CERN Accelerating science

ALICE presents latest results at Hard Probes 2015

by Andrea Rossi

ALICE, presented a wealth of scientific results. Equipped with a central barrel with high-resolution tracking systems, several detectors for identifying particles from low to high momentum, electromagnetic calorimeters, and with a muon spectrometer at forward rapidity, the ALICE detector is ideally suited for studies involving hard probes.

ALICE results indicate that partons interact strongly with the medium quarks and gluons, losing energy, as evidenced by the suppression of the yield of high momentum charged particles, p0, heavy-flavour hadrons, and jets over a broad momentum range in central Pb-Pb collisions with respect to the yields measured in proton-proton and proton-Pb collisions.

Moreover, despite the significant interaction of partons with the QGP, ALICE measurements suggest that the emerging jets do not appear as medium-modified objects and have properties similar to those measured for jets in pp collisions. This is supported by the study of the angular correlation between high momentum particle and jets. The absence of a modification of the relative yields of high momentum pion, proton, and kaons suggests that the jet hadrochemical composition remains unaltered. This is further supported by the fact that no evidence, within uncertainties, was found for an enhancement of the relative yield of L baryons with respect to K0s mesons among jet constituents, contrasting the dramatic increase characterizing the “bulk” of medium particle; effect that is usually ascribed to the hydrodynamic expansion of the medium.

The final result of the measurement of D meson suppression at high momentum as a function of the collision centrality was also presented. The comparison with the preliminary results of the suppression of J/ψ from the B-meson decay, measured by CMS, provides an indication that in-medium energy loss is larger for charm than for the more massive beauty quark

The smaller suppression of J/ψ production in central heavy-ion collisions at the higher energies of the LHC compared to observations from RHIC has been one of the most exciting results at the LHC. ALICE presented further studies for characterizing the suppression trend as a function of the collision centrality for J/ψ different momenta. The results confirm that the reduced suppression concerns mainly low momentum J/ψ, in agreement with what is expected by models including charmonia formation via recombination. This investigation brought also the unexpected observation of an excess (about a factor 7) of the J/ψ yield at transverse momenta smaller than 0.3 GeV/c in peripheral collisions with respect to what is expected by scaling the pp result according to the number of binary nucleon-nucleon collisions. Further studies are ongoing to understand the nature of this excess and the possible causes.

ALICE also presented several important new results from the p-Pb run. Since the first tantalizing observation of a double-ridge structure in the angular correlation distribution of particles produced in p-Pb events with high multiplicity. ALICE presented the result of a new analysis in which muons reconstructed in the dedicated forward spectrometer are correlated with particles reconstructed in the central barrel, thus separated by a large rapidity gap. These results are important to set constraints to models that explains the observed structure in terms of initial or final state effects.


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