Abstract
We study the relevance of experimental data on heavy-flavor [D^{0}, J/ψ, B→J/ψ and ϒ(1S) mesons] production in proton-lead collisions at the LHC to improve our knowledge of the gluon-momentum distribution inside heavy nuclei. We observe that the nuclear effects encoded in both most recent global fits of nuclear parton densities at next-to-leading order (nCTEQ15 and EPPS16) provide a good overall description of the LHC data. We interpret this as a hint that these are the dominant ones. In turn, we perform a Bayesian-reweighting analysis for each particle data sample which shows that each of the existing heavy-quark(onium) data set clearly points-with a minimal statistical significance of 7σ-to a shadowed gluon distribution at small x in the lead. Moreover, our analysis corroborates the existence of gluon antishadowing. Overall, the inclusion of such heavy-flavor data in a global fit would significantly reduce the uncertainty on the gluon density down to x≃7×10^{-6}-where no other data exist-while keeping an agreement with the other data of the global fits. Our study accounts for the factorization-scale uncertainties which dominate for the charm(onium) sector.
Highlights
Parton-distribution functions (PDFs), describing the longitudinal-momentum distributions of quarks and gluons inside hadrons, provide the essential link between the measurable hadronic cross sections and the perturbatively calculable cross sections of high-energy processes induced by quarks and gluons
This observation is striking as the used gluon Nuclear PDFs (nPDFs) were derived from totally different observables like deep-inelastic scattering (DIS) and
As for the reweighting results (gray-blue hatched bands in Figs. [1(a)–1(d)], if we could fix the scale to a single value for each particle, the LHC RpPb data for prompt D0 and J=ψ would reduce the uncertainties of the gluon density by a factor 3 for EPPS16 and 2 for nCTEQ15 down to x ≃ 7 × 10−6 [compare the gray-blue and red hatched bands in Figs. 1(a) and 1(d)]
Summary
Parton-distribution functions (PDFs), describing the longitudinal-momentum distributions of quarks and gluons inside hadrons, provide the essential link between the measurable hadronic cross sections and the perturbatively calculable cross sections of high-energy processes induced by quarks and gluons. The precise determination of PDFs of protons, fpi , is an extremely active area of research where several groups perform global analyses of a wide variety of experimental hard-process data. The modern global analyses [1,2,3,4,5,6] have evolved into impressive ventures with state-of-the-art perturbative calculations and sophisticated statistical methods to extract optimum PDFs along with their uncertainties. The situation is more challenging—but not less interesting—for PDFs of nucleons inside nuclei, f1⁄2ip;n=A, with nuclear data significantly more complex to collect and with two additional degrees of freedom, the number of protons (Z) and neutrons (N 1⁄4 A − Z) in the studied nuclei.
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