30 research outputs found
Iron Air collision with high density QCD
The color glass condensate approach describes successfully heavy ion
collisions at RHIC. We investigate Iron-air collisions within this approach and
compare results to event generators commonly used in air shower simulations. We
estimate uncertainties in the extrapolation to GZK energies and discuss
implications for air shower simulations.Comment: Proceedings of XIV International Symposium on Very High Energy Cosmic
Ray Interactions (ISVHECRI 2006), Weihai, China, 15-22 Aug 200
High energy cosmic ray air showers and small-x QCD
We discuss that hadron-induced atmospheric air showers from ultra-high energy cosmic rays are sensitive to QCD interactions at very small momentum fractions x where nonlinear effects should become important. The leading partons from the projectile acquire large random transverse momenta as they pass through the strong field of the target nucleus, which breaks up their coherence. This leads to a steeper x_F-distribution of leading hadrons as compared to low energy collisions, which in turn reduces the position of the shower maximum Xmax. We argue that high-energy hadronic interaction models should account for this effect, caused by the approach to the black-body limit, which may shift fits of the composition of the cosmic ray spectrum near the GZK cutoff towards lighter elements. We further show that present data on Xmax(E) exclude that the rapid ~ 1/x^0.3 growth of the saturation boundary (which is compatible with RHIC and HERA data) persists up to GZK cutoff energies. Measurements of pA collisions at LHC could further test the small-x regime and advance our understanding of high density QCD significantly
Effects of nucleus initialization on event-by-event observables
In this work we present a study of the influence of nucleus initializations on the event-by-event elliptic flow coefficient, v2. In most Monte-Carlo models, the initial positions of the nucleons in a nucleus are completely uncorrelated, which can lead to very high density regions. In a simple, yet more realistic model where overlapping of the nucleons is avoided, fluctuations in the initial conditions are reduced. However, v2 distributions are not very sensitive to the initialization choice
Model dependence of lateral distribution functions of high energy cosmic ray air showers
The influence of high and low energy hadronic models on lateral distribution functions of cosmic ray air showers for Auger energies is explored. A large variety of presently used high and low energy hadron interaction models are analysed and the resulting lateral distribution functions are compared. We show that the slope depends on both the high and low energy hadronic model used. The models are confronted with available hadron-nucleus data from accelerator experiments
The centrality dependence of elliptic flow, the hydrodynamic limit, and the viscosity of hot QCD
We show that the centrality and system-size dependence of elliptic flow
measured at RHIC are fully described by a simple model based on eccentricity
scaling and incomplete thermalization. We argue that the elliptic flow is at
least 25% below the (ideal) ``hydrodynamic limit'', even for the most central
Au-Au collisions. This lack of perfect equilibration allows for estimates of
the effective parton cross section in the Quark-Gluon Plasma and of its
viscosity to entropy density ratio. We also show how the initial conditions
affect the transport coefficients and thermodynamic quantities extracted from
the data, in particular the viscosity and the speed of sound.Comment: 5 pages, 2 figures. Extended discussion of the results, in particular
of lower viscosity and sound velocity required by CGC initial condition
Effects of fluctuations on the initial eccentricity from the Color Glass Condensate in heavy ion collisions
We introduce a modified form of the Kharzeev-Levin-Nardi (KLN) approach for
nuclear collisions. The new ansatz for the unintegrated gluon distribution
function preserves factorization, and the saturation scale is bound from below
by that for a single nucleon. It also reproduces the correct scaling with the
number of collisions at high transverse momentum. The corresponding Monte Carlo
implementation allows us to account for fluctuations of the hard sources
(nucleons) in the transverse plane. We compute various definitions of the
eccentricity within the new approach, which are relevant for the interpretation
of the elliptic flow. Our approach predicts breaking of the scaling of the
eccentricity with the Glauber eccentricity at the level of about 30%.Comment: 9 pages, 10 figures, Updated version as accepted by Phys.Rev.