SMuK 2021 – scientific programme
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HK: Fachverband Physik der Hadronen und Kerne
HK 22: Heavy-Ion Collisions and QCD Phases IV
HK 22.1: Group Report
Thursday, September 2, 2021, 16:30–17:00, H1
Soft dielectron production in pp and Pb–Pb collisions with ALICE — •Jerome Jung for the ALICE collaboration — IKF, Goethe University Frankfurt, Germany
The production of soft dielectrons is an exceptional and versatile tool to study the underlying mechanisms and properties of hadron-hadron and heavy-ion collisions (HIC). In HIC, the STAR collaboration observed first a clear excess of dielectrons produced at low pair momenta which exceeded the hadronic decay background. These soft dielectrons can be attributed to coherent photoproduction originating in the interaction of the highly contracted electromagnetic fields of the colliding ions, a sole QED process. In hadronic collisions, several experiments observed an excess at low momenta for real as well as virtual photons beyond hadronic decays which could not be explained by initial- and final-state bremsstrahlung either. As this soft-photon puzzle is absent in purely leptonic collisions, the origin of the effect seems to be connected to QCD.
In this talk, ALICE measurements of dielectron production in pp and (semi-) peripheral Pb–Pb collisions, will be presented. The pp collisions are recorded with a reduced magnetic field of the central barrel solenoid of 0.2 T. This enables the investigation of a kinematic domain at low invariant mass mee and pair transverse momentum pT,ee, which was previously inaccessible at the LHC. Comparison of the measured dielectron yield to the hadronic decay cocktail indicates a clear enhancement of soft dielectrons in both systems. Finally, the excess spectra are extracted and compared to theoretical model calculations.