Dielectron production in proton-proton and proton-lead collisions at sNN =5.02 TeV

The first measurements of dielectron production at midrapidity $(\left|{\ensuremath{\eta}}_{\mathrm{e}}\right|<0.8)$ in proton--proton and proton--lead collisions at $\sqrt{{s}_{\mathrm{NN}}}=5.02\phantom{\rule{0.16em}{0ex}}\mathrm{TeV}$ at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass ${m}_{\mathrm{ee}}$ and the pair transverse momentum ${p}_{\mathrm{T},\mathrm{ee}}$ in the ranges ${m}_{\mathrm{ee}}<3.5$ $\mathrm{GeV}/{c}^{2}$ and ${p}_{\mathrm{T},\mathrm{ee}}<8$ $\mathrm{GeV}/c$, 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 $\sqrt{s}=7$ and 13 TeV. The slope of the $\sqrt{s}$ 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 ${\mathrm{e}}^{+}{\mathrm{e}}^{\ensuremath{-}}$ 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 $\sqrt{{s}_{\mathrm{NN}}}$ via the dielectron nuclear modification factor ${R}_{\mathrm{pPb}}$. The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation.