Soft-dielectron excess in proton-proton collisions at s = 13 TeV

A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron (ee) invariant mass ${m}_{\mathrm{ee}}$ and pair transverse momentum ${p}_{T,\mathrm{ee}}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of ${m}_{\mathrm{ee}}$, ${p}_{T,\mathrm{ee}}$, and event multiplicity $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\ensuremath{\eta}$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\ensuremath{\eta}/{\ensuremath{\pi}}^{0}$ ratio in pp and proton-nucleus collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15<{m}_{\mathrm{ee}}<0.6\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ and for ${p}_{T,\mathrm{ee}}<0.4\text{ }\text{ }\mathrm{GeV}/c$ indicates an enhancement of soft dielectrons, reminiscent of the ``anomalous'' soft-photon and soft-dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\ifmmode\pm\else\textpm\fi{}0.13(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.17(\text{syst},\text{data})\ifmmode\pm\else\textpm\fi{}0.34(\text{syst},\text{cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in ${m}_{\mathrm{ee}}$ and ${p}_{T,\mathrm{ee}}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.