Abstract
Measurements of the per-event charged-particle yield as a function of the charged-particle transverse momentum and rapidity are performed using $p+$Pb collision data collected by the ATLAS experiment at the LHC at a centre-of-mass energy of $\sqrt{s_{NN}}=5.02$ TeV. Charged particles are reconstructed over pseudorapidity $|\eta|<2.3$ and transverse momentum between $0.1$ GeV and $22$ GeV in a dataset corresponding to an integrated luminosity of $1$ $\mu b^{-1}$. The results are presented in the form of charged-particle nuclear modification factors, where the $p+$Pb charged-particle multiplicities are compared between central and peripheral $p+$Pb collisions as well as to charged-particle cross sections measured in pp collisions. The $p+$Pb collision centrality is characterized by the total transverse energy measured in $-4.9<\eta<-3.1$, which is in the direction of the outgoing lead beam. Three different estimations of the number of nucleons participating in the $p+$Pb collision are carried out using the Glauber model and two Glauber-Gribov colour-fluctuation extensions to the Glauber model. The values of the nuclear modification factors are found to vary significantly as a function of rapidity and transverse momentum. A broad peak is observed for all centralities and rapidities in the nuclear modification factors for charged-particle transverse momentum values around $3$ GeV. The magnitude of the peak increases for more central collisions as well as rapidity ranges closer to the direction of the outgoing lead nucleus.
Highlights
Proton–nucleus collisions at ultrarelativistic energies provide an opportunity to understand the role of the nuclear environment in modifying hard scattering rates
The results presented here utilize the same centrality definition and geometric models, but build upon that work by exploring the pT, η and y∗ dependence ooff-pmear-sesveennet rcghyar√gesdNN-p=art5ic.0le2 yields in TeV and p + Pb collisions at a centrecomparing that dependence to the expectations from pp collisions through the quantities R pPb and RCP
The p + Pb collisions were recorded by the ATLAS detector in September 2012 using a trigger that selected events with at least one hit in each side of the minimum-bias trigger scintillators (MBTS), with the resulting dataset corresponding to an integrated luminosity of 1 μb−1
Summary
Proton–nucleus collisions at ultrarelativistic energies provide an opportunity to understand the role of the nuclear environment in modifying hard scattering rates. The appearance of “ridge-like” structures in high-multiplicity pp and p + Pb events [21,22,23,24,25] suggests that small collision systems have the same hydrodynamic origin as Pb + Pb events [26], or that there are already strong correlations in the initial state from gluon saturation [27] All these effects can be explored experimentally by the measurement of charged-hadron production as a function of transverse momentum. |ηCM| < 1 [40]
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