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CERN LHC Experiments and Detectors: Atlas, CMS, Alice, LHCf and TOTEM

CERN LHC Experiments and Detectors: Atlas, CMS, Alice, LHCf and TOTEM

The large hadron collider (LHC) is a gigantic scientific instrument located near Geneva, on horseback from the Franco-Swiss border, about 100 meters underground. It is a particle accelerator, with which physicists will study the smallest known particles: the fundamental components of matter. The LHC will revolutionize our understanding of the world, from the infinitely small, within the atoms, to the infinitely large of the Universe.

Two beams of subatomic particles of the “hadron” family (protons or lead ions) will circulate in the opposite direction inside the circular accelerator, storing energy each time. By bringing the two beams into a frontal collision at a speed close to that of light and at very high energies, the LHC will recreate the conditions that existed just after the Big Bang. Teams of physicists from around the world will analyze the particles resulting from such collisions using special detectors.

There are numerous theories as to the results of such collisions. In any case, physicists foresee a new era of physics, which brings new knowledge about the functioning of the Universe. For decades, physicists have relied on the standard model of particle physics to try to understand the fundamental laws of Nature. But that model is insufficient. The experimental data obtained thanks to the very high energies of the LHC will allow to expand the frontiers of knowledge, posing a challenge to those who seek to confirm current theories and those who dream of new paradigms.

Some unanswered questions

Why the LHC?

The LHC was built to help scientists answer certain essential questions of particle physics for which there is still no answer. The unprecedented energy it will reach could even reveal completely unexpected results.

Over the last decades, physicists have been able to describe more and more accurately the fundamental particles that constitute the Universe, as well as their interactions. This understanding of the Universe constitutes the standard model of particle physics. But such a model has failures and does not explain everything. To fill those gaps, scientists need experimental data and the LHC will allow them to overcome the next stage.

Newton’s unfinished work: what is mass?

Where does the dough come from? Why do these tiny particles have their own mass? The question has been the subject of debate.

The most plausible explanation could be the role of the Higgs boson, an essential particle for the coherence of the standard model. Theorized for the first time in 1964, this particle has never been observed so far.

The ATLAS and CMS experiments will look for the signals from this particle.

what is 96% of the Universe made of? An invisible problem:

Everything we see in the Universe, from ants to galaxies, is made up of ordinary particles. These particles are collectively called matter, and form 4% of the Universe. It is believed that the rest of the Universe is constituted by black matter and dark energy, but unfortunately they are difficult to detect and study if it is not through the gravitational forces they exert. The exploration of the nature of black matter and dark energy is, today, one of the greatest challenges of particle physics and cosmology.

The ATLAS and CMS experiments will look for supersymmetric particles in order to test a plausible hypothesis about the nature of black matter.

Nature’s favoritism: why is there no more antimatter?

We live in a world made of matter; Everything in the Universe, including us, is constituted by matter. Antimatter is like the twin sister of matter, but with an opposite electric charge. During the Big Bang that marked the birth of the Universe, matter and antimatter had to be produced in equal quantities. However, when the particles of matter and antimatter meet, they annihilate each other and transform into energy. In one way or another, a tiny fraction of matter had to persist to form the Universe in which we live today, and in which virtually no antimatter subsists. Why does Nature seem to have a preference for matter to the detriment of antimatter?

The LHCb experiment will look for differences between matter and antimatter and will help answer this question. Previous experiments have already revealed a slight difference in behavior, but what has been observed so far is far from sufficient to explain the apparent matter-antimatter imbalance in the Universe.

The secrets of the Big Bang: what did matter look like in the first moments of the Universe?

The matter could have as its origin a hot and dense cocktail of fundamental particles, formed a fraction of a second after the Big Bang. Physicists believe that at that moment there were more types of fundamental particles than are left today. In order to study particles that no longer exist, the ALICE experiment will use the LHC to recreate conditions similar to those that reigned just after the Big Bang. The ALICE detector has been specially designed to analyze a particular state of matter, called plasma quarks and gluons, which is believed to have existed just after the creation of the Universe.

Hidden worlds: are there really other dimensions?

Einstein showed that the three dimensions of space are linked to time. More recent theories propose the existence of other hidden spatial dimensions; string theory, for example, postulates the existence of six supplementary spatial dimensions that have not yet been observed. They could be detected at very high energies, and for that reason the data collected by all detectors will be carefully analyzed so as not to overlook any evidence of other dimensions.

LHC operation

The LHC, the largest and most powerful particle accelerator in the world, is the last link in the accelerator complex of CERN (European Nuclear Research Organization). It consists of a ring of 27 km of circumference formed by supraconducting magnets and accelerating structures that increase the energy of the particles that circulate through it. Inside the accelerator, two beams of particles circulate at very high energies and at a speed close to that of light before colliding with each other. The beams circulate in the opposite direction, in different tubes located under a high level vacuum (ultra vacuum). They are guided along the throttle ring by a powerful magnetic field, generated by supraconducting electromagnets. The latter are composed of coils of a special electrical cable that operates in a state of supraconductor, that is, conducting electricity without resistance or loss of energy. For this, the magnets must be cooled to -271° C, a cooler temperature than the intersideral space. It is the reason why a large part of the accelerator is connected to a liquid helium distribution system that cools the magnets as well as other attached systems.

Thousands of magnets of different types and dimensions are used to direct the beams along the accelerator. Among them the main magnets, among which are 1234 bipolar magnets of 15 meters in length used to bend the trajectory of the shaces, and 392 quadripolar magnets of 5 to 7 meters in length that concentrate the beams. Just before the collision, another type of magnet is used to “glue” the particles to each other, in order to increase the chances of collision. These particles are so tiny that making them collide means throwing two needles, against each other, from a distance of 10 km.

All throttle control systems and their technical infrastructure are grouped in the CERN Control Center. From there, the collisions of the beams in the center of the particle detectors will be activated.

The experiments

The six LHC experiments are international collaborations that bring together scientists from institutes around the world. Each experiment is different and is characterized by its particle detector.

The two major experiments, ATLAS and CMS, are equipped with multipurpose detectors intended to analyze the myriad of particles produced during collisions inside the accelerator, and thus study the most diverse aspects of physics. These two detectors, conceived independently, allow to locate the information in case of discovery.

Two medium-sized experiments, ALICE and LHCb, are equipped with specialized detectors and will analyze specific phenomena during collisions in the LHC.

Two other experiments of a clearly smaller dimension, TOTEM and LHCf, will study hadrons that narrowly escape a frontal collision. Indeed, when two beams that circulate in the opposite direction reach the point of collision, only some particles collide. Others rub against each other, while the vast majority continue their route without encountering other particles. Those that only rub against each other deviate very slightly from the beam path: they are the “small angle particles” analyzed by TOTEM and LHCf.

The ATLAS, CMS, ALICE and LHCb detectors are installed inside four huge caverns located along the LHC ring. The detectors of the TOTEM experiment are located near the CMS detector, and those of the LHCf experiment are near the ATLAS detector.

The experiments: ALICE

ALICE: A Large Ion Collider Experiment (Great Ion Collider Experiment)

For the ALICE experiment, the LHC will lead to collision of lead ions in order to recreate in the laboratory the conditions that reigned just after the Big Bang. The data obtained will allow studying the evolution of matter from the birth of the Universe to the present day.

All ordinary matter present in the current Universe is composed of atoms. Each atom is made up of a nucleus composed of protons and neutrons, and surrounded by a cloud of electrons. Protons and neutrons, meanwhile, are formed by quarks.

Quarks are fundamental particles. They are always found in groups of three or four, or in quark-antiquark pairs, linked together by particles called gluons. Because of that incredibly powerful link, no isolated quark has ever been observed.

The collisions that will occur in the LHC will generate temperatures more than 100,000 times higher than those that reign in the center of the Sun. Physicists hope that in this way protons and neutrons will “melt”, releasing quarks from the influence of gluons and creating a state of matter called plasma quarks and gluons. That state probably existed just after the Big Bang, when the Universe was still extremely warm. The particles found today in abundance in the Universe (protons and neutrons) would have formed in that plasma.

A collaboration of more than 1,000 scientists representing 94 institutes and 28 countries works on the ALICE experiment.

The ALICE detector:

Dimensions:16 meters high.

Weight:10,000 tons

Configuration: central barrel plus small-angle small-angle muon spectrometer.

Location: St Genis-Pouilly, France.

The experiments: ATLAS

ATLAS: A Toroidal LHC ApparatuS (Toroidal Apparatus LHC)

ATLAS is one of the two versatile detectors of the LHC. He will explore a wide range of areas of physics, from the search for the Higgs boson to that of other dimensions, through the search for particles that may constitute black matter.

ATLAS, which shares the same physics objectives as CMS, will measure comparable data on the particles created during the collisions: its trajectory, its energy and its nature. Having said that, the technical solutions and configurations selected for the magnetic systems of these two detectors are radically different.

The ATLAS detector is mainly characterized by its huge toroidal magnetic system. Said system is composed of eight coils of 25 meter long supraconducting magnets, arranged cylindrically along the beam tube whose axis constitutes the center of the detector. During the exploitation phase, the magnetic field is confined inside the central cylindrical space delimited by the coils.

More than 1,700 scientists, representing 159 institutions and 37 countries, work on the ATLAS experiment.

The ATLAS detector:

Dimensions: ATLAS is the largest detector ever built hight 25 meters,25 meters wide,46 meters long

   Weight:7,000 tons

Configuration: barrel and plugs

Location: Meyrin, Switzerland.

The experiments: CMS

CMS: Compact Muon Solenoid

The CMS experiment uses a multipurpose detector to explore a wide range of fields of physics, from the search for the Higgs boson to that of other dimensions through the search for particles that could constitute black matter. Although it pursues the same scientific objectives as the ATLAS experiment, the CMS collaboration has opted for other technical solutions and a differently designed magnetic system.

The CMS detector has been built around a huge solenoid magnet. This magnet is presented in the form of a cylindrical supraconductive coil that will generate a magnetic field of 4 teslas; approximately 100,000 times the earth’s magnetic field. The magnetic field is confined by a steel “cylinder head” which constitutes the majority of the 12,500 tons of the detector. Contrary to the other giant LHC detectors, which have been built underground, CMS has been built on the surface. Subsequently, its 15 sections were lowered to the cavern to be assembled there.

More than 2,000 scientists, representing 155 institutions and 37 countries, collaborate in the CMS experiment.

The CMS detector

Dimensions:21 meters long,15 meters wide and 15 meters high

Weight:12,500 tons

Configuration: barrel and plugs

Location: Cessy, France.

The experiments: LHCb

LHCb: Large Hadron Collider beauty (beauty of the Large Hadron Collider)

The LHCb experiment seeks to understand why we live in a Universe that seems to be made entirely of matter, without any presence of antimatter.

The experiment can explore the variations between matter and antimatter  by finding out a sort of particles referred to as “beauty quark” or “quark b”. The LHC will recreate the moments just after the Big Bang, during which the pairs of quarks by and antiquarks b would have been produced.

LHCb uses a series of sub-detectors aligned along the beam in order to mainly follow the small angle particles. The first sub-detector is installed near the collision point; the others follow each other over a length of 20 meters.

LHCb will create a wide variety of quark types before quickly disintegrating to form other particles. To intercept quarks b, the LHCb collaboration has designed and built mobile pathographs, installed as close as possible to the beam path.

The LHCb collaboration has 650 scientists, representing 48 institutions and 13 countries.

The LHCb detector

Dimensions:21 meters long,13 meters wide and 10 meters high

Weight:5,600 tons

Configuration: small angle spectrometer with planetary detectors

Location: Ferney-Voltaire, France.

The experiments: TOTEM

TOTEM: TOTal Elastic and diffractive cross section Measurement

The TOTEM experiment studies very small angle particles, a part of physics inaccessible to polyvalent experiments. Among other investigations, TOTEM will measure, for example, the dimensions of the protons and accurately assess the brightness of the LHC.

For this, TOTEM must be able to detect the particles produced as close as possible to the LHC. The experiment will include detectors protected in specially designed vacuum chambers; These detectors, called “Roman amphorae,” are connected to the beam tubes of the LHC. Eight Roman amphorae will be placed in pairs in four locations near the collision point of the CMS experiment.

Although both experiments are independent, TOTEM will complement the results obtained by the CMS detector as well as by the other LHC experiments.

The TOTEM experiment has 50 scientists representing 10 institutes and 8 countries (2006).

The TOTEM detector:

Dimensions:440 meters long,5 meters wide and 5 meters high

Weight:20 tons

Configuration: Roman amphorae with GEM detectors and cathode tape cameras

Location: Cessy, France (near CMS)

The experiments: LHCf

LHCf: Large Hadron Collider forward

The LHCf experiment uses the small angle particles created inside the LHC to simulate cosmic rays in laboratory conditions.

Cosmic rays are charged particles from interstellar space and constantly bombard the Earth’s atmosphere. When they reach the high atmosphere, these energy particles collide with nuclei of atoms, which produces a cascade of particles in the ground.

Collisions in the LHC produce similar waterfalls, which can help physicists contrast the detectors of gigantic experiments on cosmic rays (some may cover thousands of kilometers) as well as interpret their results.

The LHCf experiment has 22 scientists representing 10 institutions and 4 countries.

The LHCf detector

Dimensions: two detectors, each measuring 30 cm in length,10 cm in width and 80 cm in height

Weight:40 kg each

Location: Meyrin, Switzerland (near ATLAS)

Computer science in the LHC

When the LHC is started, about 15 petaoctets (15 million gigabytes) of data will be produced each year; the equivalent of a 20 km high CD stack. Thousands of researchers around the world will want to access that data to analyze it, and for that reason CERN decided to build a distributed infrastructure for data storage and processing: the LHC calculation network, or LCG (LHC Computing Grid).

The data from the LHC experiments will be distributed to the entire planet, while CERN will keep a primary tape backup. After the initial treatment, the data will be distributed to several large operational computer centers 24 hours a day that will have sufficient storage capacity to accommodate large amounts of them.

These centers will then make the data available to other facilities constituted by one or several calculation centers, in order to carry out specialized analysis tasks. Researchers will access these equipments individually through resources such as local terminals of university departments, or even from their individual computer, which may have regular access to the LCG network.

The LCG network collaborates closely with the other CERN network projects:

– EGEE (Enabling Grid for E-sciencE): The LCG serves as the primary production support for this European project devoted to online research and launched in April 2004 with a view to establishing a network infrastructure in a vast range of scientific fields .

– The “CERN open laboratory”(CERN openlab): the LCG also follows the evolution in the industry, in particular through the “open laboratory”, an association through which state-of-the-art computer companies test and validate cutting-edge network technologies in the LCG environment.

Security in the LHC

The Large Hadron Collider (LHC) can reach energies that no other particle accelerator has ever reached, energies that only Nature has been able to generate. Without this powerful machine, physicists could no longer probe the great mysteries of the Universe. The consequences of these high-energy particle collisions have led to misgivings. But there is no reason to worry.

Very modest energies on the scale of Nature

Accelerators recreate, in laboratory conditions, the natural phenomenon of cosmic rays, those particles produced in the intersideral space during events such as the formation of supernovae or black holes, and accelerated to energies that far exceed those of the LHC. Cosmic rays travel through the Universe and ceaselessly bombard the Earth’s atmosphere since its formation,4.5 billion years ago. Although the power of the LHC is impressive compared to that of other accelerators, the energies produced during the collisions are very weak in relation to those of some cosmic rays. The much higher energies released by the collisions that have occurred in Nature for billions of years have not had dire consequences for the Earth. Thus,

Cosmic rays do not collide only with the Earth, but also with the Moon, Jupiter, the Sun and other celestial bodies. The total number of such collisions is gigantic compared to what is expected to be achieved with the LHC. The fact that the planets and stars are still intact reaffirms us in the idea that the collisions that will occur in the LHC are safe. The energy of the LHC, huge, it is true, for an accelerator, is very modest on the scale of Nature.

Mosquitoes and TGV

The total energy of the two proton beams that circulate inside the LHC is equivalent to a 400-ton train (like the French TGV) traveling at 150 km / h. However, only a tiny part of that energy is released with each collision of particles, the approximate equivalent of the energy of 14 mosquitoes in flight … In fact, every time you try to crush a mosquito in your hands, it creates a collision energy. far superior to that of the protons in the LHC. The particularity of the LHC lies in its impressive ability to concentrate that energy inside a tiny space, at a subatomic scale. But even with that capacity, the machine only produces a pale imitation of what Nature does daily in cosmic ray collisions.

During its exploitation phase, the LHC will also bring into collision beams of lead cores that, in total, will have a greater collision energy: that of a little more than 1,000 flying mosquitoes. This energy, however, will be much less concentrated than that produced during proton collisions, and will not present any risk.

You will not be swallowed by a microscopic black hole …

In the Universe, the collapse of solid stars creates massive black holes, objects that enclose huge amounts of gravitational energy that attracts surrounding matter. The gravitational force of a black hole is related to the amount of matter or energy it contains: the less matter there is, the lower its attractive force. Some physicists think that microscopic black holes could occur during collisions inside the LHC. However, they would be created with the energies of the particles that will collide (equivalent to the energies of our mosquitoes); consequently, no microscopic black hole produced inside the LHC could generate sufficient gravitational force to absorb the surrounding matter.

If the LHC can produce microscopic black holes, cosmic rays, of much higher energy, have necessarily produced many more. And, since Earth is still here, there is no reason to think that the less energetic collisions inside the LHC are dangerous.

Black holes lose matter by emitting energy, through a process described by Srephen Hawking. Black holes that cannot attract matter to subsist, such as those that could occur in the LHC, shrink, evaporate and disappear. The smaller the black hole, the faster it fades away. If black holes were formed in the LHC, they would only exist for a fleeting moment. By the way, their existence would be so short that the only way to observe them would be to detect the products of their disintegration.

… or dragged by a strangelet

The strangelets are hypothetical bits of matter whose existence has never been proven. They are supposed to consist of “strange” quarks, heavier and more unstable relatives of the quarks that constitute stable matter. Even if there were strangelets, they would be unstable. Its electromagnetic charge would reject ordinary matter; thus, instead of combining with stable substances, they would simply disintegrate. If strangelets were produced in the LHC, they would not cause much harm … Not to mention that such strangelers would have already been created by high-energy cosmic rays, and that in that field we should not deplore any damage to this day.

Studies and evaluations

Studies on the safety of high-energy collisions inside particle accelerators have been carried out in Europe and the United States by physicists who are not involved in the LHC experiments. Their analyzes have been evaluated by experts, who have confirmed that the collisions of particles in the accelerators are safe. CERN has also commissioned a group of particle physicists, not involved in the LHC experiments, to respond to all speculation about collisions inside the LHC. It is possible to contact that group by sending a message to the following address:

Facts and figures

The biggest machine in the world …

The exact circumference of the LHC is 26,659 meters, and the machine contains a total of 9,300 magnets. Not only is the LHC the largest particle accelerator in the world, but only one-eighth of its cryogenic distribution system would be the largest refrigerator on the planet. All magnets are pre-cooled to -193.2ºC (80 K) with the assistance of ten.080 loads of nitrogen before being full of concerning sixty loads of liquid atomic number 2 which will take them to -271.3ºC (1.9 K).

The fastest circuit on the planet …

At full power, trillions of protons launched at 99.99% of the speed of light will return to the accelerator 11,245 times per second. Two beams of protons will each travel at a maximum energy of 7 TeV (teraelectronvolts), thus allowing frontal collisions of 14 TeV. This will result in about 600 million collisions per second.

The most empty space in the solar system …

In order to avoid collisions with gas molecules present in the accelerator, the particle beams travel in a cavity as empty as the interplanetary space, which is called ultra-vacuum. The internal pressure of the LHC is 10-13 atm., That is, ten times lower than the pressure that reigns on the Moon.

The warmest points of the galaxy in a ring colder than the intersideral space …

The LHC is the machine of extreme temperatures. When two proton beams collide they generate, in a tiny space, temperatures more than 100,000 times higher than those that reign in the center of the Sun. On the contrary, the cryogenic distribution system that feeds the throttle ring with superfluid helium maintains the LHC at a temperature of -271.3° C (1.9 K), colder than the intersideral space.

The largest and best performance detectors ever built …

To select and record the most interesting data among those millions of collisions, physicists and engineers have built gigantic devices that measure particle traces with precision of the order of one micron. LHC detectors such as ATLAS or CMS are equipped with electronic activation systems that measure the passage time of a particle with an accuracy of a few billionths of a second. The activation system also records the position of the particles with an accuracy of one millionth of a meter. The speed and accuracy of these systems are essential if you want to be sure that a particle registered in different layers of the detector is exactly the same.

The most powerful computer in the world …

The data recorded by each of the great experiments of the LHC could fill approximately 100,000 double-layer DVDs each year. In order to allow some 7,000 physicists around the world to participate in the analysis of the data over the next 15 years (estimated duration of the LHC’s life), tens of thousands of computers scattered around the planet will be available within the framework of a decentralized computer network called the Network.

The key dates of the LHC

Towards new frontiers

The LHC accelerator was conceived in the 1980s and its construction was approved by the CERN Council in 1994.

Civil engineering works to excavate the caverns of the experiments began in 1998. Five years later the last cubic meter of earth was extracted.

Many state-of-the-art techniques have been improved to respond to unprecedented challenges.

In anticipation of the phenomenal amount of data that the LHC experiments will produce (approximately 1% of global information production), a new approach to storage, management, distribution and data analysis has been adopted: It is the calculation network project for the LHC.

For more than a decade, all those who have worked tirelessly in the completion of the construction of the LHC have pursued a dream … that comes true.

At this time, the LHC is in full phase of repairs due to the breakage of a weld in a dipole magnet in sector 3-4.

Repair of the dipole magnet of the LHC sector 3-4

From sector 3-4 of the LHC Repair of dipole magnets 

The LHC will be launched again in 2009

LHC to restart in 2009

Geneva.- CERN confirmed today that the Large Hadron Collider (LHC) will start operating again in 2009. This news is part of an updated report, published today, on the status of the LHC that suffered a breakdown on September 19.”The high priority for CERN these days is to supply collision knowledge for the experimentsfor the experiments as soon as reasonably possible,” says general director Roberto Aymar of CERN.”This will be in the summer of 2009.”


The initial breakdown was caused by a faulty electrical connection between two dipole magnets in sector 3-4. This resulted in damage and the escape of helium from the cold zone of the magnet in the tunnel. Appropriate security procedures were in effect, security systems worked as expected, and no one was put at risk. Detailed studies of the malfunction have allowed LHC engineers to take steps to identify and take measures to prevent a similar incident from happening again in the future, and to design new machine protection systems. A total of 53 units of dipole magnets have to be removed from the tunnel for cleaning or repair, of these28 have already been brought to the surface and the first two replacement units are already installed in the tunnel.

We have a lot of work to do during the coming months,”said LHC Project Manager Lyn Evans,” but now we have the new itinerary, time and capacity to be ready to run again in the summer. We are currently at an annual stop scheduled until May, he said, hoping to have no further delays.”Full details of the schedule to restart LHC’s activity are available in the report published today.

Then we put a video of engineers, technicians etc in full repair.


LHC damaged part


Repair sector


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