Prague meeting focuses on colliders and cosmic rays

Research on elementary particles – a frontier area of physics – emerged as a distinct field during the mid-20th century, following the discovery of the pion and strange particles, and the construction of particle accelerators reaching energies of more than 100 MeV. The first high-energy physicists had grown up as nuclear or cosmic-ray scientists, but in subsequent years the liaison between cosmic-ray physics and accelerator-based elementary particle physics seemed to fade, with little communication between these two lively and interesting areas of physics. Recently this situation has begun to change, with closer interaction between the two fields. Cosmic-ray physicists need better data on particle interactions and production, and particle physicists at accelerators are interested in exploring phenomena reported from cosmic-ray studies. Also, some theorists are looking at effects that should be detectable at the future Large Hadron Collider (LHC) and may be even more notable at cosmic-ray energies.

It was in this spirit that physicists at the Institute of Physics of the Czech Academy of Sciences, the Czech Technical University and Charles University in Prague organized the conference From Colliders to Cosmic Rays (C2CR) in September 2005. Their aim was to bring together cosmic-ray and particle-accelerator physicists to discuss their latest results and problems common to both communities. An International Advisory Committee was established, representing a broad spectrum of universities and laboratories in Europe and America, with Jan Ridky of the Institute of Physics as head of the Local Organizing Committee.

A significant antecedent to the C2CR meeting was the Needs from Accelerator Experiments for the Understanding of High-Energy Extensive Air-Showers (NEEDs) workshop held in Karlsruhe in April 2002. Also relevant was the 12th (biennial) International Symposium on Very High Energy Cosmic Ray Interactions, which was held at CERN in July 2002.

A common thread in these discussions is the fact that the flux of primary cosmic rays of energies above a few hundred tera-electron-volts is so low that direct observation is not practical from balloon- or satellite-borne detectors. Our understanding of the composition and energy spectra of these cosmic rays is totally dependent on ground-level observations, and on the simulation of primary interactions and atmospheric cascades based on accelerator data. In this context, there are constant efforts to improve the interaction Monte Carlo simulation programs, which are essential components for the interpretation of the cosmic-ray data. At C2CR, Sergey Ostapchenko from the Forschungszentrum Karlsruhe presented the theoretical input into his latest version of the QGSJET program, where gluon saturation is taken into account, while his colleague Tanguy Pierog described numerical methods used in the simulation of showers that involve tens of billions of secondary particles. CERN’s Hans-Peter Wellisch also raised interest by his claim that extensive cosmic-ray air showers can be simulated within the framework of GEANT4, the latest version of the well-known toolkit for simulating the passage of particles through matter.

The experimental information available and efforts or proposals to carry out new measurements of hadronic interactions at accelerator and collider energies formed a major topic. Representatives from the experimental collaborations presented recent results from the CDF and D0 experiments at Fermilab’s Tevatron collider, from the HERA collider at DESY, and from the Hadron Production Experiment at the Proton Synchrotron accelerator at CERN. Heavy-ion physics results were also reported from experiments at CERN’s Super Proton Synchrotron and from the BRAHMS and PHOBOS experiments at Brookhaven’s Relativistic Heavy Ion Collider. Martin Block of Northwestern University presented a projection of the proton-proton total cross-section to LHC energies, and both he and Leonid Frankfurt of Tel Aviv discussed other cross-sections, such as proton-air and nucleus-nucleus, at higher energies.

The observables from cosmic-ray interactions are dominated by the most energetic final-state particles and these are mostly produced at small forward angles. Among the talks on this topic, Mark Strikman of Penn State University discussed small-x physics and forward dynamics in proton-proton and proton-nucleus ultra-high-energy collisions. Others described the discovery potential of the LHC, as well as the potential in diffraction and forward physics of the CMS/TOTEM/CASTOR experiment complex at the LHC, and LHCf, a proposed zero-degree calorimeter at the LHC.

Currently, the Karlsruhe Shower Core and Array Detector (KASCADE) with its associated muon and hadron detectors is the most sophisticated and productive cosmic-ray air-shower experiment in operation, and the Karlsruhe group was well represented in Prague. Marcus Risse, Andreas Haungs and Holger Ulrich discussed different aspects of the KASCADE data, results and interpretation, including the sensitivity of the interpretation of their data to models of the primary hadron interaction.

The intercommunication between accelerator-based and cosmic-ray physicists is perhaps nowhere more apparent than in the area of neutrino physics, with the studies of neutrino masses and mixing. The meeting heard about various aspects of neutrino physics, including results from existing detectors, theoretical ideas and plans for new detectors. For example, the Antarctic Impulsive Transient Antenna and the Salt Shower Array are planned to detect radio pulses from coherent Cherenkov radiation produced by the reaction products of ultra-high-energy cosmic-ray neutrinos interacting in ice and rock salt, respectively.

No cosmic-ray conference nowadays would be complete without some discussion of the highest-energy cosmic rays and indeed C2CR had several excellent reports. These covered the current status of the problem with ultra-high-energy cosmic rays, the latest from the High Resolution Fly’s Eye experiment, and the status and first results from the Pierre Auger Observatory. Jim Cronin of Chicago, one of the founders of the observatory and spokesman emeritus for the collaboration, was a lively and valuable participant.

An interesting cross-link between the cosmic-ray and accelerator physics communities is the use of the large detectors at colliders for studies of cosmic-ray muons. The meeting heard reports on physics results obtained with cosmic-ray muons in the detectors for the Large Electron-Positron collider at CERN. These included muon multiplicity studies and absolute differential muon spectra, both of which have been obtained with greater precision than was previously possible with detectors built for cosmic-ray studies.

Participants also heard about the latest generation of cosmic-ray detectors, with talks on Super-Kamiokande, the BAIKAL experiment, the Search for Light Magnetic Monopoles on Mount Chacaltaya in Bolivia, the AMANDA and IceCube detectors at the South Pole, the satellite-borne Cosmic Ray Energetics and Mass detector, and the planned 1 km3 Neutrino Mediterranean Observatory.

Around 60 people attended the conference, which proved a successful opportunity for the participants to learn new physics, to interact with colleagues in other areas of elementary-particle physics and particle astrophysics – and also to enjoy Prague, with a trip to the nearby Konopiste Castle over the weekend and the conference banquet aboard a river cruise boat.

During the discussions at the meeting’s final session, it was suggested that the topic of colliders and cosmic rays might appropriately become the theme of a biennial conference series. A probable site and time for the next conference is the Granlibakken Conference Center on Lake Tahoe, California, 25 February – 1 March 2007.