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Cumulants of the multiplicity distributions of identified particles measured in heavy-ion collisions by HADES

Marvin Nabroth

TL;DR

The paper addresses the search for critical behavior in QCD matter at high net-baryon density by measuring higher-order cumulants of identified-particle multiplicities in low-energy heavy-ion collisions with HADES. It introduces a probabilistic particle identification scheme to circumvent event-by-event PID, and develops efficiency corrections via distribution unfolding and direct moment corrections, along with a data-driven event-mixing approach to remove volume fluctuations, enabling robust cumulant measurements. The results show proton factorial cumulants corrected for efficiency and volume, with rapidity-acceptance dependence indicating trends toward a Poisson baseline, and a canonical ensemble with attractive correlations providing a qualitative description while comparisons to STAR extend the energy range. The work provides experimental baselines and methodological advances for fluctuation analyses at high net-baryon density, with prospects to analyze deuterons and to refine volume models for more definitive interpretation of critical phenomena.

Abstract

The HADES experiment investigates the reaction products of heavy-ion collisions at a few GeV, providing access to QCD matter at high net-baryon densities. A particular focus is the reconstruction of higher-order cumulant ratios of proton and light nuclei multiplicity distributions, which are considered sensitive probes of criticality through their connection to event-by-event (E-by-E) net-baryon number fluctuations. In this contribution, we summarize the experimental setup and outline the analysis strategy. The fluctuations are reconstructed using a novel approach that circumvents E-by-E particle identification by treating it probabilistically. We present several approaches for efficiency correction and apply a data-driven event-mixing procedure to correct for centrality fluctuations. The fully corrected normalized factorial cumulants of proton and deuteron multiplicity distributions are then presented and compared to corresponding STAR data. Our measurements extend the trends observed by STAR toward lower energies for the ratios $C_2/C_1$ and $C_3/C_1$. Finally, we show the rapidity dependence of factorial cumulant ratios and confront the results with canonical ensemble baselines that incorporate correlated particle production via local attractive interactions.

Cumulants of the multiplicity distributions of identified particles measured in heavy-ion collisions by HADES

TL;DR

The paper addresses the search for critical behavior in QCD matter at high net-baryon density by measuring higher-order cumulants of identified-particle multiplicities in low-energy heavy-ion collisions with HADES. It introduces a probabilistic particle identification scheme to circumvent event-by-event PID, and develops efficiency corrections via distribution unfolding and direct moment corrections, along with a data-driven event-mixing approach to remove volume fluctuations, enabling robust cumulant measurements. The results show proton factorial cumulants corrected for efficiency and volume, with rapidity-acceptance dependence indicating trends toward a Poisson baseline, and a canonical ensemble with attractive correlations providing a qualitative description while comparisons to STAR extend the energy range. The work provides experimental baselines and methodological advances for fluctuation analyses at high net-baryon density, with prospects to analyze deuterons and to refine volume models for more definitive interpretation of critical phenomena.

Abstract

The HADES experiment investigates the reaction products of heavy-ion collisions at a few GeV, providing access to QCD matter at high net-baryon densities. A particular focus is the reconstruction of higher-order cumulant ratios of proton and light nuclei multiplicity distributions, which are considered sensitive probes of criticality through their connection to event-by-event (E-by-E) net-baryon number fluctuations. In this contribution, we summarize the experimental setup and outline the analysis strategy. The fluctuations are reconstructed using a novel approach that circumvents E-by-E particle identification by treating it probabilistically. We present several approaches for efficiency correction and apply a data-driven event-mixing procedure to correct for centrality fluctuations. The fully corrected normalized factorial cumulants of proton and deuteron multiplicity distributions are then presented and compared to corresponding STAR data. Our measurements extend the trends observed by STAR toward lower energies for the ratios and . Finally, we show the rapidity dependence of factorial cumulant ratios and confront the results with canonical ensemble baselines that incorporate correlated particle production via local attractive interactions.
Paper Structure (8 sections, 3 figures)

This paper contains 8 sections, 3 figures.

Table of Contents

  1. Physics Motivation
  2. Experimental Setup and Analysis procedure
  3. Event and centrality selection
  4. Particle identification
  5. Efficiency corrections
  6. Controlling volume fluctuations
  7. Results
  8. Outlook

Figures (3)

  • Figure 1: Left: Explosion view of the HADES detector with its main detector components. Right: Integrated mass spectra line shapes for the selection region: $400 < p_t < 1600$$MeV/c$, $-0.4 < y_{cm} < 0.4$.
  • Figure 2: Left: Detector repose matrix depicting the relation between the number of MC (generated) protons and the number of reconstructed/identified protons. Right: Closure-Test - Unfolded proton cumulants in comparisons with the cumulants of the MC Truth.
  • Figure 3: Proton factorial cumulants as a function of rapidity acceptance w.r.t. mid-rapidity, for the pt-range ( $400$$MeV/c$$<p_{t}<$$1600$$MeV/c$). Red lines corresponds to canonical ensemble with correlated particle production based on an attractive potential with correlation coefficient $\rho$Friman:2025swg.