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.

References
ALICE collaboration, ALICE: physics performance report, volume I, J. Phys. G 30 (2004) 1517 [INSPIRE].
ALICE collaboration, ALICE: physics performance report, volume II, J. Phys. G 32 (2006) 1295 [INSPIRE].
ALICE collaboration, The ALICE experiment at the CERN LHC, 2008 JINST 3 S08002 [INSPIRE].
F. Karsch, Lattice simulations of the thermodynamics of strongly interacting elementary particles and the exploration of new phases of matter in relativistic heavy ion collisions, J. Phys. Conf. Ser. 46 (2006) 122 [hep-lat/0608003] [INSPIRE].
Wuppertal-Budapest collaboration, S. Borsányi et al., Is there still any T c mystery in lattice QCD? Results with physical masses in the continuum limit III, JHEP 09 (2010) 073 [arXiv:1005.3508] [INSPIRE].
S. Borsányi et al., The QCD equation of state with dynamical quarks, JHEP 11 (2010) 077 [arXiv:1007.2580] [INSPIRE].
A. Bazavov et al., The chiral and deconfinement aspects of the QCD transition, Phys. Rev. D 85 (2012) 054503 [arXiv:1111.1710] [INSPIRE].
P. Petreczky, Review of recent highlights in lattice calculations at finite temperature and finite density, PoS(Confinement X)028 [arXiv:1301.6188] [INSPIRE].
R. Rapp, J. Wambach and H. van Hees, The chiral restoration transition of QCD and low mass dileptons, Landolt-Bornstein 23 (2010) 134 [arXiv:0901.3289] [INSPIRE].
I. Tserruya, Electromagnetic probes, Landolt-Bornstein 23 (2010) 176 [arXiv:0903.0415] [INSPIRE].
F.-M. Liu and S.-X. Liu, Quark-gluon plasma formation time and direct photons from heavy ion collisions, Phys. Rev. C 89 (2014) 034906 [arXiv:1212.6587] [INSPIRE].
P. Petreczky, Lattice QCD at non-zero temperature, J. Phys. G 39 (2012) 093002 [arXiv:1203.5320] [INSPIRE].
C.A. Dominguez, M. Loewe and Y. Zhang, Chiral symmetry restoration and deconfinement in QCD at finite temperature, Phys. Rev. D 86 (2012) 034030 [Erratum ibid. D 90 (2014) 039903] [arXiv:1205.3361] [INSPIRE].
P.M. Hohler and R. Rapp, Is ρ-meson melting compatible with chiral restoration?, Phys. Lett. B 731 (2014) 103 [arXiv:1311.2921] [INSPIRE].
M. Gyulassy and M. Plumer, Jet quenching in dense matter, Phys. Lett. B 243 (1990) 432 [INSPIRE].
R. Baier, Y.L. Dokshitzer, A.H. Mueller, S. Peigné and D. Schiff, Radiative energy loss and p T broadening of high-energy partons in nuclei, Nucl. Phys. B 484 (1997) 265 [hep-ph/9608322] [INSPIRE].
M.H. Thoma and M. Gyulassy, Quark damping and energy loss in the high temperature QCD, Nucl. Phys. B 351 (1991) 491 [INSPIRE].
E. Braaten and M.H. Thoma, Energy loss of a heavy fermion in a hot plasma, Phys. Rev. D 44 (1991) 1298 [INSPIRE].
E. Braaten and M.H. Thoma, Energy loss of a heavy quark in the quark-gluon plasma, Phys. Rev. D 44 (1991) R2625 [INSPIRE].
V. Greco, C.M. Ko and R. Rapp, Quark coalescence for charmed mesons in ultrarelativistic heavy ion collisions, Phys. Lett. B 595 (2004) 202 [nucl-th/0312100] [INSPIRE].