Low- x QCD physics from RHIC and HERA to the LHC

Abstract

We present a summary of the physics of gluon saturation and non-linear QCD evolution at small values of parton momentum fraction x in the proton and nucleus in the context of recent experimental results at HERA and RHIC. The rich physics potential of low-x studies at the LHC, especially in the forward region, is discussed and some benchmark measurements in pp, pA and AA collisions are introduced.

Gluon saturation in dijet production inp-Pb collisions at the Large Hadron Collider

“…An interesting process in which both saturation and production at high p t can be studied, is the process of dijet production where the jets are separated by large rapidity [22][23][24][25]. More specifically, we shall focus on the case in which one jet is in the central while the other in the forward rapidity region.…”

Forward Physics at the LHC: within and beyond the Standard Model

“…HF+CASTOR in CMS). Second, low-x QCD physics [20,21] -gluon saturation, non-linear QCD evolution, small-x parton distribution functions (PDFs), multi-parton scattering -can be studied via the measurement of hard QCD cross sections in the forward direction (e.g. jets, or direct-γ in HF/FCal, CASTOR) or in exclusive photoproduction γ-p [22] and γ-A [23] processes, tagged with forward protons (neutrons) in RPs (ZDCs).…”

“…Thus, forward instrumentation provides an important lever arm for the measurement of the low-x structure and evolution of the parton densities. Three representative low-x QCD measurements at the LHC [21] are discussed next.…”

A look at the influence of the J∕ψ transverse momentum on shadowing

“…A summary of the available measurements in the (x, Q 2 ) plane used to determine the nuclear PDFs (nPDF) can be found in [15]. At a given Q 2 , the ratio R A F 2 (x, Q 2 ) of the bound to the free nucleon structure functions deviates from unity: the shadowing corresponds to the small-x region (usually x 0.1) where R A F 2 < 1, while the anti-shadowing region with R A F 2 > 1 lies at intermediate-x (usually 0.1 x 0.3).…”