Measurement of electrons from beauty-hadron decays in p-Pb collisions at \(\sqrt{s_{\rm NN}}=5.02\) TeV and Pb-Pb collisions at \(\sqrt{s_{\rm NN}}=2.76\) TeV

Abstract

The production of beauty hadrons was measured via semi-leptonic decays at mid-rapidity with the ALICE detector at the LHC in the transverse momentum interval \(1<p_{\rm T}<8\) GeV/\(c\) in minimum-bias p–Pb collisions at \(\sqrt{s_{\rm NN}}=5.02\) TeV and in \(1.3<p_{\rm T}<8\) GeV/\(c\) in the 20% most central Pb-Pb collisions at \(\sqrt{s_{\rm NN}}=2.76\) TeV. The pp reference spectra at \(\sqrt{s}=5.02\) TeV and \(\sqrt{s}=2.76\) TeV, needed for the calculation of the nuclear modification factors \(R_{\rm pA}\) and \(R_{\rm AA}\), were obtained by a pQCD-driven scaling of the cross section of electrons from beauty-hadron decays measured at \(\sqrt{s}=7\) TeV. The \(R_{\rm AA}\) is about 0.7 with an uncertainty of about 30% in the interval \(3<p_{\rm T}<6\) GeV/\(c\) and 0.47 with an uncertainty of 25% in \(6<p_{\rm T}<8\) GeV/\(c\), Below \(p_{\rm T}=3\) GeV/\(c\), the \(R_{\rm AA}\) values increase with decreasing transverse momentum with systematic uncertainties of 30-45%. The \(R_{\rm pA}\) is consistent with unity within systematic uncertainties of about 20% at high \(p_{\rm T}\), increasing at low \(p_{\rm T}\), and is well described by theoretical calculations that include cold nuclear matter effects in p-Pb collisions. The measured \(R_{\rm pA}\) and these calculations indicate that cold nuclear matter effects are small at high transverse momentum also in Pb–Pb collisions. Therefore, the observed reduction of \(R_{\rm AA}\) below unity for high \(p_{\rm T}\) can be ascribed to an effect of the hot and dense medium formed in Pb–Pb collisions.