Heavy-ion collisions at ultrarelativistic energies allow one to investigate the properties of strong interacting matter at high density and temperature. In-medium modications of meson resonances have been proposed as a possible signal of the phase transition of nuclear matter to a plasma of quarks and gluons. However, even in absence of a phase transition, modications of the properties of meson resonances can arise due to the interactions of such particles and/or their daughters with the fireball medium. Typical lifetime of such resonances are a few fm/c, comparable to the expected lifetime of the hot and dense matter produced in such collisions.

As the partonic system created in the initial stage of the collision expands and cools down, it will hadronize and reach the chemical freeze-out, which is the point where all particle abundancies are fixed. After a period of hadronic interactions, the system reaches the kinetic (thermal) freeze-out stage when all hadrons stop interacting. After the kinetic freeze-out, particles free-stream towards the detectors.

Study of short-lived resonances may probe the role of the rescattering phase between chemical and kinetic freeze-out. In fact, resonance measurements are affected by two competing effects. Some of them cannot be reconstructed due to the rescattering of their daughters in the medium; this effect depends on the time between chemical and kinetic freeze-out, the source size, the resonance phase-space distribution and the hadronic interaction cross-section of the resonance daughters. Moreover, after chemical freeze-out, pseudo-elastic interactions between particles of the same species of the resonance daughters may increase the resonance population. Thus, the study of resonances can probe the time evolution of the source from chemical to kinetic freeze-out and test dierent hadronization scenarios.

The observation of such resonances is difficult in heavy-ion experiments, because of the large background originating from the high multiplicity and from detector limitations.