The particle identification capability of the Time Projection Chamber in STAR and its large acceptance enables us to measure many hadronic resonances produced in the high energy collisions. Resonances are strongly decaying particles with lifetimes x velocity of light that are of the order of the size of the hot and dense medium produced in heavy-ion collisions. The in-medium effects related to the high density and/or high temperature of the medium can modify the properties of short-lived resonances, such as their masses, widths, and even their spectral shapes. STAR experiment has recently reported the measurement of the following resonances for colliding beam energy of 200 GeV. These are reconstructed from their hadronic decay channels using invariant mass technique in d+Au collisions - rho(770), K*(892), Delta(1232)++, Sigma(1385), Lambda(1520).
One interesting feature was observed in the transverse mass distribution of these resonances measured at midrapidity. As shown in the figure, they seem to follow a generalized scaling in d+Au collisions between transverse mass range of 1 - 2 GeV/c2. Such a scaling could be envisaged within the idea of saturation of gluon density in the nucleus for high energy collisions. However such scaling has been observed in p+p collisions at ISR, SppbarS, RHIC energies. Also the resonances in d+Au collisions do not show any difference in the shape of the transverse mass distribution between baryons and mesons at higher transverse mass. Differences were earlier observed for non-resonant particles along baryon-meson lines.
Regeneration of resonances and the re-scattering of their daughters are two competing effects that affect the interpretation of resonance production. Resonances that decay before kinetic freeze-out (vanishing elastic collisions) may not be reconstructed due to the re-scattering of the daughter particles. On the other hand, after chemical freeze-out (vanishing inelastic collisions), elastic interactions may increase the resonance population compensating for the ones that decay before kinetic freeze-out. These effects can be studied by looking at the ratio of various resonances with respect to non-resonant particles in nucleus-nucleus collisions and those from p+p collisions at the same colliding beam energy. This is shown in the second figure.
The decrease of the resonance ratios of K*/K and Lambda*/Lambda from p + p to Au+Au collisions could be explained explained by an extended lifetime of the hadronic phase where the re-scattering of the decay particles dominates over resonance regeneration. As the K*/K and Lambda*/Lambda the ratios are similar for p+p and d+Au collisions, this would suggest the absence of an extended hadronic medium in d+Au collisions. The rho0/pi-, Delta++/p and Sigma*/Lambda ratios in d+Au collisions are in agreement with their ratios measured in p+p collisions. These resonance ratios do not show any suppression from p+p to Au+Au collisions either, hence they are not sensitive to the lifetime of the hadronic medium, presumably due to their large regeneration cross-section. The rho0/pi- ratio is almost independent of dNch/deta (upto 40-80% centrality in Au+Au collisions) and is of similar order as for p+p collisions. What could be hapening is, pi+pi- re-scattering might regenerate the rho0 (reflected by mass shift of the measured rho0). In addition, one of the decay daughters might also re-scatter with other hadrons preventing the rho0 to be measured. Therefore, these two processes compete with (and balance) each other.
Enhancement has been observed in resonance production at high transverse momentum in d+Au collisions relative to p+p collisions. This effect is commonly referred to as Cronin Effect. The high statistics year 8 data will be able to extend the measurements to higher transverse momentum, reduce the statistical errors and give us a more clear picture of Cronin effect for resonances. The current measurements and the results from the high statistics d+Au data will provide reference data for resonance measurements in nucleus-nucleus collisions. Stay tuned for more on resonance measurements from nucleus-nucleus collisions and high statistics d+Au collisions.
Further details can be found in the following STAR paper -