Measurement of spectator fragment pseudorapidity distributions with the STAR Event Plane Detector

At sufficiently low center-of-mass energies, charged particles originating from the spectator region fall within the pseudorapidity acceptance ($2.14<|η|<5.09$) of the STAR Event Plane Detector (EPD). Measurements of the pseudorapidity distributions of helium spectator fragments are presented for Au+Au collisions at center-of-mass energies per nucleon pair of $\sqrt{s_{_{\mathrm{NN}}}}=$ 19.6, 14.6, 11.5, 9.2, and 7.7 GeV. The helium fragment yields in each EPD ring are extracted from fits to the deposited energy spectra using convoluted Landau distributions while accounting for the contribution from singly charged particles. The effects of the EPD detection efficiency and interactions of the fragments with the STAR beam pipe are evaluated using Geant4 Monte Carlo simulations. The resulting pseudorapidity distributions are presented as a function of collision energy and collision centrality.