Abstract
The first measurements of the production of muons and electrons from heavy-flavour hadron decays in Xe–Xe collisions at sNN=5.44 TeV, using the ALICE detector at the LHC, are reported. The measurement of the nuclear modification factor RAA is performed as a function of transverse momentum pT in several centrality classes at forward rapidity (2.5<y<4) and midrapidity (|y|<0.8) for muons and electrons from heavy-flavour hadron decays, respectively. A suppression by a factor up to about 2.5 compared to the binary-scaled pp reference is observed in central collisions at both central and forward rapidities. The RAA of muons from heavy-flavour hadron decays is compared to previous measurements in Pb–Pb collisions at sNN=5.02 TeV. When the nuclear modification factors are compared in the centrality classes 0–10% for Xe–Xe collisions and 10–20% for Pb–Pb collisions, which have similar charged-particle multiplicity density, a similar suppression, with RAA∼0.4 in the pT interval 4<pT<8 GeV/c, is observed. The comparison of the measured RAA values in the two collision systems brings new insights on the properties of the quark-gluon plasma by investigating the system-size and geometry dependence of medium-induced parton energy loss. The results of muons and electrons from heavy-flavour hadron decays provide new constraints to model calculations.
Highlights
A state of strongly-interacting matter in which quarks and gluons are deconfined, the quark–gluon plasma (QGP), is created in ultra-relativistic heavy-ion collisions at high energy density and high temperature [1,2,3,4,5]
In order to quantify the effect of parton energy loss, we employ the nuclear modification factor R AA defined as the ratio of the p T and y-differential particle yield (d2 N AA /dp T d y) in nucleus-nucleus collisions of a given centrality to the corresponding p T - and ydifferential production cross section (d2 σpp /dp T d y) in pp collisions
In the electron analysis the systematic uncertainties are evaluated considering the following contributions: i) subtraction of electrons originating from Dalitz decays and photon conversions, including variation of the number of hits for electron candidates in the Silicon Pixel Detector (SPD), ii) matching efficiency of tracks reconstructed in the Inner Tracking System (ITS) and TPC, iii) matching of reconstructed tracks between the TOF and TPC, iv) track-reconstruction and identification procedure, v) space-charge distortions in the TPC drift volume, vi) residual hadron contamination, and vii) electron contribution from J/ψ decays
Summary
A state of strongly-interacting matter in which quarks and gluons are deconfined, the quark–gluon plasma (QGP), is created in ultra-relativistic heavy-ion collisions at high energy density and high temperature [1,2,3,4,5]. This letter presents the first measurement of the production of open heavy-flavour hadrons via the muon and electron decay channels at forward rapidity (2.5 < y < 4) and midrapidity (| y | < 0.8) in Xe–Xe collisions at sNN = 5.44 TeV with the ALICE detector at the LHC. Electrons from heavy-flavour hadron decays are measured, for the first time at the LHC, down to p T = 0.2 GeV/c in the 20–40% centrality interval thanks to the reduced magnetic field of 0.2 T in the ALICE solenoid magnet, as compared to the nominal field of 0.5 T for Pb–Pb collisions. The results discussed in this letter can bring additional constraints on the model parameters sensitive to the path-length dependence of in-medium energy loss
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