Participant and spectator scaling of spectator fragments in Au+Au and Cu+Cu collisions at sqrt(sNN) = 19.6 and 22.4 GeV

TL;DR

This study measures spectator fragments in heavy ion collisions at RHIC energies, revealing their scaling behavior with spectator nucleons and participants, and how fragment deflections depend on collision centrality.

Contribution

It provides new insights into the scaling of spectator fragment yields and deflections with collision centrality and participant number in Au+Au and Cu+Cu collisions.

Findings

Fragment yields scale with spectator nucleons.

Alpha fragments are more favored in Cu+Cu collisions.

Fragment deflections increase with collision centrality.

Abstract

Spectator fragments resulting from relativistic heavy ion collisions, consisting of single protons and neutrons along with groups of stable nuclear fragments up to Nitrogen (Z=7), are measured in PHOBOS. These fragments are observed in Au+Au (sqrt(sNN)=19.6 GeV) and Cu+Cu (22.4 GeV) collisions at high pseudorapidity ($\eta$). The dominant multiply-charged fragment is the tightly bound Helium ($\alpha$), with Lithium, Beryllium, and Boron all clearly seen as a function of collision centrality and pseudorapidity. We observe that in Cu+Cu collisions, it becomes much more favorable for the $\alpha$ fragments to be released than Lithium. The yields of fragments approximately scale with the number of spectator nucleons, independent of the colliding ion. The shapes of the pseudorapidity distributions of fragments indicate that the average deflection of the fragments away from the beam direction increases for more central collisions. A detailed comparison of the shapes for $\alpha$ and Lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.