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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| ALICE experiment | 1/7 | https://en.wikipedia.org/wiki/ALICE_experiment | reference | science, encyclopedia | 2026-05-05T13:02:56.303258+00:00 | kb-cron |
A Large Ion Collider Experiment (ALICE) is one of the nine detector experiments at the Large Hadron Collider (LHC). It is designed to study the conditions thought to have existed immediately after the Big Bang, which it does by measuring the properties of quark–gluon plasma.
== Introduction == ALICE is designed to study high-energy collisions between lead nuclei. These collisions mimic the extreme temperature and energy density that would have been found in the fractions of a second after the Big Bang. This is because they form a quark–gluon plasma, a state of matter in which quarks and gluons are unbound. The results obtained by ALICE support the understanding of complex phenomena such as color confinement, chiral symmetry restoration, and how elementary particles interact. These results guide research in quantum chromodynamics (QCD), the study of the strong force. Recreating the quark–gluon plasma and understanding its evolution are expected to shed light on how matter is organized, the mechanisms that confine quarks and gluons, and the nature of the strong force and the role it plays in generating most of the mass of ordinary matter. QCD predicts that at sufficiently high energy densities, a phase transition will occur within conventional hadronic matter, where quarks, which are confined within nuclear particles, transition into a quark–gluon plasma where they are not. The reverse of this transition is believed to have occurred when the universe was approximately one microsecond (10−6 seconds) old, and transitions like it may still occur in the centers of collapsing neutron stars and other astrophysical objects.
== History == The idea of building a dedicated heavy-ion detector for the LHC was first discussed at the meeting "Towards the LHC experimental Programme", which was hosted in Évian, France, in March 1992. This meeting led to the creation of ALICE, along with other LHC programs such as ATLAS and CMS. After the Évian meeting, the ALICE collaboration was formed, and it submitted a Letter of Intent in 1993. ALICE was first proposed as a central detector in 1993. This was later complemented by an additional forward muon spectrometer designed in 1995. In 1997, the LHC Committee allowed ALICE to proceed towards final design and construction. The first 10 years of development were spent on an extensive research and development (R&D) effort. As with other LHC experiments, the challenges of heavy-ion physics at the LHC required advances in existing technology. The detector was designed to be capable of measuring a wide range of signals, with flexibility for additions and modifications as new research avenues and possibilities emerged. Various major detection systems have been added over the years, including the muon spectrometer in 1995, the transition radiation detectors in 1999, and a large jet calorimeter in 2007. In 2010, ALICE recorded data from the first lead–lead collisions at the LHC. Data sets taken during heavy-ion periods in 2010 and 2011, along with proton–lead data from 2013, provided insight into the physics of quark–gluon plasma. In 2014, the ALICE detector underwent a major consolidation program and upgrade during the long shutdown of CERN's accelerator complex. A new sub-detector, the dijet calorimeter (DCAL), was installed. All 18 of the existing sub-detectors were upgraded, and the infrastructure, including the electrical and cooling systems, underwent major renovations. In 2021, ALICE received additional sub-detectors, including a new inner tracking system, muon forward tracker, and fast interaction trigger. As of 2024, the ALICE Collaboration has more than 1,900 members from 174 institutes in 39 countries. The present detector weighs about 10,000 tons and is 26 m long, 16 m high, and 16 m wide.
== Heavy-ion collisions at the LHC == Attempts to produce quark–gluon plasma and thereby gain a deeper understanding of QCD started at CERN and Brookhaven with collisions of lighter ions in the 1980s. Present-day programs at these laboratories have moved on to ultra-relativistic collisions of heavy ions, and are only just reaching the energy threshold at which the phase transition is expected to occur. During head-on collisions of lead ions at the LHC, lead nuclei are accelerated to more than 99.9999991% of the speed of light. The particles collide at an energy of 5.36 TeV per nucleon pair, giving each lead ion an energy of 557.44 TeV. Two lead ions are accelerated in opposite directions, giving the final collision an energy of 1140 TeV. The collision heats matter in the interaction point to a temperature approximately 100,000 times higher than temperatures found in stellar cores. When the two lead nuclei collide, matter undergoes a transition to briefly form a droplet of quark–gluon plasma (QGP), the material which is believed to have filled the universe a few microseconds after the Big Bang. This quark–gluon plasma is formed as protons and neutrons "melt" into their elementary constituents. The quarks and gluons they are composed of become asymptotically free. The droplet of QGP then near-instantly cools, and the individual quarks and gluons (collectively called partons) recombine into a mixture of relatively ordinary matter that speeds away in all directions. The debris contains a great variety of particles, including mesons such as pions and kaons; baryons such as protons and neutrons; and leptons such as electrons, muons, and neutrinos. The antiparticles of these particles are also produced, possibly combining to form the nuclei of antiatoms of hydrogen or helium. The properties of QGP can be inferred by studying the distribution and energy of the collision debris.
=== First lead–lead collisions ===