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.ALICE through the phase transition, CERN Courier, 30 October 2000.
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.Panos Charitos, Interview with Krishna Rajagopal, ALICE Matters, 15 April 2013. Retrieved 20 January 2019.Panos Charitos, Interview with Johan Rafelski, ALICE Matters, 18 December 2012. Retrieved 20 January 2019.
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 New Kid on the block CERN Courier, 19 September 2008.
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.ALICE Experiment approved CERN timeline. 14 February 1997. Retrieved 20 January 2019.
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.
, 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.Experiments Revisit the Quark-Gluon Plasma CERN Courier, 26 February 2001.RHIC starts producing data CERN Courier, 10 October 2000. 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.Interview with CERN's theorist Urs Wiedemann ALICE Matters, 13 July 2012 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 leadlead collisions
The LHC operations team at CERN first recorded collisions of lead ions on at 00:30 CET.LHC begins physics with lead ions CERN Courier, 30 November 2010.
The first lead ion collisions detected in the center of the ALICE, ATLAS, and CMS detectors took place less than 72 hours after the LHC ended its first run of protons and switched to accelerating lead-ion beams. Each lead nucleus contains 82 protons, and the LHC accelerates the nucleus with an energy of 3.5 TeV per proton, thus resulting in an energy of 287 TeV per beam or a total collision energy of 574 TeV.
Up to 3,000 charged particles are emitted from each collision, shown in diagrams as lines radiating from the collision point. The colors of the lines indicate how much energy each particle carried away from the collision.
Protonlead collisions at the LHC
In 2013, the LHC collided protons with lead ions as part of the ALICE experiment.First lead-ion collisions in the LHC Symmetry Magazine, 8 November 2010. The experiment was conducted by accelerating beams of protons and lead ions in opposite directions, before colliding them when the beams reached the LHC's maximum energy. Since lead ions are heavier than protons, they entered the accelerator with different velocities, and were brought up to speed independently before colliding. This was the first successful collision of lead ions with protons.
Lead–proton experiments at the LHC lasted for one month, and data helped ALICE physicists to differentiate the effects of the quark-gluon plasma from effects that stem from cold nuclear matter effects. Lead-lead collisions form a quark-gluon plasma, while lead-proton collisions do not. By studying the effects of lead-proton collisions, physicists are able to tell what effects come from the quark-gluon plasma formed by lead-lead collisions, and what effects come from the collisions themselves, allowing for more accurate study of the quark-gluon plasma.
The ALICE detectors One of the major design goals of the ALICE experiment is to study quantum chromodynamics and quark (de)confinement under the extreme conditions of quark-gluon plasma. This is done by using particles that are created in the 'hot volume' as it expands and survive long enough to reach the detector layers around the interaction region. The ALICE experiment then has to identify the particles, through a variety of methods.Particle identification in ALICE boosts QGP studies CERN Courier, 23 August 2012.
In a "traditional" experiment, particles are identified or at least assigned to families (charged vs. neutral hadrons) by the characteristic signatures they leave in the detector. The experiment is divided into a few main components, and each component tests a specific set of particle properties. These components are stacked concentrically and the particles go through the layers sequentially from the collision point outwards: first a tracking system, then an electromagnetic calorimeter and a hadronic calorimeter and finally a muon system. The detectors are embedded in a magnetic field in order to bend the tracks of charged particles. This allows for momentum and charge determination. This method for particle identification works well only for certain particles, and is used (for example) by the large LHC experiments ATLAS and CMS. However, this technique is not suitable for hadron identification as it does not allow distinguishing the different charged hadrons that are produced in Pb–Pb collisions.
In order to identify all the particles that are coming out of the quark-gluon plasma ALICE uses a set of 18 detectorsALICE forges ahead with detector installation CERN Courier, 6 December 2006. that give information about the mass, velocity, and electrical sign of the particles.
Barrel tracking An ensemble of cylindrical barrel detectors surrounding the nominal interaction point is used to track all the particles that fly out of the hot, dense medium. The Inner Tracking System (ITS), Time Projection Chamber (TPC), and Transition Radiation Detector (TRD) measure at many points the passage of each charged particle and give precise information about the particle's trajectory. The ALICE barrel tracking detectors are embedded in a magnetic field of bending the trajectories of the particles. This field is produced by a magnetic solenoid. From the curvature of the tracks their momentum can be derived. The ITS allows identification of particles which are generated by the decay of other particles with a long life time (those able to travel ~.1 mm before decay). This is possible because it can see that they do not originate from the point where the interaction has taken place (the "vertex" of the event), but rather from a point at a distance of as small as a tenth of a millimeter. This makes it possible to measure, for example, bottom quarks, which decay into a relatively long-lived B-meson through topological cuts.
Inner Tracking System
The short-lived heavy particles cover a very small distance before decaying. The Inner Tracking System aims at identifying these decays by measuring the location where they occur with a precision of a tenth of millimetre.The Inner Tracking System arrives at the heart of ALICE CERN Courier, 4 June 2007.
ITS1 (Runs 1 & 2, 2013-2018)