The Large Hadron Collider: Inside CERN's Atom Smasher

The Large Hadron Collider (LHC) is the biggest and most powerful particle accelerator in the world. It is located at the European particle physics laboratory CERN, in Switzerland. 


Scientists use the LHC to test theoretical predictions in particle physics, particularly those associated with the "Standard Model". While the Standard Model can explain almost all results in particle physics there are some questions left unanswered such as what is dark matter and dark energy? Why is there more matter than antimatter? The LHC is designed to help answer such questions. 


The LHC can reproduce the conditions that existed within a billionth of a second of the Big Bang. The colossal accelerator allows scientists to collide high-energy subatomic particles in a controlled environment and observe the interactions. One of the most significant LHC breakthroughs came in 2012 with the discovery of the Higgs Boson. 


If you see a news headline about exotic new subatomic particles, the chances are the discovery was made at CERN, the European Organization for Nuclear Research, located near Geneva in Switzerland. 


A recent example occurred in January 2022, when CERN scientists announced "evidence of X particles in the quark-gluon plasma produced in the Large Hadron Collider." Hiding behind that technospeak is the eye-popping fact that CERN succeeded in recreating a situation that hasn't occurred naturally since a few microseconds after the Big Bang. That particular study drew on pre-existing data from the LHC, the world's biggest particle accelerator, which has been undergoing a major upgrade since 2018. 


When the LHC restarts in spring 2022 after a three-year hiatus, we can expect a whole new spate of discoveries, so it's a good time to take a closer look at what makes the LHC — and the rest of CERN — so unique. 


The LHC is a particle accelerator — a device that boosts subatomic particles to enormous energies in a controlled way so that scientists can study the resulting interactions, according to the CERN LHC fact sheet. The 'large' that the L stands for is an understatement; the LHC is by far the biggest accelerator in the world right now, occupying a circular tunnel nearly 17 miles (27 kilometers) in circumference. The middle letter, H, stands for 'hadron', the generic name for composite LHC particles such as protons that are made up of smaller particles called quarks. Finally, the C stands for ‘collider’ — the LHC accelerate two particle beams in opposite directions, and all the action takes place when the beams collide. 


Like all physics experiments, the LHC aims to test theoretical predictions — in this case, the so-called Standard Model of particle physics — and see if there are any holes in them. As strange as it sounds, physicists are itching to find a few holes in the Standard Model because there are some things, such as dark matter and dark energy, that can't be explained until they do. 



The LHC's biggest moment came in 2012 with the discovery of the Higgs boson. Although widely referred to as the "God particle", it's not really as awesome in itself as that name might suggest. Its huge significance came from the fact that it was the last prediction of the Standard Model that hadn't yet been proven. But the Higgs boson is far from being the LHC's only discovery. 


According to the physics magazine CERN Courier, the LHC has also found around 60 previously unknown hadrons, which are complex particles made up of various combinations of quarks. Even so, all those new particles still lie within the bounds of the Standard Model, which the LHC has struggled to move beyond, much to the disappointment of the numerous scientists who have spent their careers working on alternative theories. 



The first tantalizing hints that a breakthrough might be just around the corner came in 2021 when analysis of LHC data revealed patterns of behavior that indicated small but definite departures from the Standard Model. 


According to CERN, the LHC opened for business in 2009, but CERN's history goes back much further than that. The organization was established in 1954 following a recommendation by the European Council for Nuclear Research — or Conseil Européen pours la Recherche Nucléaire in French, from which it gets its name. Between its creation and the opening of the LHC, CERN was responsible for a series of groundbreaking discoveries, including weak neutral currents, light neutrinos and the W and Z bosons. As soon as the LHC is back up and running, we can expect discoveries to continue. 



As huge as it is, the LHC can't function without the help of other machines around it. Before particles, which are usually protons but for some experiments are much heavier lead ions, are injected into it, they're passed through a chain of smaller accelerators that progressively boost their speed, according to a CERN LHC report. Smaller is just a relative term; the last step in the injector chain, the Super Proton Synchrotron, is almost 4.3 miles in circumference (6.9 km). The result is two beams traveling in opposite directions around the LHC at virtually the speed of light, according to CERN. 


The beams are kept on their circular trajectories by a strong magnetic field, which has the effect of bending the path of electrically charged particles. At four points around the LHC's vast ring, the opposing beams are brought together and made to collide, and that's where all the science happens. 



Particles are smashed together with such enormous energies that the collisions create a cascade of new particles — most of them extremely short-lived. The important thing for scientists is to work out what all these particles are, and that's not an easy task. 


The LHC has an array of sophisticated particle detectors for this purpose, each made up of layers of subdetectors designed to measure certain particle properties or to look for specific types of particles. For example, calorimeters measure a particle's energy, while the curving track of a particle in a magnetic field reveals information about its electric charge and momentum. 



Two of the four collision points around the circumference of the LHC are occupied by large general-purpose detectors. These include the Compact Muon Solenoid (CMS), which can be thought of as a giant 3D camera, snapping images of particles up to 40 million times per second. 


The paths of the particles inside the detector are controlled by a gigantic electromagnet called a solenoid. Despite weighing 12,500 metric tons, it's quite compact, as the detector's name suggests. That middle word, muon, refers to an elusive particle similar to the electron but much more massive, which requires its array of subdetectors wrapped around the solenoid. 


The LHC's other general-purpose detector, ATLAS (A Toroidal LHC Apparatus), has an identical purpose to CMS but differs in the design of its detection, subsystems and magnets. It's also less compact than CMS, occupying a greater volume than any other particle detector ever built.  



Many of the LHC's most important experiments, including the discovery of the Higgs boson, utilize the general-purpose detectors ATLAS and CMS. But it also has several other more specialized detectors that can be used in specific types of experiments. 


The LHC forward (LHCf) detector, located close to the ATLAS interaction point, uses particles thrown forward in collisions as a means of simulating cosmic rays under laboratory conditions. Further, along the beam trajectory is the Forward Search Experiment (FASER), designed to look for light, weakly interacting particles that are likely to elude the larger detectors. 


A third experiment optimized for the forward direction is Total Elastic and diffractive cross-section Measurement (TOTEM), located near the CMS interaction point, which focuses on the physics of the high-energy protons themselves. 


Away from ATLAS and CMS, the LHC has two other interaction points. One is occupied by A Large Ion Collider Experiment (ALICE), a specialized detector for heavy-ion physics. The final interaction point is home to two experiments on the very cutting edge of physics: LHCb, devoted to the physics of the exotic 'beauty quark', and MoEDAL — the Monopole and Exotics Detector at the LHC. 


According to CERN, when physicists come up with new theories, they always try to make sure they can be tested experimentally. That happened in the early 1960s when Peter Higgs and others developed a theory to explain why certain force-carrier particles have non-zero mass. 


The theory predicted the existence of a previously unsuspected particle, dubbed the Higgs boson. The next step was to find the Higgs boson and thus validate the theory. As simple as that sounds, it led to a decades-long hunt around the world. The end finally came in 2012, when data from the LHC — specifically, from a combination of ATLAS and CMS measurements — proved beyond doubt that the Higgs boson had been discovered. 


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