In February, 120 physicists travelled to the mountain village of La Thuile in Italy to discuss results and perspectives in particle physics. Michael Koratzinos reports.
Now in its 19th year, the Rencontres de Physique de la Vallée d’Aoste is known for being a vibrant winter conference, where presentations of new results and in-depth discussions are interlaced with time for skiing. Taking place in La Thuile, a village on the Italian side of Mont Blanc, it consistently attracts a balanced mix of young researchers and seasoned regulars from both theoretical and experimental high-energy physics. The 2005 meeting, which took place from 27 February to 5 March, was no exception.
As well as the standard sessions on particle physics, cosmology and astrophysics typical for such a conference, the organizers always try to include a round-table session on a topical subject, as well as a session on a wider-interest topic that tackles the impact of science on society. This year, the first of these sessions was Physics and the Feasibility of High-Intensity, Medium-Energy Accelerators, and the second was The Energy Problem.
Dark energy, WIMPs and cannon balls
An increasing number of experiments are trying to answer questions in high-energy physics by taking to the skies, making the distinction between particle physics and astronomy more fuzzy. The first session of the conference presented an impressive array of experiments and results, ranging from gravitational-wave detection to gamma-ray astronomy. The team working on the Laser Interferometer Gravitational-Wave Observatory (LIGO), with two fully functioning antennas 3000 km apart, now understands the systematics and has begun the fourth period of data-taking with improved sensitivity.
In gamma-ray astronomy, ground-based detectors – which detect the Cherenkov light emitted when gamma-ray-induced particle showers traverse the atmosphere – are constantly improving. The High Energy Stereoscopic System (HESS) in Namibia became fully operational in 2004 with a threshold of 100 GeV, while new detectors with thresholds as low as 20 GeV are in the pipeline. Satellite-based gamma-ray detectors have also provided some excitement, with the Energetic Gamma Ray Experiment Telescope (EGRET) observing an excess of diffuse gamma rays above 1 GeV, uniformly distributed over all directions in the sky.
This excess could be interpreted as due to the annihilation of neutralinos. The neutralino is the supersymmetric candidate of choice as a weakly interacting massive particle (WIMP) – a popular option for the dark matter of the universe. This prompted Dmitri Kajakov of the Institute for Theoretical and Experimental Physics (ITEP), Moscow, to state that “dark matter is the supersymmetric partner of the cosmic microwave background”, since neutralinos can be thought of as spin-½ photons.
The Gamma-Ray Large Area Space Telescope (GLAST) satellite, launching in 2007, will offer an important improvement in gamma-ray astronomy, with sensitivity to 10,000 gamma-ray sources compared with EGRET’s 200.
The DAMA/NaI collaboration raised some eyebrows. It reported an annual modulation of 6.3σ significance in data observed over seven years in its nuclear-recoil experiment at the Gran Sasso National Laboratory, which stopped taking data in 2002. This modulation could be interpreted as due to a WIMP component in the galactic halo, which is seen from Earth as a “wind” with different speeds, depending on the annual cycle. The collaboration’s study of possible backgrounds has not identified any process that could mimic such a signal, but other experiments have not observed a similar effect. The new set-up, DAMA/LIBRA, which is more than twice as big and started taking data in 2003, might shed some light.
Another way of looking for WIMPs is through their annihilations that produce antimatter. Antimatter in the universe is not produced in large quantities in standard processes, therefore any excess of antimatter seen would be exciting news for WIMP searchers. The Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics (PAMELA) satellite due to be launched later this year will provide valuable data on antiproton and positron spectra.
Alvaro De Rújula of CERN, using traditional (and increasingly rare) coloured transparencies written by hand, gave an account of his theory of gamma-ray bursts (GRBs), which has now developed into a theory of cosmic rays. Central to the theory are the cosmic “cannon balls”, objects ejected from supernovae with a density of one particle per cubic centimetre, and with a mass similar to that of the planet Mercury but a radius similar to that of the orbit of Mars. These cannon balls, moving through the interstellar medium at high speeds (with initial γ factors of the order of 1000), not only explain GRBs and their afterglows in a simple way, but also explain all features of cosmic-ray spectra and composition, at least semi-quantitatively, without the need to resort to fanciful new physics. What the theory does not attempt to explain, however, is how cannon balls are accelerated in the first place.
Dark energy was reviewed by Antonio Masiero of the University of Padova. Masiero pointed out that theories that do not associate dark energy with the cosmological constant do exist. One can assume, for instance, that general relativity does not hold over very long distances, or that there is some dynamical explanation, like an evolving scalar field that has not yet reached its state of minimum energy (known as a quintessence scalar field), or even that dark energy is tracking neutrinos. With the latter assumption, he came to the interesting conclusion that the mass of the neutrinos depends on their density, and therefore that neutrino mass changes with time. The cosmological constant or vacuum-energy approach, however, offers the less exotic explanation of dark energy.
Finally, Andreas Eckart of the University of Cologne reviewed our knowledge of black holes, with emphasis on the massive black hole at the centre of our own galaxy, Sagittarius A*. He played an impressive time sequence of observations taken over 10 years of the vicinity of this black hole, showing star orbits curving around it.
The golden age of neutrino experiments
The neutrino session began with Guido Altarelli of CERN, who reviewed the subject in some depth. Although impressive progress has been made during the past decade, there are unmeasured parameters that the new generation of experiments must address. The Antarctic Muon and Neutrino Detector Array (AMANDA), which uses the clean ice of the South Pole for neutrino detection, reported no signal from its search for neutrino point-sources in the sky, but the collaboration is already excited about its sequel, IceCube.
The Sudbury Neutrino Observatory (SNO) collaboration has added salt to its apparatus, to increase the detection efficiency by nearly a factor of three compared with the earlier runs. Analysis yields slightly smaller errors on Δm13 than K2K (KEK to Kamiokande), the long-baseline experiment in Japan, which reported on the end of data-taking. K2K is now handing over to the Main Injector Neutrino Oscillation Search (MINOS) in the US, which had recorded the first events in its near detector just in time for the conference. MINOS is similar in conception to K2K, but has a magnetic field in its fiducial volume – the first time in such an underground detector – and it will need three years of data-taking to provide competitive results.
The director of the Gran Sasso National Laboratory, Eugenio Coccia, gave a status report of the activities of the laboratory, which is undergoing an important safety and infrastructure upgrade following a chemical leak. The laboratory is the host of a multitude of experiments on neutrino and dark-matter physics. These include the Imaging Cosmic And Rare Underground Signals (ICARUS) and Oscillation Project With Emulsion Tracking Apparatus (OPERA) experiments for the future CERN Neutrinos to Gran Sasso (CNGS) project and Borexino, which is the only experiment other than KamLAND in Japan that can measure low-energy solar neutrinos. The laboratory also houses neutrinoless double-beta-decay experiments.
Strong, weak and electroweak matters
In the session on quantum chromodynamics, Michael Danilov of ITEP had the unenviable task of reviewing the numerous experiments that have looked for pentaquarks. In recent years, there have been 17 reports of a pentaquark signal and 17 null results. Danilov justified his sceptical approach by pointing out various problems with the observed signals. The small width of the Θ+ is very unusual for strong decays. Moreover, this state has not been seen at the Large Electron Positron (LEP) collider, although this fact can be circumvented by assuming that the production cross-section falls with energy. However, the Belle experiment at KEK does not see the signal either, weakening the cross-section argument. The Θc is seen by the H1 experiment at HERA, but not by ZEUS or by the Collision Detector at Fermilab (CDF). Finally, many experiments have not seen the Ξ— signal. Although Danilov thinks that the statistical significance of the reported signals has been overestimated, it is still too large to be a statistical fluctuation. The question will only be settled by high-statistics experiments coming soon.
Amarjit Soni of Brookhaven summarized our knowledge of charge-parity (CP) violation by emphasizing the success of the B-factories, the fact that the Cabibbo-Kobayashi-Maskawa paradigm is confirmed, and that we now know how to determine the unitarity triangle angles α and γ, as well as the previously known angle β.
The electroweak session began with a report on new results from LEP, with LEP showing no signs that it has said its final word yet. The running of αQED has been the subject of a new analysis of Bhabha events at LEP. The results from the OPAL experiment, recently submitted for publication, give the strongest direct evidence for the running of αQED ever achieved in a single experiment, with a significance above 5σ. Regarding the W mass, the combined data error for LEP now stands at 42 MeV, whereas at the Tevatron, Run II is being analysed and the error from CDF from 200 fb-1 of data (a third of the collected data) is already less than their published result for Run I. The Tevatron collaborations expect to achieve a 30-40 MeV error on the W mass with 2 fb-1 of data. The search is on for the Higgs particle at Fermilab with a new evaluation of the Tevatron’s reach. For a low-mass Standard Model Higgs, the integrated luminosity needed for discovery (5σ) is 8 fb-1; evidence (3σ) needs 3 fb-1, while exclusion up to 130 GeV needs 4 fb-1.
From high intensity to future physics
The round-table discussion on physics and the feasibility of high-intensity, medium-energy accelerators was chaired by Giorgio Chiarelli of the University of Pisa, and after a short introduction he asked the panel members for their views. Pantaleo Raimondi of Frascati gave an overview of B and f factories and Gino Isidori, also of Frascati, pointed to a series of interesting measurements that can be performed by a possible upgrade to the Double Annular Ring For Nice Experiments (DAFNE) set-up at Frascati, where the time schedule would be a key point.
Francesco Forti of Pisa discussed the possibility of a “super B-factory”. He noted that by 2009, 1 ab-1-worth of B-physics data will be available around the world, and to have a real impact any new machine would need to provide integrated luminosities of the order of 50 ab-1. Roland Garoby of CERN talked about a future high-intensity proton beam at CERN, where the need for a powerful proton driver, a necessary building block of future projects, has been identified. Finally, Franco Cervelli of Pisa reviewed the high-intensity frontier, including prospects for the physics of quantum chromodynamics, kaons, the muon dipole-moment and neutrinos. A lively debate followed.
In the interesting science and society session on alternative energy sources, Durante Borda of the Instituto Superior Tecnico of Lisbon gave a detailed account of ITER, the prototype nuclear-fusion reactor that is expected to be the first of its kind to generate more energy than it consumes. ITER is designed to fuse deuterium (obtained from water) with tritium obtained in situ from lithium bombarded with neutrons, thereby creating helium and releasing heat (in the form of neutrons) captured through heat exchangers. It is hoped that this ambitious project, with its many engineering challenges, will pave the way for commercial fusion-power plants.
This talk was followed by presentations on geothermal, solar, hydroelectric and wind energy, covering a wide spectrum of renewable energy resources. It was clear from the presentations that the problem of future energy production is complicated, and a clear winner has yet to emerge from these alternative energy sources.
In the session on physics beyond the Standard Model, Andrea Romanino of CERN did not make many friends among the community working towards the Large Hadron Collider (LHC) at CERN. He stated that “split supersymmetry” – a variation of supersymmetry (SUSY) that ignores the naturalness criterion – pushes the SUSY scale (and any SUSY particles) beyond reach of the LHC, although within reach of a future multi-tera-electron-volt collider.
Fabiola Gianotti of CERN appeared undeterred. She closed the session and the conference by giving a taste of the first data-taking period of the LHC to come. She reminded the audience that for Standard Model processes at least, one typical day at the LHC (at a luminosity of 1033) is equivalent to 10 years at previous machines.
• The conference series is organized by Giorgio Bellettini and Giorgio Chiarelli of the University of Pisa and Mario Greco of the University of Rome.
