The precise particle-identification and momentum-measurement capabilities of the ALICE experiment allow researchers to reconstruct a variety of short-lived particles or resonances in heavy-ion collisions. These serve as a probe for in-medium effects during the last stages of evolution of the quark–gluon plasma (QGP). Recently, the ALICE collaboration has made a precise measurement of the yields (number of particles per event) of two such resonances: K*(892)0 and φ(1020). Both have similar masses and the same spin, and both are neutral strange mesons, yet their lifetimes differ by a factor of 10 (4.16±0.05 fm/c for K*0, and 46.3±0.4 fm/c for φ).
The shorter lifetime of the K*0 means that it decays within the medium, enabling its decay products (π and K) to re-scatter with other hadrons. This would be expected to inhibit the reconstruction of the parent K∗0, but the π and K in the medium may also scatter into a K∗0 resonance state, and the interplay of these two competing re-scattering and regeneration processes becomes relevant for determining the K*0 yield. The processes depend on the time interval between chemical freeze-out (vanishing inelastic collisions) and kinetic freeze-out (vanishing elastic collisions), in addition to the source size and the interaction cross-sections of the daughter hadrons. In contrast, due to the longer lifetime of the φ meson, both the re-scattering and regeneration effects are expected to be negligible.
Using lead–lead collision data recorded at an energy of 2.76 TeV, ALICE observed that the ratio K*0/K– decreases as a function of system size (see figure). In small impact-parameter collisions, the ratio is significantly less than in proton–proton collisions and models without re-scattering effects. In contrast, no such suppression was observed in the φ /K– ratio. This measurement thus suggests the existence of re-scattering effects on resonances in the last stages of heavy-ion collisions at LHC energies. Furthermore, the suppression of K*0 yields can be used to obtain the time difference between the chemical and the kinetic freeze-out of the system.
On the other hand, at higher momenta (pT > 8 GeV/c), these resonances were suppressed with respect to proton–proton collisions by similar amounts. The magnitude of this suppression for K*0 and φ mesons was also found to be similar to the suppression for pions, kaons, protons and D mesons. The striking independence of this suppression on particle mass, baryon number and the quark-flavour content of the hadron puts a stringent constraint on models dealing with particle-production mechanisms, fragmentation processes and parton energy loss in the QGP medium.
In future, it will be important to perform such measurements for high-multiplicity events in pp collisions at the LHC.