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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| ALICE experiment | 4/7 | https://en.wikipedia.org/wiki/ALICE_experiment | reference | science, encyclopedia | 2026-05-05T13:02:56.303258+00:00 | kb-cron |
The High Momentum Particle Identification Detector (HMPID) is a ring imaging Cherenkov (RICH) detector used to determine the speed of particles beyond the momentum range available through energy loss (in ITS and TPC, momenta above 600 MeV) and through time-of-flight measurements (in TOF, momenta above 1.2–1.4 GeV). Cherenkov radiation consists of photons produced by charged particles moving faster than the speed of light in a material. The angle at which these photons are released (relative to the particle's motion) depends on the particle's velocity. Cherenkov detectors detect this radiation and consist of two parts: a material the radiation is released in and a photon detector. Ring imaging Cherenkov (RICH) detectors detect the ring-shaped image this produces, allowing them to measure the angle the photons were released at and therefore the velocity of the particle that produced them. This allows for determination of the mass of the charged particle. In a dense medium with a large refractive index, only a thin radiator layer—no more than a few centimetres—is needed to emit a sufficient number of Cherenkov photons. The photon detector is positioned some distance behind the radiator (typically about 10 cm), allowing enough room for the Cherenkov light cone to expand and form the characteristic ring-shaped image. A proximity-focusing RICH detector of this type is installed in the ALICE experiment. ALICE HMPID's momentum range is up to 3 GeV for pion/kaon discrimination and up to 5 GeV for kaon/proton discrimination. It is the world's largest caesium iodide RICH detector, with an active area of 11 m2. A prototype was successfully tested at CERN in 1997 and currently takes data at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory in the US.
=== Calorimeters === Calorimeters measure the energy of particles and determine whether they undergo electromagnetic or hadronic interactions. Particle identification in a calorimeter is a destructive measurement. All particles except muons and neutrinos deposit their entire energy in the calorimeter system by producing electromagnetic or hadronic showers. Photons, electrons, and positrons deposit all their energy in the electromagnetic calorimeter. Their showers are indistinguishable, but a photon can be identified by the absence of a track in the tracking system associated with the shower. Photons (particles of light), such as those emitted by a hot object, provide information about the temperature of the system. To measure them, special detectors are required. The crystals of the Photon Spectrometer (PHOS), which are as dense as lead and as transparent as glass, measure photons with exceptional precision in a limited region. In contrast, the Photon Multiplicity Detector (PMD) and the Electro-Magnetic Calorimeter (EMCal) cover a much wider area. The EMCal also detects groups of closely spaced particles, called "jets," which retain information about the early stages of the event.
==== Photon Spectrometer ====
PHOS is a high-resolution electromagnetic calorimeter installed in ALICE to study the initial phase of the collision by measuring photons coming directly from the collision. It is made of lead tungstate crystals, similar to the ones used by CMS, read out using avalanche photodiodes. When high-energy photons strike lead tungstate, they make it glow, or scintillate, and this glow can be measured. Lead tungstate is extremely dense (denser than iron), stopping most photons that reach it. The crystals are kept at a temperature of 248 K, which helps to increase the energy resolution by decreasing noise and to optimize the response for low energies.
==== Electro-Magnetic Calorimeter ==== The EMCal is a lead-scintillator sampling calorimeter made of almost 13,000 individual towers, grouped into ten super-modules. Data from the scintillators is read out by wavelength-shifting optical fibers in a Shashlik geometry, connected to an avalanche photodiode. The EMCal covers almost the full length of the ALICE Time Projection Chamber and central detector, and a third of its azimuth placed back-to-back with the PHOS. The super-modules are inserted into an independent support frame located within the ALICE magnet, between the time-of-flight counters and the magnet coil. The support frame itself is a complex structure: it weighs 20 tons and must support five times its own weight, with a maximum deflection between being empty and being fully loaded of only a couple of centimeters. Installation of the eight-ton super-modules requires a system of rails with a sophisticated insertion device to bridge across to the support structure.
==== Photon Multiplicity Detector ==== The Photon Multiplicity Detector is a particle shower detector that measures the multiplicity and spatial distribution of photons produced in the collisions. It utilizes as a first layer a veto detector, which rejects charged particles. Photons pass through a converter, initiating an electromagnetic shower in a second detector layer, where they produce large signals on several cells of its sensitive volume. Hadrons however normally affect only one cell, and therefore produce a signal representing minimum-ionizing particles.
==== Forward Multiplicity Detector ====
The Forward Multiplicity Detector (FMD) extends the coverage for multiplicity of charge particles into the forward regions, giving ALICE the widest coverage of the 4 LHC experiments for these measurements. The FMD consists of 5 large silicon discs, each with 10,240 individual detector channels. It is used to measure the charged particles emitted at small angles relative to the beam. FMD provides an independent measurement of the position of the collisions in the vertical plane, which can be used with measurements from the barrel detector to investigate flow, jets, etc.
==== Muon Spectrometer ====