Dielectron production at midrapidity at low transverse momentum in peripheral and semi-peripheral Pb-Pb collisions at root s(NN)=5.02 TeV

The first measurement of the e(+)e(-) pair production at low lepton pair transverse momentum (p(T,ee)) and low invariant mass (m(ee)) in non-central Pb-Pb collisions at root s(NN) = 5.02TeV at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity (vertical bar eta(e)vertical bar < 0.8) as a function of invariant mass (0.4 <= m(ee) < 2.7 GeV/c(2)) in the 50-70% and 70-90% centrality classes for p(T,ee) < 0.1 GeV/c, and as a function of p(T,ee) in three m(ee) intervals in the most peripheral Pb-Pb collisions. Below a p(T,ee) of 0.1 GeV/c, a clear excess of e(+)e(-) pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The m(ee) excess spectra are reproduced, within uncertainties, by different predictions of the photon-photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the p(T,ee) spectra. The measured root < p(T,ee)(2)> of the excess p(T,ee) spectrum in peripheral Pb-Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region.

Automated summary

The first measurement of e(+)e(-) pair production at low p(T,ee) and low m(ee) in non-central Pb-Pb collisions at the LHC is presented. Excess e(+)e(-) pairs are found below a p(T,ee) of 0.1 GeV/c, compared to expectations from known sources and thermal radiation. Different predictions of photon-photon production and QED calculations provide a good description of the measured spectra.