Accessing the speed of sound in relativistic ultracentral nucleus-nucleus collisions using the mean transverse momentum
Fernando G. Gardim, Andre V. Giannini, Jean-Yves Ollitrault
Hide abstract | Show figures | Show BibTeX | Show discussion | View PDF | 2403.06052v1
It has been argued that the speed of sound of the strong interaction at high temperature can be measured using the variation of the mean transverse momentum with the particle multiplicity in ultracentral heavy-ion collisions. We test this correspondence by running hydrodynamic simulations at zero impact parameter with several equations of state, at several colliding energies from 0.2 TeV to 15 TeV per nucleon pair. The correspondence is found to be precise and robust for a smooth, boost-invariant fluid and an ideal detector. We discuss the differences between this ideal setup and an actual experiment. We conclude that the extraction of the speed of sound from data is reliable, and that the main uncertainty comes from our poor knowledge of the distribution of density fluctuations at the early stages of the collision.
/
Coherent J/ψ photoproduction and polarization in peripheral Pb−Pb collisions
Afnan Shatat
Hide abstract | Show figures | Show BibTeX | Show discussion | View PDF | 2402.12469v1
Photonuclear reactions are induced by the strong electromagnetic field generated by ultrarelativistic heavy-ion collisions. These processes have been extensively studied in ultraperipheral collisions, where the impact parameter is larger than twice the nuclear radius. In recent years, the observation of coherent J/ψ photoproduction at very low transverse momentum has been claimed in nucleus−nucleus (A−A) collisions with nuclear overlap, based on the measurement of an excess in the J/ψ yield with respect to hadroproduction expectations. Such quarkonium measurements can help to constrain the nuclear gluon distribution at low Bjorken-x and high energy. The photoproduced quarkonium is expected to preserve the polarization of the incoming photon due to the s-channel helicity conservation. In this contribution, we report on the new preliminary measurements of the y-differential cross section and the first polarization analysis of coherently photoproduced J/ψ in peripheral Pb−Pb collisions with the ALICE detector at forward rapidity in the dimuon decay channel. Comparison with models will be shown when available.
Authors' comments: 4 pages, 3 captioned figures, Quark Matter 2023-XXXth International Conference on Ultra-relativistic Nucleus-Nucleus Collisions, 3-9 Sept 2023, Houston, Texas, USA; submitted to EPJ's website
