Low-mass dielectrons in pp, p–Pb and Pb–Pb collisions measured by the ALICE Experiment

Ivan Vorobyev

doi:10.1051/epjconf/201817118011

Abstract

Dielectrons produced in ultra-relativistic heavy-ion collisions at the LHC provide a unique probe of the system evolution as they are unperturbed by final-state interactions. The dielectron continuum is extremely rich in physics sources: on top of ordinary Dalitz and resonance decays of pseudoscalar and vector mesons, thermal black-body radiation is of particular interest as it carries information about the temperature of the hot and dense system created in such collisions. The dielectron invariant-mass distribution is furthermore sensitive to medium modifications of the spectral function of short-lived vector mesons that are linked to the potential restoration of chiral symmetry at high temperatures. Correlated electron pairs from semi-leptonic charm and beauty decays provide complementary information about the heavy-quark energy loss.

Highlights

In ultra-relativistic heavy-ion collisions, hadronic matter is believed to form a new state of matter comprising deconfined quarks and gluons—the quark-gluon plasma (QGP)
We present a summary of the results from the ALICE experiment [1] in all three collisions√systems at the LHC: pp, p–Pb and Pb–Pb, including the latest analysis of Run-2 pp collisions at s = 13 TeV collected with a trigger on high charged-particle multiplicities
Event triggering is based on the information from the Silicon Pixel Detector (SPD) and/or from the V0 scintillators, the latter are used for centrality estimation in Pb–Pb collisions

Summary

Introduction

In ultra-relativistic heavy-ion collisions, hadronic matter is believed to form a new state of matter comprising deconfined quarks and gluons—the quark-gluon plasma (QGP). Electron-positron pairs are produced during all stages of such collisions and carry information to the detector unperturbed by strong final-state interactions, allowing us to study the whole space-time evolution of the system. In heavy-ion collisions thermal radiation from QGP and hadron gas contribute to the dielectron spectrum over a broad mass range. √ The dielectron invariant mass spectrum has been measured in pp collisions at s = 7 TeV and is shown in Fig. 1 (left). The contribution from the semi-leptonic decays of correlated D and B mesons has been estimated with the help of Pythia 6 simulations and scaled to the measured total charm and beauty cross sections [2]. The results are compared to the DCAee templates obtained from the Monte Carlo simulations of the ALICE detector, and the relative contributions of different sources are normalised according to the hadronic cocktail calculations. |ηe|

N ev dN dDCAee

10-2 ALICE Preliminary

Conclusion and outlook