Effective mass constraints from heavy-ion collisions*

Nucleons in dense nuclear matter appear to have reduced inertial masses due to momentum dependent interactions they experience with other nucleons. This reduction of their masses is often referred to as their effective mass, and at saturation density the masses are reduced to about 70% of their vacuum mass. In asymmetric matter the effective masses of neutrons (n) and protons (p) can be different, leading to an effective mass splitting. The sign and magnitude of this splitting is poorly constrained at densities away from saturation density.

Recent experiments at the National Superconducting Cyclotron Laboratory were performed to help constrain this effective mass splitting. By measuring the kinetic energy spectra of n and p, or analogously using measured ³He and ³H, this splitting can be extracted. Collisions of beams of ^40,48Ca at 50 and 140 MeV/A impinged on targets of ^58,64Ni and ^112,124Sn. Light charged particles were detected in the upgraded High-Resolution Array and neutrons were detected in the Large-Area Neuron Array. I will present details about the experiment setup and then discuss some results on the spectral ratios.