Multiplicity Dependence of Pion, Kaon, Proton and Lambda Production in p-Pb Collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV (1307.6796v4)

Abstract: In this Letter, comprehensive results on ${\rm\pi}^\pm$, K$^\pm$, K$^0_S$, p, $\rm\bar{p}$, $\rm \Lambda$ and $\rm \bar{\Lambda}$ 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.

• The paper demonstrates a pronounced multiplicity dependence in transverse momentum spectra, with heavier particles like protons and lambdas showing significant hardening.
• The study reports enhanced particle ratios at intermediate pT, mirroring trends in Pb–Pb collisions and indicating complex particle production mechanisms.
• The paper compares experimental data with QCD-based models, suggesting that collective effects may be relevant even in smaller collision systems like p–Pb.

Overview of Particle Production in Proton-Lead Collisions at LHC

Introduction

The paper of high-energy heavy-ion collisions enables the investigation of nuclear matter under extreme conditions, particularly with respect to the deconfined quark-gluon plasma as predicted by Quantum Chromodynamics (QCD). Such investigations necessitate a comparative analysis of different collision systems, namely proton-proton (pp), proton-nucleus (pA), and nucleus-nucleus (AA) collisions. This paper presents comprehensive measurements of particle production, specifically of pions, kaons, protons, and lambdas, in proton-lead (p--Pb) collisions at a center-of-mass energy of 5.02 TeV. The measurements were conducted using the ALICE detector at the Large Hadron Collider (LHC).

Results

The paper reports a pronounced hardening in the transverse momentum ($p_T$) distributions as a function of event multiplicity, with heavier particles exhibiting a more significant hardening. This behavior is analogous to patterns observed in pp and Pb--Pb collisions at LHC, suggesting potential common underlying mechanisms such as collective effects.

The key observations can be summarized as follows:

1. Multiplicity Dependence: A clear evolution in $p_T$ spectra was observed, becoming notably harder with increasing event multiplicity. This trend was especially pronounced for protons and lambdas.
2. Particle Ratios: Ratios such as $\pi/K$, $p/\pi$, and $\Lambda/K^0_s$ showed significant enhancements at intermediate $p_T$, resembling those seen in Pb--Pb collisions, though the magnitudes in p--Pb are notably lower.
3. Scaling Behaviors: The $p/\pi$ ratio displayed a power-law dependence on the charged particle multiplicity density ($dN/d\eta$) across different $p_T$ intervals, with similar exponents observed in both p--Pb and Pb--Pb collisions.
4. Mean Transverse Momentum: The mean $p_T$ of produced particles increased with multiplicity, demonstrating a mass-dependent order consistent with collective radial flow phenomena seen in heavy-ion collisions.
5. Model Comparisons: Comparisons were made with QCD-based theoretical models, including DPMJET, Krakow, and EPOS LHC. While EPOS provided a reasonable description of pion and proton spectra, discrepancies remained for kaons and lambdas.

Implications

The findings provide significant insights into the particle production mechanisms in p--Pb collisions, challenging the notion that final-state effects associated with dense matter can be entirely neglected in small systems such as p--Pb. The observed similarities between p--Pb and Pb--Pb collision behaviors suggest that collective effects might also be relevant in smaller systems at high multiplicities.

Theoretical and Practical Ramifications

Understanding the multiplicity dependence of particle spectra contributes to distinguishing between initial and final state effects in nuclear collisions, which is crucial for QCD theories. The experimental data serve as a testing ground for theoretical models that aim to describe the initial conditions and subsequent evolution of the collision systems, pushing forward the development of more comprehensive theoretical approaches.

Future research can extend these analyses to explore dependencies over a wider range of collision systems and energies, potentially elucidating the transition phases between pp, pA, and AA collisions. Enhanced understanding of these transitions is crucial for unifying the descriptions of matter under extreme conditions, as explored in heavy-ion physics.