What are the detectors at the LHC?
There are six experiments installed at the LHC: A Large Ion Collider Experiment (ALICE), ATLAS, the Compact Muon Solenoid (CMS), the Large Hadron Collider beauty (LHCb) experiment, the Large Hadron Collider forward (LHCf) experiment and the TOTal Elastic and diffractive cross section Measurement (TOTEM) experiment. ALICE, ATLAS, CMS and LHCb are installed in four huge underground caverns built around the four collision points of the LHC beams. TOTEM will be installed close to the CMS interaction point and LHCf will be installed near ATLAS.
What is ALICE?
ALICE is a detector specialized in analysing lead-ion collisions. It will study the properties of quark-gluon plasma, a state of matter where quarks and gluons, under conditions of very high temperatures and densities, are no longer confined inside hadrons. Such a state of matter probably existed just after the Big Bang, before particles such as protons and neutrons were formed. The international collaboration includes more than 1000 members from 104 institutes in 30 countries (October 2006).
Size |
26 m long, 16 m high, 16 m wide |
Weight |
10 000 tonnes |
Design |
central barrel plus single arm forward muon spectrometer |
Material cost |
115 MCHF |
Location |
St Genis-Pouilly, France. |
What is ATLAS?
ATLAS is a general purpose detector designed to cover the widest possible range of physics at the LHC, from the search for the Higgs boson to supersymmetry (SUSY) and extra dimensions. The main feature of the ATLAS detector is its enormous doughnut-shaped magnet system. This consists of eight 25-m long superconducting magnet coils, arranged to form a cylinder around the beam pipe through the centre of the detector. ATLAS is the largest-volume collider-detector ever constructed. The collaboration consists of more than 1900 members from 164 institutes in 35 countries (April 2007).
Size |
46 m long, 25 m high and 25 m wide |
Weight |
7000 tonnes |
Design |
barrel plus end caps |
Material cost |
540 MCHF |
Location |
Meyrin, Switzerland. |
What is CMS?
CMS is a general purpose detector with the same physics goals as ATLAS, but different technical solutions and design. It is built around a huge superconducting solenoid. This takes the form of a cylindrical coil of superconducting cable that will generate a magnetic field of 4 T, about 100 000 times that of the Earth. More than 2000 people work for CMS, from 181 institutes in 38 countries (May 2007).
Size |
21 m long, 15 high m and 15 m wide. |
Weight |
12 500 tonnes |
Design |
barrel plus end caps |
Material cost |
500 MCHF |
Location |
Cessy, France. |
What is LHCb?
LHCb specializes in the study of the slight asymmetry between matter and antimatter present in interactions of B-particles (particles containing the b quark). Understanding it should prove invaluable in answering the question: ‘Why is our Universe made of the matter we observe?’ Instead of surrounding the entire collision point with an enclosed detector, the LHCb experiment uses a series of sub-detectors to detect mainly forward particles. The first sub-detector is built around to the collision point, the next ones stand one behind the other, over a length of 20 m. The LHCb collaboration has more than 650 members from 47 institutes in 14 countries (May 2007).
Size |
21m long, 10m high and 13m wide |
Weight |
5600 tonnes |
Design |
forward spectrometer with planar detectors |
Material cost |
75 MCHF |
Location |
Ferney-Voltaire, France. |
What is LHCf?
LHCf is a small experiment that will measure particles produced very close to the direction of the beams in the proton-proton collisions at the LHC. The motivation is to test models used to estimate the primary energy of the ultra high-energy cosmic rays. It will have detectors 140 m from the ATLAS collision point. The collaboration has 21 members from 10 institutes in 6 countries (May 2007).
Size |
two detectors, each measures 30 cm long, 10 cm high, 10 cm wide |
Weight |
40 kg each |
Location |
Meyrin, Switzerland (near ATLAS). |
What is TOTEM?
TOTEM will measure the effective size or ‘cross-section’ of the proton at LHC. To do this TOTEM must be able to detect particles produced very close to the LHC beams. It will include detectors housed in specially designed vacuum chambers called ‘Roman pots’, which are connected to the beam pipes in the LHC. Eight Roman pots will be placed in pairs at four locations near the collision point of the CMS experiment. TOTEM has more than 70 members from 10 institutes in 7 countries (May 2007).
Size |
440 m long, 5 m high and 5 m wide |
Weight |
20 tonnes |
Design |
roman pot and GEM detectors and cathode strip chambers |
Material cost |
6.5 MCHF |
Location |
Cessy, France (near CMS) |
What dictates the general shape of the LHC particle detectors?
A modern general-purpose high-energy physics detector such as ATLAS or CMS, needs to be hermetic, so that there is only a small probability of a (detectable) particle escaping undetected through a region that is not instrumented. For engineering convenience, most modern detectors at particle colliders like the LHC adopt the ‘barrel plus endcaps’ design where a cylindrical detector covers the central region and two flat circular ‘endcaps’ cover the angles close to the beam (the forward region). ALICE and LHCb have asymmetric shapes as they focus on more specific areas of physics.
What will be the Higgs boson production rate at the LHC?
Although the particle collision rate at the LHC will be very high, the production rate of the Higgs will be so small that physicists expect to have enough statistics only after about 2-3 years of data taking. The Higgs boson production rate strongly depends on the theoretical model and calculations used to evaluate it. Under good conditions, there is expected to be about one every few hours per experiment. The same applies to supersymmetric particles. Physicists expect to have the first meaningful results in about one year of data taking at full luminosity.
What is the expected data flow from the LHC experiments?
The LHC experiments represent about 150 million sensors delivering data 40 million times per second. After filtering there will be about 100 collisions of interest per second.
The data flow from all four experiments will be about 700 MB/s, that is around 15 000 000 GB (=15 PB) per year, corresponding to a stack of CDs about 20 km tall each year. This enormous amount of data will be accessed and analysed by thousands of scientists around the world. The mission of the LHC Computing Grid is to build and maintain a data storage and analysis infrastructure for the entire high energy physics community that will use the LHC.
} ATLAS will produce 320 MB/s
} CMS will produce 220 MB/s
} LHCb will produce 50 MB/s
} ALICE will produce 100 MB/s
It seems that two of the experiments are very similar, namely ATLAS and CMS. If there are no significant differences, why are there two?
Technically speaking, there are quite a lot of differences between the two larger LHC experiments (ATLAS and CMS): the design of the magnetic field is different, the choice of materials for the subdetectors is different, etc. These are really 'substantial' differences. However, as you say, the type of research they will do is very similar, both equipments being adapted to a general purpose physics. Given this, statistically-wise and for an important (crucial sometimes - see what happened with the evidence of the Higgs particle shown by only one LEP experiment and not confirmed by the other three) comparability of data, two experiments are crucial for a physics discovery.