Dielectron production in proton-proton and proton-lead collisions at root s(NN)=5.02 TeV

The first measurements of dielectron production at midrapidity (vertical bar eta(e)vertical bar < 0.8) in proton-proton and proton-lead collisions at root s(NN) = 5.02 TeV at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass m(ee) and the pair transverse momentum p(T,ee) in the ranges m(ee) < 3.5 GeV/c(2) and p(T,ee) < 8 GeV/c, in both collision systems. In proton-proton collisions, the charm and beauty cross sections are determined at midrapidity from a fit to the data with two different event generators. This complements the existing dielectron measurements performed at root s = 7 and 13 TeV. The slope of the root s dependence of the three measurements is described by FONLL calculations. The dielectron cross section measured in proton-lead collisions is in agreement, within the current precision, with the expected dielectron production without any nuclear matter effects for e(+)e(-) pairs from open heavy-flavor hadron decays. For the first time at LHC energies, the dielectron production in proton-lead and proton-proton collisions are directly compared at the same root s(NN) via the dielectron nuclear modification factor R-pPb. The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation.