Measuring the temperature of the hottest medium in the universe: dielectrons in ALICE

Our research focuses on the measurement of low-mass electron pairs (dielectrons) in heavy-ion collisions with the ALICE detector at the Large Hadron Collider (LHC). These dielectrons pairs are expected to provide information about the quark-gluon plasma produced in heavy-ion collisions, such as the temperature and whether chiral symmetry is restored. Thermal dielectrons are emitted as black-body radiation from the QGP, hence one can infer the temperature of the system, the hottest ever produced, from the shape of their energy spectrum. The spontaneous breaking of chiral symmetry (an approximate symmetry of the QCD Lagrangian) in vacuum causes the mass differences of chiral partners, i.e. hadrons with opposite parity. At temperatures as high as reached inside heavy-ion collisions, one expects chiral symmetry to be restored and thus the mass splitting of chiral partners to disappear. A prime candidate for the investigation of mass effects due to chiral symmetry restoration is the ρ meson: it has a lifetime of only 1.5 fm/c which is significantly smaller than the life time of the medium (10 fm/c). Its decay channel into an electron-positron pair remains unaffected by strong final state interactions and preserves the information about the in-medium properties of the rho meson. The analysis of the a1 meson, the chiral partner of the rho, proves more challenging: it decays into three pions that will interact strongly with the surrounding medium. Furthermore, it is a much broader state and suffers significantly larger background levels so it is not clear it can ever be measured in heavy-ion collisions. Other research activities focus on the production of heavy quarks in heavy-ions collisions and their interactions with the QGP.

 For more informations contact the group leader Dr. Torsten Dahms.