The Large Hadron Collider (LHC) is three things. First, it is large – so large that it’s the world’s largest science experiment. Second, it’s a collider. It accelerates two beams of particles in opposite directions and smashes them head on. Third, these particles are hadrons. The LHC, built by the European Organisation for Nuclear Research (CERN), is on the energy frontier of physics research, conducting experiments with highly energised particles.
Currently, engineers are warming up the LHC for its third season of operations, following upgrades that will have made the collider and its detectors more sensitive and accurate than before. It will start collecting data again from mid-May.
How does the LHC work?
A typical candidate event inside the LHC, ‘seen’ by the CMS detector in which a collision between two beams has produced two high-energy photons (depicted by red towers) and other particles (yellow lines). The pale blue volume depicts the detector volume. | Photo Credit: AP Photo/CERN
A hadron is a subatomic particle made up of smaller particles. The LHC typically uses protons, which are made up of quarks and gluons. It energises the protons by accelerating them through a narrow circular pipe that is 27 km long.
Simply put, this pipe encircles two D-shaped magnetic fields, created by almost 9,600 magnets. Say there is a proton at the 3 o’clock position – it is made to move from there to the 9 o’clock position by turning on one hemisphere of magnets and turning off the other, such that the magnetic field acting on the proton causes it to move clockwise. Once it reaches the 9 o’clock position, the magnetic polarity is reversed by turning off the first hemisphere and turning on the second. This causes the proton to move in an anticlockwise direction, from the 9 o’clock back to the 3 o’clock position.
This way, by switching the direction of the magnetic field more and more rapidly, protons can be accelerated through the beam pipe. There are also other components to help them along and to focus the particles and keep them from hitting the pipe’s walls.
Eventually, the protons move at 99.999999% of the speed of light. According to the special theory of relativity, the energy of an object increases with its speed (specifically, through the equation E2 = p2c2 + m2c4, where p is momentum, equal to mass times velocity).
What happens when the particles are smashed?
A view of the LHC in its tunnel at CERN, near Geneva, Switzerland. | Photo Credit: Martial Trezzini/Keystone via AP
When two antiparallel beams of energised protons collide head on, the energy at the point of collision is equal to the sum of the energy carried by the two beams.
Thus far, the highest centre-of-mass collision energy the LHC has achieved is 13.6 TeV. This is less energy than what would be produced if you clapped your hands once. The feat is that the energy is packed into a volume of space the size of a proton, which makes the energy density very high.
At the moment of collision, there is chaos. There is a lot of energy available, and parts of it coalesce into different subatomic particles under the guidance of the fundamental forces of nature. Which particle takes shape depends on the amount and flavour of energy available and which other particles are being created or destroyed around it.
Some particles are created very rarely. If, say, a particle is created with a probability of 0.00001%, there will need to be at least 10 million collisions to observe it. Some particles are quite massive and need a lot of the right kind of energy to be created (this was one of the challenges of discovering the Higgs boson). Some particles are extremely short-lived, and the detectors studying them need to record them in a similar timeframe or be alert to proxy effects.
The LHC’s various components are built such that scientists can tweak all these parameters to study different particle interactions.
What has the LHC found?
Fabiola Gianotti, then spokesperson of the ATLAS detector at the LHC announcing the discovery of a particle consistent with the Higgs boson at CERN on July 4, 2012. | Photo Credit: AP
The LHC consists of nine detectors. Located over different points on the beam pipe, they study particle interactions in different ways. The ATLAS and CMS detectors discovered the Higgs boson in 2012 and confirmed their findings in 2013, for example.
Every year, the detectors generate 30,000 TB of data worth storing, and even more overall. Physicists pore through it with the help of computers to identify and analyse specific patterns.
The LHC specialises in accelerating a beam of hadronic particles to certain specifications and delivering it. Scientists can choose to do different things with the beam. For example, they have energised and collided lead ions with each other and protons with lead ions at the LHC.
Using the data from all these collisions, they have tested the predictions of the Standard Model of particle physics, the reigning theory of subatomic particles; observed exotic particles like pentaquarks and tetraquarks and checked if their properties are in line with theoretical expectations; and pieced together information about extreme natural conditions, like those that existed right after the Big Bang.
What is the LHC’s future?
These successes strike a contrast with what the LHC hasn’t been able to find: ‘new physics’, the collective name for particles or processes that can explain the nature of dark matter or why gravity is such a weak force, among other mysteries.
The LHC has tested some of the predictions of theories that try to explain what the Standard Model can’t, and caught them short. This has left the physics community in a bind.
One way forward, already in the works, is to improve the LHC’s luminosity (a measure of the machine’s ability to produce particle interactions of interest) by 10x by 2027 through upgrades.
Another, more controversial idea is to build a bigger, badder version of the LHC, based on the hypothesis that such a machine will be able to find ‘new physics’ at even higher energies.
While both CERN and China have unveiled initial plans of bigger machines, physicists are divided on whether the billions of dollars they will cost can be used to build less-expensive experiments, including other colliders, and with guaranteed instead of speculative results.
Published - April 12, 2023 10:30 am IST
