Measurement of D$^+_s$ production and nuclear modification factor in Pb–Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV

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–nucleon collision of $\sqrt{s_{\rm NN}} = 2.76$ 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 $\phi\rightarrow {\rm K}^−{\rm K}^+$, 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 nonstrange 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.