LHC-World's Biggest Experiment
LHC- World’s Biggest Experiment
The Large Hadron Collider (LHC) is the world’s biggest laboratory for scientific experiments. The LHC and its experiments are unmatched in size, complexity, participation and achievements by anything ever built and operated by scientists previously.
The LHC experiments on collision of high energy particles seek to unravel the mysteries of the evolution of the universe immediately after the BigBang. In a major breakthrough, scientists working at LHC confirmed the existence of Higgs boson, the particle that gives mass to other fundamental particles Scientists have now moved ahead with experiments at LHC to seek evidence for their theories and predictions on dark matter, anti-matter and super symmetry.
The gigantic LHC project is implemented and operated by the European Council for Nuclear Research (CERN). [The acronym CERN stands for its name in French language, “Counseil Europeen pour la Recherche Nucleaire”]
CERN was established in 1954, and it is completing 70 years of cutting edge experiments and fundamental research. CERN is based in Meyrin, western suburb of Geneva in Switzerland – France border.
At CERN, scientists and engineers explore the underlying structure of the universe by studying the interactions among fundamental particles that constitute matter. The world’s most complex scientific instruments – particle accelerators and particle detectors, are used in this fundamental research in physics.
CERN has a number of particle accelerators in its accelerator complex. Linear accelerator (LINAC), Proton Synchrotron (PS), Proton Synchrotron Booster (PSB), and Super Proton Synchrotron (SPS) are used for experimental research in variety of ways.
A particle accelerator is a machine that increases the velocity and energy of charged particles. This is accomplished by making the charged particles pass through a region where an electronic field is used for imparting electrical forces to them.
Accelerators can be linear or circular in geometry, depending on the trajectory chosen for the particles. In accelerators with circular geometry, the particles go around the same circular path again and again, increasing the velocity and gaining energy at each turn. Acceleration is done by making the particles pass through a cavity with an electromagnetic field, in its path. Cavities with radiofrequency waves (RF cavities) are used for this purpose. A cavity is an empty metallic chamber in which the intensity of electric field is built up due to resonance.
Magnetic fields are used for steering the charged particles into circular paths. Higher the velocity of the particles, more powerful magnetic fields are required to keep them in circular trajectories. Magnetic field is also needed to confine the particles to a beam of small cross-sectional area.
When particles collide at high energies, the energy of the collision is transferred into matter in the form of new particles. This phenomenon follows Einstein’s mass-energy relation E=mc2, according to which mass and energy are interchangeable.
The Large Hadron Collider (LHC) at CERN is the world’s largest and most powerful particle accelerator. It accelerates and collides hadrons and heavy ions. Hadrons are composite subatomic particles made of quarks held together by the strong force. Protons and neutrons are hadrons made of three quarks each. Neutrons cannot be accelerated because they have no charge.
LHC is a particle collider. Colliders are accelerators that generate collisions between accelerated particles. Other accelerators propel charged particles at high speeds towards fixed targets.
The LHC project is implemented in an underground tunnel in which CERN was earlier operating the Large Electron Positron collider (LEP). While the LEP accelerated electrons and positrons upto 100GeV of energy before collisions, LHC is designed to impart energies up to 7TeV to the colliding particles, a 70 fold increase compared to LEP.
CERN started LEP in 1989. In 2000 LEP was closed and the tunnel was made available for LHC.
The huge tunnel of LHC is 27km in circumference and lies beneath the surface at the Switzerland – France border near Geneva.
LHC being a particle collider, there are two rings with counter rotating particles Heavy particles like protons and ions are accelerated in these rings in opposite directions.
There are two transfer tunnels, each nearly 2.5km in length, linking the LHC to the CERN accelerator complex that acts as injector of charged particles. The injector chain consists of Linear accelerator (LINAC), Proton Synchrotron Booster (PSB), Proton Synchrotron (PS), and Super Proton Synchrotron (SPS).
The protons from the LINAC are first injected into the Proton Synchrotron Booster (1.4GeV) and then to the Proton Synchrotron which accelerates them to 26 GeV of energy. The protons coming out of the PS are then injected to the SPS and are accelerated to 450GeV. The LHC takes the protons from the SPS and takes their energy to the TeV level. The acceleration of protons in LHC from 450GeV to 6.5TeV takes about 20 minutes.
The actual acceleration of the particles in LHC takes place in RF cavities. 16 RF – cavities are housed in 4 cylindrical refrigerators called cryomodules, which enable them to work in the super conducting state. The total accelerating potential is 16 MV. The RF cavities are tuned to oscillate at 400MHz frequency.
LHC is the largest cryogenic system in the world. Cryogenics is the branch of physics that deals with the production and effects of very low temperatures.
At LHC, the velocity of particles can reach very close to the velocity of light. In order to keep these high velocity particles in the circular path, a powerful magnetic field of 8.33T is needed. Thousands of electromagnets are placed around the LHC tunnel to produce up to this large magnetic field. In an electromagnet, magnetic field is produced by electric currents. A current of 12000A is needed in the coils of electromagnets to reach the large magnetic field required. Coils should be of superconducting materials to avoid overheating when such large currents are flowing.
The Niobium-Titanium (NbTi) wires of the electromagnet’s coils must be kept at low temperatures to reach the super conducting state. LHC’s superconducting magnets are therefore maintained at - 271.30C (1.9K) by a closed He circuit. This is colder than outer space.
Gaseous He at atmospheric pressure changes into liquid state when the temperature is lowered to around 4.2K. If cooled below 2.17K it changes to the superfluid state. Superfluid He is an efficient thermal conductor. This makes liquefied He the best refrigerant for cooling the LHC’s super-conducting systems. 120 tonnes of liquefied He is used to maintain the LHC ring at ultra low temperature.
Perhaps more challenging than constructing a particle accelerator is devicing the detectors that will record and study the huge number of collisions that takes place between accelerated particles. In the large hadron collider the two oppositely moving beams of high energy particles are made to collide at four different locations along the ring. These are the positions of four particle detectors; ATLAS, CMS, ALICE and LHCb.
ATLAS (A Toroidal LHC Apparatus) is the largest particle detecting setup at the LHC. The giant ATLAS detector is constructed in the shape of a cylinder, 46m in length and 25m in diameter. It weighs 7000 tonnes, almost the same weight of Effel Tower. It has 6 different detecting subsystems placed concentrically in layers around the collision point.
Collisions of particles travelling at energies upto 7 trillion electron volts, and speeds upto 99.99999% that of light takes place at the centre of ATLAS. A billion such particle interactions takes place every second. Only one in a million collisions are chosen and recorded for further study. A large data analysis system consisting of 40000 CPU units is used for analyzing the data.
ATLAS is designed as a general purpose detector. It can measure a wide range of signals and it is intended to detect any new physical process or particle that may arise from the collisions. ATLAS can measure mass, momentum, energy, lifetime charge and nuclear spin of any new particle detected.
CMS (Compact Muon Solenoid) is also a general purpose detector like ATLAS. But it uses different technical solutions and magnet system design. Though smaller in size than ATLAS, it weighs 14000 tonnes.
CMS records particle collisions from all directions upto 40 million times each second. Most of the products in these collisions are unstable and they rapidly transform into stable particles. With the help of the strong magnetic field the trajectories of these particles can be analyzed. CMS is especially designed to detect particles called muons accurately.
The CMS experiment is one of the largest international scientific collaborations in history involving about 5500 scientists, engineers, technicians and students from 54 different countries.
ALICE and LHCb are detectors designed for specific enquires. ALICE (A Large Ion Collider Experiment) is dedicated to heavy-ion physics. It studies the phase of matter called quark gluon plasma. Collisions at LHC generates temperatures 100000 times hotter than the centre of the Sun. At this high temperature similar to that existed just after the Big Bang protons and neutrons ‘melt’ forming the quark-gluon plasma.
The Large Hadron Collider beauty (LHCb) experiment specializes in investigating the slight differences between matter and antimatter by studying the beauty quark (b quark)
The biggest achievement of LHC so far is the discovery of Higgs boson in 2012. In 1964, British theoretical physicist Peter W Higgs proposed a mechanism by which fundamental particles acquire mass. He proposed that all space is filled with a certain scalar field, the Higgs field. It was also suggested that fundamental particles acquire mass by interacting with the Higgs field. Collisions of high energy heavy particles should produce vibrations in this Higgs field, which should lead to the creation of a heavy particle with zero spin. This is the Higgs boson.
Experiments at the LHC using the ATLAS and CMS detectors found the Higgs boson with a mass of 125 GeV among the products of numerous proton-proton collisions. On July 4th 2012, scientists at CERN officially announced that they have confirmed the existence of Higgs boson. This is a momentuous discovery. It confirms the standard model of particle physics as the authentic theory of all known fundamental particles, and it illuminates our understanding of physical processes immediately after the Big Bang.
The Large Hadron Collider is currently into its third run. LHC experiments have made tremendous progress in the understanding of the properties of Higgs Boson. Its properties may give useful indications about physics beyond the standard model. Extensive studies are done on the ways in which Higgs boson decays into other particles.
The rare decay of Higgs boson into bottom quarks is one of them. The production of Higgs boson along with top quarks is also studied.
LHC experiments have also led to the discovery of new particles like pentaquarks.
Once the third run of the Large hadron collider is completed in 2026, the stage will be set for the High Luminosity Large Hadron Collider (HL-LHC) which will be operational from 2029. The new accelerator will have luminosity almost 10 times that of the current LHC. Luminosity is proportional to the number of collisions in a given cross sectional area that occur in a given time. Higher the luminosity, more will be the number of collisions, and more data will be available for analysis.
The now running LHC and the future High Luminosity LHC experiments are designed to expand the frontiers of our understanding of fundamental physics. Physicists are actively looking into frontier areas of research like dark matter, antimatter, super symmetry and many others.
Supersymmetry is an extension of the standard model. It predicts a partner particle for each particle in the standard model. If the theory is correct supersymmetric particles are expected to appear in high energy collisions at the LHC.
Appearance of Supersymmetric particles may solve the mystery of dark matter that makes up most of the universe. It is hypothesized that dark matter could contain supersymmetric particles.
Every fundamental particle of matter has a corresponding anti-particle. The BigBang should have created equal amounts of matter and anti-matter. It is inexplicable that now the universe has far more matter than antimatter. High energy collisions at the accelerator will produce more antiparticles and scientists can check if their properties differ from those of matter particles.
The physics of quark-gluon plasma is another field of interesting studies. In the first few microseconds after the Big Bang the Universe was filled with an extremely hot and dense soup dominated by quarks and gluons moving at speeds very near to the speed of light. In this quark gluon plasma, quarks and gluons were bond only weekly, and were free to move on their own. The high energy collision experiments at LHC are expected to provide opportunities for detailed studies of quark – gluon plasma.
By Dr.Jolly K John
Editor-in-Chief
Amazingphysics.in
