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Quark matter conference highlights RHIC results

cernquark1_10-02 — CERN research director Claude Détraz (left) with QM 2002 conference organizer Hans Gutbrod, who put the host town Nantes firmly on the heavy-ion physics map.

At a seminar in February 2000, spokespersons from CERN’s heavy-ion experiments presented compelling evidence for the existence of a new state of matter in which quarks, instead of being bound up into more complex particles such as protons and neutrons, are liberated to roam freely. The seminar marked a turning point in heavy-ion research, with the spotlight turning to the other side of the Atlantic, where the Brookhaven Laboratory’s Relativistic Heavy-Ion Collider (RHIC) was about to switch on. Would the RHIC data corroborate or contradict CERN’s findings at the lower-energy programme? Most of the hadronic signals measured at RHIC so far have confirmed the CERN data and their interpretation, but new findings on high transverse momentum (p_T) physics at RHIC have opened up a new avenue of enquiry. The impressively rich harvest of data from the first running periods of RHIC made QM 2002 a major milestone in the quark matter (QM) conference series.

The French town of Nantes was chosen to host QM 2002 last July in recognition of its importance as a focus for French heavy-ion research. As the home of the SUBATECH laboratory, founded in 1995, Nantes leads French participation in the ALICE heavy-ion experiment in preparation for the Large Hadron Collider (LHC) at CERN. The six-day conference programme saw 46 plenary talks and debates, 100 parallel session talks and more than 150 posters. Many of the speakers were appearing for the first time at a QM conference, the vanguard of a new generation to carry forward the physics of this field. The 700 participants came from all over the world.

Claude Détraz of CERN formally opened the conference, in the presence of the president of the Pays de la Loire regional council, with a look back at more than 20 years of heavy-ion history at CERN. It all began with the proposal of a US-German collaboration to bring a heavy-ion injector to CERN to do physics at the laboratory’s proton synchrotron. This idea evolved into a physics programme at the super proton synchrotron (SPS), leading to a lead-ion beam being built by a consortium from France, Germany, Holland, India, Italy and Sweden, together with CERN. The result was a fully-fledged research activity at CERN that will continue with the LHC.

Results from CERN

The search for a new state of strongly interacting matter at CERN started with beams of oxygen in 1986, and has continued with lead beams since 1994. The SPS fixed-target programme still has several experiments taking data, and many of those that have completed data-taking are still delivering physics results. Measurements of hadronic observables (strange and non-strange particles, flow and interferometry) are now becoming complete thanks to the measurements of the NA49, CERES and NA57 experiments, which were discussed by Christoph Blume of the German GSI laboratory, Marco van Leeuwen of Holland’s NIKHEF, Johannes Wessels of GSI, and Vito Manzari of the Italian INFN laboratory in Bari. Many of these studies have revealed interesting phenomena, which are also observed at RHIC.

New measurements at lower SPS energies allow excitation functions to be studied. These reveal interesting features, such as an indication of a maximum strangeness enhancement in an energy range between that achieved by Brookhaven’s Alternating Gradient Synchrotron and the top SPS energies. Statistical effects dominate fluctuations to a large extent, but detailed studies have revealed small non-statistical fluctuations. This was discussed by Bedanga Mohanty of Calcutta. Stefan Bathe of Münster presented results on high p_T studies from the WA98 experiment. These do not exclude small jet-quenching effects (suppression of jets through medium-induced energy loss of hard-scattered partons) for very central lead-lead collisions compared with peripheral collisions. However, the clear suppression seen at RHIC is not observed at the SPS. Jet-quenching appears to be a signal that will be clearly accessible only at the colliders.

The most striking observation relevant to quark-gluon plasma formation is the suppression of J/y particles as measured by the NA38 and NA50 experiments at CERN. Luciano Ramello of the University of Piedmont Orientale presented improved data on J/y from NA50, with much better quality for peripheral reactions. Moreover, detailed momentum distributions of J/y are now available to constrain theoretical calculations. Philippe Crochet of Clermont Ferrand pointed out that data on J/y deserve a fresh look to understand all the details. Final results from the improved CERES experiment and the measurements of open charm, which will be performed by NA60, are eagerly awaited.

Results from RHIC

While the SPS heavy-ion programme has reaped most of its harvest, results from the RHIC experiments only started to appear at last year’s QM conference at Stony Brook, US, and became plentiful at this conference. Results on hadronic observables were presented by all four RHIC experiments in talks by Ian Bearden of the Niels Bohr Institute (NBI) for BRAHMS, Tatsuya Chujo of Tsukuba for PHENIX, Mark Baker of Brookhaven for PHOBOS, and by Lenny Ray of the University of Texas and Gene van Buren of Brookhaven for STAR. Their contributions took in global observables; spectra and yields of identified hadrons; interferometry; momentum, isospin and charge-fluctuations; and elliptic flow.

Full justice can only be done to all the new results from RHIC in the conference proceedings, but it is worthwhile drawing attention to some highlights. The ratios of produced particles can be described very accurately by statistical models of hadronization as discussed by van Buren, Chujo and Bearden for the experiments, and from the theory side by Andrzej Bialas of Krakow’s Jagellonian University, Johann Rafelski of Arizona, and Volker Koch of Berkeley. This was already seen at the SPS, and can also be successfully applied to lower-energy heavy-ion reactions and to proton-proton and electron-positron reactions. Whether these statistical distributions are of thermal origin, or whether they are merely related to phase space dominance is not at all clear. Thermal concepts may be applicable to high-energy heavy-ion reactions, but this appears much more unlikely for electron-positron reactions. Interestingly, the associated “chemical temperature” for high-energy nuclear reactions is very close to the postulated phase-transition temperature.

Interferometry of charged pions leaves us with a few unresolved puzzles, as discussed by Ray and Chujo. The duration of emission of the most abundant particles appears to be very short. The apparent size of connected regions of the expanding fireball has similar dimensions in the radial and the tangential directions, unlike the predictions of most models for the expansion. The phase space density of pions is apparently very high, which can be interpreted in terms of very low entropy per pion.

The asymmetry of particle emission in the transverse plane (known as elliptic flow), which showed up in non-central heavy-ion reactions at lower energies, has not only survived at RHIC, but is even stronger as was demonstrated in the talks by Ray, Chujo and Baker. Since hydrodynamical models are so far the most successful in describing the features of this asymmetry, this is one of the strongest hints of an early local equilibration of the system. The strength of the asymmetry is still finite for very large transverse momenta, where one would expect production from hard scattering to dominate over any equilibrated contribution. This may be seen as a hint of asymmetric jet-quenching in the reaction zone; the quantitative interpretation is, however, not yet settled.

Fluctuations and multiparticle correlations may help in finding hints of new physics. Results on various different approaches were presented, with Ray giving one example from STAR. He proposed an interpretation in terms of very late hadronization in central collisions.

Jet-quenching

cernquark2_10-02 — Figure 1 Theoretical fits to the nuclear modification factor (R_AA) for neutral pions in central collisions of gold nuclei at RHIC describe the data better where energy loss in the medium is included.

At QM 2002, it became even clearer than at last year’s QM conference that RHIC has truly opened up the domain of hard scattering. This is particularly interesting, as it should allow the predicted jet-quenching to be used as a signature of the hot, dense medium. Rudolf Baier of Bielefeld discussed jet-quenching in detail. Hadron spectra have been measured by all experiments and were presented in the talks by Saskia Mioduszewski of Brookhaven for PHENIX, Gerd Kunde of Yale for STAR, Christof Roland of MIT for PHOBOS, and Claus Jørgensen of NBI for BRAHMS. Only PHENIX and STAR have measured out to truly high momenta. Their spectra are compared to the expectation from proton-proton reactions. This is commonly done by calculating a so-called nuclear modification factor (R_AA), the ratio of spectra in nucleus-nucleus collisions to those in proton-proton collisions scaled by the number of possible binary nucleon-nucleon collisions within a heavy-ion reaction. Of particular importance was the measurement of neutral pions out to p_T = 10 GeV/c by PHENIX. This measurement is important, because the neutral pion can be well identified out to such high p_T and because PHENIX has also measured the spectra for proton-proton reactions within the same apparatus. The nuclear modification factor, which should be equal to 1 without any nuclear effects, is significantly below 1 up to the highest momenta analyzed (figure 1). Also shown in figure 1 are results of theoretical calculations with and without energy loss (jet-quenching). Calculations without energy loss miss the data completely. Calculations including energy loss provide the right order of magnitude for R_AA, but still fail to describe the data exactly.

While the suppression in inclusive spectra shown in figure 1 is quantitatively the best-established indication of jet-quenching, it is much more fascinating to see hints of jet structures themselves. Indications of such angular correlations relative to a high-momentum trigger particle, both at small angles and back to back, were discussed by David Hardtke of Berkeley for STAR and by Mioduszewski for PHENIX. STAR presented the most advanced analysis of these correlations, and was able to demonstrate modifications of the jet structures for central gold-gold collisions (figure 2). The trigger jet appears to be almost unaltered. The counter jet, however, is completely suppressed in central collisions. The quenching of jets relative to the expectation from the proton-proton case appears to be established. Nevertheless, the interpretation of this phenomenon is not completely clear. One of the major open questions remaining is that of the composition of the high p_T distributions of different particle species. Results from PHENIX seem to indicate that pions make up only around half of all charged hadrons even at very high p_T, in contradiction to expectations from perturbative quantum chromodynamics (QCD).

The study of rare probes at RHIC, such as J/y and direct photon production, is still under way. PHENIX collaboration members James Nagle of Columbia and Klaus Reygers of Münster presented the status of these analyses. Such results are likely to contribute to the experimental highlights of the next QM conference.

Looking ahead

cernquark3_10-02 — Figure 2 The azimuthal correlation of charged particles relative to a high p_T trigger particle. The counter jet at Df = p in central collisions of gold nuclei at RHIC (black crosses) is suppressed compared to expectations from the proton-proton case (red histogram), while the trigger jet itself (Df = 0) is unaffected.

The experiments have provided theorists with a huge amount of data to contemplate. Currently there is no comprehensive theory in sight that could describe all the data or even a larger fraction of them, as Brookhaven’s Robert Pisarski pointed out. Theories or models can describe limited sets of data, but for some observables even such attempts fail. For example, the precise shape of the nuclear modification factor observed at RHIC cannot be described. A lot of effort is needed in the coming years to close the gap between theory and experiment. Progress has so far been made in areas of limited scope. Pasi Huovinen of Minnesota pointed out how hydrodynamical models are successful in describing certain bulk properties of the reactions. Direct photon calculations have been stimulated by the available measurement of WA98, which apparently requires a hot initial state to be explained. François Gélis of Orsay discussed new ideas, including the precise treatment of the Landau-Pomeranchuk-Migdal effect, which predicts a suppression of low-energy photon production in a dense medium. Frithjof Karsch of Bielefeld said that the possible use of the maximum entropy method to calculate production rates is very promising. The idea of saturation has been developed to very fruitful theoretical recipes, as presented by Edmond Iancu of Saclay. Lattice calculations approach realistic parameter settings, but as Tsukuba’s Kazuyuki Kanaya pointed out, the question of the nature of the phase transition remains open. Zoltan Fodor of Eötvös University discussed the possibility of calculating properties of QCD at finite baryochemical potential on the lattice.

Future directions were the subject of the final sessions. The major avenue of quark matter research leads to the LHC heavy-ion programme, which was presented by Helsinki’s Keijo Kajantie and Paolo Giubellino of INFN Turin, where still higher-energy density and temperature at negligible net baryon density are expected. An important follow-up on the lower-energy front was presented by Jochen Wambach of Illinois, who discussed a new facility at GSI that aims to study the highest net baryon densities.

Workshops focus on photon-hadron collisions

cernwork1_10-02 — Adrian Melissinos (left) and Ingvar Lindgren discuss strong fields during a break at the Erice workshop.

In 1924 Enrico Fermi travelled to Leiden, the Netherlands, to visit Paul Ehrenfest on a three-month fellowship from the International Education Board, which had been founded the year before by John D Rockefeller Jr for the “promotion and advancement of education throughout the world”. Fermi was just 23 years old. During his stay in Leiden, he developed a method to calculate the interactions of charged particles with matter, which he called “äquivalente strahlung”. This was extended to the relativistic case in the 1930s by Carl Friedrich von Weizsäcker and E J Williams, who realized that charged particles in cosmic rays could deliver the highest-energy (virtual) photon beams to study pair production of the positron. Today, the technique is known as the equivalent photon approximation, and it provides a powerful tool for fundamental physics.

In an echo of the insight of von Weizsäcker and Williams, a growing community of physicists is looking to CERN’s forthcoming Large Hadron Collider (LHC) as a new source of very-high-energy photon-hadron interactions. When the LHC stores colliding lead beams at a centre of mass energy of 5.4 TeV per nucleon, for example, lead ions in grazing collisions will be bombarded with photons of energy extending to hundreds of tera-electron-volts in their rest frame. Similarly, photon-photon invariant masses in the range of 100 GeV will be created in this way.

Such collisions are known as ultraperipheral, and have the general feature of an absence of particles produced along at least one beam direction (Bjorken’s “rapidity gap”), because the photon is neutral. In many cases the produced system can be measured in a rather clean way in any of the LHC’s detectors. The strong field of these “quasi-real photons” has found many interesting applications in atomic, nuclear and particle physics.

In October of Fermi’s centenary year, 2001, a workshop in Erice, Sicily, focused on ultraperipheral collisions. It was followed by a second meeting at CERN last March. The Erice workshop – Electromagnetic Probes of Fundamental Physics – was organized by Bill Marciano and Sebastian White from the US Brookhaven National Laboratory, and featured sessions on the muon g-2, ultraperipheral collisions at Brookhaven’s Relativistic Heavy-Ion Collider (RHIC) and the LHC, as well as topics in strong field research.

Francis Farley of Yale University, US, gave a review of 44 years of g-2, while David Hertzog of Illinois, US, presented the latest results from Brookhaven. Andreas Höcker of Orsay, France, Simon Eidelman of Novosibirsk, Russia, and Marciano discussed the theoretical side, with an emphasis on hadronic corrections. Since the meeting, higher-precision results on g-2 have been announced. Teams from Brookhaven, Novosibirsk and the ALEPH experiment at CERN’s large electron-position collider presented results on the related analyses of electron-positron annihilation to hadrons and semi-leptonic tau lepton decays. An intriguing discrepancy between the electron-positron and tau analyses is now the focus of the interpretation of g-2 as a potential indicator of physics beyond the Standard Model. New data from the Belle, BaBar and Kloe experiments at Japan’s KEK laboratory, SLAC in the US and Frascati in Italy are eagerly awaited.

Ilya Ginzburg and Valery Serbo of Novosibirsk presented the related topic of future photon-photon colliders. The spontaneous breakdown of the vacuum by intense lasers was discussed by Adrian Melissinos of Rochester University, US, while Andreas Ringwald of Germany’s DESY laboratory talked about the science that will be accessible with the X-ray free-electron lasers being developed by the DESY-led TESLA project. The sources and interactions of ultra-high-energy cosmic rays were the subject of talks from Alex Kusenko of UCLA and Steve Reucroft of Boston’s Northeastern University, US.

Carlos Bertulani of Michigan State University, US, and Spencer Klein of Berkeley, US, along with Kai Hencken of Basel University, Switzerland, and Gerhard Baur of Jülich, Germany, reviewed the theoretical background of ultraperipheral collisions, while also pointing out the opportunities in both photon-photon and photon-hadron physics.

After presentations on recent experimental results from RHIC by Jim Thomas and White of Brookhaven, and Pablo Yepes of Rice University, US, the discussion turned to a prioritized list of the opportunities at RHIC and the LHC. These are summarized in an Erice white paper on hot topics in ultraperipheral collisions, which is included in the workshop proceedings (see Further reading).

The white paper was adopted as the point of departure for an exploratory meeting on ultraperipheral relativistic heavy-ion collisions that was held at CERN in March. This meeting, organized by Hencken and Yepes, was well attended not only by theoreticians, but also by experimentalists from the LHC collaborations interested in ultraperipheral collisions.

cernwork2_10-02 — The strong Coulomb field of the fast-moving heavy ions is an abundant source of quasi-real photons. At the LHC, a range of photon-photon and photon-hadron collisions will occur at high photon energies.

SLAC’s Stan Brodsky gave the introductory talk, followed by Baur. Both presented a range of interesting applications from a theoretical point of view for photon-photon and photon-hadron collisions at RHIC and the LHC. Leonid Frankfurt of Tel Aviv University, Israel, discussed photoproduction of vector mesons on protons and ions. The first results from the STAR experiment at RHIC for coherent rho-meson production were presented by Falk Meissner of Berkeley. Joakim Nystrand of Lund University, Sweden, showed that interesting interference phenomena, due to the fact that each ion can either be the source of the photon or the target, can be studied in these collisions. Highlights from the rich photon-hadron programme at DESY’s HERA electron-proton collider were presented by Giuseppe Iacobucci of the INFN in Bologna, Italy.

The strong electromagnetic excitation of heavy ions in ultraperipheral collisions is largely due to the strong source of low-energy photons. As they subsequently decay, heavy-ion fragments can be used for luminosity monitoring by detecting the neutrons in a zero-degree calorimeter, as discussed by White, Igor Pshenichnov of Moscow’s Institute for Nuclear Research and Vladimir Korotkikh from Moscow State University, Russia.

Physics opportunities

A range of physics opportunities was discussed at the CERN workshop. Berkeley’s Ramona Vogt pointed out the possibility of deducing the gluon structure function within the nuclear medium using heavy-quark production in photon-gluon fusion. Krzysztof Piotrzkowski from Louvaine-la-Neuve, Belgium, described how the possibility of tagging protons in a forward detector after photon emission permits the study of photon-photon collisions at invariant masses beyond the Z mass. This would allow researchers to look at the electromagnetic coupling of W bosons and also to search for new physics. Brookhaven’s Anthony Baltz and Hencken, and Frank Krauss of Cambridge University, UK, discussed aspects of lepton pair production. These include bound-free pair production and the production of muonium.

Studies on detecting ultraperipheral collisions within the LHC detectors were presented by Serguei Sadovsky of Protvino, Russia, for ALICE and Yepes for CMS, both of whom emphasized the importance of having a trigger for these events. Daniel Brandt of CERN discussed the possibilities of different ion beams at the LHC, where electromagnetic processes can limit the maximum beam luminosity.

One tangible outcome of the workshop is the formation of a working group on ultraperipheral collisions, paying particular attention to the potential for this physics at the LHC. Another meeting is scheduled to take place at CERN on 11-12 October. Its goal will be to start work on a report giving full details of the physics of ultraperipheral heavy-ion collisions accessible within the LHC detectors as currently conceived. With such an intense source of photons, the conclusion of the workshops was that the future is bright for ultraperipheral collisions at the LHC.

PASI studies new states of matter

cernmatter1_10-02 — The Leão da Montanha (Mountain Lion) hotel in Campos do Jordão played host to the PASI on new states of matter in hadronic interactions. (Takeshi Kodama.)

The search for a phase transformation between hadronic matter and a state of deconfined colour charge, generically known as quark-gluon plasma (QGP), began in the mid-1980s with experiments at CERN’s Super Proton Synchrotron (SPS) in Europe, and Brookhaven’s Alternating Gradient Synchrotron in the US. In 2000, the search moved onto the Relativistic Heavy Ion Collider (RHIC) at Brookhaven. A deconfined quark phase is predicted by numerical solutions of quantum chromodynamics (QCD) at finite temperature, and so its identification would be a dramatic manifestation of the theory that governs strong interactions.

The idea of organizing a Pan American advanced study institute (PASI) under the auspices of the US National Science Foundation’s Americas programme was first advanced in early 2000. A number of heavy-ion researchers recognized that such an institute could help emerging research groups in Latin America, and would serve as a focus for potential new participants from widely scattered and distant locations in the region. Planned as an advanced school rather than a conference or workshop, the objective of the institute was primarily pedagogical, although the presentation of the most recent results in both theory and experiment was also on the agenda. Of equal importance, however, was PASI’s role as a meeting place where participants could forge new working relationships, and the selection of the small Brazilian mountain resort of Campos do Jordão as the venue was made with this in mind.

The participation reflected a good regional balance, with 20 US and European physicists, and 28 Latin Americans. There were 5 US/European and 16 Latin American postdocs, and 23 US/European and 36 Latin American students. The primary lecturers delivered hot-off-the-press scientific material on all aspects relevant to heavy-ion research, including the theory of strong interactions and experimental heavy-ion collision results. Lectures ranged from elementary tutorial-style introductions to technical seminars at the forefront of the subject. The study of the properties of QGP and the confining vacuum structure was at the heart of the meeting. Daily discussion sessions allowed the principal lecturers and the participants to interact. One afternoon was devoted to a survey of research activities in Latin America. The school was supported by the US National Science Foundation and Department of Energy, Brazilian federal and state agencies (CNPq, FAPERJ, FAPESP, the Federal University of Rio de Janeiro and the University of Sao Paulo), and by Germany’s GSI laboratory.

Broad programme

cernmatter2_10-02 — Poster sessions gave room for lively discussion. Left to right: Guido Altarelli, Robert Thews and Thomas Trainor. (Takeshi Kodama.)

Wit Busza of MIT presented a general introduction course to the physics of heavy-ion collisions. His lectures were complemented by several brilliant tutorials on the foundations of this interdisciplinary field. These were given by Guido Altarelli of CERN on QCD, Takeshi Kodama of Rio de Janeiro on relativistic gases, Berndt Muller of Duke University on the properties of quark matter, Johann Rafelski of Arizona on hadrochemistry, and Columbia’s Bill Zajc on the correlation observables such as Hanbury-Brown and Twiss (HBT) intensity interferometry.

The experimental programmes were thoroughly covered in several courses. John Harris of Yale and Bill Zajc gave a survey of the RHIC physics programme and surveyed the latest results from the Brookhaven collider’s STAR and PHENIX experiments. Christof Roland of MIT presented the latest work of another RHIC experiment, PHOBOS. CERN’s SPS experimental programme was reported by Federico Antinori of Padova, while Karel Safarik of CERN took the students into the instrumental realm of particle tracking in a detailed and fascinating lecture. In his presentation, Seattle’s Tom Trainor clearly showed the complexity of some experimental results.

The study of vacuum structure, confinement, lattice gauge theories and quantum transport was addressed by Dima Kharzeev of Brookhaven, Adriano Di Giacomo of Pisa, Frithjof Karsch of Bielefeld, and Hans-Thomas Elze of Rio de Janeiro. Bira van Kolck of Arizona and Brookhaven introduced the power of effective theories in the study of strong interactions, while Jörg Aichelin of Nantes showed how this method can be used in the study of phase transition dynamics.

These hard-core theoretical topics were accompanied by more gentle phenomenological courses by Bob Thews of Arizona on charm, Klaus Werner of Nantes on event generators, and Laszlo Csernai of Bergen on hydrodynamics and flow. Frankfurt’s Horst Stöckers’ survey of QGP signals relied on exotica such as stable black hole formation in heavy-ion collisions, quite in contrast to the more traditional discussion of strangeness and entropy evidence for the formation of deconfined states, offered by Arizona’s Johann Rafelski.

Among more than 15 seminars and lectures reporting the latest progress were many notable theoretical and experimental contributions. Frederique Grassi of Sao Paulo introduced open issues in particle emission, US PASI student Paul Sorensen from UCLA surveyed azimuthal anisotropy in strange particle production, and Latin American PASI student Javier Castillo discussed multistrange particle production at RHIC.

Constantino Tsallis of Rio de Janeiro gave the institute’s final lecture on non-extensive statistics and its applications, leading to a discussion which carried on well into dinner. Rainy weather hampered the programme of excursions to the local countryside, but ensured that the lecture rooms were always full of enthusiastic students, which was both stimulating for the lecturers and very promising for the future of the field.

Muon magnetism study reveals new twist

cernnews6_9-02 — The muon storage ring used for Brookhaven’s dedicated muon g-2 experiment. (BNL.)

In the latest twist to the US Brookhaven laboratory’s high-precision muon magnetism experiment, researchers announced in July a result slightly at odds with the Standard Model of particle physics.
The Brookhaven experiment measures the quantity g-2 for the muon, where g is the particle’s so-called gyromagnetic ratio – the ratio of its magnetic moment to its spin angular momentum. According to classical Dirac quantum theory, this should be exactly two. However, additional small effects can change this value, leading to a non-zero g-2.

The quantity g-2 is a very sensitive probe for testing the Standard Model, so when in 2001 the Brookhaven team announced a result at odds with Standard Model calculations, the particle physics world took notice. However, when the theoretical calculations were redone, a small error was found and agreement between theory and experiment was restored.

The original Brookhaven measurement was based on an analysis of 10⁹ muon decays accumulated in 1999. The result announced in July includes a second data sample, accumulated in 2000, which contains four times as much data as the first. The new measured value of g-2 reinforces the earlier result with a total error of 0.7 ppm compared with the 1.3 ppm for the 1999 data measurement. Along with further refinement of the theoretical calculations, it shows a slight discrepancy with the current Standard Model value, differing by between 1.6 and 2.6 times the estimated error of the measurement. This is too small a discrepancy to claim new physics. Given the precision achieved by the Brookhaven experiment, however, it is bound to attract speculation.

Supergravity celebrates quarter of a century

cernsuper1_9-02 — Supergravity researchers gathered last December at Stony Brook, where it all began a quarter of a century ago.

The development of supergravity is a landmark in the intertwined histories of gauge field theory and quantum gravity. Culminating the drive towards higher symmetry within quantum field theory, it opens a door to the unification of all forces, and the possibility for extra dimensions. As a candidate theory of everything, supergravity was eclipsed by strings, yet in recent years it has re-emerged as a key component of “modern” string theory, playing an essential role in connecting field theories with string theories, and string theories with each other. Supergravity is as much on theorists’ minds now as it was in the mid-1970s. On 3-4 December 2001, an international meeting, “Supergravity at 25”, was hosted by the C N Yang Institute for Theoretical Physics at the Stony Brook campus of the State University of New York, to commemorate the anniversary of this protean theory, and to assess its ongoing role today.

The modern era of field theory began with the discovery of nonabelian gauge invariance by Chen Ning Yang and Robert L Mills in 1954. A decade and a half later, Gerard ‘t Hooft and Martinus Veltman famously proved that a large class of nonabelian gauge theories can be quantized and renormalized consistently.

Before long, the elements of the Standard Model had fallen into place, and theorists hurried forward toward a “grand” unification of the strong and electroweak interactions.

Charmed circle

Gravity, however, remained outside this charmed quantum circle. The force in quantum gravity is carried by the spin-2 graviton, and could not immediately be unified with Standard Model forces, carried by the spin-1 photon, weak bosons and gluons. The same spin-2 graviton leads to particularly aggressive high-energy behaviour. This results in a hopelessly ambiguous theory, through the proliferation of new infinities at each level of calculation. In technical terms, Einstein’s gravity is not renormalizable. The first inspirations for supergravity were to address these two problems. The novel element was another form of unification, supersymmetry, which relates bosons and fermions. According to this theory, there exists for every boson in nature a fermionic partner, and vice versa. The fermionic partner of the graviton is the gravitino. CERN will search for superpartners of Standard Model particles at the Large Hadron Collider. Within supersymmetry, it was hoped that gravity would be united with other forces, and, through a special combination of particles and interactions, would even turn out to be finite.

cernsuper2_9-02 — Fig. 1: the progression of field theory from QED to supergravity.

In this context, the progression of field theory from quantum electrodynamics (QED) to supergravity is illustrated in figure 1. The basic interaction of QED is the emission of a photon (g) from a charged particle, like a quark (q), with an amplitude proportional to the quark’s electric charge, (Q_q in figure 1a). We say that the photon is the gauge particle of the electric current. The photon itself, however, is electrically neutral. At the next level of complexity, nonabelian gauge theories like quantum chromodynamics (QCD) introduce an array of “colour” charges, each of which is conserved. In this case, the gluon (G) is the gauge particle of the colour currents, and carries them as well. Thus, when a gluon is emitted (figure 1b), it connects quarks of different colour, with an amplitude proportional to the strong coupling (g_s). All of the colour charges are conserved in the full system of quarks and gluons, and the gluon is represented as a double line to emphasize its colour structure.

In supergravity, currents that describe the flow of energy, momentum and spin combine into a set, called the supercurrent multiplet, analogous to the currents of electric and colour charges in QED and QCD. Part of this multiplet is the energy-momentum tensor, but part is the “supercurrent” itself, a hybrid field with a vector and a spinor index, related to the energy-momentum tensor by a supersymmetry transformation. The spin-2 graviton is the gauge particle of the energy-momentum tensor, while its “superpartner” the spin-3/2 gravitino (y_m) is the gauge particle of the supercurrent. The gravitino is emitted with an amplitude proportional to the energy carried by this current, multiplied by the square root of Newton’s constant (figure 1c). The gravitino (with an extra arrow in the figure to emphasize its spin structure) carries no electric and colour charges, but connects particles with different spin, in this case a quark and scalar supersymmetric partner “squark” (~q). It thus reveals the underlying unity of the quark and squark within their own supermultiplet, in much the same way that the gluon connects quarks that are unified within a multiplet of three colours.

The development of supergravity 25 years ago may be thought of as the exercise of identifying a minimal set of interactions between gravitons and gravitinos that respects general co-ordinate invariance and makes supersymmetry a gauge symmetry. Today this is as routine as writing down the Lagrangian for a Yang-Mills theory. In 1976, however, it was not even clear that it was possible. The task of formulating the minimal supergravity theory was accomplished by Sergio Ferrara, then at the Ecole Normale Supérieure, and Daniel Freedmann and Peter van Nieuwenhuizen of Stony Brook (see further reading list). At the December meeting, they recalled days of alternating hope and despair, which reached a climax one evening in the spring of 1976, when 2000 terms generated by an infinitesimal supersymmetry transformation were miraculously cancelled by computer. With this result, supergravity moved from conjecture to consistency. Their approach, which they called the “Noether method”, was based on building the correct transformation laws by retracing the reasoning of Emmy Noether’s famous theorem connecting symmetries and conservation laws. Shortly afterwards, Stanley Deser of Brandeis University and Bruno Zumino of CERN gave a useful reformulation (see further reading list), and in the early months and years of supergravity, other approaches gave further insights and dramatic simplifications. A systematization of what quickly became a veritable zoo of supergravity theories was provided by the superspace approach of Julius Wess and Zumino, developed by S James Gates, Jr and Warren Siegel, and the tensor calculus developed by Ferrara and van Nieuwenhuizen, and independently by Kellogg S Stelle of Imperial College, London, and Peter West of King’s College, London.

Supergravity today

Participants at the symposium included the original authors, along with others, such as Marc Grisaru of Brandeis University, who recalled classic investigations into the new theory. Many presentations, however, addressed the role of supergravity today. It fell to string theory to provide a finite quantum theory of gravity, matter and other forces. Our grasp of string theory is incomplete, however, because we lack an understanding of its ground (vacuum) state, and in the larger sense, its non-perturbative spectrum of states. This is one area where supergravity is central to the study of string theory. At the meeting, aspects of the dualities between different string theories were discussed by Bernard Julia of the Ecole Normale Supérieure, Igor Klebanov of Princeton, and West. These dualities have led to a compelling conjecture that all the consistent (10-dimensional) string theories are actually different vacua of a single underlying theory, M-theory, whose low-energy limit is 11-dimensional supergravity. Properties of 11-dimensional supergravity do indeed shed light on string theories in 10 dimensions. In addition, many special solutions to the supergravity equations of motion can be identified with objects in string theory called D-branes, a theme discussed at the conference by Gary Gibbons of Cambridge University, Pietro Fré of Turin, and Kostas Skenderis of Princeton. D-branes are central to a program described by Ashoke Sen of India’s Harish-Chandra Research Institute, for a closed-string theory based on open strings, and aspects of D-brane dynamics were discussed by Michigan’s Michael Duff, Stockholm’s Ulf Lindström, and John Schwarz of Caltech. Bernard de Wit of Utrecht and Ferrara discussed recent developments in supergravities with more than one supersymmetry.

Supergravity is also central to a remarkable discovery called the AdS/CFT correspondence, which relates supergravity in higher-dimensional anti-de Sitter (AdS) space-time (a space-time with constantly negative curvature) to strongly coupled gauge field theories (CFT). This correspondence, which relates quantum correlations in the field theory to classical solutions of supergravity, was discussed by Freedman, Klebanov, Emery Sokatchev from CERN, Ergin Sezgin from Texas A&M, Arkady Tseytlin from Ohio State and Nicholas Warner from the University of Southern California (USC). Supergravity’s possible role in cosmology was discussed by Renata Kallosh of Stanford.

In part, the meeting celebrated the influence of supergravity (thousands of papers have the word in their title, and thousands more list it as a keyword). Even more impressively, it demonstrated its vitality. Though supergravity is 25 years old, the conference had the excitement and energy characteristic of a recent discovery. Only half in jest, some participants looked forward to new and unexpected developments to be celebrated on supergravity’s 50th birthday.

Testing models for quantum gravity

The theory of general relativity provides an appealing way of understanding gravitational dynamics, by perceiving the nature of the gravitational force as being due to a non-trivial geometry of space-time.

The predictions of this revolutionary theory were verified experimentally almost immediately after its proposition, making Einstein an instant success. However, general relativity is a purely classical theory. Quantizing it has so far proved to be a formidable task, which is far from complete. Due to the theory’s unconventional form, as compared with the rest of the fundamental interactions in nature, a mathematically consistent and complete theory of quantum gravity remains elusive.

Theoretical approaches

The discovery of string theory opened up novel and unconventional ways of attacking the problem. By viewing gravitons, the hypothesized carriers of the gravitational interaction, as one of the excitations of the (closed) superstring ground state, the first mathematically consistent framework of unification of gravity with the rest of the fundamental interactions (strong and electroweak) could be achieved. However, this approach is also incomplete. The last decade has revealed a much richer structure of the theory, consisting not only of one-spatial-dimensional objects (strings), but also of higher-dimensional membrane solitonic structures (such as D-branes) whose dynamics are still not well understood. String theory itself, however, has not yet given a complete answer to the question of what happens when quantum matter interacts with singular space-time backgrounds such as black holes.

cernquant1_9-02 — Fig. 1. An artist’s impression of space–time foam in some models of quantum gravity: tiny (average size 10^–35 m) and short-lived (average lifetime 10^–43 s) fluctuations of space–time, with non-trivial topology and/or singular geometry (such as black holes), give the ground state of quantum gravity the nature of a stochastic medium. Propagation of ordinary matter in such backgrounds may result in non-trivial “optical properties” such as a refractive index, modifications of dispersion relations, deviations from normal quantum-mechanical evolution, light-cone fluctuations and violations of Lorentz invariance.

In the framework of local field theory, John Wheeler and Stephen Hawking have suggested that microscopic quantum space-time fluctuations of black hole type, with size of the order of the Planck length (10^-35 m), characterize the quantum-gravity vacuum, giving it a “space-time foamy” nature (figure 1). Interaction of matter with such backgrounds may result in the loss of quantum coherence. This in turn could lead to significant deviations from the standard quantum-mechanical behaviour of matter particles, even at scales much lower than the characteristic quantum-gravity (Planck) energy scale of 10¹⁹ GeV. This is because gravity is a non-renormalizable interaction, with a dimensionful coupling constant, and so it may manifest itself at much lower scales. An example of such a phenomenon is provided by the so-called charge-parity (CP) discrete symmetry, whose violation manifests itself at scales much lower than the characteristic scale of the underlying weak interactions responsible for the effect.

However, the existence of space-time foam effects has been questioned, in particular by string theorists. In superstrings, for certain specific classes of black holes, it is possible to count precisely the microstates that constitute the internal black hole structure by virtue of string dualities (certain discrete gauge symmetries of strings). This has prompted the conjecture that in string theory there is no loss of information during the interaction of string matter with singular space-time backgrounds, and therefore no loss of quantum coherence. If this is true for every singular space-time background, it would be a manifestation of the so-called holographic conjecture of ‘t Hooft and Susskind, according to which any information that enters the horizon of a black hole (a sort of space-time boundary) is encoded on the boundary and no information is lost.

Unfortunately, the situation may not be that simple. Physicists have so far been unable to demonstrate the holographic principle rigorously outside certain restricted classes of stringy black hole backgrounds. Moreover, within the modern context of brane theory there have been theoretical arguments demonstrating that it is impossible to describe the formation of a black hole by collapsing string matter, or the final stage of its evaporation by purely unitary quantum methods.

Liouville strings

It is at this point that alternative paths within the string framework have been proposed. One is the so-called Liouville (non-critical) string, a sort of non-equilibrium string theory allowing for a stochastic description of space-time foam backgrounds. In such models, gravitational degrees of freedom, unobservable by low-energy observers, constitute an “environment” that results in the non-equilibrium nature of the underlying string theory. Technically speaking, the foam background is not conformal (its world-sheet dynamics are not invariant under special local-scale transformations). Nevertheless, with certain restrictions such theories are still mathematically consistent.

Time plays a particularly special role in Liouville strings, where it is identified with a world-sheet renormalization-group scale, the Liouville mode itself. In such a picture, the irreversibility of the temporal flow is guaranteed by powerful remormalization-group-flow theorems of the 2D world-sheet geometry. Such an irreversible flow has immediate consequences for entropy production, associated with the non-equilibrium nature of the Liouville string.

On the other hand, it has been argued that a Liouville-string irreversible time flow may play an important role in cosmological scenarios, in particular in relaxation models of the universe, allowing for the possibility of an exit from an accelerating universe (de Sitter) phase, as well as a relaxing-to-zero vacuum energy. Such models provide a natural explanation for the smallness of the present-era value of the cosmological constant, consistent with recent astrophysical claims. It is therefore interesting to note that the same mathematical framework of the Liouville string, which provides a picture for the space-time foam, can also describe a cosmological model of the universe with correct phenomenology.

The fundamental issue of the nature of time probably holds the key to unravelling a consistent theory of quantum gravity. The relevant phenomenology, therefore, which has already enjoyed almost two decades of intense research, may also contribute significantly to an understanding of such fundamental questions.

For the time being, however, string theories seem to suffer from an important drawback, which probably prevents a complete understanding of quantum gravity issues within this framework. This is their background dependence – the fact that the whole formalism of strings, at least so far, is based on specific space-time backgrounds. A satisfactory theory of quantum gravity is expected to be capable of describing the generation of the space-time structure in a dynamical way from more fundamental units. This may be possible in string field theory, but the subject is at present in its infancy.

This property of background independence seems to characterize another major approach towards the quantization of gravity, the so-called loop quantum gravity. This is a way of canonically quantizing gravity starting from a theory of abstract “spin-foam networks”. These are the fundamental units from which the fabric of space-time is generated dynamically in a mathematically self-consistent and elegant formalism, and hence the approach is background independent by design. At present it is not known whether the theory is related to (Liouville) string theory. Moreover, no significant progress has been made towards an understanding of issues relating to information loss in quantum black hole backgrounds or the unification of gravity with the rest of the interactions. There is intense ongoing research in this field, and such questions may soon be tackled. For our purposes it is sufficient to mention that loop quantum gravity, like Liouville strings, appears to provide a theoretical framework for a discussion of stochastic space-time foam dynamics.

Experimentally falsifiable predictions

It is of primary importance to attempt to make physical, experimentally falsifiable predictions from these different frameworks of quantum gravity. At first sight this may seem wishful thinking for any theory of quantum gravity, whose fundamental length scale is far too small to be directly visible. However, since gravity is a non-renormalizable interaction, effects associated with it might be visible at energy scales much lower than the Planck scale.

cernquant2_9-02 — CERN’s CPLEAR experiment displayed sensitivity not far from the theoretically estimated magnitude of CPT violating effects in minimal-suppression quantum-gravity models. A new generation of experiments, at Frascati and Fermilab, should reach the required level of sensitivity.

One possible imprint of quantum gravity at lower scales might be the loss of quantum coherence that characterizes certain models of quantum gravity. The imprint would be a violation of CPT symmetry (T=time reversal), whose conservation is a theorem of any local unitary field theory without gravity, but whose failure is almost guaranteed in a theory with highly curved space-time structure such as a space-time foam. The violation of such symmetry occurs through a modification of the standard quantum-mechanical evolution of matter, which is somewhat similar in nature to the behaviour characterizing open systems in stochastic media. One of the most sensitive probes for such effects is neutral kaons and other mesons. The recently completed CPLEAR experiment at CERN has sensitivity not far from the theoretically estimated magnitude of such CPT violating effects in minimal-suppression models. If the Planck energy scale minimally suppresses quantum-gravity effects, then such effects could be falsified at new experimental meson facilities, for example at Frascati in Italy and Fermilab in the US.

A second effect, which may also be falsified in the immediate future, is a sort of mean-field effect of quantum-gravity foamy situations, according to which the propagation of ordinary particles in this medium results in a modification of the particle’s dispersion relation. This effect was first predicted in the framework of Liouville strings. A similar effect has since been shown to characterize the loop-gravity approach. Modified dispersion relations have also been postulated independently at a phenomenological level by researchers proposing a thermal-like nature of the quantum gravity environment or inspired by condensed matter situations. A modification of the dispersion relation for photons or other particles with no mass implies a non-trivial refractive index in vacuo, in other words a frequency-dependent velocity of light. The origin of this effect can be traced back to the spontaneous violation of Lorentz invariance that may characterize the ground state of such theories.

Such effects are also known to describe local field theory situations in non-trivial vacua, such as thermal vacua, or quantum field theories in the vicinity of black hole backgrounds, even allowing for superluminal signals without violations of causality. However, the effect predicted by the stringy (Liouville) approach to quantum gravity has two distinctive features: the effect always leads to subluminal signals, and it is enhanced with increasing energy of the particle probe, in contradistinction to the field-theoretical cases where the effect decreases with energy. In contrast to the stringy case, the loop gravity approach predicts superluminal signals as well.

The Liouville approach to quantum gravity also predicts stochastic light-cone fluctuations. The latter have also been conjectured to characterize some non-trivial theories of local quantum gravity involving coherent gravitons. Light-cone fluctuations imply stochastic fluctuations of the speed of light in vacuo, which will imply stochastic fluctuations in the arrival times of photons of the same frequency in contrast to the refractive index effect, which implies arrival-time fluctuations for photons of different frequencies. The stochastic effect in string theory is suppressed (as compared with the refractive index effect) by powers of the (weak) string coupling expressing the strength of the interactions of string matter.

Astrophysical probes

cernquant3_9-02 — Fig. 2. A typical search for microstructure in the light curves of a GRB can be used for placing bounds on (or detecting) the non-trivial refractive index and light-cone fluctuation effects of some models of stochastic quantum gravity. In Liouville-string models, higher-energy channels are delayed more, due to a subluminal refractive index effect of quantum gravity, and the width of the GRB pulse is wider, due to stochastic fluctuations, compared with lower-energy channels. The linear-regression plot shows the correlation of the difference in arrival times of the peaks of the GRB between two energy channels Δt_p with the redshift z, which is a characteristic feature of such effects. At present, the number of GRBs whose distances are known is statistically insignificant, but it is hoped that new GRB dedicated searches will be able to provide statistically significant populations of GRB with known redshifts. This would allow more stringent bounds, or detection, of stochastic quantum-gravity effects.

One of the first probes suggested for the experimental falsification of the refractive index and the stochastic light-cone fluctuation effects was gamma-ray bursts (GRBs). Experimental tests of such effects would look for microstructure in the arrival time of light beams from a GRB (figure 2) and a correlation with distance (redshift). If the emission of photons at various energies were simultaneous within the standard quantum mechanical limits, then a refractive index effect would imply differences in the arrival times of photons at different energies. In Liouville string models the subluminal nature of the effect implies that the higher energies would be delayed more. In contrast, loop-gravity-inspired models are characterized by both superluminal as well as subluminal propagation, and as a result there would be birefringence effects, which are absent in the stringy models. On the other hand, as far as stochastic light-cone fluctuations are concerned, there would be fluctuations in the arrival times of photons at the same energy. A systematic theoretical error analysis is needed, however, given that it is possible that photons at different energies are not emitted simultaneously.

Some of the GRBs observed so far are characterized by microstructures in their light curves of less than a millisecond, and are likely to emit gamma rays in the GeV or even TeV energy regions. Since many of these GRBs are known to be at cosmological distances (of the order of 3000 Mpc or larger), the sensitivity of these high-energy channels to the quantum-gravity refractive index and stochastic effects is such that they can be falsified with the next generation of satellite GRB-dedicated facilities, such as NASA’s Gamma-ray Large Area Space Telescope (GLAST) and the AMS experiment. Similar sensitivities might be achieved by terrestrial and extraterrestrial interferometric devices, which are also capable of detecting these stochastic quantum-gravity effects as part of the noise in the resulting interference patterns.

cernquant4_9-02 — Experiments such as NASA’s GLAST will be able to differentiate between some quantum gravity models by studying gamma-ray bursts. (Hytec.)

Other astrophysical probes of the stochastic quantum-gravity effects may be provided by ultra-high-energy cosmic rays (UHECR) with energies above 10¹⁹ eV, as well as by TeV photons. The presence of such events seems puzzling from the point of view of Lorentz invariance – standard kinematics imply the existence of energy thresholds, the Greisen, Zatsepin, Kuzmin (GZK) cut-off, above which certain reactions would prevent such energetic particles from reaching the observation point, assuming an extra-galactic origin. Some exotic suggestions have been made to relate Lorentz invariance violation associated with the quantum-gravity-induced modification of the particle’s dispersion relations with the existence of UHECR or TeV photons, in the form of an abolition of the GZK cut-off in such models.

High-energy cosmic neutrinos are also sensitive probes of quantum-gravity effects. For instance, if a minimally suppressed refractive index for neutrinos is valid, then ultra-high-energy neutrinos of 10¹⁹ eV, which may be emitted from GRBs, will be completely dispersed and thus unobservable, according to models with minimal Planck-scale suppression. The observation of such energetic particles would therefore exclude such models right away. Minimal suppression models of quantum-gravity foam with baryon-number violating interactions are in fact already excluded on the basis of solar and extra-galactic neutrino observations, since such effects would lead to neutrino oscillations in direct conflict with the experiment.

Last, but not least, bounds on stochastic effects significantly more stringent than those placed by astrophysical observations may be obtained by means of atomic physics experiments. Such experiments are mainly concerned with measurements of quantum-gravity induced corrections to the gyromagnetic ratio of charged particles, which can be measured with high accuracy.

In summary, a plethora of relatively low-energy measurements can be made, some of which are already on the verge of excluding Planckian physics models. Any future experimental attempt to bind models of quantum gravity should therefore be encouraged, since they present the only way to arrive at a true theory of quantum gravitation, which will probably lead to a better understanding of the structure of space-time itself. The concept of a phenomenology of quantum gravity may soon no longer be considered oxymoronic.

Cosmic-ray apparatus images Moon’s shadow

cernnews2_7-02 — The deficit of primary cosmic rays is clear in these pictures, covering two muon energy ranges. In each, a circle indicates the real position of the Moon. The axes correspond to the parallel and perpendicular directions of the deflection in degrees. The Moon's shadow is displaced more in the sample corresponding to the lower-energy cosmic rays (left).

A dedicated cosmic-ray experiment making use of the muon identification system of CERN’s L3 experiment collected around 12 x 10⁹ cosmic ray muons between April 1999 until L3 finished taking data in 2000. The goal of the so-called L3 plus cosmics (L3+C) experiment is to study the cosmic muon spectrum precisely over the range 20-2000 GeV/c, allowing for normalization of the calculated muon-neutrino spectrum. Other topics under analysis by the experiment include the search for point-source burst signals, exotic events and studies of the composition of very-high-energy primary cosmic rays around 10¹⁵ eV, made possible by coupling the L3+C apparatus to a surface scintillator array.

Also interesting is the ability of the apparatus to image the shadow of the Moon. High-energy muons indicate the direction of the primary cosmic-ray particle. Since the Moon can absorb primaries – usually protons – on their way to Earth, the L3+C apparatus saw fewer muons originating from that direction and so observed a shadow. Moreover, since the Earth’s magnetic field deviates the course of charged primaries, positive particles are shifted eastwards and the corresponding shadow is shifted to the west. The extent of the shift and the shape of the shadow depend on the momentum of the primary and the size and the direction of the field along its path.

Antiprotons, if any, would be deviated in the opposite direction, giving rise to an “anti-shadow”. No such feature is seen. Work is continuing to set a limit to the antiproton to proton ratio in an energy range around 1 TeV, well above the present range of direct measurements, which extend up to 50 GeV.

Fermilab experiment hints at double charm

cernnews4_7-02 — Two of the SELEX candidates have spin 1/2 and fit the description of singly and doubly charged X particles. Their mass difference, however, is larger than expected.

The SELEX experiment at Fermilab has announced three candidates for doubly charmed baryons. In a presentation at the US laboratory at the end of May, collaboration co-spokesman Jim Russ of Carnegie Mellon University described the painstaking analysis of data recorded in 600 GeV proton, pion and sigma hyperon collisions with a diamond target in 1996. The observation comes as a surprise to the collaboration, which did not expect to see doubly charmed baryons at all, and there remain uncertainties over the conclusion to be drawn. The SELEX candidates have the right characteristics to be up-charm-charm and down-charm-charm combinations, but the mass difference between the two states is larger than expected. Like the proton and neutron, the candidate particle pair is related by the replacement of an up by a down quark, so the mass difference was naively expected to be similar. SELEX, however, sees a mass difference 60 times larger.

The collaboration chose to make its announcement in the hope that doing so would spur the broader community to take up the search. In particular, data from the BaBar and Belle collaborations at the SLAC and KEK B-factories in the US and Japan would be good places to look for doubly charmed baryons.

Multiply charmed baryons are a natural consequence of the quark model of hadrons, and it would be surprising if they did not exist. Whether or not the high-mass states that SELEX reports turn out to be the first observation of doubly charmed baryons, studying their properties is important for a full understanding of the strong interaction between quarks.

New dipole moment centre inaugurated

cernnews6_7-02 — Research fellow Jonathan Hudson is seen here at work at the University of Sussex’s new Centre for the Measurement of Particle Electric Dipole moments.

A new laboratory inaugurated in May at the University of Sussex, UK, aims to shed light on matter-antimatter asymmetry by measuring the electric dipole moment of the neutron. The new Centre for the Measurement of Particle Electric Dipole Moments has been created thanks to a £1.7 million (€2.6 million) award from the UK’s Joint Infrastructure Fund.

The B-ology of physics

cernbphys1_7-02 — The muon pair spectrometer which discovered the upsilon resonances at Fermilab in 1977. (Fermilab Visual Media Services.)

A quarter of a century ago, particle physicists were accustomed to making a major discovery more or less every year. Some of the greatest drama was provided by the highly unexpected announcement of the J/psi particle in 1974 – the “November Revolution”. After the initial amazement had died down, physicists learned that nature contained a fourth “charm” quark that augmented the traditional up-down-strange triplet.

However, even before this discovery, Makoto Kobayashi and Toshihide Maskawa had pointed out that more than four quarks were possible. To explain the mysterious violation of CP symmetry, they had suggested that six types of quark could be present. And so it came to be.

But in 1974, many physicists had trouble digesting a fourth quark, and initially paid little attention to a call for any more. In the newly extended four-quark picture, the charm quark was paired with the strange quark to form a second doublet, a heavier counterpart of the up-down pair which make up the protons and neutrons of stable nuclear matter. These two quark doublets could also be associated with two then-known electrically charged weakly interacting particles (leptons), the muon and the electron. The muon, being heavy, went with the heavier charm-strange quark pair, while the electron was associated with the lighter doublet.

Then, in the mid-1970s, a group led by Martin Perl working at the SPEAR electron-positron collider at the Stanford Linear Accelerator Center (SLAC) discovered a third electrically charged lepton, the tau, which was much heavier than the muon. It took time for the idea to be accepted, but a third charged lepton called for a third doublet of quarks, making the sextet suggested by Kobayashi and Maskawa.

Talk of heavier quarks became commonplace, and the shadowy third doublet was called either “top” and “bottom” (reflecting the “up” and “down” of everyday quarks) or “truth” and “beauty” by the more romantic. (The “top” label has now eclipsed “truth”, but “beauty” and “bottom” are both widely used. In either case the labels of the third quark doublet can be abbreviated to “t” and “b”.)

cernbphys3_7-02 — Leon Lederman’s insistent study of muon pairs led to the discovery of a fifth kind of quark. (Fermilab Visual Media Services.)

The first awareness that leptons could be served in different ways had come with Lederman, Schwartz and Steinberger’s 1962 neutrino beam experiment at Brookhaven, which showed that neutrinos (electrically neutral leptons) come in two kinds – one associated with electrons, the other with muons. (Describing this discovery in the March 1963 issue of Scientific American, Lederman wrote: “These days, the discovery of a new elementary particle is scarcely news…”)

Neutrino beams became a new physics tool, and one idea was to use them to uncover the W and Z carrier particles of the weak force, which are analogous to the photon carrier of electromagnetism. The signature of a W, the charged carrier of the weak force, would then be a pair of oppositely charged leptons (electrons or muons). This search soon bypassed the need for a neutrino beam altogether, and focused instead on the production of charged lepton pairs at high energy.

The upsilon

The W was not found via this route, but in the late 1960s Lederman’s team at Brookhaven uncovered a bump in the spectrum of muon pairs. The reason for this mysterious effect was not immediately clear, but Lederman’s curiosity was aroused. Physicists had learned that charged lepton pairs (either muons or electrons) could be a pointer to other photon-like particles, created in the annihilation of quarks and antiquarks deep inside subnuclear processes.

In this way, Sam Ting codiscovered the J/psi particle at Brookhaven in 1974. This, with a parallel experiment by Burt Richter at SLAC’s SPEAR ring, catalysed the November revolution. The J/psi is a tightly bound charmed quark-antiquark pair.

Having missed out on the J/psi discovery, which earned a Nobel prize for Richter and Ting, Lederman diligently continued his study of muon pairs, this time in a new energy domain using the Fermilab synchrotron. An initial sighting near 6 GeV triggered a discovery action plan, with the Greek letter upsilon being reserved for a possible new particle. This signal went away, but in 1977, muon pairs began to accumulate at 9.5 GeV. This time it was a new particle.

The upsilons (on close examination, there were several of them) are b quark-antiquark pairs, in the same way that the J/psi is made up of a charmed quark and antiquark. Unlike the J/psi, the upsilon was not discovered via the electron-positron annihilation route, as in 1977 no electron-positron collider had enough energy. Nearest in energy was the DORIS machine at DESY, Hamburg, and following the upsilon news the DORIS beam energy was turbocharged in a crash programme. By the following summer, the PLUTO and DASP detectors at DORIS had seen their first upsilons.

cernbphys2_7-02 — In the 1990s, Cornell’s CESR electron-positron collider became a focus for the physics of particles containing the b quark.

The lightest upsilons are tightly bound and therefore cannot decay into their component b quarks. The next collider to arrive on the B physics scene was Cornell’s CESR electron-positron collider, which came into operation in 1979, and with its higher energy could reveal a full array of upsilon resonances. By the following year, the CLEO and CUSB detectors at CESR were seeing the first B particles (containing the b quark) produced via the decay of the heaviest (4S) upsilon.

For the next few years, these detectors, together with the ARGUS detector at DORIS, were major players in B physics, which went on to become mainstream science at major machines all over the world. In a major effort, the spectroscopy of B particles took shape, and the parameters of the b quark were documented.

In the 1970s, the economical SPEAR electron-positron collider at SLAC showed how effective these machines could be. Bigger colliders – PETRA and PEP – were proposed at DESY and SLAC, respectively. Even bigger was the TRISTAN machine at the Japanese KEK laboratory. Unfortunately, the energy ranges covered by these machines were not as rich in discovery potential as SPEAR’s had been.

B-factories

The next chapter in the B physics saga is being written by a pair of new high-intensity machines which mass-produce B particles – PEP-II at SLAC and KEKB at KEK. Proposed in the mid-1990s, these machines are now making their first precision measurements, and provide the right conditions to explore CP violation in a new setting – B physics. The wheel has turned full circle – after providing the first indication that a third generation of quarks exists, CP violation is now being measured using those quarks.

Eclipsed by the new B-factories after making a decade of milestone contributions, notably with a line of CLEO detectors, the CESR machine at Cornell stopped for B physics in 2001. The physics of b quarks is far from being fully understood, and major mysteries remain. For the future, B physics will continue to be a major focus, notably at Fermilab’s Tevatron collider and with the LHCb detector at CERN’s LHC collider. (The “top” or t quark, the companion of the b quark in the third quark doublet, and the heaviest quark of all, was discovered at Fermilab in 1995, giving that laboratory a proprietary interest in the heaviest quark pair.)

Further information

Twenty years

The 20th anniversary of B physics in 1997 was marked by a symposium at the Illinois Institute of Technology in Chicago, US. The proceedings of this meeting, edited by Ray Burnstein, Daniel Kaplan and Howard Rubin, and published in the American Institute of Physics Conference Proceedings Series (Volume 424), provide a valuable pointer to early developments in B physics. Its title – “Twenty Beautiful Years of Bottom Physics” – underlines the continuing confusion about what to call the fifth (b) quark.

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