Abstract
This paper presents a measurement of forward-forward and forward-central dijet azimuthal angular correlations and conditional yields in proton-proton ($pp$) and proton-lead ($p$+Pb) collisions as a probe of the nuclear gluon density in regions where the fraction of the average momentum per nucleon carried by the parton entering the hard scattering is low. In these regions, gluon saturation can modify the rapidly increasing parton distribution function of the gluon. The analysis utilizes 25 pb$^{-1}$ of $pp$ data and 360 $\mu \mathrm{b}^{-1}$ of $p$+Pb data, both at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, collected in 2015 and 2016, respectively, with the ATLAS detector at the LHC. The measurement is performed in the center-of-mass frame of the nucleon-nucleon system in the rapidity range between $-$4.0 and 4.0 using the two highest transverse momentum jets in each event, with the highest transverse momentum jet restricted to the forward rapidity range. No significant broadening of azimuthal angular correlations is observed for forward-forward or forward-central dijets in $p$+Pb compared to $pp$ collisions. For forward-forward jet pairs in the proton-going direction, the ratio of conditional yields in $p$+Pb collisions to those in $pp$ collisions is suppressed by approximately 20%, with no significant dependence on the transverse momentum of the dijet system. No modification of conditional yields is observed for forward-central dijets.
Highlights
Studies of particle collisions at accelerators have contributed significantly to an improved understanding of the strong interaction in quantum chromodynamics (QCD) and to the knowledge of the parton distribution functions (PDFs) of the proton
The uncertainties associated with unfolding, fitting, the acceptance correction, and the additional jet energy scale (JES) uncertainties associated with the differences between the detector response and its simulations in p + Pb collisions compared to pp collisions are taken to be uncorrelated between the two collision systems and are added in quadrature
The results of the measurement of conditional yields I12 in p + Pb and pp collisions are shown in the right panels of Fig. 5
Summary
Studies of particle collisions at accelerators have contributed significantly to an improved understanding of the strong interaction in quantum chromodynamics (QCD) and to the knowledge of the parton distribution functions (PDFs) of the proton. Global QCD analyses of structure functions in deep-inelastic lepton-nucleon scattering at HERA, as well as jet and hadron cross sections at the Large Hadron Collider (LHC), Tevatron, and Relativistic Heavy Ion Collider (RHIC) were performed in a wide kinematic range, providing several new sets of PDFs with the highest degree of precision reached so far [1,2,3,4] These analyses constrain quark and gluon contributions over a wide range of the Bjorken variable x: The longitudinal-momentum fraction of a nucleon carried by its constituent partons. The ATLAS and CMS analyses of inclusive jet production did not find significant evidence of nuclear modification [15,16] Another approach to probe gluon saturation in nuclear gluon densities was proposed in the framework of the color glass condensate (CGC) model [17] by studying the modifications of dijet azimuthal angular distributions in pp and p + Pb collisions at forward rapidities at xA down to 10−5 [18]. The results of the measurement are presented both without any requirement on pT and with a requirement of pT > 3 GeV
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