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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| ALICE experiment | 3/7 | https://en.wikipedia.org/wiki/ALICE_experiment | reference | science, encyclopedia | 2026-05-05T13:02:56.303258+00:00 | kb-cron |
ITS1 was replaced during the LHC's Long Shutdown 2 (2018–2021) by a new 7 layer monolithic active pixel sensor-based detector with the aim of improving several parameters: the determination of the impact parameter to the primary vertex, tracking efficiency at low transverse momentum, and readout rate capabilities. The upgraded ITS was expected to allow the study of the thermalization of heavy quarks in the medium by measuring heavy charm and beauty flavored baryons and allowing these measurements to be made with very low transverse momentum for the first time. It was also expected to give a better understanding of the dependence of energy loss in the medium on quark mass, and to offer a unique capability of measuring the beauty quarks while also improving the reconstruction of the beauty decay vertex. Finally, the upgraded ITS was expected to allow for the characterization of the thermal radiation coming from the quark–gluon plasma and the in-medium modification of hadronic spectral functions as related to chiral symmetry restoration.
==== Time Projection Chamber ====
The ALICE Time Projection Chamber (TPC) is a large space filled with gas as a detection medium and is the main particle tracking device in ALICE. The manner in which fast charged particles ionize the matter they pass through can be used to identify them. Charged particles crossing the gas of the TPC ionize the gas atoms along their path, freeing electrons, which drift towards the end plates of the detector. The Bethe formula describes how these particles lose energy. Multiwire proportional counters or solid-state counters are used as detection media, because they provide signals with pulse heights proportional to ionization strength. An avalanche effect in the vicinity of the anode wires strung in the readout chambers gives the necessary signal amplification. The positive ions created in the avalanche induce a positive current signal on the pad plane. The readout is performed by the 557,568 pads, located at the end plates, which form the cathode plane of the multi-wire proportional chambers. This gives the radial distance and azimuth to the beam. The last coordinate necessary to locate the ion, the distance along the beam direction, is given by the drift time. Since the measurements can vary considerably, a great may are taken to provide optimal resolution. Almost the entirety of the TPC's volume is sensitive to charged particles passing through. The TPC is ideal for environment like heavy-ion collisions, in which the number of particles to be tracked can easily be in the thousands. ALICE's TPC samples the ionization strength of all particle tracks up to 159 times. This allows it to have an ionization measurement resolution as low as 5%.
==== Transition Radiation Detector ====
Electrons and positrons can be distinguished from other charged particles by detecting transition radiation, that is, X-rays emitted when the particles cross many layers of thin materials. This allows for electrons and positrons to be identified by a transition radiation detector (TRD). To develop the TRD for ALICE many detector prototypes were tested, using mixed beams of pions and electrons.
=== Particle identification with ALICE === ALICE is also intended to determine the identity of each particle it detects. This can be accomplished by determining their mass and charge. The mass cannot be directly measured, but can be determined from the momentum and velocity, both of which can be measured. Velocity can be determined by any of four methods based on time-of-flight, ionization, transition radiation, and Cherenkov radiation. ALICE often combines these methods when making measurements. Momentum, as well as whether the charge is positive or negative, can be determined by observing how the particle's path bends in a magnetic field. In addition to the information given by ITS and TPC, more specialized detectors are needed: the TOF measures the time that each particle takes to travel from the vertex to reach it, allowing determination of its speed. This measurement is precise to less than 10 nanoseconds. The High Momentum Particle Identification Detector (HMPID) measures the faint light patterns generated by fast particles, and the transient radiation detector (TRD) measures the radiation very fast particles emit when crossing different materials, thus allowing it to identify electrons. Muons are measured by exploiting the fact that they penetrate matter more easily than most other particles. In the forward region a very thick and complex absorber stops all other particles and muons are measured by a dedicated set of detectors called the muon spectrometer.
==== Time of Flight ==== ALICE's TOF system measures the velocity of charged particles by measuring how long it takes them to go a given distance along their trajectory. Using the tracking information from other detectors every track firing a sensor is identified. Provided the momentum is also known, the mass of the particle can then be derived from these measurements. The TOF detector is based on multigap resistive plate chambers, or MRPCs. These pads are distributed over a 141 m2 cylindrical surface with an inner radius of 3.7 m. The MRPCs are made of sheets of standard window glass, separated by fishing line. These provide narrow gaps for gas, across which high electrical fields are applied. Each MRPC has 10 gas gaps. This system is highly efficient, nearing 100% detector efficiency. The simplicity of the MRPCs allow for the relatively cheap construction of a large number of them. Despite being relatively cheap, they have a TOF resolution of 80ps, making it possible to distinguish kaons, pions, and protons at momenta up to a few GeV/c. Combining these measurements with those from the TPC has proven a particularly effective technique for distinguishing different types of particles.
==== High Momentum Particle Identification Detector ====