Soft-Dielectron Excess in Proton-Proton Collisions at sqrt[s]=13  TeV

Area of Science:

• High-energy particle physics
• Quantum chromodynamics
• Hadron spectroscopy

Background:

• Proton-proton collisions at the Large Hadron Collider (LHC) provide a unique environment to study fundamental particle interactions.
• Previous experiments have reported an excess of soft dileptons in hadron-hadron collisions, hinting at phenomena beyond the Standard Model.
• Investigating dielectron production at low invariant mass and transverse momentum is crucial for understanding these anomalies.

Purpose of the Study:

• To measure dielectron production in proton-proton collisions at a center-of-mass energy of 13 TeV.
• To explore a previously inaccessible kinematic region at the LHC, characterized by low dielectron invariant mass and pair transverse momentum.
• To compare the measured dielectron yield with theoretical predictions, specifically the hadronic cocktail, and quantify any observed enhancement.

Main Methods:

• Utilized the ALICE detector at the CERN LHC, recording data with a reduced magnetic solenoid field.
• Studied the cross section for dielectron production as a function of invariant mass (m_{ee}), pair transverse momentum (p_{T,ee}), and event charged particle multiplicity (dN_{ch}/dη).
• Employed a parametrization of the measured eta/pi^{0} ratio for the hadronic cocktail, assuming minimal dependence on collision energy at low transverse momentum.

Main Results:

• An enhancement of soft dielectrons was observed in the kinematic region of 0.15 < m_{ee} < 0.6 GeV/c² and p_{T,ee} < 0.4 GeV/c.
• The measured dielectron yield exceeds the hadronic cocktail prediction by a factor of 1.61 ± 0.13 (stat) ± 0.17 (syst,data) ± 0.34 (syst,cocktail) within the ALICE acceptance.
• Extracted acceptance-corrected excess spectra were compared with theoretical calculations of dielectron production from hadronic bremsstrahlung and thermal radiation.

Conclusions:

• The observed enhancement of soft dielectrons suggests the presence of contributions beyond standard hadron decays.
• This finding aligns with previous reports of anomalous soft dilepton excesses in hadron-hadron collisions.
• Further theoretical and experimental investigations are needed to fully interpret the origin of this enhancement and its implications for particle physics.