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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| ALICE experiment | 2/7 | https://en.wikipedia.org/wiki/ALICE_experiment | reference | science, encyclopedia | 2026-05-05T13:02:56.303258+00:00 | kb-cron |
The LHC operations team at CERN first recorded collisions of lead ions on 7 November 2010 at 00:30 CET. 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.
=== Proton–lead collisions at the LHC ===
In 2013, the LHC collided protons with lead ions as part of the ALICE experiment. 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. 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 detectors 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 0.5 T 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.
===== ITS1 (Runs 1 & 2, 2013-2018) =====
The first Inner Tracking System (ITS1) consisted of six cylindrical layers of silicon detectors. The layers surrounded the collision point and measured the properties of the particles emerging from the collisions, pin-pointing their position of passage to a fraction of a millimetre. With the help of the ITS, particles containing heavy charm and bottom quarks can be identified by reconstructing the coordinates at which they decay. The ITS1 consisted of six layers, listed here outward from the interaction point:
2 layers of Silicon Pixel Detector, 2 layers of Silicon Drift Detector, 2 layers of Silicon Strip Detector. The ITS1 was inserted at the heart of the center experiment in March 2007 following a large phase of R&D. With almost 5 m2 of double-sided silicon strip detectors and more than 1 m2 of silicon drift detectors, it was the largest system using both types of silicon detector.
===== ITS2 (Run 3, 2021–present) =====