Spectroscopy of resonance decays in high-energy heavy-ion experiments

Abstract

Invariant mass distributions of the hadronic decay products from resonances formed in relativistic heavy-ion collision (RHIC) experiments are investigated with a view to disentangle the effects of thermal motion and the phase space of decay products from those of intrinsic changes in the structure of resonances at the freeze-out conditions. Analytic results of peak mass shifts for the cases of both equal and unequal mass decay products are derived. The shift is expressed in terms of the peak mass and width of the vacuum or medium-modified spectral functions and temperature. Examples of expected shifts in meson (e.g., $\ensuremath{\rho},$ $\ensuremath{\omega},$ and $\ensuremath{\sigma})$ and baryon (e.g., $\ensuremath{\Delta})$ resonances that are helpful to interpret recent RHIC measurements at BNL are provided. Although significant downward mass shifts are caused by widened widths of the $\ensuremath{\rho}$ meson in medium, a downward shift of at least 50 MeV in its intrinsic mass is required to account for the reported downward shift of 60--70 MeV in the peak of the $\ensuremath{\rho}$ invariant mass distribution. An observed downward shift from the vacuum peak value of the $\ensuremath{\Delta}$ distinctively signals a significant downward shift in its intrinsic peak mass, since unlike for the $\ensuremath{\rho}$ meson, phase space functions produce an upward shift for the $\ensuremath{\Delta}$ isobar.