When protons collide

I work on the Large Hadron Collider, which is the world’s largest particle collider. What it does is simple and brutal. It takes particles called protons and smashes them into one another with lots of energy.

There’s a long tradition in particle physics of these sorts of experiments where you smash particles into one another. The reason for doing this is perhaps not what you would expect. Particle colliders are sometimes described as atom smashers, which conveys the idea that we’re using these machines to smash atoms apart to see what’s inside them. But that’s not really what colliders are for, because we know what’s inside atoms. We discovered that in the 20th century.

View of the detector LHCb. Courtesy of CERN PhotoLab.

Colliders like the Large Hadron are better described as matter factories. You accelerate particles to very high speeds because, at those speeds, particles carry huge amounts of energy. When you collide them, that kinetic energy is converted into new matter. So, you’re actually making matter or making new particles from energy. When two protons collide, many of the particles coming out of that collision didn’t exist before; they were created in the collision.

In other words, this is a way of studying the sorts of objects that can exist in the universe. When you get to really high energies, you’re probing energy densities, temperatures and conditions that haven’t existed in the universe in any large amounts since about a trillionth of a second after the Big Bang. You’re probing the kinds of things that were going on in the very first moments of the universe. This is why these experiments are so interesting.

CERN on July 4th

More than 200 Fermilab researchers and staffers crowded into an auditorium at 2 a.m. EDT July 4 . Wikimedia Commons. Public Domain.

If you’re a particle physicist, July 4th, 2012 was a really exciting day: a special event was called at CERN to announce results from two experiments at the Large Hadron Collider called ATLAS and CMS. These are two huge particle detectors, each with around 3,000 physicists working on them, sitting on opposite sides of this 27-kilometre ring. Both experiments announced that they had seen conclusive evidence of the existence of a new fundamental particle that had been hypothesised almost 50 years earlier in the mid-1960s; they had found the Higgs boson.

Everyone had been looking for the Higgs since the 1960s. It was the holy grail of particle physics. That’s because it plays a crucial role in our understanding of particle physics. It’s like the keystone in an arch, where the arch is the Standard Model of particle physics, our best description of the universe. The Higgs sits right at the centre of that. Its basic role is connected to why the elementary particles in the universe possess mass.

Higgs, mass and a stable universe

The idea that Peter Higgs and several other theorists came up with in the mid-1960s was that mass arises because particles like electrons and quarks interact with a quantum field – an invisible cosmic energy field that fills the whole universe. When particles like electrons or quarks move through this field, it imbues them with the property of mass.

This idea, written in mathematical language, was proposed to solve a particular type of theoretical problem with our understanding of particle physics in the 1960s. The problem was this: you’ve invented this new invisible field. But how do you know it’s there? One way is to hit this field with such force that you make it resonate or ripple, which shows up as a new particle called the Higgs boson.

So, the Higgs boson is really important and interesting because its existence demonstrates that the associated Higgs field is also there. It’s the Higgs field that gives mass to the quarks and electrons that make up our bodies. If it wasn’t there, atoms couldn’t form because electrons and quarks would never bind together to form stable atoms. It plays an absolutely crucial role in shaping all of the ingredients of our universe and making our universe somewhere where things made of atoms like stars and people can exist.

The “God Particle”?

The Higgs boson is so important to our understanding of particle physics that it was hyperbolically referred to as the “God Particle”. This originated in a book by Leon Lederman, a very eminent particle physicist. His story is that his publisher wanted to call the book The God Particle because he knew it would sell more copies. Lederman joked that it should be known as the “The Goddamn Particle” because it was so hard to find.

Most physicists, including myself, don’t really like this association. The Higgs boson has nothing to do with divinity or God or anything like that. On the other hand, it is an incredibly important ingredient of our universe and central to our understanding of particle physics. What’s more, it’s also connected to some of the big outstanding mysteries about the basic building blocks of the universe.

Why dark matter matters

From astronomy, we now suspect that about 85% of the matter in the universe is invisible. It’s what we call dark matter, which is really just a term to cover our ignorance. It’s some kind of substance that’s all around us, but there is no particle that we know about in the Standard Model that can explain what dark matter is.

Another problem with the Standard Model of particle physics is that it tells us that in the very early universe, around a millionth of a second after the Big Bang, all the matter in the universe should have been annihilated in a reaction with its mirror opposite, something called antimatter. Essentially, the theory tells us that the material world should not exist.

These are reasons for thinking there should be new things to discover at the Large Hadron Collider. In a way, the discovery of the Higgs boson closes the story of 20th century physics; it’s the last missing piece of a theory that was developed in the 1960s and 1970s. But it also potentially opens up new avenues for discovery because the Higgs boson is connected to some of the really big problems facing particle physics. The hope is that by studying the Higgs boson very accurately, either at the LHC or some future machine, we will start to get clues about whether there are other ingredients of our universe that we haven’t seen so far.

A lot of the work at the LHC is about finding signs of new particles, forces of nature and phenomena that we’ve never seen. So far, at least, we haven’t seen any of the predicted new particles that we might have hoped to see. This is worrying, of course, though we’ve seen an increasing number of what we call anomalies in the last few years. These are when experimental data subtly disagree with the theory, and it’s looking increasingly likely that we might be seeing the first signs of something altogether new, although it’s too early to say. The next few years will be exciting as we produce more data and learn more about the basic building blocks of our world.

An engineering marvel

The Large Hadron Collider is an engineering marvel. It’s the biggest scientific instrument ever built. By some measures, it’s the biggest machine ever built.

Aerial View of the CERN, 15 July 2008. Photo by Maximilien Brice (CERN). Wikimedia Commons. Public Domain.

It’s housed in a 27-kilometre-long circular tunnel buried roughly 100 metres beneath the Swiss-French countryside, outside Geneva. The machine itself is mostly made up of magnets. Imagine a great long pipe that goes round in a circle. You fire particles into it, they go around the ring and, at a certain stretch, only about 30 metres long, they get hit by a very powerful 2-million-volt electric field which accelerates them. Every time the particles orbit, they get faster. You have these very powerful superconducting magnets that are chilled to -271 degrees Celsius, just a couple of degrees above absolute zero. These incredibly powerful magnetic fields are needed to bend the fast-moving particles back around the rings so that they can be accelerated over and over again.

A view of the Large Magnet Facility in Building 180. Courtesy of CERN PhotoLab.

Building this machine was both a technical and a political feat. It involved fundraising from numerous countries all around the world because no single country could afford to build the LHC on its own. You had to manufacture thousands of these extremely powerful magnets. When the machine was being designed, the technology to build such things didn’t even exist. It had to be developed. Huge amounts of work had to be done not just on the physical infrastructure but also on the computing side of things, because the LHC generates vast amounts of data. One of the big challenges is how you cope with that flood of data. The solution is a global network of computing centres all over the world whose job it is to analyse the data.

Beauty, bottom and the search for new particles

What we call the experiments at the LHC are basically the detectors, where the particles collide. You can think of these as like gigantic three-dimensional digital cameras made up of millions of precision-engineered components. Their job is to record the particles as they fly out from the collision point.

Each of these experiments has several hundreds, or often thousands, of physicists working on them. My particular experiment is called LHCb, which is one of the four big detectors on the LHC. The “b” in LHCb stands for “beauty”, also sometimes referred to as “bottom”, which is the name of one of the six quarks that we know about. What LHCb does is study these beauty quarks very precisely. We do that because the way these beauty quarks behave can be affected by the existence of new forces of nature or particles that we haven’t yet discovered. By measuring their properties very accurately and comparing that to what our Standard Model tells us should be happening, we can get indirect evidence for the existence of new forces, new particles and so on.

In some sense, the Higgs is the end of one story and the beginning of another. It’s the end of our understanding of 20th century particle physics, the last missing piece of the Standard Model, but it’s also an unusual, unique particle that comes with a whole set of other questions and problems. It’s going to be one of the main areas of study in particle physics for the coming decades.

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what happens when particles collide

Cliff, H. (2021). How to Make an Apple Pie from Scratch: In Search of the Recipe for Our Universe. Picador.

The Higgs boson, dark matter and other mysteries of the universe