How does a particle accelerator work?

Image credit: US Department of Energy Science

Take a trip into a particle accelerator and discover the experiments that are solving the mysteries of the universe

Deep underground in the US Midwest, groundbreaking projects are utilising advanced particle technology to examine tiny subatomic matter. From a subterranean facility, powerful particle pulses are sent across states every second at almost the speed of light. The reason? To try and find out why we exist. Welcome to Fermilab.

The Fermi National Accelerator Laboratory (Fermilab) near Chicago, Illinois, is the US’ premier laboratory for high-energy particle physics. It began operations on 15 June 1967 and it’s one of 17 national laboratories mananged by the US Department of Energy. Its staff are currently on a mission to locate and study mysterious particles called neutrinos.

Neutrinos are subatomic elementary particles, similar to an electron or proton, except they have less mass and no electrical charge. You can’t see, feel, hear or smell them, but neutrinos are all around us. And they’re passing through your body at a rapid rate – 100,000 billion of them every second actually, give or take.

Solving their mysteries could potentially increase our knowledge of the origins of matter. Neutrinos can’t be seen with the naked eye, but they could be vital to how the universe works. It’s thought there were equal amounts of matter and antimatter (a partner particle with the same mass but opposite electric charge) shortly after the Big Bang that formed the universe. Then matter became much more abundant than antimatter, allowing for the formation of atoms, stars, planets and humans. The particle accelerators at Fermilab can send out both neutrinos and antineutrinos, their antimatter counterpart, so if a difference is found in how neutrinos and antineutrinos behave, it may help to explain how the universe evolved to end up without antimatter. 

Neutrinos are created naturally in huge nuclear reactions in the Sun, or when a star supernovas, but can also be made in nuclear power plants and using particle accelerators, like at Fermilab. To try and analyse these rare particles, various projects have been set up at Fermilab. The first was DONUT in the late 1990s, which was followed by MINOS in 2005. The NOvA Neutrino Experiment began in 2014 and upped the ante – it’s one of the largest experiments of all time.

The particle accelerator used in NOvA fires a beam of protons to a detector more than 800 kilometres away in Ash River, Minnesota. The particles don’t need a tunnel to travel through as they simply go straight through the Earth. This 14,000-ton detector is filled with light-conducting fibres that record the energy from neutrinos colliding with other particles. Along with the fibres, there are 344,000 cells of reflective plastic that are packed with 11 million litres of clear liquid that illuminate when particles come into contact. The facility uses cryogenic technology to keep the machines at -15 degrees Celsius, its optimum operating temperature. The detector is so gargantuan that a unique transport machine was required to move the 28 200-ton blocks that make it.

NOvA analyses how neutrinos change or ‘oscillate’ into different types. Neutrinos leave almost no trace and rarely interact with each other or other particles. The particle accelerator shoots protons, which then slam at very high energies into the target at Ash River. This creates shortlived particles that then decay to produce neutrinos. When neutrinos collide with other particles, the traces of the interactions are received by a detector, are examined by physicists and compared to previous statistics. Scientists are looking for trends in the data to decipher what neutrinos do and how they act.

One of the key breakthroughs that physicists have made is that there are different types, or ‘flavours’, of neutrino – each named after which electric-charged particle it collides with.Neutrinos are from the lepton family of particles, and like leptons there are three types – muon neutrinos, electron neutrinos and tau neutrinos. Electron neutrinos are produced when a neutrino slams into an electron, for example. As neutrinos blast through the beam, they change between the three types frequently. Starting off as a muon neutrino, they often oscillate to electron and tau neutrinos. Neutrino oscillation is like a piece of fruit changing into a vegetable when you leave the supermarket, or this magazine changing into a book before you get home. Understanding why this happens will be key to understanding neutrinos.

Is there an endgame for NOvA? It has helped to increase scientific knowledge of neutrino oscillation and furthered the search for a fourth neutrino type. University College London and Sussex University in the UK are both collaborating with the project by helping to analyse the oscillations. NOvA will collect data until 2024, ten years after it was first switched on, when it will be replaced by an all-new project called DUNE.

Work on DUNE, or ‘Deep Underground Neutrino Experiment’, began in July 2017. The project will be the largest international science experiment to take place in the US. It will be the strongest particle beam in the world, sending particles 1,300 kilometres to the Sanford Underground Research Facility in Lead, South Dakota. Excavation for the Long-Baseline Neutrino Facility, which will house it, begins later this year and is planned to be up and running by 2022. The European Organization for Nuclear Research, or CERN, which houses the Large Hadron Collider, has its own, slightly smaller detector, which went online in September 2018. A second detector is also on the way.

DUNE will benefit from a significant upgrade to Fermilab’s accelerators, with the Proton Improvement Plan II (PIP-II). PIP-II will provide a new particle accelerator that will generate a proton beam with 60 per cent more power than before. The mechanism will be made from superconducting materials with no electrical resistance, resulting in even more power for a lower cost, and there will be more neutrinos to study than ever. DUNE will also have more sensitive detectors, using liquefied argon operating at -185 degrees Celsius. By 2026 the project will be fully operational.

Physicists at Fermilab will continue to study neutrinos to try and unlock the secrets they hold. Neutrinos can travel vast distances over the universe quickly, as few other particles, including those in magnetic fields, interfere with them. Because they are so difficult to locate, neutrinos could expose aspects of nature that are unknown to science, and potentially reveal the reasons why the universe is made of matter. We’re only just beginning to understand the potential wonders of neutrinos and, as technology improves and our knowledge grows, there could be some startling revelations around the corner. 

This article was originally published in How It Works issue 123, written by Jack Griffiths

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