Resonance Production in STAR
By Patricia Fachini
Resonances are hadrons that decay via the strong nuclear force within 10−23 second. The first resonance was discovered in 1960 and in 1968 a Nobel Prize was awarded to Luis Walter Alvarez for his work, which included the discovery of many resonances . The discovery of resonances such as the Σ(1385), K*(892), and ρ0 helped confirming the quark model.
Using the STAR (Solenoidal Tracker At RHIC) detector, resonance production is measured at RHIC (Relativistic Heavy Ion Collider) via their hadronic decay channels. Resonances decay into long lived particles like kaons, pions and protons that leave measurable tracks in the detector. So far the resonances observed in the STAR detector are: ρ0(770), K*(892)0, K*(892)±, f0(980), φ(1020), ∆(1232)++, f2(1270), Σ(1385)±, Λ(1520), and their corresponding anti-particles.
Since resonances have lifetimes that are comparable to the lifetime of the hot and dense matter produced in heavy-ion collisions (~few fm), their measurement provide detailed information about the dynamics in relativistic heavy-ion collisions.
The regeneration of resonances and the re-scattering of their daughters are two competing effects that make the interpretation of resonance production difficult. Resonances that decay before kinetic freeze-out (vanishing elastic collisions) may not be reconstructed due to the re-scattering of the daughter particles. In this case, the resonance survival probability is important and depends on the time between chemical and kinetic freeze-outs, the source size, and the resonance transverse momentum (pT). 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. The case of resonance regeneration depends on the hadronic cross-section of their daughters. Therefore, the study of resonances can provide an independent probe of the time evolution of the source from chemical to kinetic freeze-outs and detailed information on hadronic interaction at later stages. Figure 1 presents the schematics of the collision evolution between chemical and kinetic freeze-out for the K*, where we can visualize the regeneration and the daughter re-scattering. The measurement of the K* has provided a time between chemical and kinetic freeze-outs of ~2 fm, if the regeneration process is negligible .
The ρ0 meson measured in the dilepton channel probes all stages of the system formed in relativistic heavy-ion collisions because the dileptons have negligible final state interactions with the hadronic environment. Heavy-ion experiments at CERN and RHIC observed an enhanced dilepton production cross section in the invariant mass range of 200-600 MeV/c2, showing that the ρ0 seems to be broadened rather than shifted [3,4,5]. The hadronic decay measurement at RHIC by the STAR collaboration, ρ(770)0 --> π+π−, was the first of its kind in heavy-ion collisions. Since the ρ0 lifetime of cτ = 1.3 fm is small with respect to the lifetime of the system formed in Au+Au collisions, the ρ0 meson is expected to decay, regenerate, and rescatter all the way through kinetic freeze-out. Therefore, the measured ρ0 mass at RHIC should reflect conditions at the late stages of the collisions.
Figure 2 presents the π+π− invariant mass distribution measured in peripheral Au+Au collisions, where we can see the peaks corresponding to K0S, ω, and the resonances ρ0(770) and f0(980). We can also see a very small contribution from K*(892)0, where the K* decays into a pion and a kaon, and in this case the kaon is misidentified as a pion by the detector. From this measurement we observed that the rho0 mass is shifted by ~70 MeV/c2. This shift is possibly due to dynamical interactions with the surrounding matter, interference between various scattering channels, phase space distortions due to the rescattering of pions forming and Bose-Einstein correlations between decay daughters andpions in the surrounding matter .
Recently, we have measurements of and in Cu+Cu collisions that are compared to Au+Au, d+Au, p+p and collisions in different energies for a study of energy and system size dependence. In addition, there are observations of ρ0 at high pT, where we compare to the results at low pT to study the effects of different production mechanisms on the resonance properties. All these new results will be presented at the upcoming 20th Conference on Nucleus Nucleus Collisions (QM2008) .