by Ivan Vorobyev. Published: 25 September 2018

On September 4th, Ivan Vorobyev, from the Technische Universität München, gave a talk within the LHC seminar series presenting recent results on low-mass dielectron production measured by ALICE in pp and Pb−Pb collisions. In this article, the author provides a summary of his seminar.

Ivan Vorobyev giving his talk in the CERN Main Auditorium.

Electron – positron pairs (dielectrons) have established themselves as a particularly useful experimental tool to study thehot and dense medium created in ultra-relativistic heavy-ion collisions. In contrast to strongly interacting hadronic probes, dielectrons provide an unperturbed view into the quark-gluon plasma (QGP) produced in these collisions. They allow to determine the initial temperature of the QGP generated and to study effects of the predicted restoration of chiral symmetry.

Recently, the ALICE Collaboration has analysed data of proton–proton (pp) collisions at centre-of-mass energies of √s = 7 and 13 TeV and lead–lead (Pb–Pb) collisions at √sNN= 2.76 TeV. The results have been submitted for publication. In the figure here below, the measured dielectron invariant-mass spectra are shown for all three collision systems. The data are compared with the expected spectra of dielectrons from decays of light mesons and J/ψ as well as semi-leptonic decays of correlated heavy-flavour hadrons. The Pb–Pb results are not yet sensitive enough to quantify the presence of thermal radiation and signs of chiral symmetry restoration on top of the “vacuum” expectation.

Dielectron cross section as a function of invariant mass in pp collisions at √s = 7 TeV (red) and 13 TeV (blue) and in central Pb—Pb collisions at √sNN = 2.76 TeV (green) measured by ALICE. The results are scaled by the number of binary collisions (Ncoll) and compared to a cocktail of known hadronic sources shown as bands.

In inelastic pp collisions, the dielectron yield can be well described by the expectation from hadronic cocktail calculations. The heavy-flavour production cross-sections have been measured by fitting the dielectron spectrum in the intermediate mass region between the ϕ and J/ψ masses with two different models of heavy-flavour production, PYTHIA 6.4 and POWHEG. The substantial difference between the results obtained with two Monte Carlo generators, observed at both investigated collision energies, reflects different kinematic correlations of the heavy-quark pairs implemented in the two event generators. Dielectron measurements are uniquely sensitive to the heavy quark production mechanisms and provide crucial input to improve models of charm production that is complementary to single charmed-hadron measurements. In addition, the distance of closest approach of the electrons to the collision vertex has been successfully used in the analysis of pp collisions at √s= 7 TeV to distinguish displaced dielectrons from open-heavy flavour decays and prompt decays of light hadrons.

In pp collisions at √s= 13 TeV, the dielectron production has been studied as a function of the charged-particle multiplicity at mid-rapidity. The observed increase of dielectron production is well reproduced by hadronic cocktail calculations based on previous measurements of light- and heavy-flavour production in pp collisions as a function of multiplicity. The results indicate for the first time that open beauty production scales similarly with multiplicity as open charm production.

In continuation of a systematic investigation of dielectron production, ALICE will study dielectrons in pp, p−Pb and Pb−Pb collisions collected in LHC Run 2 (2015−2018) and Run 3 (2021−2023). One of the main objectives of the ALICE physics programme for the LHC Run 3 and beyond will be the precise measurement of dielectron production in heavy-ion collisions. Part of data will be recorded with the magnetic field of the central barrel solenoid reduced from 0.5 T to 0.2 T in order to further increase the acceptance of dielectrons with low mass and transverse momentum. After a major upgrade of main detector systems, about 100 times more data in central Pb−Pb collisions will allow ALICE to study the low-mass region and the in-medium properties of the ρ meson in detail. The initial system temperature is expected to be measured with a precision of 10−20% from an exponential fit to the intermediate mass region of the dielectron spectrum. Here, the impact parameter of the electrons will be a crucial tool to isolate a thermal signal from open heavy-flavour decays.

Further reading:

  1. ALICE Collaboration, “Dielectron production in proton−proton collisions at √s= 7 TeV”, accepted by JHEP, arXiv:1805.0439
  2. ALICE Collaboration, “Dielectron and heavy-quark production in inelastic and high- multiplicity proton−proton collisions at √s= 13 TeV”, submitted to Phys. Lett. B, arXiv:1805.04407
  3. ALICE Collaboration, “Measurement of dielectron production in central Pb−Pb collisions at √sNN= 2.76 TeV”, submitted to Phys. Rev. C, arXiv:1807.00923