Particles meet cosmology and strings in Boston

1 March 2005

PASCOS 2004 is the latest in the symposium series that brings together disciplines from the frontier areas of modern physics.

PASCOS 2004 is the latest in the symposium series that brings together disciplines from the frontier areas of modern physics.

The Tenth International Symposium on Particles, Strings and Cosmology took place at Northeastern University, Boston, on 16-22 August 2004. Two days of the symposium, 18-19 August, were devoted to the Pran Nath Fest in celebration of the 65th birthday of Matthews University Distinguished Professor Pran Nath. The PASCOS symposium is the largest interdisciplinary gathering on the interface of the three disciplines of cosmology, particle physics and string theory, which have become increasingly entwined in recent years.


Topics at PASCOS 2004 included the large-scale structure of the universe, cosmic strings, inflationary models, unification scenarios based on supersymmetry and extra dimensions, M-theory and brane models, and string cosmology. Experimental talks discussed data from the Wilkinson Microwave Anisotropy Probe (WMAP), neutrino physics, the direct and the indirect detection of dark matter, B-physics and data from the CDF and D0 detectors at Fermilab’s Tevatron.

Cosmology and quantum gravity

The issue of dark matter in the universe and prospects for the future were reviewed by Joseph Silk of Oxford and Margaret Geller of the Harvard-Smithsonian Center for Astrophysics. Geller observed that, while the cosmic microwave background combined with large redshift surveys suggests that the critical matter density of the universe is Ωm ~ 0.3, direct dynamical measurements combined with the estimates of the luminosity density indicate Ωm = 0.1-0.2. She suggested that the apparent discrepancy may result from variations in the dark-matter fraction with mass and scale. She also suggested that gravitational lensing maps combined with large redshift surveys promise to measure the dark-matter distribution in the universe. The microwave background can also provide clues to inflation in the early universe. Eva Silverstein from SLAC discussed a new mechanism for inflation that results from a strong back-reaction on rolling scalar-field dynamics near regions with extra-light states. She claimed that this leads to a distinctive non-Gaussian signature in the cosmic microwave background, which can distinguish this mechanism from traditional slow-roll inflation.


Cosmology and particle physics connected again in a talk at the Nath Fest by Steven Weinberg of the University of Texas, Austin. He spoke on the analogy between perturbations to the Friedmann-Robertson-Walker cosmology and the Goldstone bosons of particle physics in his talk “Goldstone Bosons Through the Ages”. Ali Chamseddine of the Center for Advanced Mathematical Sciences, American University of Beirut, showed that consistency problems on the action for massive coloured gravitons can be resolved by employing spontaneous symmetry-breaking to give masses to gravitons.

In his talk on quantum gravity, Lee Smolin of Perimeter Institute described rigorous results and the possibility of testing them experimentally. He discussed possible violations of the Greisen-Kuzmin-
Zatsepin bound on the upper energies of cosmic rays, which may be observed by the Pierre Auger Observatory, and possible variations of the speed of light with energy, which would be observable by the GLAST gamma-ray observatory. Dark energy in the universe formed part of the talk by Gregory Tarlé of Michigan reviewing the SNAP (Supernova Acceleration Probe) satellite observatory.

Supersymmetry and strings

Strings featured at the symposium on both the cosmic and the fundamental particle scales. In a talk on cosmic strings, Alexander Vilenkin of Tufts presented their current status in view of recent developments in string cosmology. At the opposite end of the scale, other speakers discussed string- and brane-based models in particle physics. Mary K Gaillard of the University of California, Berkeley, presented results from studies of effective Lagrangian theories that arise from compactification of the weakly coupled heterotic string. Models based on D-branes and their implications were discussed by Mirjam Cvetic of Pennsylvania, while Richard Arnowitt from Texas A&M examined the gravitational forces felt by point particles on two 3-branes (the Planck brane and the tera-electron-volt brane) bounding a 5D anti de Sitter (AdS) space with S1/Z2 symmetry.


Nima Arkani-Hamed of Harvard and Michael Dine of the University of California, Santa Cruz, discussed string-based landscape scenarios from two different perspectives: whether the landscape does or does not predict low-energy supersymmetry. Arkani-Hamed argued for a high scale for supersymmetry or split supersymmetry while Dine said that, under rather mild assumptions, the landscape seems to favour a low and possibly even a very low scale for supersymmetry breaking. In considering the possibility for inflation in string theory, Boris Kors from MIT discussed a Stückelberg extension of both the Standard Model and the Minimal Supersymmetric Standard Model, recently introduced in collaboration with Pran Nath. In this extension, the vector bosons become massive without spontaneous symmetry-breaking, via condensation of Higgs scalar fields. Furthermore, such an extension implies the existence of a sharp Z boson and may lead to a new lightest supersymmetric particle composed mainly of Stückelberg fermions. In this case, the signals of supersymmetry will change in a significant way and the Stückelberg fermion may become the new candidate for dark matter.

Experiment and phenomenology

A number of talks dealt with supersymmetry phenomenology, specifically with regard to searches for supersymmetry at particle colliders and in dark matter. Howard Baer of Florida State described the possibilities for direct and indirect detection of supersymmetric dark matter, as well as searches at colliders, within the minimal supergravity grand unification (mSUGRA) paradigm. Searches at colliders were also discussed by Xerxes Tata of Hawaii, this time in the light of data from WMAP and other experimental constraints on weakly interacting massive particles (WIMPs). On the experimental side, Rupak Mahapatra of the University of California, Santa Barbara, reported on the world’s lowest exclusion limits on the coherent WIMP-nucleon scalar cross-section for WIMP masses above 13 GeV/c2 based on data from the Cryogenic Dark-Matter Search experiment at the Soudan Underground Laboratory. These results rule out a significant part of the parameter space of supersymmetric models.

David Cline of UCLA presented the current ZEPLIN II programme for the direct detection of dark matter as a prototype of large liquid- xenon detectors. He then described ZEPLIN IV and other 1 t liquid xenon detectors, and discussed the limiting backgrounds for such detectors in exploring the full range of the SUSY parameter space. Stefano Lacaprara of INFN, Padua, looked at the prospects for dark-matter searches at the Large Hadron Collider, and Rita Bernabei from INFN Rome reviewed the observation of dark-matter signals using the low-background NaI(Tl) detector of the DAMA dark-matter project in the Gran Sasso Laboratory.

Neutrinos and other particles

Several speakers at the symposium emphasized the promising future for neutrino physics and astrophysics. Vernon Barger from Wisconsin gave an in-depth presentation about the status and future prospects of precision neutrino physics. Haim Goldberg of Northeastern discussed galactic and extra-galactic neutrino sources, and Sandip Pakvasa from Hawaii showed how high-energy astrophysical neutrinos can provide information about neutrino lifetimes and mass hierarchies. Tom Weiler of Vanderbilt reviewed the particle physics and astrophysics information encoded in the energy spectrum, arrival directions and the flavour content of such cosmic neutrinos.

The detection of high-energy neutrinos was discussed by Stefan Schlenstedt of DESY-Zeuthen, who gave an update on the AMANDA experiment at the South Pole and the construction of the IceCube experiment for the observation of high-energy neutrinos. Luis Anchordoqui of Northeastern University gave an overview of the current status of the Pierre Auger Observatory being built to detect the highest-energy cosmic rays.

At lower energies, there are new measurements of the solar neutrino spectrum at the Sudbury Neutrino Observatory, using salt to enhance the detection of neutral currents. These were presented by José Maneira of Queen’s University, who also described the prospects for using strings of 3He proportional counters to increase the sensitivity by a factor of two. Nikolai Tolich from Stanford presented the improved measurement from KamLAND of Δm2 versus sin22θ for neutrino oscillations, while Ion Stancu of Alabama covered the status of the MiniBooNE neutrino oscillation experiment. Hans Volker Klapdor-Kleingrothaus of MPI-Heidelberg discussed the evidence for neutrinoless double-beta-decay using data from the Heidelberg-Moscow experiment, which shows a signal at the 4.2 σ level, and discussed its consequences for particle physics.

Other aspects of particle physics were not neglected. Shiro Suzuki from Saga University presented new results from the Belle experiment at KEK on the measurement of time-dependent charge-parity (CP) violation in b→s penguin processes. These yield in an average value 2.4 σ away from the Standard Model value.

Continuing with B-physics, Stefano Passaggio of INFN Genova reported the direct observation of CP violation at BaBar in B→K+π at a confidence level of 4.2 σ. Results from DESY’s HERA collider and prospects for HERA II were reviewed by Chiara Genta of INFN Florence, while electroweak results from LEP2, the upgraded Large Electron Positron collider at CERN, were summarized by Roberto Chierici of CERN. Markus Schumacher from Bonn presented results of searches for new physics by the LEP experiments. Recent results from the D0 experiment at Fermilab were presented by Pushpalatha Bhat from Fermilab and Nick Hadley of Maryland. Those from CDF were presented by Un-ki Yang of Chicago and Dmitri Tsybychev from SUNY, Stonybrook. Ernst Sichtermann of Lawrence Berkeley National Laboratory gave the latest status of the muon g-2 experiment at Brookhaven, and William Marciano of Brookhaven reviewed the theoretical implications of the g-2 results.

Other talks dealt with a range of interdisciplinary topics. In his status report on using lattice quantum chromodynamics (QCD) in the calculation of light quark masses and the CP-violation parameter BK, Rajan Gupta of Los Alamos was able to weave in some early history of the lattice gauge calculations from his time at Northeastern University in the early 1980s. Roman Jackiw of MIT discussed the consequences of a vanishing Cotton tensor, which ensures that the 3D gravitational Chern-Simons term is stationary. He showed that this condition leads to kink solutions and that the effective theory is a new type of dilaton gravity.

• PASCOS 2005 will be held in the 1600-year-old ancient Korean town of Gyeong-Ju.

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