Quarkonia production and elliptic flow in small systems measured with ALICE

The production of quarkonia in hadronic collisions provides a unique testing ground for understanding quantum chromodynamics (QCD) since it involves both the perturbative and non-perturbative regimes of this theory. Given that a satisfactory description of quarkonia production has not yet been achieved, new measurements that can provide new insights, helping to constrain models, are needed. The ALICE apparatus allows to measure inclusive J/$\psi$ production, as well as to separate prompt charmonia from those originating from b-hadron decays. The study of the azimuthal correlation of the emitted particles, e.g. via the measurement of the elliptic flow ($v_2$), in high multiplicity proton-proton (pp) collisions can probe collective behaviour in small systems. In this contribution, we present new measurements of the inclusive, prompt and non-prompt J/$\psi$ production in pp collisions at different collision energies, together with the J/$\psi$ $v_2$ in high multiplicity pp collisions at $\sqrt{s}$=13 TeV.Comment: 6 pages, 3 figures. 41st International Conference on High Energy physics - ICHEP2022, Bologna, Italy, 6-13 July, 202

Dilepton polarization as a signature of plasma anisotropy

We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By constrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio $\eta/s$ in the early stages of the collision.Comment: 6 pages, 3 figure

Sonder le plasma de quarks et de gluons au LHC : étude des charmonia avec le détecteur ALICE et phénoménologie des dileptons thermiques

Ultrarelativistic heavy-ion collisions allow to study the behavior of strongly interacting matter at high temperature. In such conditions, the quarks and gluons are no longer confined into hadrons, but form a quark-gluon plasma (QGP). Charmonia, bound states of charm and anti-charm quarks, are important probes of the formation of such a state of matter. In particular, the production and transport of J/psi particles in heavy-ion collisions are impacted by the interaction of the charm quarks with the QGP.In 2021, a new secondary vertexing detector called the Muon Forward Tracker (MFT) was installed in the ALICE experiment. It now allows to separate non-prompt J/psi, originating from the decay of hadrons containing beauty quarks, from those produced promptly in the collisions, in the forward rapidity region of this experiment. This separation allows to further assess the effect of heavy quarks interaction with the QGP. First preliminary studies presented in this thesis show that this separation is indeed achievable in proton-proton collisions using the MFT, opening the door to pursue this analysis in heavy-ion data.Beside charmonia, another significant probe of the medium produced in heavy-ion collisions are thermal dileptons, pairs of electron-positron or muon-anti-muon radiated by the QGP. In particular, dileptons can be used to gain insight into the first instants of heavy-ion collisions and give keys to understand to what extent thermalization can be achieved in such collision. In a phenomenology study, we compute the dilepton production including the contribution from the early stages of the QGP, and show that their invariant mass, transverse mass and polarization distributions can give direct access to the thermalization time and early-time properties of the QGP.Les collisions ultrarelativistes d'ions lourds permettent d'étudier le comportement de la matière en interaction forte à haute température. Dans ces conditions, les quarks et les gluons ne sont plus confinés dans des hadrons, mais forment un plasma de quarks et de gluons (QGP). Les charmonia, états liés de quarks et antiquarks de saveur charm, sont des sondes importantes de la formation d'un tel état de la matière. En particulier, la production et le transport du charmonium J/psi dans les collisions d'ions lourds sont influencés par l'interaction des quarks charm avec le QGP. En 2021, l'expérience ALICE s'enrichit d'un nouveau détecteur appelé Muon Forward Tracker (MFT). Celui-ci permet désormais de séparer les J/psi non prompts, issus de la désintégration des hadrons composés de quarks beauty, de ceux produits directement lors des collisions, dits prompts, dans la région de rapidité à l'avant de l'expérience ALICE. Cette séparation permet de mieux évaluer l'effet de l'interaction des quarks lourds avec le QGP. Des études préliminaires présentées dans cette thèse montrent que cette séparation est effectivement réalisable dans les collisions proton-proton à l'aide du MFT, ce qui ouvre la voie à la poursuite de cette analyse dans les données d'ions lourds.Au côté des charmonia, les dileptons thermiques, paires électron-positron ou muon-anti-muon émises par le QGP, constituent une autre sonde remarquable du milieu produit dans les collisions d'ions lourds et de son évolution. Ils sont particulièrement sensibles aux premiers instants de ces collisions et donnent des clés pour comprendre dans quelle mesure l'équilibre thermique peut y être atteint. Dans une étude phénoménologique, nous calculons la production de dileptons thermiques en prenant en compte la contribution des premiers instants de la formation du QGP, et nous montrons que leurs distributions de masse invariante, de masse transverse et de polarisation peuvent donner un accès direct au temps de thermalisation et aux propriétés du QGP lors de ses premiers instants

Sonder le plasma de quarks et de gluons au LHC : étude des charmonia avec le détecteur ALICE et phénoménologie des dileptons thermiques

Ultrarelativistic heavy-ion collisions allow to study the behavior of strongly interacting matter at high temperature. In such conditions, the quarks and gluons are no longer confined into hadrons, but form a quark-gluon plasma (QGP). Charmonia, bound states of charm and anti-charm quarks, are important probes of the formation of such a state of matter. In particular, the production and transport of J/psi particles in heavy-ion collisions are impacted by the interaction of the charm quarks with the QGP.In 2021, a new secondary vertexing detector called the Muon Forward Tracker (MFT) was installed in the ALICE experiment. It now allows to separate non-prompt J/psi, originating from the decay of hadrons containing beauty quarks, from those produced promptly in the collisions, in the forward rapidity region of this experiment. This separation allows to further assess the effect of heavy quarks interaction with the QGP. First preliminary studies presented in this thesis show that this separation is indeed achievable in proton-proton collisions using the MFT, opening the door to pursue this analysis in heavy-ion data.Beside charmonia, another significant probe of the medium produced in heavy-ion collisions are thermal dileptons, pairs of electron-positron or muon-anti-muon radiated by the QGP. In particular, dileptons can be used to gain insight into the first instants of heavy-ion collisions and give keys to understand to what extent thermalization can be achieved in such collision. In a phenomenology study, we compute the dilepton production including the contribution from the early stages of the QGP, and show that their invariant mass, transverse mass and polarization distributions can give direct access to the thermalization time and early-time properties of the QGP.Les collisions ultrarelativistes d'ions lourds permettent d'étudier le comportement de la matière en interaction forte à haute température. Dans ces conditions, les quarks et les gluons ne sont plus confinés dans des hadrons, mais forment un plasma de quarks et de gluons (QGP). Les charmonia, états liés de quarks et antiquarks de saveur charm, sont des sondes importantes de la formation d'un tel état de la matière. En particulier, la production et le transport du charmonium J/psi dans les collisions d'ions lourds sont influencés par l'interaction des quarks charm avec le QGP. En 2021, l'expérience ALICE s'enrichit d'un nouveau détecteur appelé Muon Forward Tracker (MFT). Celui-ci permet désormais de séparer les J/psi non prompts, issus de la désintégration des hadrons composés de quarks beauty, de ceux produits directement lors des collisions, dits prompts, dans la région de rapidité à l'avant de l'expérience ALICE. Cette séparation permet de mieux évaluer l'effet de l'interaction des quarks lourds avec le QGP. Des études préliminaires présentées dans cette thèse montrent que cette séparation est effectivement réalisable dans les collisions proton-proton à l'aide du MFT, ce qui ouvre la voie à la poursuite de cette analyse dans les données d'ions lourds.Au côté des charmonia, les dileptons thermiques, paires électron-positron ou muon-anti-muon émises par le QGP, constituent une autre sonde remarquable du milieu produit dans les collisions d'ions lourds et de son évolution. Ils sont particulièrement sensibles aux premiers instants de ces collisions et donnent des clés pour comprendre dans quelle mesure l'équilibre thermique peut y être atteint. Dans une étude phénoménologique, nous calculons la production de dileptons thermiques en prenant en compte la contribution des premiers instants de la formation du QGP, et nous montrons que leurs distributions de masse invariante, de masse transverse et de polarisation peuvent donner un accès direct au temps de thermalisation et aux propriétés du QGP lors de ses premiers instants

Quarkonia production and elliptic flow in small systems measured with ALICE

International audienceThe production of quarkonia in hadronic collisions provides a unique testing ground for understanding quantum chromodynamics (QCD) since it involves both the perturbative and non-perturbative regimes of this theory. Given that a satisfactory description of quarkonia production has not yet been achieved, new measurements that can provide new insights, helping to constrain models, are needed. The ALICE apparatus allows to measure inclusive J/$\psi$ production, as well as to separate prompt charmonia from those originating from b-hadron decays. The study of the azimuthal correlation of the emitted particles, e.g. via the measurement of the elliptic flow ($v_2$), in high multiplicity proton-proton (pp) collisions can probe collective behaviour in small systems. In this contribution, we present new measurements of the inclusive, prompt and non-prompt J/$\psi$ production in pp collisions at different collision energies, together with J/$\psi$ $v_2$ in high multiplicity pp collisions at $\sqrt s$ = 13 TeV

Initial Stages 2021

Early time dynamics of heavy-ion collisions represents an important quest to connect the initial conditions with the hydrodynamic evolution and to understand thermalisation in general. Directly sensitive measurements have remained elusive so far. Electromagnetic radiation is a sensitive probe to study QCD systems throughout the full time evolution including the very early stages. In this contribution, we propose an approach to estimate the production of dilepton pairs from the pre-equilibrium phase of heavy-ion collisions. We are employing a generalization of the thermal dilepton production rates for anisotropic systems, where the pre-equilibrium evolution of the system is described by non-equilibrium attractors that connect the highly anisotropic initial state at early times with the late stage viscous hydrodynamic evolution. Based on this framework, we investigate the sensitivity of the dilepton production yields on the macroscopic features of the pre-equilibrium stage; the required kinematics and open points will be pointed out. Finally, we will address the main background and the measurement feasibility with dimuons at forward rapidity for the future LHCb U2 set-up

Intermediate mass dileptons as pre-equilibrium probes in heavy ion collisions

Winn M, Schlichting S, Du X, Ollitrault J-Y, Coquet M. Intermediate mass dileptons as pre-equilibrium probes in heavy ion collisions. Physics Letters B . 2021;821: 136626.The production of dileptons with an invariant mass in the range 1 GeV < M < 5 GeV provides unique insight into the approach to thermal equilibrium in ultrarelativistic nucleus-nucleus collisions. In this mass range, they are produced through the annihilation of quark-antiquark pairs in the early stages of the collision. They are sensitive to the anisotropy of the quark momentum distribution, and also to the quark abundance, which is expected to be underpopulated relative to thermal equilibrium. We take into account both effects based on recent theoretical developments in QCD kinetic theory. We argue that the dilepton mass spectrum provides a measure of the shear viscosity to entropy ratio that controls the equilibration time. We evaluate the background from the Drell-Yan process and argue that future detector developments can suppress the additional background from semileptonic decays of heavy flavors. (C) 2021 The Author(s). Published by Elsevier B.V

Dilepton Polarization as a Signature of Plasma Anisotropy

Coquet M, Winn M, Du X, Ollitrault J-Y, Schlichting S. Dilepton Polarization as a Signature of Plasma Anisotropy. Physical Review Letters . 2024;132(23): 232301.We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By contrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio eta/s in the early stages of the collision

Dilepton polarization as a signature of plasma anisotropy

International audienceWe propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By constrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio $\eta/s$ in the early stages of the collision