Bettini from the Galileo Galilei physics department at Padua University pointed out: “From 1962 neutrinos were used to look into the Sun’s core, but their behaviour was totally unexpected.” This led to the case for neutrino oscillations – a phenomenon that the Italian Laboratori Nazionali Gran Sasso (LNGS) is studying through the CERN Neutrinos to Gran Sasso project, which started in August 2006. “The observation of neutrino oscillations has now established beyond doubt that neutrinos have mass and mix,” claimed Eugenio Coccia, director of LNGS, during his talk. “This existence of neutrino masses is the first solid experimental fact requiring physics beyond the Standard Model.”
The physics of neutrinos is also linked to the unseen matter of the universe. In 1933, Fritz Zwicky, on measuring the mean quadratic velocity of galaxies, proposed the existence of a kind of “invisible matter” – he named it dark matter – that could have neither electromagnetic nor strong nuclear interactions. Neutrinos became the obvious candidates for dark-matter particles, but the study of the evolution of large-scale structures in the universe has unexpectedly shown that the contribution of neutrinos must be extremely small, if it exists at all. Indeed, no Standard Model particle can be considered as the dominant component of dark matter. One new particle candidate is the sterile neutrino, as Lisa Randall from Harvard University explained last year. “This new ‘flavour’ of neutrino could be trapped, like gravitons, in a different brane from the one we live on,” she said. “For this reason we have not observed it directly so far. But the LHC should manage to see many particles that were created during the dawn of the universe and disappeared soon after the Big Bang.”
There are many questions in particle physics that the LHC could help to solve, which the 46th course will again discuss this year. A key question is whether the expectations from the LHC predictable.
To answer this, during his talk at the 45th course, Zichichi recalled a front-line scientist of the 20th century, whose birth centenary was celebrated last year at the World Nuclear Physics Conference in Tokyo. In 1935 Hideki Yukawa proposed the existence of a particle with a mass between that of the light electron and the heavy nucleon – the first meson. “No-one was able to predict the ‘gold mine’ hidden in the production, decay and intrinsic structure of the Yukawa particle,” said Zichichi. “This gold mine is still being explored today, and its present frontier is the quark–gluon-coloured world.” Zichichi also pointed out: “It is considered standard wisdom that nuclear physics is based on perfect theoretical predictions, but people forget the impressive series of unexpected events with enormous consequences [UEEC] discovered inside the Yukawa gold mine.”.
Such UEEC events are a common feature of the greatest scientific discoveries and the most important historical facts. However, there is a difference. Analysing history on the basis of “what if?” leads historians to conclude that the world would not be as it is if one or any number of “what if?” events had not occurred. This is not the case for science, as Zichichi underlines: “The world would have exactly the same laws and regularities, even if Galileo Galilei or somebody else had not made their discoveries.”
UEEC events will be crucial evidence for understanding the existence of complexity at the elementary level. “No one could predict a UEEC event on the basis of present knowledge,” Zichichi pointed out. “In fact predictions are based on the mathematical description of UEEC events, so they come only after a UEEC event. Moreover, we should be prepared with powerful experimental instruments, technologically at the frontier of our knowledge, to discover all the pieces of the Yukawa gold mine.”
With the advent of the LHC at CERN, a new supercollider will study the properties of a “new world” produced in collisions between heavy nuclei (208Pb82+) at the maximum energy so far available (1150 TeV). This world is the quark–gluon-coloured world, totally different from all that we have so far been dealing with.
As Aymar underlined: “If new physics is there, the LHC should find it.” There is nothing left for us but to await the unexpected.
Researchers at the Jefferson Lab have found that neutron–proton pairs in the ground state carbon-12 nucleus are far more common than proton–proton pairs and neutron–neutron pairs. As many as 18% of the nucleons are involved in proton–neutron short-range correlations (SRCs), a result that could have implications for neutron stars.
In a typical nucleus, nucleons maintain an average distance of 1.7 fm. However, roughly one-fifth of nucleons are involved in short-range correlations, where two nucleons come to within a femtometre of each other. These pairs can create local densities five times that of average nuclear matter, thus providing a glimpse of dense nuclear matter as found in neutron stars.
Now a team working in Jefferson Lab’s Hall A has made the first simultaneous measurement of SRCs and their constituents. The experiment used an incident electron beam of 4.627 GeV and a carbon-12 target. Proton-knockout events were defined by the two High-Resolution Spectrometers (HRS) in Hall A. The left HRS detected scattered electrons and the right HRS detected knock-out protons. A large acceptance spectrometer (BigBite) and a neutron array detected correlated high-momentum recoiling protons and neutrons, respectively.
The experiment selected (e,e’p) events with high missing momentum, greater than 300 MeV/c, and revealed that the missing momentum was balanced almost entirely by a single recoiling nucleon. This nucleon was initially back to back with the knock-out proton. The team found that 90% of these SRCs involved proton–neutron pairs. The remaining 10% were split between proton–proton and neutron–neutron pairs (Subedi et al. 2008). Calculations of this effect in recent theoretical work indicate that the large prevalence of neutron–proton pairs over proton–proton and neutron–neutron pairs is a result of the nucleon–nucleon tensor force (Sargsian et al. 2005 and Schiavilla et al. 2007).
Together with previous work, including cross-section ratio measurements at Jefferson Lab and proton-knockout experiments at Brookhaven National Laboratory, the new result yields a consistent picture of the short-distance structure of nuclear systems, from light nuclei to neutron stars. Most accepted models of neutron stars assume a make-up of 95% neutrons and 5% protons at the core. The presence of strong short-range, neutron–proton pairing could alter assumptions about the protons’ momenta, thus affecting calculations of the density and/or lifetime of neutron stars.
The Sun, a typical middle-aged star, is the most important astronomical body for life on Earth, and since ancient times its phenomena have had a key role in revealing new physics. Answering the question of why the Sun moves across the sky led to the heliocentric planetary model, replacing the ancient geocentric system and foreshadowing the laws of gravity. In 1783 a sun-like star led the Revd John Mitchell to the idea of the black hole, and in 1919 the bending of starlight by the Sun was a triumphant demonstration of general relativity. The Sun even provides a laboratory for subatomic physics. The understanding that it shines by nuclear fusion grew out of the nuclear physics of the 1930s; more recently the solution to the solar neutrino “deficit” problem has implied new physics.
This progress in science, triggered by the seemingly pedestrian Sun, seems set to continue, as a variety of solar phenomena still defy theoretical understanding. It may be that one answer lies in astroparticle physics and the curious hypothetical particle known as the axion. Neutral, light, and very weakly interacting, this particle was proposed more than 25 years ago to explain the absence of charge-parity (CP) symmetry violation in the strong interaction.
So what are the problems with the Sun? These lie, perhaps surprisingly, with the more visible, outermost layers, which have been observed for hundreds, if not thousands, of years.
First, why is the corona – the Sun’s atmosphere with a density of only a few nanograms per cubic metre – so hot, with a temperature of millions of degrees? This question has challenged astronomers since Walter Grotrian, of the Astrophysikalisches Observatorium in Potsdam, discovered the corona in the 1930s. Within a few hundred kilometres, the temperature rises to be about 500 times that of the underlying chromosphere, instead of continuing to fall to the temperature of empty space (2.7 K). While the flux of extreme ultraviolet photons and X-rays from the higher layers is some five orders of magnitude less than the flux from the photosphere (the visible surface), it is nevertheless surprisingly high and inconsistent with the spectrum from a black body with the temperature of the photosphere (figure 1). Thus, some unconventional physics must be at work, since heat cannot run spontaneously from cooler to hotter places. In short, everything above the photosphere should not be there at all.
Another question is how does the corona continuously accelerate the solar wind of some thousand million tonnes of gas per second at speeds as high as 800 km/s? The same puzzle holds for the transient but dramatic coronal mass ejections (CMEs). How and where is the required energy stored, and how are the ejections triggered? This question is probably related to the mystery of coronal heating. And what is it that triggers solar flares, which heat the solar atmosphere locally up to about 10 to 30 million degrees, similar to the high temperature of the core, some 700,000 km beneath? These unpredictable events appear to be like violent “explosions” occurring near sunspots in the lower corona. This suggests magnetic energy as their main energy source, but how is the energy stored and how is it released so rapidly and efficiently within seconds? Even though many details are known, new observations call into question the 40-year-old standard model for solar flares, which 150 years after their discovery still remain a major enigma.
On the Sun’s surface, what is it that causes the 11-year solar cycle of sunspots and solar activity? This seems to be the biggest of all solar mysteries, since it involves the oscillation of the huge “magnets” of a few kilogauss on the face of the Sun, ranging from 300 to 100,000 km in size. The origin of sunspots has been one of the great puzzles of astrophysics since Galileo Galilei first observed them in the early 1600s. Their rhythmic comings and goings, first measured by the apothecary Samuel Heinrich Schwabe in 1826, could be the key to understanding the unpredictable Sun, since everything in the solar atmosphere varies in step with this magnetic cycle.
Beneath the Sun’s surface, the contradiction between solar spectroscopy and the refined solar interior models provided by helioseismology has revived the question about the heavy-element composition of the Sun, with new abundances some 25 to 35% lower than before. Abundances vary from place to place and from time to time in the Sun, and are enhanced near flares, showing an intriguing dependence on the square of the magnetic intensity in these regions. The so-called “solar oxygen crisis” or “solar model problem” is thus pointing at some non-standard physical process or processes that occur only in the solar atmosphere, and with some built-in magnetic sensor.
These are just some of the most striking solar mysteries, each crying out for an explanation. So can astroparticle physics help? The answer could be “yes”, using a scenario in which axions, or particles like axions, are created and converted to photons in regions of high magnetic fields or by their spontaneous decay.
The expectation from particle physics is that axions should couple to electromagnetic fields, just as neutral pions do in the Primakoff effect known since 1951, which regards the production of pions by high-energy photons as the reverse of the decay into two photons. Interestingly, axions could even couple coherently to macroscopic magnetic fields, giving rise to axion–photon oscillation, as the axions produce photons and vice versa. The process is further enhanced in a suitably dense plasma, which can increase the coherence length. This means that the huge solar magnetic fields could provide regions for efficient axion–photon mutation, leading to the sudden appearance of photons from axions streaming out from the Sun’s interior. The photosphere and solar atmosphere near sunspots are the most likely magnetic regions for this process to become “visible”, as the material above is transparent to emerging photons.
According to this scenario, the Sun should be emitting axions, or axion-like particles, with energies reflecting the temperature of the source. Thus one or more extended sources of new low-energy particles (below around 1 keV), and the ubiquitous solar magnetic fields of strengths varying from around 0.5 T, as measured at the surface, up to 100 T or much more in the interior, might together give rise to the apparently enigmatic behaviour of a star like the Sun.
Conventional solar axion models, inspired by QCD, have one small source of particles in the solar core, with an energy spectrum that peaks at 4 to 5 keV. They therefore exclude the low energies where the solar mysteries predominantly occur. This immediately suggests an extended axion “horizon”. Experiments to detect solar axions – axion helioscopes such as the CERN Solar Axion Telescope (CAST) – should widen their dynamic range towards lower energies, in order to enter this new territory.
The revised solar axion scenario must also accommodate two components of photon emission, namely, a continuous inward emission together, occasionally, with an outward radiation pressure. Massive and light axion-like particles, both of which have been proposed, can provide these thermodynamically unexpected inward and outward photons respectively. They offer an exotic but still simple solution, given the Sun’s complexity.
The emerging picture is that the transition region (TR) between the chromosphere and the corona (which is only about 100 km thick and only some 2000 km above the solar surface) is the manifestation of a space and time dependent balance between the two photon emissions. However, the almost equally probable disappearance of photons into axion-like particles in a magnetic environment must also be taken into account in understanding the solar puzzles. The TR could be the most spectacular place in the Sun, since it is where the mysterious temperature inversion appears, while flares, CMEs and other violent phenomena originate near the TR.
Astrophysicists generally consider the ubiquitous solar magnetism to be the key to understanding the Sun. The magnetic field appears to play a crucial role in heating up the corona, but the process by which it is converted into heat and other forms of energy remains an unsolved problem. In the new scenario, the generally accepted properties of the radiative decay of particles like axions and their coupling to magnetic fields are the device to resolve the problem – in effect, a real “απó μηχανηζ θεóζ” (the deus ex machina of Greek tragedy). The magnetic field is no longer the energy source, but is just the catalyst for the axions to become photons, and vice versa.
The precise mechanism for enhancing axion–photon mutation in the Sun that this picture requires remains elusive and challenging. One aim is to reproduce it in axion experiments. CAST, for example, seeks to detect photons created by the conversion of solar axions in the 9 T field of a prototype superconducting LHC dipole. However, the process depends on the unknown mass of the axion. Every day the CAST experiment changes the density of the gas inside the two tubes in the magnet in an attempt to match the velocity of the solar axion with that of the emerging photon propagating in the refractive gas.
It is reasonable to assume that fine tuning of this kind in relation to the axion mass might also occur in the restless magnetic Sun. If the energy corresponding to the plasma frequency equals the axion rest mass, the axion-to-photon coherent interaction will increase steeply with the product of the square of the coherence length and the transverse magnetic field strength. Since solar plasma densities and/or magnetic fields change continuously, such a “resonance crossing” could result in an otherwise unexpected photon excess or deficit, manifesting itself in a variety of ways, for example, locally as a hot or cold plasma. Only a quantum electrodynamics that incorporates an axion-like field can accommodate such transient brightening as well as dimming (among many other unexpected observations).
These ideas also have implications for the better tuning not only of CAST, but also of orbiting telescopes such as the Japanese satellite Hinode (formerly Solar B), NASA’s Reuven Ramaty High Energy Solar Spectroscopic Imager and the NASA–ESA Solar and Heliospheric Observatory, which have been transformed recently to promising axion helioscopes, following suggestions by CERN’s Luigi di Lella among others. The joint Japan–US–UK mission Yohkoh has also joined the axion hunt, even though it ceased operation in 2001, by making its data freely available.
The revised axion scenario therefore seems to fit as an explanation for most (if not all) solar mysteries. Such effects can provide signatures for new physics as direct and as significant as those from laboratory experiments, even though they are generally considered as indirect; the history of solar neutrinos is the best example of this kind.
Following these ideas and others on millicharged particles, paraphotons or any other weakly interacting sub-electron-volt particles, axion-like exotica will mean that the Sun’s visible surface – and probably not its core – holds the key to its secrets. As in neutrino physics, the multifaceted Sun, from its deep interior to the outer corona and the solar wind, could be the best laboratory for axion physics and the like. The Sun, the most powerful accelerator in the solar system, whose working principle is not yet understood, has not been as active as it is now for some 11,000 years. Is this an opportunity not to be missed?
Milagro – Spanish for miracle – was the first of a new generation of extensive air shower (EAS) detectors. Traditionally, EAS arrays have been composed of a discrete set of small detectors, spread over large areas. Typically active over approximately 1% of the enclosed area only, they were sensitive to cosmic gamma rays with energies of around 100 TeV and above. The combination of steeply falling source spectra and the absorption in flight of these high-energy gamma rays via interactions with the cosmic microwave background radiation meant that this first generation of instruments did not succeed in detecting any astrophysical sources. In contrast, imaging atmospheric Cherenkov telescopes (IACT), pioneered by Trevor Weekes at Mount Hopkins, led to the discovery of several tera-electron-volt gamma-ray sources, the first of which was the Crab Nebula, the remnant of a supernova that occurred in 1054 (Weekes et al. 1989). More recently an array of such detectors, the HESS telescopes in Namibia, have demonstrated the richness of the tera-electron-volt sky.
Despite these difficulties, the advantages of EAS arrays, with their large instantaneous field of view (around 2 sr) and continuous operation, provided strong motivation to improve the technique. The key to success was to lower the energy threshold and simultaneously improve the ability to reject the abundant cosmic-ray background. Water Cherenkov technology, developed for underground proton-decay physics experiments such as the Irvine Michigan Brookhaven and Kamiokande detectors, led the way to this success.
When employed above ground as an EAS array, water Cherenkov technology enables the construction of an array that is sensitive over its entire area. The Cherenkov angle in water is 41° so an array of photomultiplier tubes (PMTs) placed at a depth comparable to their spacing can detect the Cherenkov light emitted from any electromagnetic particle entering the water volume. Moreover, the composition of an EAS at ground level is predominantly photons (which are around six times as numerous as electrons and positrons), and, as the depth of water above the PMTs is sufficient to convert these gamma rays to charged particles, these photons can also be detected by the PMTs.
The Milagro detector is located in the Jemez Mountains of northern New Mexico. It is operated by the Los Alamos National Laboratory in partnership with the National Science Foundation and the US Department of Energy Office of Science. Milagro uses a covered water reservoir that contains 2.5 × 107 litres of water and measures 80 m × 60 m, with a depth of 8 m. The reservoir is instrumented with 750 PMTs deployed in two layers. The top layer of 450 PMTs is beneath 1.5 m of water with a spacing of 2.8 m. This layer is used to reconstruct the direction of the primary gamma ray or cosmic ray by measuring the relative arrival time of the shower front to around 0.5 ns. The second layer of PMTs, beneath 6 m of water, is used to detect the penetrating component of any EAS initiated by hadronic cosmic rays. An array of 175 water tanks surrounds the central water reservoir. Each is 1 m high and 3 m in diameter and is lined with reflective Tyvek. A single 8 inch PMT mounted at the top of each tank looks down into the water volume.
After seven years of operation, four of which included the array of outrigger water tanks, Milagro ceased operation in April this year. Its results have been impressive and ushered in a new era for ground-based gamma-ray astrophysics at tera-electron-volt energies, where the role of the EAS arrays is now clearly established.
The figure above shows a region around the galactic plane as observed by Milagro, where the median energy of the detected gamma rays is 20 TeV (Abdo et al. 2007b). It contains several noteworthy features. The sources marked JXXXX+YY, where XXXX and YY are the right ascension and declination, respectively, are three new sources that Milagro discovered. MGRO J2031+41 and MGRO J2019+37 lie within the Cygnus region of the galaxy. This direction points into our spiral arm and is rich with possible cosmic-ray acceleration sites, such as Wolf–Rayet stars, OB associations (a sign of star formation) and supernova remnants. The locations of these two sources are coincident with sources of giga-electron-volt gamma rays discovered by the Energetic Gamma Ray Emission Telescope (EGRET) on NASA’s Compton Gamma Ray Observatory (the squares mark the locations of gamma-ray sources of more than 100 MeV reported in the 3rd EGRET catalogue). However, the true nature of the sources is still to be determined.
The third new source shown in the figure above is MGRO J1908+06. This was subsequently observed by HESS, which measured a “hard” energy spectrum, falling more or less with the square of the energy. Preliminary analysis of Milagro data indicates that this source may be emitting gamma rays with energies in excess of 100 TeV, which would make it the highest-energy gamma-ray source detected to date and a likely site of cosmic-ray acceleration.
In addition to these three sources, there are four other regions in Milagro’s view of the galaxy that are likely to be sources of tera-electron-volt gamma rays. The image above shows three of these regions: C1, C3 and C4. C2, which is not indicated, lies just above C1.
The source candidate C4 is coincident with the Boomerang pulsar wind nebula, and the shape seen in tera-electron-volt gamma rays is similar to that observed at 100 MeV. C3 is coincident with the Geminga pulsar (although no pulsed emission is observed at tera-electron-volt energies), which, at a distance of 180 pc, is the closest pulsar to the Earth and the brightest source of giga-electron-volt gamma rays visible in the northern sky. Finally, C1 has no giga-electron-volt source in the vicinity and its nature is at present completely unknown. The air shower array operating at Yangbajing cosmic-ray observatory in Tibet has confirmed this source, in addition to the two others that lie in the Cygnus region. One interesting feature of these is that they appear to be extended, with diameters ranging from 0.25° to more than 1°. Large sources are difficult for IACTs to detect, possibly explaining why they have eluded detection until now, despite the fact that these regions had been examined by past IACT arrays, such as the Whipple Observatory and the High Energy Gamma Ray Astronomy experiment.
The second image also shows a diffuse glow visible around the galactic plane, especially in the Cygnus region and at lower galactic longitude. This arises from the interaction of hadronic cosmic rays and high-energy cosmic-ray electrons with matter and radiation in the galaxy. The interaction of cosmic-ray protons with matter leads to the production of neutral pions that subsequently decay into gamma rays. The high-energy electrons interact with low-energy (optical, infrared and cosmic microwave background) photons through Compton scattering to produce high-energy gamma rays. Prior to Milagro’s measurements, EGRET observed this galactic diffuse radiation up to about 30 GeV and discovered an excess of diffuse emission over predictions based on the known matter density in the galaxy and the cosmic-ray rate and spectrum measured at the Earth. The explanation for this excess is still a matter of debate, with possible solutions including the annihilation of dark matter. A much greater intensity of high-energy electrons throughout the galaxy than is measured at the Earth, and a miscalibration of the EGRET response at high energies, are also possible explanations.
The third image shows Milagro’s measurement of the diffuse emission at 12 TeV in the Cygnus region (Abdo et al. 2007a). This measurement indicates that at tera-electron-volt energies the excess over expectations is even larger than it is at giga-electron-volt energies. While the cause of this excess is a matter of debate, possible explanations include cosmic-ray acceleration sites in the region, unresolved sources of tera-electron-volt gamma rays in the region, and the presence of very-high-energy electrons in the region. The resolution of this puzzle will require more detailed observations. Whatever the final explanation, it is clear that gamma-ray astronomy is an important tool in answering the nearly century-old problem of the origin of cosmic radiation.
While observations with Milagro have drawn to a close, plans for a new instrument are proceeding. A joint US–Mexico collaboration has proposed the High Altitude Water Cherenkov (HAWC) telescope to be located at Volcà n Sierra Negra (Tliltepetl) near the site of the Large Millimeter Telescope in Mexico. At 4100 m above sea level (compared with 2600 m above sea level for Milagro) and with a dense sampling detector that encloses around 22,000 m2, HAWC is expected to be about 15 times as sensitive as Milagro and have an energy threshold of less than 1 TeV. Unlike Milagro, it will comprise 900 individual water tanks. Each tank will be 5 m in diameter and 4.6 m tall – much larger than those used by Milagro or the Pierre Auger Observatory in Argentina – and would have a PMT at the bottom looking up into the water volume. If built, the complete array will have an unprecedented level of sensitivity to the highest-energy particle accelerators in our galaxy, as well as the sensitivity needed to detect short flares from active galaxies and the ability to make a detailed map of the diffuse gamma-ray emission in our galaxy.
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