Using the data on coherent J/ψ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) obtained in Runs 1 and 2 at the Large Hadron Collider (LHC), we determined with a good accuracy the nuclear ...suppression factor of SPb(x) in a wide range of the momentum fraction x, 10−5≤x≤0.04. In the small-x region x<10−3, our χ2 fit favors a flat form of SPb(x)≈0.6 with approximately a 5% accuracy for x=6×10−4−10−3 and a 25% error at x=10−4. At the same time, uncertainties of the fit do not exclude a slow decrease of SPb(x) in the small-x limit. At large x, SPb(x) is constrained to better than 10% precision up to x=0.04 and is also consistent with the value of SPb(x) at 〈x〉=0.042, which we extract from the Fermilab data on the A dependence of the cross section of coherent J/ψ photoproduction on fixed nuclear targets. The resulting uncertainties on SPb(x) are small, which indicates the potential of the LHC data on coherent charmonium photoproduction in Pb-Pb UPCs to provide additional constraints on small-x nPDFs. We explicitly demonstrate this using as an example the EPPS16 and nCTEQ16 nuclear parton distribution functions, whose uncertainties decrease severalfold after the Bayesian reweighting of the discussed UPC data.
We make predictions for cross sections of ρ and ϕ vector meson photoproduction in ultraperipheral Xe–Xe collisions at sNN=5.44TeV. Analyzing the momentum transfer distribution of ρ mesons in this ...process, we explore the feasibility of extracting the nuclear density of 129Xe, which is needed in searches for dark matter with Xenon-based detectors.
Abstract
Thermal photons serve as valuable probes of the hot and dense medium produced in heavy ion collisions. The effective thermal photon temperature measured at RHIC and LHC energies far exceeds ...the temperature predicted for the phase space transition into the deconfined state of quarks and gluons, known as quark-gluon plasma (QGP). Direct photon measurements in heavy ion collisions at the future NICA collider may help to estimate the effective temperature of the produced medium at lower energies and trace the transition from QGP to the hadron gas state. In this contribution, we present feasibility studies on the thermal photon measurements in AuAu collisions using the photon conversion method in the MPD experiment at NICA.
The MPD experiment is aimed to study strongly interacting matter in nucleus-nucleus collisions at the future NICA collider in the energy range 4-11 GeV per nucleon pair. Neutral pion and η ...measurements via two-photon decays will allow the MPD experiment to extend the variety of registered particle species and will provide important information on dynamics and properties of the medium created in heavy ion collisions at NICA energies. The measurement of neutral pion and η meson spectra is also important as a first step towards the analysis of thermal photons sensitive to the temperature of the produced medium. In this contribution, the feasibility of neutral meson production measurements in the two-photon channel via photon conversions will be presented, and implications for the study of thermal photons will be discussed.
We show that the recent ALICE measurements of exclusive J/ψ production in ultraperipheral PbPb collisions at 2.76 TeV provide the first direct experimental evidence for the strong nuclear gluon ...shadowing in lead at x∼10−3. The evidence is based on the comparison of the nuclear suppression factor S(x≈0.001)=0.61−0.04+0.05 found in the analysis of the coherent J/ψ photoproduction cross sections measured by ALICE with the nuclear gluon shadowing predicted by the global fits of nuclear parton distributions and by the leading twist theory of nuclear shadowing.
We argue that the STAR data on J/ψ photoproduction in d + Au ultraperipheral collisions (UPCs) at the momentum transfer t ≠ 0 primarily probes elastic and possibly nucleon-dissociative J/ψ ...photoproduction on quasifree nucleons, while being essentially insensitive to expectedly weak nuclear modifications of the gluon distribution in the deuteron. Analyzing the STAR detector capabilities for selection of UPC events, we explore the possibility that the contribution of nucleon-dissociative J/ψ photoproduction is negligibly small and obtain a very good description of the t dependence of the STAR total cross section.
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