Multiplicity dependence of $Ξ_c^+$ and $Ξ_c^0$ production in pp collisions at $\sqrt{s} = 13$ TeV

Abstract

The first measurement at midrapidity ($|y| < 0.5$) of the production yield of the strange-charm baryons $Ξ_c^+$ and $Ξ_c^0$ as a function of transverse momentum ($p_{\rm T}$) in different charged-particle multiplicity classes in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ALICE experiment at the LHC is reported. The $Ξ_c^+$ baryon is reconstructed via the $Ξ_c^+ \rightarrow Ξ^-π^+π^+$ decay channel in the range $4 < p_{\rm T} < 12$ GeV/$c$, while the $Ξ_c^0$ baryon is reconstructed via both the $Ξ_c^0 \rightarrow Ξ^-π^+$ and $Ξ_c^0 \rightarrow Ξ^-e^+ν_e$ decay channels in the range $2 < p_{\rm T} < 12$ GeV/$c$. The baryon-to-meson ($Ξ_c^{0,+}/{\rm D}^0$) and the baryon-to-baryon ($Ξ_c^{0,+}/Λ_{\rm c}^+$) production yield ratios show no significant dependence on multiplicity. In addition, the observed yield ratios are not described by theoretical predictions that model charm-quark fragmentation based on measurements at $e^+e^-$ and $e^-$p colliders, indicating differences in the charm-baryon production mechanism in pp collisions. A comparison with different event generators and tunings, including different modelling of the hadronisation process, is also discussed. Moreover, the branching-fraction ratio of BR($Ξ_c^0 \rightarrow Ξ^-e^+ν_e$)/BR($Ξ_c^0 \rightarrow Ξ^-π^+$) is measured as 0.825 $\pm$ 0.094 (stat.) $\pm$ 0.081 (syst.). This value supersedes the previous ALICE measurement, improving the statistical precision by a factor of 1.6.

Figures (6)

• Figure 1: Invariant-mass distributions of signal candidates for the hadronic decays of $\Xi_{\rm c}^{+}$ in 4 $<$$p_{\rm T}$$<$ 6 GeemV/$c$ (left), and $\Xi_{\rm c}^{0}$ in 6 $<$$p_{\rm T}$$<$ 8 GeemV/$c$ (right), in the high-multiplicity class. The blue solid curve shows the total fit, and the red dashed curve shows the combinatorial background.
• Figure 2: Invariant-mass distribution of $\rm e\Xi$ pairs for $\Xi_{\rm c}^{0}\xspace \rightarrow \Xi^- {\rm e}^+ \rm \nu_{\rm e}$ in 4 $<$$p_{\rm T}$$<$ 6 GeemV/$c$, in the high- (left) and low- (right) multiplicity classes. The blue filled distribution shows the total fit and the coloured lines indicate the different sources contributing to the fit.
• Figure 3: $p_{\rm T}$-differential per-event yield of prompt $\Xi_{\rm c}^{0}$ (left) and $\Xi_{\rm c}^{+}$ (right) baryons measured in the different multiplicity classes in pp collisions at $\sqrt{s}=13$ TeV at midrapidity ($|y| < 0.5$), along with the corresponding ratios to the multiplicity-integrated (INEL $>$ 0) class in the bottom panel. The values shown in the legend, denoted as $\left< \mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta\xspace \right>_{|\eta|<0.5}$, correspond to the average charged-particle multiplicity at midrapidity for the respective multiplicity classes, as introduced in Table \ref{['tab:multiplicity class']}. The statistical and systematic uncertainties are shown as bars and open boxes, respectively. The shaded boxes indicate the uncertainty of the branching ratio.
• Figure 4: The prompt production yield ratios between $\Xi_{\rm c}^{0,+}$ and $\rm D^{0}$ mesons (top) and $\Xi_{\rm c}^{0,+}$ and $\Lambda_{\rm c}^{+}$ baryons (bottom) measured in the same multiplicity classes in pp collisions at $\sqrt{s}=13$ TeV ALICE:2021npz. The statistical and systematic uncertainties are shown as bars and open boxes, and the uncertainty from BR is represented in shaded boxes, respectively.
• Figure 5: The baryon-to-meson ratios (top) and baryon-to-baryon ratios (bottom), measured in the low- (left) and high- (right) multiplicity classes. The measurements are compared with the predictions from two event generators: PYTHIA 8 with different tunes (namely Monash Skands_2014, CR-BLC Christiansen:2015yqa Mode 0, 2, and 3) and EPOS4HQ EPOS4HQ.