Gluon saturation in $pA$ collisions at energies available at the CERN Large Hadron Collider: Predictions for hadron multiplicities

The upcoming $p+\text{Pb}$ run at the Large Hadron Collider (LHC) will probe the nuclear gluon distribution at very small Bjorken $x$ (from $x\sim {10}^{-4}$ at midrapidity down to $x\sim {10}^{-6}$ in the proton fragmentation region) and will allow testing of approaches based on parton saturation. Here, we present the predictions of the Kharzeev-Levin-Nardi model for hadron multiplicities and multiplicity distributions in $p+\text{Pb}$ collisions at a center-of-mass energy of 4.4 TeV. We also compare the model to the existing $pp$, $dA$, and $AA$ data from the Relativistic Heavy Ion Collider and LHC.

Figure 1

Rapidity distribution of charged particles in minimum-bias $d+\text{Au}$ collisions at $W=200$ GeV. PHOBOS and BRAHMS data from Refs. [27, 28].

Figure 2

Centrality dependence of the charged-hadron multiplicity at $\eta =0$ in Au+Au collisions at $W=200$ GeV [14] and Pb+Pb collisions at $W=2.76$ TeV [17, 29].

Figure 3

Top: Rapidity distribution of charged particles in $pp$ collisions at $W=900$ GeV. Bottom: Charged-particle multiplicity distribution. ALICE and CMS data from Refs. [30, 31].

Figure 4

Top: Rapidity distribution of charged particles in $pp$ collisions at $W=2360$ GeV. Bottom: Charged-particle multiplicity distribution. ALICE and CMS data from Refs. [30, 31].

Figure 5

Top: Rapidity distribution of charged particles in $pp$ collisions at $W=7000$ GeV. Bottom: Charged-particle multiplicity distribution. ALICE and CMS data from Refs. [30, 31].

Figure 6

Top: Rapidity distribution of charged particles in minimum-bias $p+\text{Pb}$ collisions at $W=4400$ GeV. An $\sim$10$\%$ overall normalization uncertainty is not shown explicitly. Bottom: Charged-particle multiplicity distribution.

Figure 7

Rapidity distribution of charged particles in $p+\text{Pb}$ collisions at $W=4400$ GeV for minimum-bias trigger and for the 0–50 $\%$ centrality or multiplicity class. An $\sim$10$\%$ overall normalization uncertainty is not shown explicitly.