Abstract
One of the common methods to measure the centrality of nucleus-nucleus collision events consists of detecting forward spectator neutrons. Because of non-monotonic dependence of neutron numbers on centrality, other characteristics of spectator matter in 197Au–197Au collisions at NICA must be considered to improve the centrality determination. The numbers of spectator deuterons and α-particles and the forward–backward asymmetry of the numbers of free spectator nucleons were calculated with the Abrasion–Ablation Monte Carlo for Colliders (AAMCC) model as functions of event centrality. It was shown that the number of charged fragments per spectator nucleon decreases monotonically with an increase of the impact parameter, and thus can be used to estimate the collision centrality. The conditional probabilities that a given event with specific spectator characteristics belongs to a certain centrality class were calculated by means of AAMCC. Such probabilities can be used as an input to Bayesian or other machine-learning approaches to centrality determination in 197Au–197Au collisions.
Highlights
It is expected that hot and dense matter is created in the domain of overlap of colliding relativistic nuclei
It is expected that light spectator fragments, in particular, deuterons and α-particles can be registered in addition to spectator nucleons in the forward calorimeters Forward Hadron Calorimeters (FHCal) designed for the MPD experiment at NICA [2]
The posterior probabilities P(Ci |αneutr ) and P(Ci |αnucleons ) of a 197 Au–197 Au collision event at s NN = 11 GeV with a given value of forward–backward asymmetry αneutr of spectator neutrons or unbound spectator nucleons αnucleons to belong to specific centrality interval: 0–20%, 20–40%, 40–60%, 60–80% or 80–100% were calculated with Ablation Monte Carlo for Colliders (AAMCC)
Summary
It is expected that hot and dense matter is created in the domain of overlap of colliding relativistic nuclei. Some of spectator nucleons and nuclear fragments will remain undetected by Forward Hadron Calorimeters (FHCal) designed for the MPD experiment at NICA facility [2] This is because of void central sections of FHCal which make possible the passage of collider beams. By studies of multifragmentation of spectator matter in collisions of 197 Au with light and heavy nuclei [6] it was demonstrated that the multiplicity of intermediate-mass fragments (IMF) with 3 ≤ Z ≤ 30 is large in semi-central events in contrast to central and peripheral ones This all means that the measurements of multiplicities of unbound spectator nucleons as well as IMF are not sufficient for unambiguous centrality determination in the whole range of b.
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