by Panos Charitos. Published: 24 April 2015

Leonardo Milano is a research fellow in the ALICE Experiment working on data analysis aimed at measuring the production of identified charged hadrons and their correlations in p-Pb, Pb-Pb and p-p collisions. These different colliding systems make possible a lot of new exciting physics analyses. In his presentation he discussed a number of interesting topics and latest results from ALICE. We thank him for this interview.

 

ALICE has published a wealth of scientific results, which were presented at the recent LHCC meeting. The simultaneous study of three different colliding systems (i.e. Pb-Pb, p-Pb, and pp) offers a broad understanding of ultra relativistic heavy-ion collisions.

The first result presented at the meeting was a recently published paper on K* and φ resonances These particles are important because the lifetime of the system created during the collision (i.e. QGP) is thought to be of the same order of their lifetime. After the collision, the system is believed to be formed by free quarks and gluons up to hadronization, where all the particles are created. The abundances of all particles can be predicted using statistical tools.

The collision happens at a given time (t=0), the system reached quickly an equilibrium state, before two important moments in the timeline of the collision occur: chemical freeze-out, in which the abundances of all the hadrons are frozen including the K*0 and φ, and kinetic freeze-out that occurs a bit later in the evolution of the collision. Between these two stages, the system is still evolving (i.e. we can still apply hydrodynamic calculations) but the degrees of freedom are not quarks and gluons anymore, but hadrons. Two different mechanisms affect the production of the K*0 and φ resonances. On the one hand, the decays products may interact with the medium, leading to a suppression of the yield (rescattering). On the other hand, the resonances may be formed out of the interaction among of other particles (regeneration). The study of these two particles allows a better understanding of this phase of the collision.

Measurement of the K/φ ratio shows that the φ seems to decay mainly outside the medium also revealing that the time between the chemical and the kinetic freeze-out is much shorter than its lifetime. This is not the case for the K*0, and the fact that we see less K*0 with respect to Kaons in central collisions means that the re-scattering is dominant with respect to regeneration. Using an extension of the statistical hadronization model, it is possible to study in detail the evolution of the system between the chemical and the kinetic freeze-out.

Particle ratios K*0/K- and φ/K- as a function of (dNch/dη)1/3 [31, 44] for Pb– Pb collisions at √sNN = 2.76 TeV and pp collisions at √s = 7 TeV. Statistical uncertainties are shown as bars. The shaded boxes show systematic un- certainties that are not correlated between centrality intervals, while the open boxes show the total system- atic uncertainties including both correlated and uncor- related sources. The values given by a grand-canonical thermal model with a chemical freeze-out temperature of 156 MeV are also shown.

Notably, the paper also discusses the comparison of the Φ with the proton, since they have a similar mass, but the Φ has two quarks and the proton, three. In central collisions the ratio between the two is flat as a function of pt, as predicted by hydrodynamics, in which the mass is the main parameter. This result confirms that the mass is dominant as predicted by hydrodynamic models. This is not the case in peripheral collisions, where the dynamics are not only dominated by the mass. There is a clear change in dynamics with the centrality of the collision. In peripheral collisions  other parameters apart from the mass may play an important role.

This observation is also confirmed by examining the v2 of different particles. The above figure shows results from the π that is light and has two quarks, the φ that is heavier but has two quarks, and finally the proton that has similar mass to the φ but three quarks. In particular φ and π have a similar number of quarks, while φ and π have a similar masse. In semi-central collisions it was found that at low p2 the v2 of the φ is close to the one of the proton, which means that even for non-central collisions at low pt is still the mass the dominates the mechanism, while at higher pt we see that indeed the φ follows the π, hence the quark content might become more important. This is not the case in central collisions where the measurement is dominated by the mass in the full transverse momentum range.

Another interesting measurement is the RΑΑ of jets, following previous measurements done only with charged particles measured in the TPC. Neutral particles are also included in the new measurement thanks to the EMCAL that allowed a better reconstruction of the jets by measuring all the particles and energy. Thanks to the installation of the DCAL, ALICE will also be able to study back-to-back jets and learn more about jets produced in heavy-ion collisions.

RAA for R = 0.2 jets with the leading track requirement of 5 GeV/c in 0–10% (left) and 10–30% (right) most central Pb–Pb collisions compared to calculations from YaJEM and JEWEL. The boxes at RAA = 1 represent the systematic uncertainty on TAA.

Moreover, the measurement of the RpPb of charged jets in proton-lead collisions is close to one showing that there is no suppression of jets in proton-lead, which means that there is a sign of strong energy loss effects in such collisions. It should be noted that these results were done for minimum-bias events. This is important, as in Pb-Pb the suppression is larger in central collision compared to peripheral collision, where the system that is created lives less and particles have to cross a smaller volume. Furthermore, it is important to mention that the same ratio (RpPb) has been measured for different particles and it is always close to one.

The measurement of RpPb in different multiplicities is another important result analysed during the presentation. In p-Pb collisions, the relative multiplicity fluctuations are large, hence it is not easy to define the number of binary collisions (Ncoll) in a given multiplicity class. This is the so-called multiplicity bias that has to be taken into account along with two other mechanisms: the jet veto bias and the geometric bias. These three factors combined can explain a deviation from unity of the RpPb that is not related to the nuclear effects. The effects of these biases are smaller if the Zero-Degree-Calorimeter is used to select the events, since it is not sensitive to the multiplicity fluctuation at midrapidity.

Scaling properties of different observables are used to calculate NColl, and when we apply them in the calculation of the RpPb, it is close to one, which means that there is no effect from the medium, even when we look at different centrality classes.

Results from two-pion femtoscopy in p-Pb collisions were also presented during the meeting. In lead-lead collisions, the radius extracted from femtoscopic correlation scales linearly with the cubic root of the multiplicity that is usually taken as a proxy for the size of the system. However, results from p-p, p-Pb, and Pb-Pb from ALICE indicate a different trend between the different systems for the same multiplicities. This seems to indicate that (as previous measurements of the double ridge) the p-Pb system differs from the pp, and we should further study the results from these collisions to deepen our understanding of the nuclear matter.

Comparison of femtoscopic radii (Gaussian), as a function of the measured charged-particle multiplicity density, measured for various collision systems and energies by CERES, STAR, PHENIX, and ALICE.

Finally, a number of results from pp data were produced and will serve as reference for all the lead-lead measurements, including those that we will get from the next run.