PASCOS 2010 explores the interface of particle physics, string theory and cosmology.
The guiding spirit behind the series of annual symposia on Particles, Strings and Cosmology (PASCOS) is the unification of the microcosm with the macrocosm. It follows from the basic principles of uncertainty and mass-energy equivalence, which imply that when we probe deep inside subatomic space, we inevitably come across states of very high energy and mass that would have abounded in the early history of the universe. Recreating them in the laboratory is like recreating dinosaurs as in Jurassic Park, but is much more significant because it helps us to trace the very early history of the universe.
Making heavy particles such as the W and Z bosons and the top quark, as well as studying their interactions in the laboratory, helps us retrace the history of the universe to within a few picoseconds of its beginning. The discovery of the Higgs boson(s) and supersymmetric (SUSY) particles should, likewise, throw light on the nature of the phase transition that the universe experienced during those first few picoseconds – as well as on the nature of the cold dark matter (CDM) that permeates the universe today as a relic of its early history. But the story does not end there. We would like to follow the history of the universe right back to the instant of the Big Bang – and even beyond, where the standard tool of quantum field theory breaks down. The recent developments in string theory offer us the best hope of addressing these issues.
The interface of particle physics, string theory and cosmology is thus a highly active field of research at the frontiers of human knowledge. The PASCOS series of international symposia was started in the early 1990s in the US to recognize this interplay. The meetings strive to bring together researchers from these three areas to facilitate their mutual interaction and the cross-fertilization of ideas. After circulating round the US during its first decade, the series is now global, having visited India, South Korea, the UK, Canada and Germany.
PASCOS 2010, the 16th symposium in the series, took place on 19–23 July in the Spanish city of Valencia and was organized by the Instituto de Fisica Corpuscular (IFIC), which is the largest particle-physics laboratory of the Spanish National Research Council (CSIC) and is jointly operated by the University of Valencia. This year’s symposium, attended by more than 160 participants from around the world, was of particular significance because it came in the wake of the start-up of the LHC and the launch of the Planck satellite. In general, plenary sessions were held in the mornings, while the afternoons were devoted to parallel sessions focusing on the three areas of particles, strings and cosmology.
The first day’s proceedings started with an overview of the status of the LHC by Richard Hawkings of CERN. This first run of the LHC should accumulate an integrated luminosity of 1 fb–1 at a total energy of 7 TeV by the end of 2011. After a shutdown for a year to increase the total energy to 14 TeV, the next run is scheduled to start in early 2013. In talks on phenomenology, Manuel Drees of Bonn University and Werner Porod of the University of Würzburg and IFIC Valencia said that a meaningful SUSY search could already begin with the 1 fb–1 data at 7 TeV. However, a meaningful search for Higgs bosons will require about 10 fb–1 of data at 14 TeV, as Howard Haber of the University of California, Santa Cruz, discussed. If the Higgs does not show up then other physics may come into play, as Francesco Sannino of the University of Southern Denmark noted. In looking at alternative mechanisms he argued that a successful technicolour theory requires near-conformal dynamics.
Meanwhile experiments at the Tevatron have results based on 5 fb–1 of data, which Vadim Rusu of Fermilab presented. These include the recent result from DØ on CP violation in B–B mixing in the like-sign di-muon channel, which disagrees with the Standard Model by 3.2σ. Searches for a Standard Model Higgs particle continue at the Tevatron with a very complex multichannel analysis that leads to a small window, only a few giga-electron-volts wide, of excluded masses around 165 GeV, with the window set to widen as the luminosity increases beyond 10 fb–1 per experiment through 2011.
Assuming that SUSY does appear at the LHC, then quantitative predictions will be valuable. Kiwoon Choi, of the Korea Advanced Institute of Science and Technology, discussed SUSY-breaking from the perspective of string theory, including dilaton/moduli mediation, gauge mediation, anomaly mediation and D-term breaking. He argued that in string theory it is quite plausible that all mediation schemes could be present and give comparable contributions. Angel Uranga, of the Instituto de Física Teórica UAM/CSIC Madrid pointed out that, although string theory is unique (all versions being related by dualities), the way that the extra six dimensions are compactified is far from unique, leading to different low-energy physics. Current approaches include heterotic models, (intersecting) D-brane models and F-theory constructions, with concrete models leading to predictions for the spectrum for SUSY and exotic particles at the LHC.
An exciting possibility, discussed by Ignatios Antoniadis of CERN, is that the string scale is at energies of tera-electron-volts. This could in principle solve the gauge-hierarchy problem and provide an explanation of the weakness of gravity, provided that the extra dimensions perpendicular to the D-brane on which the Standard Model lives are large, which would lead to spectacular missing energy signals at the LHC. The extra tera-electron-volt-scale dimensions parallel to the brane also lead to Kaluza-Klein excitations of gauge bosons, and string/strong gravity effects, including the possible production of micro-black holes. “Are these ideas physical reality or imagination?” asked Antoniadis, replying that “the LHC will explore physics beyond the Standard Model”.
The only solid new microscopic physics in the past dozen years has been the discovery of neutrino mass and mixing. Yoichiro Suzuki of the University of Tokyo tracked the progress of atmospheric and solar-neutrino experiments over this period, focusing on the results from SuperKamiokande. This talk was complemented by that of Mayly Sanchez of Iowa State University/Argonne National Laboratory, who reported on the latest results from accelerator and reactor experiments. These included the antineutrino results from the MINOS experiment that show a 2σ discrepancy with the neutrino results, and the OPERA detector’s first observation of a τ event in the beam of muon neutrinos sent from CERN to the Gran Sasso National Laboratory.
Steve King of the University of Southampton considered the antineutrino results from MiniBooNE at Fermilab, which show an excess consistent with oscillations of the kind reported some years ago by the LSND collaboration at Los Alamos, while the neutrino results do not. He advocated a “wait and see” approach to these data and focused instead on the paradigm of three active neutrinos, as well as ideas such as SUSY R-parity violation and the several different types of see-saw mechanism that have been proposed and studied by José Valle’s group at IFIC. King also discussed the exciting observation that accurate tribimaximal lepton mixing suggests a non-Abelian discrete family symmetry that might unlock the long-standing flavour puzzle, which began with the discovery of the muon in 1937. These ideas may be incorporated into a complete SUSY grand unified theory (GUT) of flavour, as Eduardo Peinado and Stefano Morisi of IFIC, and Reiner de Adelhart Toorop of Nikhef also discussed. This could include new SUSY GUT relations presented by Stefan Antusch of Max Planck Institut für Physik, Munich.
Carlos Frenk of Durham University set the agenda for the challenges facing the macrocosm with an entertaining talk on “The Standard Model of cosmogony: what next?”. After reviewing this “a priori implausible model but one which makes definite predictions and is therefore testable,” he focused on the prospects for testing the three assumptions that underpin the ΛCDM model: dark energy density Λ with negative pressure; structure seeded by quantum fluctuations during inflation; and CDM particles.
If CDM turns out to arise from weakly interacting massive particles, such as predicted for example in SUSY models with conserved R-parity, they could soon be discovered in direct detection experiments at underground laboratories.
Although the equation of state for dark energy can be constrained, with current combined limits giving the ratio of the pressure to the density, w = –0.97±0.05, Frenk regarded the prospects for understanding the nature of dark energy as questionable. However in a later talk on the self-tuning cosmological constant, Jihn Kim of Seoul National University reported on progress in the search for a non-anthropic solution to this big problem, including ideas such as inflation, the wave-function of the universe and “quintessential axions”. By contrast, in talking of the holographic principle and the surface of last scatter, Paul Frampton of the University of North Carolina at Chapel Hill attempted to dispense with dark energy altogether, starting from the observation that the observable universe is close to being a black hole. He argued that from our viewpoint, the apparent acceleration of the universe arises as a consequence of information storage on the surface of the visible universe because of the entropy of the black hole. The contrasting nature of these talks perhaps underscores Frenk’s point that we are a long way from understanding dark energy.
Alessandro Melchiorri of Sapienza Università di Roma reviewed the latest results from seven years of observation by the Wilkinson Microwave Anisotropy Probe and presented the First Light Survey from the Planck satellite from September 2009. The polarization of the cosmic microwave background (CMB) will be measured accurately with Planck, including the curl-free E-mode and the divergenceless B-mode, which at large angular scales are produced only by gravitational waves and provide a key signature of inflation. Planck will measure the gravitational wave background at 3σ if the tensor-to-scalar ratio r = 0.05, as could be the case in some of the inflation models discussed by Philipp Kostka and Jochen Bauman of the Max Planck Institut für Physik at Munich, as well as Lancaster University’s Anupam Mazumdar and others. Qaisar Shafi of the University of Delaware also reviewed some recent ideas including gauge singlet Higgs inflation and Standard Model inflation, including a non-minimal coupling of the Higgs field to gravity, where the gravitational couplings can have desirable effects if their magnitude is tuned to be very large.
If CDM turns out to arise from weakly interacting massive particles (WIMPs), such as predicted for example in SUSY models with conserved R-parity, they could soon be discovered in direct detection experiments at underground laboratories. For example, the Cryogenic Dark Matter Search in the Soudan Mine has seen two candidate events, although, as both Jodi Cooley-Sekula of Southern Methodist University and Andrea Giammanco of the Catholic University of Louvain pointed out, neither of them are “golden events”. Other direct detection experiments such as DAMA in Gran Sasso and CoGeNT, again in the Soudan Mine, also have candidate events, as Nicolao Fornengo of INFN/Torino described. He also talked about indirect WIMP detection signals that could be observed via annihilation radiation. Dark-matter effects in gamma rays could be seen by the Fermi Gamma-Ray Space Telescope, and leptonic anomalies in cosmic rays studied by the PAMELA satellite experiment, Fermi, the HESS Cherenkov telescope array and future experiments such as the Alpha Magnetic Spectrometer. Aldo Morselli of INFN Roma Tor Vergata showed that the positron excess observed by PAMELA is, however, well fitted by the assumption of nearby pulsar(s) and the electron discrepancies observed by Fermi are now being used to help constrain the pulsar models. A potentially clean signal of WIMPs could come as gamma-ray spectral lines from dwarf spheroidal galaxies, which are dark-matter dominated systems with low astrophysical background, but Fermi has not yet detected such signals.
The microcosm-macrocosm connection
Bhaskar Dutta of Texas A&M University illustrated the connection between particle colliders and dark matter in the framework of minimal supergravity (mSUGRA). The favoured CDM regions of mSUGRA, such as the co-annihilation region, imply distinctive signatures for gluinos produced at the LHC, including two jets, two τ leptons and missing energy. By suitable choices of kinematic variables, the SUSY particle masses can be reconstructed and the mSUGRA parameters determined to check for a consistent CDM region. John Gunion of University of California at Davis also discussed such connections in the next-to-minimal supersymmetric Standard Model (NMSSM), motivated by data from the Large Electron–Positron (LEP) collider, which prefers a Higgs mass at around 100 GeV. This is possible in the NMSSM if the dominant Higgs decays are to pairs of CP-odd Higgs bosons that are sufficiently light that they do not decay to b-quark pairs so as to escape LEP limits. Such a scenario with the lightest neutralinos at 5–10 GeV might also account for recent results from the CoGENT experiment. However these findings are already challenged by first data from the XENON 100 experiment in the Gran Sasso laboratory, with the next results from this powerful experiment eagerly expected soon.
In the quest to discover the particle responsible for dark matter, which experiment will be first, the LHC or XENON 100? Whatever happens, it is clear that these and other experiments will all be required in order to unveil the complete theory at the heart of both the microcosm and the macrocosm.