Workshop on Hadron-Hadron & Cosmic-Ray
Interactions at multi-TeV Energies

ECT* - Trento, Nov 29th - Dec 3rd, 2010

Cosmics Rays - The origin and nature of cosmic rays (CRs) with energies between 10^{15} eV and the so-called Greisen-Zatsepin-Kuzmin (GZK) cut-off at about 10^{20} eV, recently measured by the HiRes and Auger experiments, remains a central open question in high-energy astrophysics. It has with very interesting connections to particle physics and, in particular, to Quantum-Chromo-Dynamics (QCD) at the highest energies ever studied. One key to solving this question is the determination of the elemental composition of cosmic rays in this energy range. The candidate particles, ranging from protons to nuclei as massive as iron, generate ``extensive air-showers'' (EAS) in interactions with air nuclei when entering the Earth's atmosphere. The determination of the primary energy and mass relies on hadronic Monte Carlo (MC) models which describe the interactions of the primary cosmic-ray in the upper atmosphere.

QCD - The bulk of particle production in such high-energy hadronic collisions can still not be calculated within first-principles QCD. General principles such as unitarity and analyticity (as implemented in Regge-Gribov theory) are often combined with perturbative QCD predictions for high-p_{T} processes, constrained by the existing collider data (E_{lab} < 10^{15} eV). Important theoretical issues at these energies are the understanding of diffractive and elastic hadronic scattering contributions, the description of hadronic forward fragmentation and multi-parton interactions (``underlying event''), and the effect of high parton density (``gluon saturation'') effects at small values of parton fractional momentum x=p_parton/p_proton. Indeed, at these energies, the relevant x values are as low as 10^{-7}, where effects like gluon saturation and multi-parton interactions, particularly enhanced with nuclear targets, are expected to dominate the early collision dynamics.

LHC - The coming energy frontier for hadron collisions in the laboratory will be reached at the Large Hadron Collider (LHC), currently running at CERN. The measurement of inclusive hadron production observables in proton-proton, proton-nucleus, and nucleus-nucleus collisions, at LHC energies (equivalent to E_{lab} ~ 10^{17} eV) will thus provide very valuable information on high-energy multiparticle production, and allow for more reliable determinations of the CR energy and composition around the GZK cutoff. In the high luminosity phase of LHC, each bunch crossing will lead to several proton-proton interactions, increasing even more the importance of understanding the background from diffractive and soft particle production. Semi-hard particle physics will allows us to test the boundaries of the applicability of perturbative QCD in the region where low-x gluon saturation phenomena become increasingly important and may even dominate the characteristics of particle production.

All LHC experiments feature detection capabilities with a wide phase-space coverage without parallel - in particular in the forward direction - compared to previous colliders. Such capabilities will allow for a (fast) measurement of global hadron-hadron collisions properties (inelastic - including diffractive - cross sections, particle multiplicity and energy flows as a function of pT and pseudorapidity, ...) and validation and tuning of the MC hadronic models - even with the limited statistics expected in a first p-p run.