Un colloque fait le lien entre vide et Univers
Le premier colloque Autriche–France–Italie (AFI) intitulé « Du vide à l’Univers » a eu lieu à octobre 2007 à l’Université d’Innsbruck. Inspiré par les dernières avancées en physique des particules et en astrophysique, il s’est intéressé à l’exploration de la physique du vide, ses manifestations dans le monde subatomique et ses conséquences pour la structure à grande échelle de l’Univers. Le confinement des quarks, le boson de Higgs et d’autres recherches liées au LHC, les neutrinos, les rayons cosmiques et la recherche de la matière noire en astrophysique – autant de sujets qui pourraient apporter des éléments neufs sur la structure de l’Univers, que ce soit à l’échelle de la chromodynamique quantique ou à celle des téra-électron-volts.
The first Austria–France–Italy (AFI) symposium, From the Vacuum to the Universe, took place on 19–20 October at the University of Innsbruck. Inspired by developments in particle and astrophysics, it explored the physics of the vacuum, its manifestations in the subatomic world and its consequences for the large-scale structure of the universe. Studies of quark confinement; searches for the Higgs boson and other LHC physics; neutrinos; cosmic rays; and astrophysical probes of dark matter – all promise to reveal vital information about the structure of the universe, from the scale of QCD to tera-electron-volts.
The physical world is built from spin-1/2 fermions interacting through the exchange of gauge bosons: massless spin-1 photons and gluons; massive W and Z bosons; and gravitational interactions. The Pauli exclusion principle (PEP), which says that two identical fermions cannot exist in the same quantum state, is responsible for the stability of the physical world and is a pillar of chemistry. Further ingredients are needed to allow the formation of large-scale structures on the galactic scale and to explain the accelerating expansion of the universe. These are the mysterious dark matter and dark energy, respectively. Current observations point to an energy budget of the universe where just 4% is composed of atoms, 23% involves dark matter (possibly made of new elementary particles) and 73% is dark energy (the energy density of the vacuum perceived by gravitational interactions).
The AFI meeting, with a mix of colloquium talks and discussion sessions, deliberated the interplay of this physics and possible synergies between different methods to learn about the physics of the vacuum. It also considered the use of particle physics to understand problems in astrophysics and the large-scale structure of the universe.
The vacuum is associated with various condensates. The QCD scale associated with quark and gluon confinement is around 1 GeV, while the electroweak mass scale associated with the W and Z boson masses is around 100 GeV. These scales are many orders of magnitude less than the Planck-mass scale of around 1019 GeV, where gravitational interactions are supposed to be sensitive to quantum effects. The vacuum energy density associated with dark energy is characterized by a scale around 0.002 eV, typical of the range of possible light neutrino masses, and a cosmological constant, which is 54 orders of magnitude less than the value expected from the Higgs condensate and no extra new physics. Finally, the mass scale associated with dark matter remains to be determined. The physics of confinement, the origin of electroweak symmetry breaking, the nature of dark matter and why the dark-energy scale is finite and so much less than the electroweak and QCD scales, are fundamental questions for sub-atomic physics and its consequences for the macroscopic world.
For fermions, the VIP Collaboration at Frascati and Gran Sasso is performing precise new tests of the PEP for electrons, as Johann Marton of the Austrian Academy of Science described. These experiments look for anomalous 2p → 1s X-ray transitions in copper. Recent results have reduced the probability of a violation of the PEP by two orders of magnitude, with results of tests to a further two orders of magnitude expected shortly. The parameter characterizing possible PEP violation is currently measured to be β2/2 < 6 × 10–29.
The origin of mass is a fundamental problem in QCD and electroweak physics. In QCD the coupling constant that describes the strength of quark–gluon interactions (and gluon–gluon) grows in the infrared. It becomes so large that the quarks and gluons are confined, and in isolation particles carrying the colour quantum number can propagate a maximum distance of only around 1 fm. Reinhard Alkofer of Karl-Franzens University, Graz, explained that recent studies suggest that confinement works differently in the pure gluon theory and in QCD with light quarks. Ghost loops seem to be important. The physical-confinement mechanism is associated with dynamical breaking of the chiral symmetry between left and right-handed quarks; 98% of the proton’s mass is produced by the binding energy between quarks.
The subtle role of spin-1/2 quarks in the proton is further highlighted by the proton-spin problem, as Fabienne Kunne of CEA/Dapnia described. Polarized deep inelastic scattering experiments at CERN, DESY and SLAC have revealed that only about 30% of the spin of the proton comes from the intrinsic spin of the quarks that it contains. Where is the "missing" spin and why is the quark contribution so small? Possibilities include a topological effect where the spin becomes in part delocalized in the proton, or sea quarks polarized against the direction of the spin of the proton. The COMPASS experiment at CERN, as well as spin experiments at RHIC and Jefferson Lab, are currently investigating these issues.
QCD and electroweak interactions are governed by Yang–Mills fields – the gluons and W and Z bosons, respectively. The interactions appear fundamentally different because of the large mass of the W and Z bosons. This means that the electroweak force has a short range of around 0.01 fm, which stops the electroweak coupling from increasing to be large enough in the infrared to produce confinement: electrons and neutrinos are not confined. Electroweak interactions are also characterized by parity violation and CP violation. Furthermore, only neutrinos with left-handed chirality are observed.
The origin of the W and Z boson masses is believed to be associated with the Higgs mechanism, a major target for LHC physics. The LHC’s 14 TeV collisions will eventually cover the entire mass range, with an integrated luminosity of around 30 fb–1. Joachim Mnich of DESY, Hamburg, presented the status of the collider and early expectations. The LHC experiments will also look for new physics such as the lightest supersymmetric-particle (LSP) candidate for dark matter, possible extra dimensions, and strong WW scattering if the Higgs mechanism proves to be an electroweak dynamical effect – topics described by Caroline Collard of the Laboratoire de l’Accélérateur Linéaire, Orsay. LHC physics and its interface with gravitational interactions pose many challenges. The Higgs mechanism required to explain the W and Z boson masses with no additional physics yields a cosmological constant larger than the observed value by a factor around 1054.
Silvia Pascoli of Durham University talked about the neutrino sector, where evidence from solar, atmospheric and reactor experiments points to oscillations with a different mixing pattern from that of quarks. Oscillations between different neutrino species require small but finite neutrino masses. Open questions for future experiments include possible CP violation for neutrinos, the order of masses (is the flavour hierarchy the same as for quarks?), the absolute mass determination, and whether neutrinos are their own antiparticles.
The origin of cosmic radiation has been a mystery since its discovery by Victor Hess in 1912. Neutrinos have no electromagnetic interaction and do not bend in magnetic fields in space. Neutrino telescopes that look for point sources of neutrinos in space are probing the origin of cosmic rays, complementing studies at the Pierre Auger Observatory. These use kilometre-scale detectors in the sea or ice, which act as transparent media. Mieke Bouwhuis of Nikhef and Carlos de los Heros of Uppsala University presented the status and plans for ANTARES in the Mediterranean and IceCube at the South Pole, respectively.
These experiments, as well as those at the LHC, will look for new particles that help to explain the mysterious dark matter, described by Antonaldo Diaferio of Torino, which is needed to account for structure formation in galaxies and the large-scale structure of the universe. Galaxy rotation curves reveal that the variation of the velocity, v, of the stars with the distance, r, from the centre of the galaxy is approximately flat, rather than v2 falling off as 1/r, which should occur if gravity couples only to the visible matter. Extra mass must be present and to explain this, either extra matter or some modification to gravity over large distances is required. It is a mystery whether this dark matter is made of fermions, bosons or of both. Possible candidates for dark matter include weakly interacting massive particles with no electro-magnetic interactions, which behave almost like collisionless particles and yield cold dark matter in the outer halos of galaxies. Celine Boehm of the Laboratoire d’Annecy-le-Vieux de Physique Théorique described how, for dark matter at the tera-electron-volt scale, the LHC collisions might produce and reveal the conjectured fermionic LSP. If the dark matter is bosonic, new particles of lighter mass are possible. The 511 keV positron-annihilation radiation observed from the centre of the galaxy could be evidence for light-mass dark matter.
The nature of the missing galaxy mass and its connection to possible new physics is undoubtedly an open question. While the masses of the known fermions may depend on the same mechanism of electroweak symmetry breaking that produces the W and Z boson masses, the origin of dark-matter mass will involve new physics. The connections between particle physics and gravitation, taking us from the very small to the very large, promise to inspire much experimental and theoretical investigation in the decades ahead.
• The AFI symposium was organized in collaboration with the Frankreich Schwerpunkt and Italien Zentrum of the University of Innsbruck whose mandates are to develop and promote scientific and cultural relations between the West Austrian University and French and Italian experts and institutes. It was further supported by the BMWF, the Austrian Science Fund FWF and the University of Innsbruck. For more information, see www.uibk.ac.at/italienzentrum/italienzentrum/afi-meeting.html.