Multiplicity Dependence of Pion, Kaon, Proton and Lambda Production in p-Pb Collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

TL;DR

This study measures identified hadron production ($\pi^{\pm}$, K$^{\pm}$, K$^{0}_{S}$, p, $\bar{p}$, $\Lambda$, $\bar{\Lambda}$) at midrapidity in p--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV as a function of event multiplicity ($\langle dN_{\rm ch}/d\eta\rangle$). The $p_T$ spectra harden with increasing multiplicity, especially for heavier particles, and the baryon-to-meson ratios show an enhancement at intermediate $p_T$, consistent with radial-flow-like dynamics in a small system; mean $p_T$ rises with multiplicity following a mass hierarchy. Blast-wave fits and model comparisons indicate final-state interactions are needed to describe the data, with hydrodynamic-inspired descriptions and color-reconnection effects providing the best agreement in different regions. The results supply critical constraints for p--Pb modeling and contribute to the broader understanding of collective phenomena across pp, p--Pb, and Pb--Pb collisions. All mathematical notation is presented with proper delimiters, e.g., $p_T$, $dN_{\rm ch}/d\eta$, $T_{kin}$, and $\langle \beta_T \rangle$.

Abstract

In this Letter, comprehensive results on ${\rmπ}^\pm$, K$^\pm$, K$^0_S$, p, $\rm\bar{p}$, $\rm Λ$ and $\rm \barΛ$ production at mid-rapidity ($0 < y_{\rm cms} < 0.5$) in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, measured by the ALICE detector at the LHC, are reported. The transverse momentum distributions exhibit a hardening as a function of event multiplicity, which is stronger for heavier particles. This behavior is similar to what has been observed in pp and Pb-Pb collisions at the LHC. The measured $p_{\rm T}$ distributions are compared to results at lower energy and with predictions based on QCD-inspired and hydrodynamic models.

Figures (7)

• Figure 1: (color online) Invariant $p_{\rm T}$-differential yields of $\pi^{\pm}$, K$^{\pm}$, ${\rm K}^{0}_{S}$, p($\rm\overline{p}$) and $\Lambda$($\bar{\Lambda}$) in different V0A multiplicity classes (sum of particle and antiparticle states where relevant) measured in the rapidity interval $0 < y_{\rm CMS} < 0.5$. Top to bottom: central to peripheral; data scaled by $2^{n}$ factors for better visibility. Statistical (bars) and full systematic (boxes) uncertainties are plotted. Dashed curves: blast-wave fits to each individual distribution.
• Figure 2: (color online) Ratios ${\rm K}/\pi$ = (K$^{+}$ + K$^{-}$)/($\pi^{+}$ + $\pi^{-}$), ${\rm p}/\pi$ = (p + $\rm\overline{p}$)/($\pi^{+}$ + $\pi^{-}$) and $\Lambda$/${\rm K}^{0}_{S}$ as a function of $p_{\rm T}$ in two multiplicity bins measured in the rapidity interval $0 < y_{\rm CMS} < 0.5$ (left panels). The ratios are compared to results in Pb--Pb collisions measured at midrapidity, shown in the right panels. The empty boxes show the total systematic uncertainty; the shaded boxes indicate the contribution uncorrelated across multiplicity bins (not estimated in Pb--Pb).
• Figure 3: (color online) ${\rm p}/\pi$ ratio as a function of the charged-particle density $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$ in three $p_{\rm T}$ intervals in p--Pb (measured in the rapidity interval $0 < y_{\rm CMS} < 0.5$) and Pb--Pb collisions (measured at midrapidity). The dashed lines show the corresponding power-law fit (top). Exponent of the ${\rm p}/\pi$ (middle) and $\Lambda$/${\rm K}^{0}_{S}$ (bottom) power-law fit as a function of $p_{\rm T}$ in p--Pb and Pb--Pb collisions. The empty boxes show the total systematic uncertainty; the shaded boxes indicate the contribution uncorrelated across multiplicity bins (not estimated in Pb--Pb).
• Figure 4: (color online) Mean transverse momentum as a function of $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$ in each V0A multiplicity class (see text for details) for different particle species measured in the rapidity interval $0 < y_{\rm CMS} < 0.5$. The $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$ values of ${\rm K}^{0}_{S}$ are shifted for visibility. The empty boxes show the total systematic uncertainty; the shaded boxes indicate the contribution uncorrelated across multiplicity bins (not estimated in Pb--Pb).
• Figure 5: (color online) Particle yields d$N$/d$y$ of kaons, protons, and lambdas normalized to pions as a function of $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$ in each V0A multiplicity class (see text for details) measured in the rapidity interval $0 < y_{\rm CMS} < 0.5$. The values are compared to results obtained from Pb--Pb collisions at the LHC and Au--Au and d--Au collisions at RHIC measured at midrapidity. The empty boxes show the total systematic uncertainty; the shaded boxes indicate the contribution uncorrelated across multiplicity bins (not estimated in Pb--Pb).