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Soft-dielectron excess in proton-proton collisions at = 13 TeV
A measurement of dielectron production in proton-proton (pp) collisions at
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 and pair transverse momentum that was previously inaccessible at the LHC. The cross section for
dielectron production is studied as a function of , ,
and event multiplicity . The expected
dielectron rate from hadron decays, called hadronic cocktail, utilizes a
parametrization of the measured 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 GeV/
and for GeV/ 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 in the ALICE acceptance. Acceptance-corrected
excess spectra in and are extracted and compared
with calculations of dielectron production from hadronic bremsstrahlung and
thermal radiation within a hadronic many-body approach.Comment: 19 pages, 4 captioned figures, authors from page 14, submitted to
PRL, figures at http://alice-publications.web.cern.ch/node/629
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