# 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 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.

Figures

## Figure 1

 Invariant-mass distributions of $\rm D_s^+$ candidates and charge conjugates in the three considered $\pt$ intervals in the 10% most central Pb-Pb collisions.

## Figure 2

 Left: Acceptance-times-efficiency for $\rm D_s^+$ mesons in the 10% most central Pb--Pb collisions. The efficiencies for prompt (solid lines) and feed-down (dotted lines) $\rm D_s^+$ mesons are shown. Also displayed, for comparison, the efficiency for prompt $\rm D_s^+$ mesons without PID selections (dashed lines). Right: Relative variation of the prompt $\rm D_s^+$-meson yield in the 0-10% centrality class as a function of the hypothesis on $\RAA^{\rm{feed-down}}/\RAA^{\rm prompt}$ for the B feed-down subtraction approach based on Eq. (2).

## Figure 3

 Transverse momentum distributions $\rm{d} N/\rm{d}\pt$ of prompt $\rm D_s^+$ mesons in the 0-10% (left panel) and 20-50% (right panel) centrality classes in Pb-Pb collisions at $\snn=2.76~\rm TeV$. Statistical uncertainties (bars), systematic uncertainties from data analysis (empty boxes) and systematic uncertainties due to beauty feed-down subtraction (shaded boxes) are shown. The reference pp distributions $\langle T_{\rm AA}\rangle \rm{d}\sigma/\rm{d}\pt$ are shown as well.

## Figure 4

 Left: $\RAA$ of prompt $\rm D_s^+$ mesons in the 0-10% and 20-50% centrality classes as a function of $\pt$. For the 20-50% case, the symbols are displaced horizontally for visibility. Right: $\RAA$ of prompt $\rm D_s^+$ mesons compared to non-strange D mesons(average of $\Dzero$, $\Dplus$ and $\Dstar$) in the 0-10% centrality class. Statistical (bars), systematic (empty boxes), and normalisation (full box) uncertainties are shown.

## Figure 5

 Ratios of prompt D-meson yields ($\rm D_s^+$/$\Dzero$ and $\rm D_s^+$/$\Dplus$) as a function of $\pt$ in the 10% most central Pb-Pb collisions at $\snn=2.76~\rm TeV$ compared to the results in pp collisions at $\sqrt{s}=7~\rm TeV$. Statistical (bars) and systematic (boxes) uncertainties are shown.

## Figure 6

 $\RAA$ of prompt $\rm D_s^+$ and non-strange D mesons (average of $\Dzero$, $\Dplus$ and $\Dstar$) in the 0-10% centrality class compared to predictions of the TAMU model [61]. The bands shown for the TAMU predictions encompass the charm-shadowing uncertainty.