Direct-photon and heavy-flavour production in proton-proton collisions at $\sqrt{s} = 7$ TeV with ALICE

Abstract

Low-mass dielectron measurements play an essential role in the study of the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. They are produced in all stages of the collision and are not affected by final-state interactions. Thus, they provide a penetrating probe of the created medium. In the dielectron intermediate-mass region a measurement of the thermal radiation from the QGP gives information on the medium temperature. However, in this region the main component of the dielectron continuum stems from correlated semi-leptonic decays of charm and beauty hadrons. Therefore, it is crucial to understand the heavy-flavour (HF) production in vacuum and to find a way to separate this contribution from the thermal dielectron signal of the QGP. In this paper we will present the production of correlated $\rm e^+e^-$ pairs in pp collisions at $\sqrt{s} = 7$ TeV with the expectations from known hadronic sources as a function of $m_{\rm ee}$, $p_{\rm T,ee}$, and the pair distance of closest approach to the primary vertex $\rm DCA_{ee}$. The extraction of the ratio of inclusive photons to decay photons and the charm and beauty cross sections from a fit of the data with different Monte-Carlo generators are presented, providing insight into the mechanisms of heavy-flavour production. In particular, we demonstrate how the $\rm DCA_{ee}$ variable will allow prompt and non-prompt dielectron pairs to be separated and thus help to disentangle the contribution of thermal radiation from the contribution of charm and beauty. Finally, we present the prospect of extending this analysis to proton-lead collisions.