Hadron spectra probe nature of matter in Pb–Pb collisions

In current understanding, the matter created in heavy-ion collisions – the quark–gluon plasma (QGP) – behaves as a nearly perfect liquid. The confirmation of this hydrodynamic behaviour, previously observed at Brookhaven’s Relativistic Heavy Ion Collider (RHIC), was one of the most eagerly awaited results from the first Pb–Pb collisions at the LHC. One of the crucial measurements for the characterization of the fireball produced in the collisions centres on the spectra of identified hadrons, which encode the collective expansion velocity in the QGP and hadronic stages. Moreover, their overall abundances are believed to be fixed at hadronization.

The ALICE detector was designed to perform these measurements with a unique combination of detectors for particle identification (PID): the silicon inner-tracking-system, the time-projection chamber and the time-of-flight detector. The collaboration has used these to measure the production of pions, kaons and protons in the range in transverse-momentum, p_t, where most of the particles are produced (0.1 to about 3 GeV/c).

The figure shows the results compared with the expectation from a hydrodynamic model, revealing a good agreement with the predicted shapes (Floris 2011). Together with the results on the azimuthal anisotropy also reported by ALICE and the other LHC experiments, this represents the most direct confirmation of the hydrodynamic interpretation at the LHC. On an absolute scale, however, the model calculations shown in the figure significantly over-predict the production of protons – a surprise revealed by the first LHC data.

The production of soft hadrons (p_t < 1–2 GeV/c) is generally described in a statistical language: it is assumed that particles are created in thermal equilibrium. This idea, dating back to a classic 1950 paper by Enrico Fermi, has proved successful over a range of collision energies (√s ˜ 2 GeV – 200 GeV) and provides a possible link to the temperature of the hadronization (or deconfinement phase transition).

At present, however, the yield ratios measured by ALICE seem to challenge both previous experiments and theory. While the K/π, Ξ/π and Ω/π ratios are compatible with the expectations from the thermal model with T ≈ 165 MeV, as in previous observations, the p/π ratio points to a significantly lower temperature. On the experimental side, there are indications of a similar effect at lower energies, which call for further investigations. On the theoretical side, a number of different possibilities are being investigated, none of them conclusive at the moment.

The unique PID capabilities of the ALICE experiment will continue to be crucial for the characterization of the deconfined matter produced in Pb–Pb collisions at the LHC. They also pave the way for a rich programme in proton–proton physics, especially in the soft physics domain, e.g. with the forthcoming measurement of fragmentation constraints with identified particles and spectra in high-multiplicity events.