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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| ALICE experiment | 7/7 | https://en.wikipedia.org/wiki/ALICE_experiment | reference | science, encyclopedia | 2026-05-05T13:02:56.303258+00:00 | kb-cron |
Quarkonia are bound states of heavy flavour (charm or bottom) quarks and their antiquarks. Two types of quarkonia have been extensively studied: charmonia, which consist of a charm quark and an anti-charm, and bottomonia, made of a bottom and an anti-bottom quark. Charm and anticharm quarks in the presence of QGP, in which there are many free color charges, are not able to "see" each other any more and therefore they cannot form bound states. The "melting" of quarkonia into the QGP manifests itself in the suppression of the quarkonium yields compared to the production rates without the presence of the QGP. The search for quarkonia suppression as a QGP signature started 25 years ago. The first ALICE results for charm hadrons in Pb-Pb collisions at a center-of-mass energy of 2.76 TeV indicate strong energy loss in the medium for charm and strange quarks. This is an indication of the formation of the hot medium of QGP. As the temperature increases so does the colour screening, resulting in greater suppression of the quarkonium states as it is more difficult for charm–anticharm or bottom–antibottom pairs to form new bound states. At very high temperatures no quarkonium states are expected to survive; they melt in the QGP. Quarkonium sequential suppression is therefore considered a QGP thermometer, as states with different masses have different sizes and are expected to be screened and dissociated at different temperatures. However, as the collision energy increases, so does the number of charm-anticharm quarks that can form bound states, and a balancing mechanism of recombination of quarkonia may appear at higher energies. The results from the first ALICE run differ from observations at lower energies. While a similar suppression is observed at LHC energies for peripheral collisions, with increasingly head-on collisions – as quantified by the increasing number of nucleons in the lead nuclei participating in the interaction – the suppression no longer increases. Therefore, despite the higher temperatures attained in the nuclear collisions at the LHC, more J/ψ mesons are detected by the ALICE experiment in Pb–Pb collisions than in p–p collisions. Such an effect is likely to be related to a regeneration process occurring at the temperature boundary between the QGP and a hot gas of hadrons. The suppression of charmonium states was also observed in proton–lead collisions at the LHC, in which quark-gluon plasma is not formed. This suggests that the observed suppression in proton–nucleus collisions is due to cold nuclear matter effects. Today there is a large amount of data available from RHIC and LHC on charmonium and bottomonium suppression, and ALICE is used in attempts to distinguish between effects due to the formation of the QGP and those from cold nuclear matter effects.
=== Turning lead into gold ===
In May 2025, the ALICE experiment team at the Large Hadron Collider announced that nearly 260 billion gold-203 nuclei (a very small amount massing about 90 picograms) were produced from induced photon swaps in deliberate near-miss collisions of lead-208 nuclei during three experiment runs in the late 2010s.
== Upgrades and future plans ==
=== Long Shutdown 1 (2013-2015) === The main upgrade activity performed on ALICE during LHC's Long Shutdown 1 was to be the installation of the dijet calorimeter (DCAL), an extension of the preexisting EMCAL system that added 60° of azimuthal acceptance opposite the preexisting 120° of the EMCAL's acceptance. This new subdetector was to be installed on the bottom of the solenoid magnet, which previously housed three modules of the photon spectrometer (PHOS). Moreover, an entirely new rail system and cradle was to be installed to support the three PHOS modules and eight DCAL modules, which together weigh more than 100 tonnes. The installation of five modules of the TRD was to completed this complex detector system, which consists of 18 units. In addition to these mainstream detector activities, all of the 18 ALICE subdetectors were to undergo major improvements during LS1 while the computers and discs of the online systems were replaced, followed by upgrades of the operating systems and online software. All of these efforts were to ensure that ALICE was in good shape for the three-year LHC running period after LS1, with heavy-ion collisions at the top LHC energy of 5.5 TeV/nucleon at luminosities in excess of 1027 Hz/cm2.
=== Long Shutdown 2 (2018–2021) === The ALICE collaboration planned for a major upgrade during Long Shutdown 2 (LS2), which was scheduled for 2018. Then the entire silicon tracker was to be replaced by a monolithic-pixel tracker system built from ALPIDE chips; the time-projection chamber was to be upgraded with gaseous electron-multiplier (GEM) detectors for continuous read-out and the use of new microelectronics; and all of the other subdetectors and the online systems were to prepare for a 100-fold increase in the number of events written to tape.
== References ==
== External links == Media related to ALICE at Wikimedia Commons Official ALICE Public Webpage Archived 2011-02-21 at the Wayback Machine at CERN Interactive Timeline for ALICE 20th anniversary Archived 22 November 2019 at the Wayback Machine ALICE section on US/LHC Website Aamodt, K.; et al. (The ALICE Collaboration) (2008). "The ALICE experiment at the CERN LHC". Journal of Instrumentation. 3 (8) S08002. Bibcode:2008JInst...3S8002A. doi:10.1088/1748-0221/3/08/S08002.