Low mass dilepton production in pp, p–Pb and Pb–Pb collisions with ALICE

Alessandro De Falco

doi:10.1051/epjconf/20159700012

Abstract

Low mass vector meson ( ρ, ω, ϕ ) production provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions (called Quark-Gluon Plasma, QGP). Strangeness enhancement is one of the possible signatures of the Quark-Gluon Plasma formation and can be accessed through the measurement of ϕ meson production with respect to ρ and ω mesons. The production dynamics and hadronization process in relativistic heavy-ion collisions can be probed through the measurement of the ϕ nuclear modification factor.We present results on the low mass dimuon analysis in pp, p–Pb and Pb–Pb collisions. In pp collisions at √ s = 2.76 TeV the ϕ differential cross section as a function of the transverse momentum has been measured, serving as a baseline for Pb–Pb data.The ϕ yield and the nuclear modification factor R pPb at forward and backward rapidity have been measured for p–Pb collisions at √ s = 5.02 TeV. At forward rapidity, R pPb increases as a function of p T , saturating for p T > 3 GeV/ c at R pPb ~ 1. At backward rapidity, R pPb shows an increase as a function of the transverse momentum up to a factor of 1.6 for p T ~ 3-4 GeV/ c , followed by a decrease at higher p T .The ϕ yield and nuclear modification factor R AA have been measured for Pb–Pb collisions at √ s NN = 2.76 TeV in the intermediate p T region 2 p T c , as a function of the number of participating nucleons. Remarkable differences are observed in the comparison between the results at forward rapidity in the dimuon channel and the ones measured in the same p T range at midrapidity in the hadronic channel ϕ → KK .In the dielectron channel at midrapidity, the invariant mass distributions in the range 0 m ee c 2 are compared with the expected hadronic sources for pp collisions at √ s = 7 TeV, and for p–Pb collisions at √ s NN = 5.02 TeV. Latest results of the analysis of Pb–Pb collisions at √ s NN = 2.76 TeV are presented.

Highlights

Vector meson production in pp collisions provides a reference for the determination of the nuclear modification factor
It consists of a hadron absorber, a set of cathode pad chambers for the track reconstruction in a dipole magnetic field, an iron wall acting as a muon filter and two stations of two resistive plate chambers for the muon trigger
The centrality is determined with the VZERO detector, that consists of two arrays of plastic scintillators placed at 3.4 m and -0.9 m from the interaction point (IP)

Summary

Introduction

The ALICE experiment at the LHC can access low-mass dileptons both at forward rapidity, in the dimuon channel, and at midrapidity in the dielectron channel The focus of this contribution will be on vector meson production in the dimuon channel. The measurement in the dimuon channel was performed using the forward muon spectrometer, that covers the pseudorapidity range −4 < η < −2.5 ( in the ALICE reference frame the muon spectrometer covers a negative η range, we chose to present our results in symmetric systems with a positive y notation) It consists of a hadron absorber, a set of cathode pad chambers (five stations, each one composed of two chambers) for the track reconstruction in a dipole magnetic field, an iron wall acting as a muon filter and two stations of two resistive plate chambers for the muon trigger. The centrality is determined with the VZERO detector, that consists of two arrays of plastic scintillators placed at 3.4 m and -0.9 m from the interaction point (IP)

Results from pp collisions

Results from Pb–Pb collisions

Results from p–Pb collisions p–Pb collisions by the ALICE experiment in

Conclusions