Observation of a multiplicity dependence in the $p_{\rm T}$-differential charm baryon-to-meson ratios in proton-proton collisions at $\sqrt{s} = 13$ TeV

The production of prompt $D^{0}$, $D^{+}_{\rm s}$, and $\Lambda_{\rm c}^{+}$ hadrons, and their ratios, $D^{+}_{\rm s}$/$D^{0}$ and $\Lambda_{\rm c}^{+}$/$D^{0}$, are measured in proton-proton collisions at $\sqrt{s}$ = 13 TeV at midrapidity ($|y| <~0.5$) with the ALICE detector at the LHC. The measurements are performed as a function of the charm-hadron transverse momentum ($p_{\rm T}$) in intervals of charged-particle multiplicity, measured with two multiplicity estimators covering different pseudorapidity regions. While the strange to non-strange $D^{+}_{\rm s}$/$D^{0}$ ratio indicates no significant multiplicity dependence, the baryon-to-meson $p_{\rm T}$-differential $\Lambda_{\rm c}^{+}$/$D^{0}$ ratio shows a multiplicity-dependent enhancement, with a significance of 5.3$\sigma$ for $1<~ p_{\rm T} <~ 12$ GeV/$c$, comparing the highest multiplicity interval with respect to the lowest one. The measurements are compared with a theoretical model that explains the multiplicity dependence by a canonical treatment of quantum charges in the statistical hadronisation approach, and with predictions from event generators that implement colour reconnection mechanisms beyond the leading colour approximation to model the hadronisation process. The $\Lambda_{\rm c}^{+}$/$D^{0}$ ratios as a function of $p_{\rm T}$ present a similar shape and magnitude as the $\Lambda/K^{0}_{s}$ ratios in comparable multiplicity intervals, suggesting a potential common mechanism for light- and charm-hadron formation, with analogous multiplicity dependence. The $p_{\rm T}$-integrated ratios, extrapolated down to $p_{\rm T}$=0, do not show a significant dependence on multiplicity within the uncertainties.

 

Submitted to: PLB
e-Print: arXiv:2111.11948 | PDF | inSPIRE
CERN-EP-2021-245

Figure 1

Transverse-momentum spectra of \Dzero, \Ds, and \Lc hadrons measured in \pp collisions at \thirteen for different multiplicity classes selected with the \ntrkl estimator at midrapidity. The corresponding ratios to \inel are shown in the bottom panels.

Figure 2

The \DsDzero (top) and \LcDzero (bottom) ratios measured in \pp collisions at \thirteen for different multiplicity classes at mid- (left) and forward (right) rapidity.

Figure 3

The \DsDzero (top) and \LcDzero (bottom) ratios measured in \pp collisions at \thirteen for the lowest (left) and highest (right) multiplicity classes at midrapidity. The measurements are compared to PYTHIA predictions with the Monash~ and the CR-BLC tunes~, and the CE-SH model~, estimated in similar multiplicity classes. The uncertainty bands for the PYTHIA predictions are the statistical uncertainties on the simulations, while for the CE-SH model they refer to the variation of the branching ratios of the additional charm-baryon states from RQM~.

Figure 4

The baryon-to-meson ratios in the light-flavour, based on measurements from Ref.~ and charm sector measured in \pp collisions at \thirteen for similar low- and high-multiplicity classes at mid- (left) and forward (right) rapidity.

Figure 5

Ratios of \pt-integrated yields of \Lambdac and \Dzero hadrons as a function of \dNchdeta in pp collisions at \thirteen. Measurements performed in pp and p--Pb collisions at \fivenn from Ref.~ are also shown. Statistical and systematic uncertainties are shown by error bars and empty boxes, respectively. Shaded boxes represent the extrapolation uncertainties. The corresponding PYTHIA predictions~ are also shown.