Beauty in the quark–gluon plasma
[News published in the CERN Courier, issue March 2017]
Nuclear modification factors of electrons from beauty-hadron decays at mid-rapidity for p–Pb and central Pb–Pb collisions.
In high-energy nucleus–nucleus collisions, heavy-flavour quarks (charm and beauty) are produced on a very short time scale in initial hard-scattering processes and thus they experience the entire evolution of the collision. Such quarks are valuable probes to study the mechanisms of energy loss and hadronisation in the hot and dense matter, the quark–gluon plasma, formed in heavy-ion collisions.
To investigate these effects, proton–proton (pp) and proton–lead (p–Pb) collisions are measured as a reference. While the former allows the study of heavy-flavour production when no medium is formed, the latter gives access to cold nuclear matter effects, namely parton scattering in the initial state and modifications of the parton densities in the nucleus.
The excellent electron identification capabilities and track impact parameter resolution of the ALICE detector enable measurements of electrons from heavy-flavour hadron decays at mid-rapidity. To study the predicted quark mass dependence of the parton energy loss, the contributions of electrons from charm- and beauty-hadron decays are statistically separated using the different impact parameter distributions as a proxy for their decay length and empirical estimations of the background.
The measurement of electrons from heavy-flavour hadron decays in p–Pb collisions shows no indication of a modification of the production with respect to pp collisions at high transverse momentum (pT), indicating that cold nuclear matter effects are small. The observed reduction in yield at high pT in central Pb–Pb collisions relative to pp interactions can thus be attributed to the presence of the hot and dense medium formed in Pb–Pb collisions. This implies that beauty quarks interact with the medium.
The larger suppression of electrons from both charm- and beauty-hadron decays compared with the beauty-only measurement is consistent with the ordering of charm and beauty suppression seen previously in the comparison of prompt D mesons (measured by ALICE) and J/ψ from B meson decays (measured by CMS). The larger samples of Pb–Pb collisions in Run 2 will improve the precision of the measurements and will make it possible to determine if beauty quarks participate in the collective expansion of the quark–gluon plasma.
ALICE Collaboration 2016 arXiv:1609.03898.
ALICE Collaboration 2016 arXiv:1609.07104.
ALICE Collaboration 2015 JHEP 1511 205.