What Is the Large Hadron Collider? CERN Restarts the World's Largest Particle Accelerator

After more than three years, the Large Hadron Collider was reactivated Friday.

Flora Graham
8 min read
The Large Hadron Collider outside Geneva, Switzerland.

The Large Hadron Collider outside Geneva, Switzerland.

Getty Images

After more than three years of inactivity, the Large Hadron Collider located on the French-Swiss border outside Geneva restarted on Friday shortly after 12 p.m. local time. Two beams of protons circulated in opposite directions around the particle collider, according to CERN, the European Organization for Nuclear Research.

Nearly 17 miles in circumference, the LHC is the world's highest-energy particle collider. But it has sat unused since December 2018, when it was shut down for maintenance. 

"The machines and facilities underwent major upgrades during the second long shutdown of CERN's accelerator complex," CERN's director for accelerators and technology, Mike Lamont, said in a statement. "The LHC itself has undergone an extensive consolidation program and will now operate at an even higher energy and, thanks to major improvements in the injector complex, it will deliver significantly more data to the upgraded LHC experiments."

But what is the Large Hadron Collider and what is its mission? Read on to find out. 

What is the Large Hadron Collider?

There are 17 known fundamental particles -- six quarks, six leptons and five bosons (not counting the theoretical Higgs boson) -- and their corresponding anti-particles. They are called "fundamental" or "elementary" particles because they have no smaller constituent parts. Quarks combine in various combinations to form other particles, such as protons and neutrons. 

Collectively, all the particles that are made up of quarks are called "hadrons."  

The Large Hadron Collider is a particle accelerator. It does just what it says on the box: it smashes hadrons together -- in this case, protons, which are a type of hadron particle -- at very high speeds.

Workers position a magnet inside the Large Hadron Collider

Nearly 17 miles in circumference, the Large Hadron Collider is the world's largest particle accelerator.

Lionel Flusin/Getty Images

The protons are created when hydrogen atoms, which consist of one electron orbiting a single proton, are stripped of their electron. The protons are divided into two streams, made up of clusters of about 100 billion protons, and sped up by a series of smaller accelerators before being injected into the main LHC ring.
Superconducting electromagnets guide the two streams in opposite directions around the ring and an electric field boosts their energy until they are traveling at 99.99% the speed of light.

After about 20 minutes -- and 13.5 million trips around the LHC -- the two streams are brought together in an enormous collision at one of four detector sites along the LHC: AtlasAliceCMS and LHCb.

These protons are so small that most of them fly right past each other -- there are only 20 collisions each time two bunches of 100 billion protons are brought together. But there are so many in the beam, that's still 600 million collisions per second.

At the point of collision, each proton has 7 tera-electronvolts (TeV) of energy. One TeV is about the energy of a flying mosquito, but a proton is a trillion times smaller.

When the protons smash together, it creates a shower of energetic particles that shoot off in all directions. Detectors measure the energies, directions and velocities of these particles, and the resulting data is fed to a supercomputer for analysis.

Bosons are force-carrying particles: Examples of bosons are the photon and the hypothetical Higgs boson, which is being sought in  the Atlas and CMS experiments at the LHC. 

All this knowledge about the fundamental particles and how they interact is called the "standard model." But there's more to discover, and that's the goal of the LHC.

What is CERN?

CERN, the Conseil Européen pour la Recherche Nucléaire, or European Council for Nuclear Research, oversees the LHC. The council researches particle physics, which means studying the fundamental particles -- the basic constituents of matter.  

Its goal is to find out what the universe is made of and how it works.

Established in 1952, CERN is governed by 20 European member states, though many non-European countries are also involved. Scientists come from around the world to use CERN's facilities. The agency employs around 2,500 people with roughly 8,000 visiting scientists, half of the world's particle physicists, coming to CERN for their research.

The Globe of Science and Innovation at CERN

The Globe of Science and Innovation at CERN.

Betul Yuruk/Getty Images

The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they're made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions. From these results, scientists learn about a particle's properties, such as mass and charge.

In 1995, CERN made the first antimatter atoms. They were anti-hydrogen atoms, there were nine of them, and they only lasted about forty billionths of a second before annihilating with ordinary matter. Since then, they've created thousands more.

CERN's first accelerator was the Synchrocyclotron (SC), which was completed in 1957 and stayed in use until 1990.

The Super Proton Synchrotron (SPS) was the first of CERN's giant rings. It was started in 1976 and was about 4.4 miles in circumference. Among other experiments, it was the collider that smashed together protons and anti-protons: In 1983 it was used to discover the W and Z particles, which provide the weak force.

The Large Electron-Positron (LEP) collider was commissioned in 1989 and, at 16.6 miles in circumference, is the largest electron-positron accelerator ever built. Prior to the Channel Tunnel, the excavation of the LEP tunnel was Europe's largest civil engineering project. 

LEP was decommissioned in 2000 to make way for the construction of the Large Hadron Collider in the same tunnel.

In 2020, CERN announced it plans to build a second collider that'll be almost four times the size of the LHC and could cost up to $23 billion. 

What is the Higgs boson?

We don't know what causes these fundamental particles to have mass. There's a theory that mass comes from the particles interacting with a particle called the Higgs boson -- after physicist Peter Higgs, one of the scientists who proposed its existence in 1964. The more they interact, the heavier they become, whereas particles that never interact are left with no mass at all.

Nicknamed "the God particle," the Higgs boson was finally detected in 2012.

An artist's rendering of a Higgs boson particle.

An artist's rendering of a Higgs boson particle.

Tobias Roetsch/Getty Images

The questions being asked at the LHC

What is mass? Why do particles weigh the amount they do? Why do some particles have no mass at all? There are no established answers to these questions, but one explanation is the Higgs boson. First hypothesized in 1964, it has yet to be observed.

The Atlas and CMS experiments will be searching for signs of the Higgs boson.

What is dark matter/dark energy? Everything we see in the universe is made up of ordinary matter, but that forms only 4% of the universe. Dark matter and dark energy are believed to make up the remaining 96%, but they're incredibly difficult to detect and study, other than through the gravitational forces they exert.

The Atlas and CMS experiments will look for supersymmetric particles to test a likely hypothesis for the make-up of dark matter.

Why is there no more antimatter? Antimatter is like a twin version of matter, but with an opposite electric charge. At the birth of the universe, equal amounts of matter and antimatter should have been produced in the Big Bang. But when matter and antimatter particles meet, they annihilate each other, transforming into energy. Somehow, a tiny fraction of matter must have survived to form the universe we live in today, with hardly any antimatter left. 

Why does nature appear to have this bias for matter over antimatter? The LHCb experiment will be looking for differences between matter and antimatter to help answer this question. Previous experiments have already observed a tiny behavioral difference, but what has been seen so far is not nearly enough to account for the apparent matter-antimatter imbalance in the universe.

How did quarks form particles just after the Big Bang? Protons and neutrons (which make up the atomic nucleus) are made of quarks, which are bound together by other particles called gluons. The bond is so strong that quarks are never seen in isolation. Collisions with lead ions in the Alice detector will generate temperatures more than 100,000 times hotter than the Sun's core, possibly freeing the quarks from their bonds with the gluons. This should create a state of matter called quark-gluon plasma, which probably existed just after the Big Bang.

The Alice experiment will recreate conditions similar to those just after the Big Bang to study the quark-gluon plasma as it expands and cools, observing how it progressively gives rise to the particles that constitute the matter of our Universe today.

Are there more than three dimensions? Einstein showed that the three dimensions of space are related to time. Subsequent theories propose that further hidden dimensions of space may exist -- for example, string theory implies that there are additional spatial dimensions yet to be observed. These may become detectable at very high energies, so data from all the detectors will be carefully analyzed to look for signs of extra dimensions.

Peter Higgs in front of a photograph of the Large Hadron Collider

Peter Higgs stands in front of a photograph of the Large Hadron Collider at the Science Museum in London.

Peter Macdiarmid/Getty Images

Fun facts about the LHC

The largest machine in the world: The LHC is 17 miles in circumference and runs about 330 feet under the surface, spanning the border of France and Switzerland. It's the world's largest particle accelerator, taking 20 years of planning and $10 billion to complete.

The most powerful supercomputer in the world: The LHC will generate 40,000GB of data each day -- this would fill 20 million CDs a year. To handle this data, tens of thousands of computers located around the world are being connected over the Internet in a distributed computing network called the Grid -- a virtual supercomputer.

The Compact Muon Solenoid detector assembly in a tunnel of the Large Hadron Collider.

A picture of the Compact Muon Solenoid detector assembly in a tunnel of the Large Hadron Collider.

Valentin Flauraud/Getty Images

The emptiest space in the Solar System: To avoid colliding with gas molecules inside the accelerator, the beams of particles travel in an ultra-high vacuum -- a cavity as empty as interplanetary space. The internal pressure of the LHC is 10-13 atmospheres, ten times less than the pressure on the Moon.

Hotter than the Sun: When two beams of protons collide, they will generate temperatures more than 100,000 times hotter than the core of the Sun, concentrated within a minuscule space.

Colder than outer space: The LHC's 9,300 magnets will be pre-cooled to -193.2°C (80K) using 10,080 tonnes of liquid nitrogen, before they are filled with nearly 60 tonnes of liquid helium to bring them down to -271.3°C (1.9K) -- colder than outer space.

The experiments: Atlas, Alice, CMS and LHCb

There are four main experiments (and four smaller ones) at the LHC, each with its own detector. The detectors are installed in four huge underground caverns located around the ring of the LHC.

Atlas (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid): Both Atlas and CMS are general-purpose detectors. They will investigate the myriad particles produced by the collisions in the accelerator. They will search for the Higgs boson, extra dimensions, and particles that could make up dark matter. Having two independently designed detectors is vital for cross-confirmation of any new discoveries. They are huge: CMS is double the mass of Atlas at 12,500 tonnes; Atlas is eight times the volume of CMS and as big as St Paul's cathedral.

The ALICE detector in the Large Hadron Collider ring

Alice (A Large Ion Collider Experiment) cavern and detector in the Large Hadron Collider ring.

Dean Mouhtaropoulos/Getty Images

Alice (A Large Ion Collider Experiment): Alice will investigate the quark-gluon plasma, looking at free quarks for the first time and how quarks combine to form particles such as protons and neutrons (which make up the atomic nucleus).

LHCb (Large Hadron Collider beauty): Although absent from the Universe today, 'beauty' (also called 'bottom' or 'b') quarks were common in the aftermath of the Big Bang, and will be generated in their billions by the LHC, along with their antimatter counterparts, anti-beauty quarks. They decay rapidly into a range of other particles, and the LHCb will compare these decays to find out why nature prefers matter over antimatter.