Heavy Ions at Ultra-Relativistic Energies at LHC: ALICE

ALICE is now a collaboration of more than 1300 people, including scientists and engineers from 131 institutions in 32 different countries. The component includes the Italian University and INFN Sections of Alessandria, Bari, Bologna, Cagliari, Catania, LNF Frascati, Legnaro LNL, Padova, Salerno, La Sapienza Rome, Torino, Trieste, for a total of 200 physicists and technologists, including 22 PHD students.

The experiment studies the interactions between lead ions accelerated by the LHC at ultra-relativistic energies. His goal is the study of Quark Gluon Plasma and the associated phenomena, and thanks to the remarkable and peculiar characteristics of the detector, contributes significantly to the results of the physics of p + p interactions at energies of several TeV reached by the LHC.

 

   

Fig. 1

 

The apparatus (Fig. 1) includes a magnetic solenoid and a variety of different detectors, for the most part contained within the magnet and willing to a cylindrical symmetry around the axis of the LHC beams. This allows us to detect, with high efficiency, the multitude of different particles produced in the ion-ion collisions at very high energy (Fig. 2).

 

Fig. 2

 

The whole apparatus is approximately 20 meters in length and 16 in height and width. Starting from the volume close to the beam-interaction zone and moving radially outward, there are: the vertex detector (ITS) consists of six cylindrical layers of silicon detectors, respectively, two silicon pixels (SPD), two silicon drift (SDD) and two silicon microstrip (SSD), a tracking gas chamber (TPC), detectors for electrons, positrons and other particles with high momenta, a time of flight detector (TOF), a spectrometer for photons, an electromagnetic calorimeter (EMCal). Besides the magnet there are one muon spectrometer, detectors dedicated to the trigger and calorimetric measurements at small angles, detectors monitor for cosmic rays.

The detectors with a qualifying and sometimes almost total Italian participation are: the vertex detector ITS, for which the researchers of Trieste are giving ​​a fundamental contribution from the original design, especially with regard to SDD and SSD (Fig. 3); the TOF, EMCal, HMPID, the muon spectrometer, the ZDC. Among the Italian researchers there are also responsible people for many of the study groups for physics and analysis.

Fig. 3

 

The results of the study of ALICE on the phenomena associated with the Quark Gluon Plasma, the strongly interacting deconfined medium produced at LHC in Pb-Pb collisions, contribute to a better and broader understanding of the nature of the strong interaction. This can not be obtained with reactions involving only few nucleons in the input channel. They also provide important results with which to compare the predictions of the evolution of the Universe during its first microseconds after the Big Bang. The lead ion collisions at high energy are basically Little Bang or Big Bang on a tiny scale (Fig. 4).

Fig. 4

 

Currently the LHC, and the experiments along it, are in a phase of maintenance and partial upgrade, which began in February 2013 and will continue until the end of 2014. Over the last year and a half, the analysis of the Pb-Pb and p-p data collected by progresseded with the achievement of important new results.

Among them, especially notable are the measurements of nuclear modification factor (RAA) for particles with open charm (D0, D +, D * +) and closed (J/ψ).

This factor quantifies the deficit in the production of these particles in Pb-Pb collisions than expected climbing those measured in p-p interactions, based on the number of binary nucleon-nucleon collisions. The factor RAA of D mesons (Fig. 5) is sensitive to color charge dependence of energy loss of the partons (quarks or gluons) through the matter deconfined, while the dependence on the centrality of the collision factor RAA of J/ψ, much less pronounced than at lower energy (Fig. 6), could signal the preponderance, at the energies of LHC, of regeneration of J/ψ due to pairs for recombination of c-cbar not originating from the same interaction vertex.

Fig. 5

 

 

Fig. 6

The measurement of the azimuthal asymmetry (v2) in the production of these particles, should allow to further constrain theoretical models. It is also worth mentioning the extraction, in Pb-Pb collisions, of the decay signals of the rare charmed-strange meson Ds(Fig. 7), whose study should provide valuable information on the mechanisms of hadronization of deconfined medium.

 

Fig. 7

As for the study of p-p interactions, we report the measurement of the polarization of the J/ψ, the first carried out at the LHC, which indicates as favored, the models based on Non-Relativistic Quantum Cromo Dynamics (NRQCD).

In the period 2012-2013 the analysis of Pb-Pb and p-p data already collected, were completed and enriched. The contribution of initial and final state on measurements in Pb-Pb has been studied, based on data collected during the data taking dedicated to p-Pb collisions, which concluded the first long campaign of measurements with LHC, before closing until 2014.

Among the important physics topics studied by ALICE there are light hypernuclei, with the search for exotic strange states (Λ-p and Λ-Λ), and anti-hypernuclei and hypernuclei produced in Pb-Pb collisions. The group of Trieste is a leader on this, and it also proposes further in view of the ALICE upgrade.

In view of the future further upgrade of LHC, which will lead to much higher intensity beams and to the nominal energies, also ALICE has in fact provided a broad and varied program of upgrade, in order to fully exploit the characteristics of the future ion and proton beams of LHC. The detectors of the apparatus involved are ITS, which will be completely redesigned and rebuilt, the TPC that will be deeply renewed, the muon spectrometer, which will be enriched with a forward silicon tracker. Above all, electronics and readout systems will be updated, to allow the apparatus to withstand the high frequency of interactions that will be associated with the increased luminosity of the more intense beams of ions and protons. The Italian researchers and also the group of Trieste, are heavily involved, especially in the upgrade of the future ITS and in the proposition of the new physics topics.

The ALICE collaboration publishes with continuity: 56 articles published since the beginning of the LHC beams, 6 being published and 12 more about ready to be submitted during 2013, as well as many ongoing analysis that will result in papers to be published in the coming months. These articles have high citation indexes, and this testifies to the originality, quality and stimulated interest in the scientific community.

Head of Research Group
Head: 
Stefano Piano & Giacomo Vito Margagliotti

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Last update: 10-15-2019 - 18:50