First measurement of $Λ_\mathrm{c}^{+}$ production down to $p_\mathrm{T} = 0$ in pp and p-Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV

The production of prompt $\mathrm {\Lambda_{c}^{+}}$ baryons has been measured at midrapidity in the transverse momentum interval $0<~p_{\rm T}<~1$ GeV/$c$ for the first time, in pp and p-Pb collisions at a centre-of-mass energy per nucleon-nucleon collision $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The measurement was performed in the decay channel ${\rm \Lambda_{c}^{+}\to p K^{0}_{S}}$ by applying new decay reconstruction techniques using a Kalman-Filter vertexing algorithm and adopting a machine-learning approach for the candidate selection. The $p_{\rm T}$-integrated $\mathrm {\Lambda_{c}^{+}}$ production cross sections in both collision systems were determined and used along with the measured yields in Pb-Pb collisions to compute the $p_{\rm T}$-integrated nuclear modification factors $R_{\rm pPb}$ and $R_\mathrm{AA}$ of $\mathrm{\Lambda_{c}^{+}}$ baryons, which are compared to model calculations that consider nuclear modification of the parton distribution functions. The $\mathrm{\Lambda_{c}^{+}/D^0}$ baryon-to-meson yield ratio is reported for pp and p-Pb collisions. Comparisons with models that include modified hadronisation processes are presented, and the implications of the results on the understanding of charm hadronisation in hadronic collisions are discussed. A significant ($3.7\sigma$) modification of the mean transverse momentum of $\mathrm {\Lambda_{c}^{+}}$ baryons is seen in p-Pb collisions with respect to pp collisions, while the $p_{\rm T}$-integrated $\mathrm{\Lambda_{c}^{+}/D^0}$ yield ratio was found to be consistent between the two collision systems within the uncertainties.

 

Submitted to: PRC
e-Print: arXiv:2211.14032 | PDF | inSPIRE
CERN-EP-2022-261
Figure group

Figure 1

Distributions of the BDT output probabilities for \LctopKs signal (red) and background (blue) candidates in \pPb collisions for \lowptbin. The shaded regions represent the output of the training sample, and the markers are the results after applying the model on the test sample.

Figure 2

Invariant mass distributions of \LctopKs candidates in \lowptbin, in pp (left) and \pPb (right) collisions at \sqrtsNNfive. The red dashed curves represent the background fits, and the blue curves the total fits. The lower panels show the distributions after subtracting the background estimated with the fit.

Figure 3

The \pt-differential \Lc production cross sections in pp and \pPb collisions at \sqrtsNNfive , including the new measurements in \lowptbin as open markers. The lower panels show the ratios of the measurements to POWHEG+PYTHIA6, with EPPS16 nPDF calculations included for \pPb collisions .

Figure 4

Nuclear modification factor \RpPb of prompt \Lc baryons in \pPb collisions at \sqrtsNNfive as a function of \pt, compared with model calculations .

Figure 5

The \pt-integrated nuclear modification factors of prompt \Lc baryons and \Dzero mesons measured in \pPb and Pb–Pb collisions at \sqrtsNNfive . Statistical (bars) and systematic and extrapolation (brackets) uncertainties are shown. The measurements are compared with calculations from the theoretical models nCTEQ15  and EPPS16  that include only initial-state effects. The uncertainty bands on the models represent the 90\% confidence level.

Figure 6

Left: \LcD ratio in pp and \pPb collisions as a function of \pt, compared with the QCM model . Right: \LcD ratio as a function of \pt in pp collisions at \sqrtsfive, including comparisons with models .