Low-x QCD with CMS at the LHC

Abstract

The physics of gluon saturation and non-linear evolution at small values of parton momentum fraction x in the proton and nucleus is briefly discussed in the context of experimental results at HERA and RHIC. The rich physics potential of low-x QCD studies at the LHC is presented and some measurements in pp, pA and AA collisions accessible with the Compact Muon Solenoid (CMS) experiment are discussed.

Quark–gluon matter

“…In such a kinematic range non-perturbative effects can blur a simple interpretation based on partonic degrees of freedom alone. Following equation ( 6), the relevance of low-x QCD effects will certainly be enhanced at the LHC due to the increased centre-of-mass energy, nuclear radius (A 1/3 ) and rapidity of the produced partons [76,91]. At the LHC, the saturation momentum Q 2 s ≈ 5-10 GeV 2 [34] will be more clearly perturbative and the relevant x values in AA and pA collisions will be 30-70 times lower than at RHIC: x 2 ≈ 10 −3 (10 −5 ) at central (forward) rapidities for processes with a hard scale p T ∼ 10 GeV/c.…”