Speaker
Description
In heavy-ion collisions (A-A) at the CERN Large Hadron Collider (LHC) energies, a strongly coupled Quark Gluon Plasma (QGP) is produced which gives rise to collective phenomena whose signatures can be retrieved in final state hadronic observables. Recent observations in small systems, such as pp collisions, show remarkable similarities among these systems, which are highly suggestive of the presence of collectivity. Current research therefore tries to identify whether a unified description of the pp and A-A data can be established.
Hydrodynamic and recombination models are tested against the measured hadron spectral shapes at low and intermediate transverse momenta ($p_{\rm T}$). In particular, most of them have problems with the correct prediction of very low $p_{\rm T}$ spectra of pions. The problem may be solved by assuming that the matter at LHC energies is produced out of chemical equilibrium. The chemical non-equilibrium model predicts that the pion abundances are characterized by the non-zero value of the chemical potential which is very close to the critical value for the Bose-Einstein condensation. The crucial point is the measurement of pions at very low $p_{\rm T}$ (< 200 MeV/$\textit{c}$), as the onset of pion condensation would manifest itself as an excess in the low $p_{\mathrm{T}}$ pion yield while the spectra of kaons and protons remain unaltered.
The ALICE Collaboration at the CERN LHC recently collected for the first time data in Xe-Xe collisions at $\sqrt{s_{\rm NN}}$ = 5.44 TeV with a low magnetic field (B = 0.2 T) as well as in pp collisions at the highest LHC energy of 13 TeV.
An overview of the new ALICE results which contribute to the understanding of collective phenomena will be presented. Pion, kaon and proton $p_T$-spectra are presented and compared to the main hydrodynamical models.
Thanks to the lower magnetic field in Xe-Xe collisions, the pion spectra can be measured down to 80 MeV/$\textit{c}$ with the Inner Tracking System (ITS), allowing the search for pion condensation effects. The search for an enhancement of pions is carried out also in very high multiplicity pp events down to the lowest $p_{\rm T}$ possible with the ALICE detector.
