Finding the fundamentals of matter

Trillions of tiny objects are rushing through your body right now, and yet you probably have no idea...
21 November 2021


Trillions of tiny objects are rushing through your body right now, and yet you probably have no idea...

These objects, which have names like electrons, photons and positrons, are known collectively as “fundamental particles”. They’re named as such because, as far as we know, they cannot be divided into smaller chunks. It’s currently believed that they are the most elementary entities in the universe, from which everything else is ultimately formed.

However, not only do these particles influence how things are constructed on the smallest scales, but on the biggest scales too. That's because the way particles behaved in the early universe is responsible for how the universe looks today. Understanding particles, then, is crucial for understanding the rules our universe abides by, on both the very smallest and the very biggest of scales.

The question is: is our picture complete?

The Standard Model and why it’s not enough

How did we get to where we are now? “The quest to understand what matter is made of goes back to the ancient Greeks, who postulated that everything was made of atoms. And a century ago, that's what we thought the answer was: that the atoms of the elements, hydrogen, helium through to uranium, are what made everything,” explains Frank Close, Emeritus Professor of Physics at Oxford University.

But atoms aren’t the end of the story. “They're made of smaller pieces: there's a central nucleus made of protons and neutrons, around which circle tiny electrons. And so electrons, protons and neutrons are the basic constituents of all atoms.” However, like peeling an onion, more layers keep appearing. “Although electrons appear to be a basic seed to everything, the protons and neutrons are made of smaller bits, little particles called quarks.” As far as we know, it’s these particles - electrons, quarks and the like - from which all matter we know of is made.

However, as Frank points out, these three basic, stable particles that make up you and me - the electron and the two quarks that make up protons and neutrons - aren’t the full picture. Nature wasn’t happy to stop there. “It made two heavier versions involving things called muons, which are like heavy electrons, charm and strange quarks, which are heavier than the ones that make protons and neutrons, and top and bottom quarks, which are even heavier still. In addition to those, there are some ghostly, electrically-neutral particles called neutrinos.” Finally, there are some particles that allow all of these particles to talk to each other via forces - so called “interaction particles”.

This categorisation of fundamental particles is the result of the careful piecing-together of experimental information from the past half a century. It has culminated in a theory of how these tiny objects behave, called The Standard Model of Particle Physics. It’s one of science's most accurate and well-tested theories, and its final missing piece was discovered in 2012 - the Higgs boson. That said, its discovery does not mean that the job of the particle physicist is now over. Rather, it signals that the way particle physicists have seen the world up to now makes sense.

“We know a huge amount about 5% of everything. The reason being that the stuff that makes up you and me is but flotsam on a sea of what's called Dark Matter. We know from the behaviour of the galaxies - the way that the galaxies tug on one another - that there's more gravitational pull than we can account for based on what we see through our telescopes. So, this dark stuff is called Dark Matter. There seems to be no real doubt that it exists, but what it consists of at the particle level, we don't know.”

Dark matter is just one member of a set of things that The Standard Model can’t explain. “There are many unknowns. It's a bit like trying to get to the end of the rainbow. You don't know how many things are on the way, but as you try to get there, you pass a lot of things that you didn't know were there before.”

To the frustration of many particle physicists, the field is at a curious juncture: there’s clear evidence that there’s something new to explain, and yet The Standard Model is proving robust. After around 10 years of particle experiments around the world producing vast amounts of data since the discovery of the Higgs boson, no new fundamental particles have been found.

“It isn't complete, but there is nothing in our model that we have yet found which is ‘wrong’. And, in fact, that is part of the problem - that we know that this model cannot be the final answer, because there are many things about it we don't understand. So, there is something beyond The Standard Model. The answers are clearly out there somewhere, but how far we have to travel to find them, we just don't yet know.”

As for where to look, Close considers how experimentalists can approach the problem. “You can do strategic things by designing your experiments cleverly to look for certain things that theorists think might be there, but serendipity happens, so it's quite possible that discoveries come out of the blue, and that's one of the excitements of science. We don't know what it will be, but if there is something out there, we will certainly have a new way of looking at the universe. And, hopefully, that something will be the explanation of why things are as they are.”

The Big Bang in a lab: the LHC

One approach to looking for new particle species, like dark matter, is to try to produce them in a laboratory. One method to do just this is to smash well understood particles, like protons, together at extremely high speeds. By increasing the energy of these collisions, which amounts to moving closer to the conditions just after the Big Bang, when particles were phenomenally energetic, scientists hope to produce exotic, never-before-seen matter. This is what’s being done at the CERN Large Hadron Collider (LHC), a 27 km-long machine buried deep under Geneva.

However, trying to produce evasive matter like dark matter in a lab is tricky, the main reason being that it prefers to interact gravitationally. This means that, while it exerts a big force on cosmological scales, where gravity dominates, and from which its existence has been inferred, it probably interacts very weakly with the particles scientists work with in the lab.

According to Sarah Williams, who is searching for dark matter with the ATLAS experiment at the LHC, the power of the LHC is useful in this situation. “The nice thing about these LHC collisions is they happen so frequently - we're talking about colliding bunches of protons 40 million times a second - and that means that, even if we'd only have very rare or weak interactions between the Standard Model particles and dark matter, in our very enormous data sets we might be able to see interesting hints that we've produced dark matter particles in these collisions.”

While the LHC hasn’t shown hints of dark matter in the 10 years Sarah’s been working on ATLAS, there’s nevertheless hope for the future. “This information allows us to refine our future searches and gives us a better idea of where to look. Actually, one of the exciting things at the moment is that when we start taking data again next year, we've got a few more years of data-taking ahead of us, and we can use the lessons learnt so far to really design our searches to target some of the areas of possible dark matter space that could lead us to a discovery.”

Extreme precision: the magnetism of muons at g-2

Hoping to produce new particles directly isn’t the only way to look for them. Scientists at Fermilab’s g-2 experiment in the US are taking a different approach. They’re trying to better quantify a particle discovered way back in the 1930s - the muon - which is a heavy version of an electron. They’ve chosen to measure just one property of muons - how magnetic they are - but incredibly precisely. That means that at g-2, scientists are on the hunt for miniscule deviations from the Standard Model expectations.

Brendan Casey is one of the team working on the project. “This is an experiment people do every day around the world. Every time you pull a compass out of your pocket and look at which way the needle points, the needle points north. So, that's the same experiment we're doing, but we're doing it with muons.”

And last summer, they announced some intriguing findings. “When we do this experiment with electrons, the compass points north, but when we do the experiment with muons, for some reason the needle is not pointing north. And so, it's a huge mystery.”

While the result is not yet significant enough to be considered a discovery within the field, Brendan is confident in it. “There's something big out there which is turning the needle of muons, and it seems like it's just happening to muons. It's very strange... We know it's happening.” As for what could be causing it - a new force perhaps - the jury is still out. “It could be Bigfoot for all we know!”

More exciting still, recent results from the LHCb experiment at the LHC suggest something strange is going on with muons there too. In essence, they’re seeing fewer muons arising from their collisions than The Standard Model tells us we should see. Could it be two sides of the same coin? “It could easily be the same thing. It could be that there's something big out there that likes to knock muons around at Fermilab and it likes to gobble up muons at the LHC! When you see these two things, it's natural to try and put them together,” says Casey.

If, when more data are collected at LHCb and g-2, these tantalising results are bolstered, the race will be on to explain what’s causing them. Like a torch in the darkness, these results could show the way for the theorists to find an explanation. “It is a very important tool that we have for any model that people come up with.” Whatever new theory is created, whether describing dark matter or anything else, it would have to explain these anomalies in the muons. “Because we see such a large effect, it makes it this kind of bright candle that's sitting out there in the wilderness, that we have to be able to explain if we're going to be able to explain all these other things as well.”

Harnessing the power of space: IceCube

While scientists at the LHC try to create the most energetic particles possible in a lab, scientists at IceCube in Antarctica are trying to capture extremely energetic particles from far-flung corners of space. These particles, known as neutrinos, can travel vast distances uninterrupted, because, like dark matter, they interact so weakly with matter. This property, which means that they can tell scientists about exciting, distant objects in space, makes them incredibly hard to detect. Nevertheless, understanding neutrinos better might lead to some of the answers particle physicists are looking for.

Member of IceCube, Summer Blot, explains how the detector works. “IceCube is the world's largest neutrino telescope. It is actually buried into the glacier at the South Pole in Antarctica.” The neutrinos that the experiment aims to detect are those that naturally happen to travel through the glacier, on their journey to Earth from millions of light years away.

How is it that a particle detector can be built into a glacier? “We buried about 5,000 sensors down into the glacier. They start at about 1,500 metres under the surface, and then go down for another 1,000 metres. In total, these sensors instrument a cubic kilometre of ice. They then pick up the light signals that are produced when a neutrino naturally interacts in the glacier.”

Scientists decided that it was worth braving the Antarctic conditions to do this experiment for a couple of main reasons. “The benefit is that the glacier provides an incredibly large amount of matter for the neutrinos to interact with. [In addition, ] the optical properties of the ice are simply such that the light signals that neutrinos produce when they've interacted in the glacier, they can travel really, really far distances before they're absorbed in the ice itself. So, that gives us the ability with our sensors to pick up more information.”

By measuring the pattern of light formed by an incoming neutrino in their 3D array of sensors, researchers can pinpoint the direction the neutrino came from. Linking this with data from conventional telescopes, they can work out where the neutrino most likely came from. Using these techniques, scientists at IceCube have been able to verify that these cosmic neutrino sources do exist. “When we built IceCube, we thought they were out there, but we hadn't actually discovered them. At this point in time, we've detected enough of these neutrinos that it's pretty well established that they exist. What we still don't know is exactly how they're being produced or where they're being produced.”

With there still being plenty of work left to do to realise the full potential of this experiment, it’s hoped that IceCube will receive an upgrade that would see its volume increase by eight times. “This would allow us to detect even more neutrinos at even higher energies and really start to do much more precise measurements of neutrino production in these very extreme distant sources.”

Whichever route leads to a better understanding of our universe on its most fundamental scales, whether it’s through work at the LHC, g-2, IceCube or somewhere else entirely, what’s clear is that the next discovery of a fundamental particle is likely to usher in a new era of our understanding of the universe.


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