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

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Summary

Apparatus and data sample

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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