Soft-dielectron excess in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Abstract

A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s}=13$ 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 invariant mass $m_{\mathrm{ee}}$ and pair transverse momentum $p_{\mathrm{T},\mathrm{ee}}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\mathrm{ee}}$, $p_{\mathrm{T},\mathrm{ee}}$, and event multiplicity $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta /{\pi }^0$ ratio in pp and proton-nucleus (p-A) 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 $0.15<m_{\mathrm{ee}}<0.6$ GeV/$c^2$ and for $p_{\mathrm{T},\mathrm{ee}}<0.4$ GeV/$c$ indicates an enhancement of soft dielectrons, reminiscent of the 'anomalous' soft-photon and -dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\pm 0.13(stat.)\pm 0.17(syst.,\mathrm{data})\pm 0.34(syst.,\mathrm{cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in $m_{\mathrm{ee}}$ and $p_{\mathrm{T},\mathrm{ee}}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.

arXiv comment20 pages, 4 captioned figures, authors from page 15, published version, figures at http://alice-publications.web.cern.ch/node/6295

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