Constraining Gluon Distributions in Nuclei Using Dijets in Proton-Proton and Proton-Lead Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

The pseudorapidity distributions of dijets as functions of their average transverse momentum ($p_{\mathrm{T}}^{\text{ave}}$) are measured in proton-lead ($p\mathrm{Pb}$) and proton-proton ($pp$) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. A significant modification of the $p\mathrm{Pb}$ spectra with respect to the $pp$ spectra is observed in all $p_{\mathrm{T}}^{\text{ave}}$ intervals investigated. The ratios of the $p\mathrm{Pb}$ and $pp$ distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken $x$ in lead ions is strongly suppressed with respect to the PDF in unbound nucleons.

Figure 1

Top: correlation between $x_{\mathrm{Pb}}/x_p$ and dijet pseudorapidity ${\eta }_{\text{dijet}}$. The dashed line corresponds to the expected correlation without ISR or FSR effects. Bottom: mean Bjorken $x$ of the parton from the lead ion $⟨x_{\mathrm{Pb}}⟩$ obtained from pythia 6 events as a function of ${\eta }_{\text{dijet}}$ in different dijet $p_{\mathrm{T}}^{\text{ave}}$ intervals.

Figure 2

Ratio of $p\mathrm{Pb}$ to $pp$ ${\eta }_{\text{dijet}}$ spectra compared to NLO pQCD calculations with DSSZ [18] and EPS09 [14] nPDFs, using CT14 [58] as the baseline nucleon PDF. Red boxes indicate systematic uncertainties in data and the height of the NLO pQCD calculation boxes represent the nPDF uncertainties.

Figure 3

Ratio of theory to data, for the ratio of the $p\mathrm{Pb}$ to $pp$ ${\eta }_{\text{dijet}}$ spectra for $115<p_{\mathrm{T}}^{\text{ave}}<150\mathrm{GeV}$. Theory points are from the NLO pQCD calculations of DSSZ [18], EPS09 [14], nCTEQ15 [15], and EPPS16 [16] nPDFs, using CT14 [58] as the baseline PDF. Red boxes indicate the total (statistical and systematic) uncertainties in data, and the error bars on the points represent the nPDF uncertainties.