Large Hadron Collider

The Large Hadron Collider (LHC) is a powerful particle accelerator built by the European Organization for Nuclear Research, better known as CERN. At a circumference of 16.8 miles (27 kilometers) across, it is the largest machine in human history. It is located about 300 feet (91 meters) underground at the French-Swiss border. The LHC helps scientists study the fundamental building blocks of matter by accelerating subatomic particles close to the speed of light and observing what happens when they collide. The project began in 1989 and became operational in 2008 at a cost of about $10 billion. The LHC paid dividends in 2012 when it detected evidence of a mysterious particle believed to be a key component of all the matter in the universe.

Background

An atom is the basic unit of the chemical elements that make up everything in the universe. Atoms are very small, measuring on average about a million times thinner than a human hair. They are composed of three main subatomic particles called protons, neutrons, and electrons. The nucleus, or center, of an atom is made up of protons and neutrons, while the electrons orbit around the nucleus. Protons, neutrons, and electrons are further divided into two groups of particles called fundamental particles. One of those groups, the fermions, consists of numerous smaller particles such as quarks and leptons. Fermions are the primary building blocks that make up all matter. The other fundamental particle is a boson, a force-exerting particle that holds matter together.

To learn more about the smallest fundamental particles, scientists must find a way to increase the energy of a beam of electrons or protons. They do this by accelerating the particles close to the speed of light, or about 186,000 miles (299,700 kilometers) per second. When particle beams moving at such high speeds collide, they give off radiation that scientists can observe to study fundamental particles.

American scientist Ernest Lawrence invented one of the first particle accelerators in the 1930s. The device, called a cyclotron, used magnetic and electrical fields to propel electrons to the required speeds necessary to penetrate an atomic nucleus. Larger accelerators allow the beams to travel faster and produce more energy. Lawrence's first cyclotron was small, but by the end of the decade, he constructed an accelerator that was 184-inches (467-centimeters) wide and required its own building.

Overview

The European Organization for Nuclear Research is an international scientific organization created in the years after World War II (1939–1945). Its acronym, CERN, comes from the organization's original French name, the Conseil Européen pour la Recherche Nucléaire. CERN built its first particle accelerator in 1957 at its research facility in Geneva, Switzerland. By 1965, the particle accelerator required more space, and CERN extended its facilities across the border into neighboring France.

The project that would become the Large Hadron Collider (LHC) began in 1989 as the Large Electron-Positron (LEP) Collider. The LEP was built in a 16.8-mile (27-kilometer) ringed tunnel below the French-Swiss border. It operated until 2000 when scientists disassembled the LEP and began work on the LHC. The new particle accelerator used the same tunnel complex as the LEP. It was finished in 2008 and cost about $10 billion. It became operational that same year, but an explosion severely damaged it just nine days after it was turned on. It was brought back online in 2009.

The LHC gets its name from a type of subatomic particle called a hadron. The LHC uses about 9,600 powerful magnets of different sizes to accelerate the particles close to the speed of light and steer them through the accelerator. To avoid overheating and to maximize efficiency, the magnets are kept at a supercooled temperature of -456.3 degrees Fahrenheit (-271.3 Celsius). A distribution system of liquid helium pumped throughout the complex maintains these low temperatures.

To perform experiments, scientists use hydrogen atoms from a bottle of hydrogen gas. An electrical field strips the electrons from the atoms to create a particle beam of protons. Each beam has about 3,000 groups, or bunches, of particles, and each bunch can contain about 100 billion particles. These numbers may seem high, but the particles are so small that collisions between them are relatively rare. CERN estimates about 40 collisions occur between 200 billion particles. The protons in the beams make about 11,200 circuits around the collider per second. By operating the LHC continuously for months at a time, scientists are able to generate enough collisions to observe data.

The beams traveling around the LHC can collide at four particle detectors stationed at certain points along the large underground ring. Each detector is used to perform a specific experiment. The Large Hadron Collider beauty (LHCb), named after a type of quark called a "beauty quark," studies the difference between matter and antimatter. Antimatter has an opposite charge and spin than regular matter. A Large Ion Collider Experiment (ALICE) is designed to study the physics of extremely dense matter. The Compact Muon Solenoid (CMS) is a general-purpose detector that performs several functions. A Toroidal LHC ApparatuS (ATLAS) is another general-purpose detector named for its large size. The cavern that houses ATLAS is large enough to fit a twelve-story building.

On July 4, 2012, the LHC detected evidence of a long-predicted fundamental particle called the Higgs boson. The Higgs boson was a missing piece in a scientific theory called the standard model. The model suggests that everything in the universe is made up of twelve types of matter particles held together by four forces—gravity, electromagnetic, strong, and weak. The theory holds that each of these has a corresponding boson that provides the force to hold matter together. In the 1960s, British physicist Peter Higgs and Belgian physicist François Englert proposed that a boson is responsible for generating a field that gives all the particles in the universe their mass. Named the Higgs boson, this particle was just a theory until the ATLAS and CMS detectors found evidence of its existence in 2012.

The LHC was shut down for renovations in 2013. When it came back online in 2015, scientists were able to use its improved capabilities to confirm the discovery of several new subatomic particles. They also were able to reconfirm data on the Higgs boson.

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