Abstract
The production of prompt D$_s^+$ mesons was measured for the first time in collisions of heavy nuclei with the ALICE detector at the LHC. The analysis was performed on a data sample of Pb-Pb collisions at a centre-of-mass energy per nucleon pair, $\sqrt{s_{\rm NN}}$, of 2.76 TeV in two different centrality classes, namely 0-10% and 20-50%. D$_s^+$ mesons and their antiparticles were reconstructed at mid-rapidity from their hadronic decay channel D$_s^+\rightarrow\phi\pi^+$, with $\phi\rightarrow$K$^-$K$^+$, in the transverse momentum intervals $4< p_{\rm T}<12$ GeV/$c$ and $6< p_{\rm T}<12$ GeV/$c$ for the 0-10% and 20-50% centrality classes, respectively. The nuclear modification factor $R_{\rm AA}$ was computed by comparing the $p_{\rm T}$-differential production yields in Pb-Pb collisions to those in proton-proton (pp) collisions at the same energy. This pp reference was obtained using the cross section measured at $\sqrt{s}= 7$ TeV and scaled to $\sqrt{s}= 2.76$ TeV. The $R_{\rm AA}$ of D$_s^+$ mesons was compared to that of non-strange D mesons in the 10% most central Pb-Pb collisions. At high $p_{\rm T}$ ($8< p_{\rm T}<12$ GeV/$c$) a suppression of the D$_s^+$-meson yield by a factor of about three, compatible within uncertainties with that of non-strange D mesons, is observed. At lower $p_{\rm T}$ ($4< p_{\rm T}<8$ GeV/$c$) the values of the D$_s^+$-meson $R_{\rm AA}$ are larger than those of non-strange D mesons, although compatible within uncertainties. The production ratios D$_s^+$/D$^0$ and D$_s^+$\D$^+$ were also measured in Pb-Pb collisions and compared to their values in proton-proton collisions.
Highlights
On a data sample of Pb-Pb collisions at a centre-of-mass energy per nucleon pair, sNN, of 2.76 TeV in two different centrality classes, namely 0–10% and 20–50%
JHEP03(2016)082 decay leptons [7,8,9,10], D mesons [11, 12] and non-prompt J/ψ [13] in Au-Au and Pb-Pb collisions at RHIC and LHC energies as compared to proton-proton collisions. This modification is usually quantified by the nuclear modification factor RAA, defined as the ratio between the yield measured in nucleus-nucleus collisions and the cross section in pp interactions scaled by the average nuclear overlap function
The information provided by the following detectors was utilised to perform the analysis presented in this paper: the Inner Tracking System (ITS), the Time Projection Chamber (TPC) and the Time Of Flight (TOF) detector were used to reconstruct and identify charged particles at mid-rapidity, while the V0 scintillator detector provided the information for triggering, centrality determination and event selection
Summary
The ALICE detector and its performance are described in detail in refs. [46] and [47], respectively. To select D+s mesons decaying in the considered φπ+ mode, with φ → K−K+, candidates were rejected if none of the two pairs of opposite-charged tracks had an invariant mass compatible with the PDG world average for the φ mass (1.0195 GeV/c2 [52]). Signal candidates with wrong mass assignment to the same-sign tracks would give rise to a contribution to the invariant-mass distributions that could potentially introduce a bias in the measured raw yield of D+s mesons It was verified, both in data and in simulations, that this contribution is reduced to a negligible level by the particle identification selection and by the requirement that the invariant mass of the two tracks identified as kaons is compatible with the φ mass. The background was evaluated by integrating the background fit functions in ±3σ around the centroid of the Gaussian
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