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Register a new userCoherent J/$psi$ photoproduction in Pb-Pb collisions with nuclear overlap studied with ALICE at the LHC
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L. Massacrier
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The photoproduction of heavy vector mesons in the electromagnetic interactions of ultra-relativistic nuclei is sensitive to the gluon distribution in the nucleus and thus to cold nuclear matter effects like shadowing or parton saturation. Besides the well known observations of vector meson production in ultra-peripheral collisions, first observations of an excess over the expected hadronic J/$psi$ production at very low transverse momentum ($p_T < $~0.3 GeV/$c$) in peripheral and semi-central nucleus-nucleus collisions both at LHC and RHIC energies were interpreted as the first sign of coherent J/$psi$ photoproduction occurring in Pb-Pb collisions with nuclear overlap. The ALICE Collaboration published the J/$psi$ coherent photoproduction cross sections in peripheral and semi-central Pb-Pb collisions at $sqrt{s_{rm NN}}$ = 2.76~TeV and forward rapidity ($2.5<y<4.0$). Using the LHC Run-2 data, ALICE presents preliminary results in peripheral Pb-Pb collisions at $sqrt{s_{rm NN}}$ = 5.02 TeV at mid-rapidity ($|y|<0.9$) and forward rapidity. Thanks to the very good tracking resolution of the central barrel, the extraction of the $p_T$-differential cross section was also possible, strengthening the photoproduction origin of the observed J/$psi$ excess. The quantitative understanding of this low-$p_T$ excess poses significant theoretical challenges since the J/$psi$ photoproduction depends on the collision dynamics as well as on the photon-flux and the photonuclear cross section. In this proceeding, we present the latest ALICE measurements on J/$psi$ photoproduction cross section in peripheral Pb-Pb collisions, with emphasis on the new forward measurement in the dimuon decay channel at $sqrt{s_{rm NN}}$ = 5.02~TeV. These results will be discussed and compared to several model calculations of J/$psi$ photoproduction in Pb-Pb collisions with nuclear overlap.
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Recent results for high multiplicity pp and p-Pb collisions have revealed that they exhibit heavy-ion-like behaviors. To understand the origin(s) of these unexpected phenomena, event shape observables such as transverse spherocity ($S_{rm 0}^{p_{rm T} = 1}$) and the relative transverse activity classifier ($R_{rm{T}}$) can be exploited as a powerful tools to disentangle soft (non-perturbative) and hard (perturbative) particle production. Here, the production of light-flavor hadrons is shown for various $S_{rm 0}^{p_{rm T} = 1}$ classes in pp collisions at $sqrt{s}$ = 13 $textrm{TeV}$ measured with the ALICE detector at the LHC are presented. The evolution of average transverse momentum ($langle p_{rm T}rangle$) with charged-particle multiplicity, and identified particle ratios as a function of $p_{rm T}$ for different $S_{rm 0}^{p_{rm T} = 1}$ are also presented. In addition, the system size dependence of charged-particle production in pp, p-Pb, and Pb-Pb collisions at $sqrt{s_{rm NN}}$ = 5.02 TeV is presented. The evolution of $langle p_{rm T}rangle$ in different topological regions as a function of $R_{rm{T}}$ are presented. Finally, using the same approach, we present a search for jet quenching behavior in small collision systems.
We present the first results from the ALICE experiment on the nuclear modification factors for heavy-flavour hadron production in Pb-Pb collisions at sqrt{s_NN}=2.76 TeV. Using proton-proton and lead-lead collision samples at sqrt{s}=7 TeV and sqrt{s_NN}=2.76 TeV, respectively, nuclear modification factors R_AA(pt) were measured for D mesons at central rapidity (via displaced decay vertex reconstruction), and for electrons and muons, at central and forward rapidity, respectively.
By generalizing the statistical model for particle production to the spin degree of freedom of initially produced J/psi, we study the spin projection J_y of J/psi perpendicular to the reaction plane in peripheral heavy ion collisions at the LHC energy that leads to a strong, albeit of short duration, magnetic field. We find that for J/psis produced directly from charm and anticharm quarks in the color singlet state, like that in the Color-Singlet Model, their yield in the presence of the magnetic field is larger for J_y=0 than for J_y=1 or -1. This leads to a spin asymmetry of finally produced J/psi even after including their final-state scattering in the produced quark-gluon plasma.
One of the key results of the LHC Run 1 was the observation of an enhanced production of strange particles in high multiplicity pp and p--Pb collisions at $sqrt{s_mathrm{NN}}$ = 7 and 5.02 TeV, respectively. The strangeness enhancement is investigated by measuring the evolution with multiplicity of single-strange and multi-strange baryon production relative to non-strange particles. A smooth increase of strange particle yields relative to the non-strange ones with event multiplicity has been observed in such systems. We report the latest results on multiplicity dependence of strange and multi-strange hadron production in pp collisions at $sqrt{s} = $ 13 TeV with ALICE. We also presented recent measurements of mesonic and baryonic resonances in small collision systems like pp and p--Pb at $sqrt{s_mathrm{NN}}$ = 13 and 8.16 TeV, respectively. The system size dependent studies in pp and p-Pb collisions have been used to investigate how the hadronic scattering processes affect measured resonance yields and to better understand the interplay between canonical suppression and strangeness enhancement. The measurement of the $phi(1020)$ meson as a function of multiplicity provides crucial constraints in this context.
Heavy quarks, i.e. charm and beauty, are produced on a shorter time scale with respect to the strongly-interacting matter produced in high-energy heavy-ion collisions. Therefore, they are unique probes to study the mechanisms of parton energy loss, hadronisation and thermalization in the hot and dense state of matter. The nuclear modification factor ($R_{rm AA}$) and the elliptic flow ($v_{2}$) are two of the main experimental observables that allow us to investigate the interaction strength of heavy quarks with the medium. The most recent results on heavy-flavour production and elliptic flow measured by the ALICE collaboration in Pb--Pb collisions at $sqrt{s_mathrm{NN}}$ = 2.76 TeV will be discussed.
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