In giant detectors that are capable of reconstructing what happened during collisions, and all this in a very high collision rate environment, the events (an event is a collision with all of its resulting particles) are studied. They can be compared to extremely large three-dimensional digital cameras that can take 40 million “sequences” per second (digitised by tens of millions of sensors). The detectors are built in layers, each layer having certain functionality. The inner ones are the least dense, while the outer ones are the densest and most compact.
The very massive particles that scientists hope to create have a very short life, decaying into others that are lighter and already known. After a collision, hundreds of these light particles, such as electrons, muons, and photons, but also protons, neutrons, and others, fly through the detector at speeds close to the speed of light. Detectors use those light particles to deduce the brief existence of the new, heavy ones produced.
The trajectories of the charged particles are curved by magnetic fields, and the radii of curvature are used to calculate their moments: the higher the energy, the more open the curvature. Therefore, particles with a lot of kinetic energy have a sufficient path through the detector to measure their radius of curvature and therefore their momentum.
Other parts of the detector are calorimeters intended to measure the energy of particles (both charged and uncharged). Calorimeters should also be large enough to absorb as much energy as possible. These two are the reasons that LHC detectors are so large. Detectors surround the interaction point to collect all the energy of the particles and the balance of the moments of each event to reconstruct it in detail.
The particles – electrons, protons, and muons – leave traces by ionization. The electrons are very light and thus very easily lose their energy, while the protons penetrate into the detector more deeply. The photons do not leave traces by themselves but in the calorimeter, they become electron-positron pairs, whose energies can be measured. Neutron energy can be measured indirectly from the transfer of energy to protons. Muons are the only particles which enter and are detected by the detector’s outermost layers.
Via thousands of cables, each part of the detector is connected to an electronic reading system. The moment an impulse is produced, the system records the exact time and place, sending the information to the computer. Hundreds of computers work together to combine that information. At the top of the computational hierarchy it is decided in a fraction of a second which event is interesting and which is not. There are various criteria for selecting potentially significant events, thus reducing the number of events from the 600 million produced to a few hundred that will be investigated in detail.
The LHC detectors were designated, built and carried out by international collaborations from all over the world. ATLAS, CMS, LHCb, and ALICE were the four large experiments whereas TOTEM, LHCf, and MoeDAL smaller ones. Twenty years were necessary for the design and construction of the detectors and the duration of the experiments will be of the order of 15 years. This is equivalent to the total career of a physicist.
The construction of these detectors is the result of what could be called “group intelligence”: while all the scientists participating in a detector generally understand the functions of the apparatus, none knows precisely the details and the precise function of all the parts of the detector. In such a collaboration, each scientist contributed their knowledge in the experiment to total success.
Read more: LHC and other particle accelerators
ATLAS (LHC Detectors)
The ATLAS (A Toroidal LHC Apparatus) detector is the largest general-purpose detector in Particle Physics (designated to “see” a wide range of particles and phenomena produced in collisions at the LHC). It is 46 metres in length, 25 metres in height and 25 metres in width. It weighs 7000 tonnes and is made up of 100 million sensors to quantify the particles at the LHC that will occur from proton-proton collisions. The first piece of ATLAS was installed in 2003 and the last one was downloaded in March 2008, thus completing the gigantic puzzle.
ATLAS could answer the mysterious “dark matter and energy” and search for extra dimensions in space-time. It is designed to be able to discover new particles and expected new phenomena as extensions of the Standard Model: supersymmetry or the Higgs Boson.
If the Higgs field is not the desired response to understand the mass of the particles, the ATLAS experiment is expected to guide physicists in the right direction.
The ATLAS (A Toroidal LHC Apparatus) detector offers a hybrid system of four superconducting magnets: a center solenoid surrounded by 2 extreme toroids (End-cap) and a “barrel” (BT) toroidal system. With its close to 2 GJ of stored energy, it is truly the world’s largest superconducting magnet.
The central solenoid, 5.5 tons in weight, 2.5 m in diameter and 5.3 m in length, provides a 2 T axial magnetic field in the center of the ATLAS tracking area. Since this solenoid precedes the argon-liquid electromagnetic calorimeter (LAr), its thickness should be as small as possible to allow maximum response from the calorimeter. It contains 9 km of superconducting cables cooled by liquid helium and an 8000 A electric current flows through it.
ATLAS also has a huge superconducting toroidal magnetic system (Barrel Toroid – BT) with dimensions of 25 m long and 22 m in diameter. This toroidal system provides the magnetic field for muonic detection areas. The toroid is made up of 8 structures of 25m x 5m through which they circulate. 20,500 A superconducting currents.
Its total mass is 850 t.
More about ATLAS at http://www.lhc-closer.es/taking_a_closer_look_at_lhc/1.home
ATLAS is a global collaboration involving some 2,100 scientists and engineers from 167 institutions in 38 countries. They are Argentina, Armenia, Australia, Austria, Azerbaijan, Belarus, Brazil, Canada, Chile, China, Colombia, Czech Republic, Denmark, France, Georgia, Germany, Greece, Hungary, Israel, Italy, Japan, Morocco, Holland, Norway, Poland, Portugal, Romania, Russia, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Taiwan, Turkey, the United Kingdom and the USA.
CMS (LHC Detectors)
CMS (Compact Muon Solenoid) is, along with ATLAS, a “general purpose” detector designed to explore physics at the TeV scale over a wide range of particles and phenomena produced by collisions at the LHC. It is hoped to find answers to questions such as: Are there still undiscovered fundamental principles? Is the Higgs mechanism responsible for the visible mass of the universe? How can we solve the mystery of dark energy? Are there extra dimensions in space? How was the universe created? Is the Higgs Boson behind the mass of the particles?
The main body of the CMS detector is a multilayer cylinder about 21 m long and 16 m in diameter, with a total weight of more than 13000 tons. The innermost layer is the silicon-based particle tracker (surrounded by the scintillating crystal electromagnetic calorimeter), which in turn is covered by the sampling calorimeter for hadrons ( sample calorimeter for hadrons) measuring the energy of the particles. All these sub-detectors are located inside the superconducting central solenoid (3.8 Tesla), 13 m long and 6 m in diameter, which will allow the momentum of the charged particles to be measured. Outside the solenoid are the large muon detectors.
The CMS (Compact Muon Solenoid) detector is a 12,500-ton instrument (the iron core – in red in the image – of the magnetic system contains more iron than the Eiffel Tower).
The magnet is made up of three parts: the superconducting coil, the vacuum tank and the iron core. The coil produces the axial field while the core is responsible for the return of the magnetic flux on the outside of the solenoid. This flow return is what makes up the set of lines of force that fill the detector in all its volume parallel to the axis, and that will curve the trajectories of the particles that are produced due to the collisions in the centre of the detector.
The Solenoid consists of 5 modules of 2.5 m long each.
Each module is made up of an aluminium cylinder with four internal winding layers, 109 turns each.
The CMS collaboration comprises 2,300 scientists from 159 institutions in 37 countries.
88 Spanish researchers participate in CMS. CIEMAT) has participated in the development and manufacture of superconducting magnets for the accelerator, as well as in the design and construction of 70 CMS muons (25% of the total) and in the manufacture of the reading electronics for these cameras. CIEMAT and the Institute of Physics of Cantabria (IFCA), a joint centre of the CSIC and the University of Cantabria, are responsible for the alignment system and the associated electronics of the CMS muon chambers.
The University of Oviedo and the Autonomous University of Madrid also collaborate in this system, also involved in the development of the data selection system or “Trigger”. Spanish participation in the LHC is promoted through the Consolider-Ingenio 2010 CPAN project (National Center for Particle Physics)
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