The ALICE Collaboration has submitted for publication a review article “The ALICE experiment – a journey through QCD”, which summarizes the results achieved in the last 13 years of measurements, from the start of LHC activities at the end of 2009.
The ALICE experiment mainly studies the results of the interactions between heavy nuclei accelerated to high energies at the Large Hadron Collider (LHC) at CERN. The extreme conditions of the collision mimic those of the primordial universe up to few microseconds after the Big Bang, when the matter-constituent quarks and gluons were free, in a quark-gluon plasma state, rather than confined in protons and neutrons as they are nowadays.
The measurements based on Run 1 and Run 2 data-taking (the first two LHC periods, covering the years 2009-2018) allowed for a significant advancement in our understanding of quark-gluon plasma physics, and of other aspects of Quantum Chromodynamics (QCD) studied in proton-proton, proton-nucleus and nucleus-nucleus collisions at the LHC.
“This review article marks an important milestone in the lifetime of the ALICE experiment,” says Andrea Dainese (Padua INFN), current ALICE physics coordinator. “We have taken stock of a decade of physics studies, which range from the characterization of the quark-gluon plasma with a multitude of probes in lead-lead collisions, to the surprising observation of QCD collectivity even in proton-proton collisions, and to unique contributions in areas of high-energy QCD and hadronic physics.”
“Our local ALICE team had a leading role in measurements of charm and beauty quark production in proton-proton, proton-lead, and lead-lead collisions. Some of these were pillar measurements for understanding how quarks interact in the QGP. Other results, as the discovery that charm-quark fragmentation changes significantly already in proton-proton collisions with respect to e+e– collisions, came really unexpected and defined new goals and directions for understanding hadron formation in all collision systems” says Andrea Rossi, coordinator of the Padua ALICE team, which includes several researchers from the local INFN division and the Physics and Astronomy Department.
This work represents an important achievement at half of the ALICE journey in QCD. After a major detector upgrade, ALICE entered a new phase in 2022, taking data at a 500 times larger interaction rate in proton-proton collisions. For Pb-Pb collisions, the goal is to collect in next 10 years, during Run 3 and Run 4 at the LHC, a data sample that for most physics analyses will be larger by a factor of 50 with respect to that available from Run 1 and Run 2. Furthermore, the improved spatial resolution on the particle trajectories provided by the new Inner Tracking System will allow new and high-precision measurements for charm and beauty probes of the QGP. A first test of the upgraded detector with lead-lead collisions has recently been carried out.
This new phase will conclude in 2032, with the beginning of the LHC Long Shutdown 4. After that, the ALICE Collaboration aims at a real quantum leap. This week, the Letter Of Intent of ALICE 3, has been submitted to arXiv. The letter, recently endorsed by the LHC Committee, proposes a completely new apparatus, largely based on frontier silicon sensor technologies, which will enable novel studies of the quark-gluon plasma and of the formation and interactions of hadrons starting from 2035.
“ALICE 3 will be a game changer for the investigation of the properties of QCD matter”, says Federico Antinori (INFN Padua), chair of the editorial committee of the Letter of Intent and previous ALICE spokesperson. “We shall be able to measure the electromagnetic radiation emitted by the quark-gluon plasma throughout its expansion, to observe the evolution of quarks as they propagate through the dense QCD medium and to study with an unprecedented precision the way they freeze into hadrons at the end of the deconfined phase” .