Abstract
The first measurement of e+e− pair production at mid-rapidity (|ηe| < 0.8) in pp collisions at sqrt{s}=7 TeV with ALICE at the LHC is presented. The dielectron production is studied as a function of the invariant mass (mee < 3.3 GeV/c2), the pair transverse momentum (pT,ee < 8 GeV/c), and the pair transverse impact parameter (DCAee), i.e., the average distance of closest approach of the reconstructed electron and positron tracks to the collision vertex, normalised to its resolution. The results are compared with the expectations from a cocktail of known hadronic sources and are well described when PYTHIA is used to generate the heavy-flavour contributions. In the low-mass region (0.14 < mee < 1.1 GeV/c2), prompt and non-prompt e+e− sources can be separated via the DCAee. In the intermediate-mass region (1.1 < mee < 2.7 GeV/c2), a double-differential fit to the data in mee and pT,ee and a fit of the DCAee distribution allow the total mathrm{c}overline{mathrm{c}} and mathrm{b}overline{mathrm{b}} cross sections to be extracted. Two different event generators, PYTHIA and POWHEG, can reproduce the shape of the two-dimensional mee and pT,ee spectra, as well as the shape of the DCAee distribution, reasonably well. However, differences in the mathrm{c}overline{mathrm{c}} and mathrm{b}overline{mathrm{b}} cross sections are observed when using the generators to extrapolate to full phase space. Finally, the ratio of inclusive to decay photons is studied via the measurement of virtual direct photons in the transverse-momentum range 1 < pT < 8 GeV/c. This is found to be unity within the statistical and systematic uncertainties and consistent with expectations from next-to-leading order perturbative quantum chromodynamic calculations.
Highlights
The fi√rst measurement of e+e− pair production at mid-rapidity (|ηe| < 0.8) in pp collisions at s = 7 TeV with ALICE at the LHC is presented
To single out the interesting signal characteristics of the QGP, it is crucial to understand the dielectron yield from primordial heavy-flavour production. The latter can be studied in proton-proton collisions, which serve as a reference for the analysis of heavy-ion collisions and provide a test for perturbative QCD calculations and Monte Carlo (MC) event generators
The results are presented as a function of the invariant mass mee (0 < mee < 3.3 GeV/c2) of the e+e− pair, its transverse momentum pT,ee (0 < pT,ee < 8 GeV/c), and the pair transverse impact parameter DCAee
Summary
The ALICE apparatus and its performance are described in detail in [1,2,3, 59]. In the following, only the subsystems relevant for the dielectron analysis are briefly discussed. The charged-particle identification capability of the TPC and ITS is supplemented by the TOF [62], which is located at a radial distance of 3.7 m from the beam axis It provides a measurement of the time of flight for particles from the interaction point up to the detector itself. The event collision time is either measured with the T0 detector, which consists of two arrays of Cherenkov counters located at z = +375 cm and z = −72.7 cm from the nominal interaction point, or estimated using the particle arrival times at the TOF for events with sufficiently large multiplicity [59] Due to their curved paths in the magnetic field of the solenoidal magnet, charged particles need a minimum pT of about 300 MeV/c to reach the TOF detector.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
