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Unlike the names of other U. That thermal history can tell us something about where the comet was formed and traveled in its orbits. If we know that a comet never got above a certain temperature, we can infer that it never got closer than a certain distance from the sun or, for that matter, any other star. These markers are a special group of volatile gases, called noble gases which include helium, neon, argon, and krypton. Sometimes they are called inert gases because they do not interact with other chemicals to form new ones.

They remain as they were when they first appeared on the comet. Alan Stern and his colleagues discovered the first noble gas ever found on a comet argon in spectrographic studies of the comet Hale-Bopp.

ALICE enhances hospitality

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Sep 6, Exclude some folders from downloading into vendor with Composer Aug 4, Oct 20, Replace some assertEquals by their native counterpart Dec 24, Test against PHP 7. Mar 29, In addition to the information given by ITS and TPC, more specialized detectors are necessary to reveal the particle's identity. The TOF measures, with precision greater than a tenth of a billionth of a second, the time that each particle needs to travel from the vertex to reach it, so that its speed can be measured.

The HMPID measures the faint light patterns generated by fast particles and the TRD measures the special radiation emitted by very fast particles when they cross different materials, thus allowing the identification of electrons. Muons are measured by exploiting the fact that they penetrate matter easier 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: the muon spectrometer.


To measure photons special detectors are necessary: the crystals of the PHOS, which are as dense as lead and as transparent as glass, measure them with fantastic precision in a limited region, while the PMD and in particular the EMCal measures them over a very wide area. ITS consists of six cylindrical layers of silicon detectors and is used to track the particles produced in the collisions before they quickly decay.

Using the smallest amounts of the lightest material, ITS has been made as lightweight and delicate as possible. The main tasks of the ITS are to reconstruct the primary and secondary vertices, to track and identify charged particles with a low pt cutoff and to improve the momentum resolution at high pt. Read more about ITS.

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The FMD extends the coverage for the multiplicity of charged particles into the forward regions with high resolution - giving ALICE the widest coverage of the four LHC experiments for these measurements. It is designed to measure the charged particles produced in the collisions and emitted at small angles relative to the beam direction. The FMD also provides an independent measurement of the orientation of the collisions in the vertical plane, which can be used with measurements from the barrel detector to investigate flow, jets, etc.

Read more about the FMD. The measured time of arrival of the charged particles at the surface of the detector is used in order to discriminate parasitic collisions of the beam with the remaining gas in the LHC beam-pipe. Heavy nuclei are extended objects and collisions between them do not always happen at the centre. The centrality is obtained by summing up the energy deposited in the two disks of the V0 detector. This observable scales directy with the number of primary particles generated in the collision and therefore to the centrality.

The detector participates in the measurement of the charged particle multiplicity density and the orientation of the reaction plane in collisions of heavy ions. Read more about V0. T0 serves as a start, trigger, and luminosity detector allowing fast particle identification at ALICE. It gives the key trigger and timing signals, measures on-line vertex position and gives rough centrality. There are many technical challenges, such as the dead time below 25 ns needed to cope with the bunch crossing rate in p-p collisions, sustained count rate of 0.

Data from T0 are crucial not only for extraction of the precise interaction time but also for the normalization between proton-proton and heavy ion runs. The main device in ALICE's central barrel for the tracking and identification of charged particles, coping with unprecedented densities. Charged particles crossing the TPC ionize the gas atoms along their path, liberating electrons that drift towards the readout chambers covering the end plates of the cylinder.

Read more about the TPC. Electrons and positrons coming out of the collision points of heavy ions can be distinguished from other charged particles by the emission of transition radiation. The identification of electrons and positrons is achieved by a transition radiation detector TRD.


Inside this detector, fast charged particles cross the boundaries between materials with different dielectric constants, which leads to the emission of TR photons with energies in the X-ray range. The effect is tiny and the radiator has to provide hundreds of material boundaries to achieve a high enough probability to produce at least one photon.

The ALICE TRD was designed to derive a fast trigger for charged particles with high momentum and can significantly enhance the recorded yields of vector mesons and jets. The TOF measures the flight times of charged particles over a given distance to determine their velocity and identify them. The time measurement with the TOF, in conjunction with the momentum and track length measured by the tracking detectors is used to calculate the particle mass.

Read more about TOF. The HMPID identifies charged particles with large momentum, which is a physical quantity related to the particle's mass and speed. Particle identification plays a key role in the complete understanding of heavy-ion collisions at the LHC. The EMCal is a lead-scintillator sampling calorimeter with large acceptance degrees azimuthal, from The towers are read out by wavelength-shifting optical fibers in a shashlik geometry coupled to avalanche photodiodes.

The complete EMCal contains , individual scintillator tiles and kilometers of optical fiber. EMCal weighs in total about tons. It is an apparatus able to measure the energy of the particles, in particular of photons and electrons. Photons tell us about the temperature of the system. To measure them, special detectors are necessary: the crystals of the PHOS calorimeter. PHOS is a high-resolution electromagnetic calorimeter which measures the photons coming out of the extremely hot plasma created in the lead-lead collisions at the LHC.

PHOS covers a limited acceptance domain at central rapidity. 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 and stops most photons that reach it.

The crystals are kept at a temperature of K, which helps minimize the deterioration of the energy resolution due to noise and optimize the response for low energies. Read more about PHOS The ALICE underground cavern provides an ideal place for the detection of high energy atmospheric muons coming from cosmic ray showers.

ALICE project-team

It is optimized for the detection of heavy quark resonances that are an important tool for the study of the early and hot stage of heavy-ion collisions. The Muon Tracking Chambers detect the muons produced in the forward region. The chambers are located before and after the Muon Magnet which bends the trajectories of high momentum muons. The Muon Filter is an additional absorber to suppress left over hadrons. Finally the Muon Trigger signals the passage of the muons.

The AD detector will help study such processes. Diffractive and photon induced physics is a research area with a remarkable discovery potential at the LHC. ALICE has started a programme to exploit its unique capabilities to study the subject in both proton-proton and heavy ion collisions. The new sub-detector for diffractive physics would enhance the performance of ALICE to address some relevant topics on this matter.

It consists of four stations of scintillator pads that would tag the diffractive gap more efficiently.