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Horst Sebastian Scheid

Publications and source records attributed to Horst Sebastian Scheid.

3 recordsLinked to original sources

Measurement of heavy-quark production via dielectrons in pp and p-Pb collisions with ALICE at the LHC

Heavy quarks are useful probes to investigate the properties of the Quark-Gluon Plasma (QGP) produced in heavy-ion collisions at the LHC, since they are produced in initial hard scattering processes. To single out the signals that are characteristic of the QGP, it is nevertheless crucial to understand the primordial heavy-quark production in vacuum, and to disentangle hot from cold nuclear matter effects. Moreover, observations of collective effects in high-multiplicity pp and p-Pb collisions show surprising similarities with those in heavy-ion collisions. Heavy-flavour production in such collisions could give further insight into the underlying processes. The heavy-flavour production can be studied with $\rm e^{+}e^{-}$ pairs from correlated semileptonic decays of heavy-flavour hadrons. Compared to single heavy-flavour measurements, the dielectron yield contains information about the initial kinematical correlations between the charm and anti-charm quarks, which is otherwise not accessible, and is sensitive to soft heavy-flavour production. We report results on correlated $\rm e^{+}e^{-}$ pairs in pp collisions recorded by the ALICE detector at different collision energies. The production of heavy quarks is discussed by comparing the yield of dielectrons from heavy-flavour hadron decays as a function of invariant mass, pair transverse momentum and distance of closest approach to the primary vertex with different Monte Carlo event generators. The heavy-flavour production cross sections are also presented. Results from high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV and the status of the p-Pb analysis at $\sqrt{s_{\rm NN}} = 5.02$ TeV are reported as well.

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Dielectron production in proton-proton collisions at $\sqrt{s} = 7$ TeV with ALICE

The ALICE Collaboration measured dielectron production as a function of the invariant mass ($m_{\rm ee}$), the pair transverse momentum ($p_{\rm T,ee}$) and the pair distance of closest approach ($\rm DCA_{ee}$) in pp collisions at $\sqrt{s} = 7$ TeV. Prompt and non-prompt dielectron sources can be separated with the $\rm DCA_{ee}$, which will give the opportunity in heavy-ion collisions to identify thermal radiation from the medium in the intermediate-mass range dominated by contributions from open-charm and beauty hadron decays. The charm and beauty total cross sections are extracted from the data by fitting the spectra with two different MC generators, i.e. PYTHIA a leading order event generator and POWHEG a next-to-leading order event generator. Significant model dependences are observed, reflecting the sensitivity of this measurement to the heavy-flavour production mechanisms.

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

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.

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