After years without particle physics making the news, recent announcements suggest a breakthrough. Could a new fundamental force also explain the mystery of the three generations of matter? Harry Cliff weighs up the case.
Most of my colleagues would probably admit, at least in private, that it’s been an anxious time to be a particle physicist. Thirteen years ago, when the world’s largest (and most expensive) scientific instrument, the Large Hadron Collider (LHC), fired up for the first time, hopes were high that we would soon discover new particles and forces that could help address some of the most profound mysteries in science.
Things got off to a spectacular start with the discovery of the long-sought Higgs boson in 2012, but momentous as its discovery was, the Higgs belongs to the well-established ‘standard model’ of particle physics, which took shape more than half a century ago in the 1960s and 70s. Now, I don’t want to do the standard model down. It is without a doubt the most successful scientific theory ever devised, describing everything we know about the fundamental building blocks that makes up the world around us with stunning precision. You could make a good case for it being the greatest intellectual achievement of humankind. But we know it can’t be the end of the story.
The standard model has no solutions for numerous thorny problems, including how matter survived annihilation during the Big Bang, or indeed why we observe the set of particles that we do. Perhaps its most glaring omission is its failure to account for a whopping 95% of universe, which astronomy tells us is dominated by enigmatic substances known as dark matter and dark energy. So, when the LHC switched on in September 2008, particle physicists like me were itching to see something altogether new, something that might show us the way to an expanded picture of the subatomic world.
Yet almost a decade later, after literally thousands of searches performed by the four big LHC experiments, nature has stubbornly refused to give up its secrets. After the discovery of the Higgs, the LHC experiments continued to verify the predictions of the standard model, while ruling out a whole host of speculative new theories that were intended to extend it into new territory.
SUGGESTED READING What Do We Know? A Quantum Perspective By Juha Saatsi Some began to talk about a crisis in particle physics. Could it be that the long quest for an ever-deeper understanding of the fundamental constituents of our universe had reached a dead end? However, amid the gathering gloom, a series of unexpected chinks of light were beginning to appear.
Once again, particle physics made headline news around the world. Major discoveries seemed to be arriving like buses.
The LHCb experiment, one of the four giant detectors that study particle collisions produced by the LHC and the experiment on which I work, was reporting a growing number of ‘anomalies’; measurements that seemed to be in tension with the predictions of the standard model. While intriguing, for a long while these deviations were too subtle for physicists to have much confidence that they were anything other than random statistical wobbles in the data. That is until the 23rd March of this year.
On that day, my colleagues at LHCb announced they had found firm evidence for exotic particles known as beauty quarks decaying in ways that the standard model can’t explain. If borne out, these results suggest the existence of a brand-new force of nature, which would make it arguably the most momentous scientific discovery of the 21st century so far. The story broke out into the mainstream media, quickly making it one of the most widely covered particle physics stories since the discovery of the Higgs in 2012.
Then, just two weeks later on the 7th April, a completely different experiment at Fermilab in the United States announced a second result that seemed to suggest that fundamental particles called muons were also experiencing the tug of a hitherto undiscovered force. Once again, particle physics made headline news around the world. Major discoveries seemed to be arriving like buses.
So, what is going on? Are we really on the brink of a breakthrough, or are we being led astray by cruel quirks of the data? To answer that question, we need to delve a little deeper into what exactly these experiments have seen.
The b in LHCb stands for ‘beauty’, the name given to one of the six fundamental particles known as ‘quarks’. The two lightest quarks, known rather unimaginatively as ‘up’ and ‘down’, make up protons and neutrons, which in turn make up the nucleus of every atom. For reasons we don’t yet understand, nature also provides us with two additional sets of copies of the up and down quarks – the charm and strange quarks, and the top and bottom quarks – which are heavier, but unstable, meaning that once produced they quickly decay and hence don’t hang around in the universe to form atoms. Rather confusingly, the bottom quark (the heaviest copy of the down quark) is also referred to as ‘beauty’. There was originally an effort to name the heaviest two quarks ‘truth’ and ‘beauty’, but physicists plumped for the more prosaic top and bottom. However, on LHCb we’d rather be known as beauty physicists than bottom physicists.
Beauty quarks are produced in huge numbers by the collisions at the LHC and are fascinating to study because their properties, particularly how they decay, can be influenced by the existence of particles that we’ve never seen before, potentially giving us indirect evidence of new phenomena that might help address some of the aforementioned big mysteries. To understand why this is the case, we first need to consider what we mean by a ‘particle’.
Despite the study of the fundamental building blocks of our world being called ‘particle physics’, particles are not actually thought to be truly fundamental objects. Instead, the modern view of particle physics describes particles as disturbances or vibrations in invisible, ever-present objects called ‘quantum fields’.
Since the standard model forces treat electrons and muons identically, this result suggests that some new force of nature, one that interacts with electrons and muons differently, is interfering in the decay and changing how often they happen.
Formally, a field is a rather abstract concept – a mathematical object which has a value at every point in space and time. However, fields are undeniably physical things. If you’ve ever held two north poles of two magnets close to each other, you’ll have felt a repulsive force pushing them apart. You are feeling a magnetic field. You can even trace out its invisible influence by wobbling the poles of the magnet around, as if you’re touching the edges of some invisible, and yet undeniably physical thing.
So, fields are far more than just abstractions. In fact, modern particle physics describes all the known particles as vibrations in corresponding quantum fields. The ‘quantum’ bit refers to the fact that there is a minimum size of vibration that the field can sustain – in other words, the quanta or particle of that field. The photon – the particle of light – is a quantised vibration in the electromagnetic field. Likewise, an electron is a quantised vibration in something called the ‘electron field’, the quarks are vibrations in the corresponding ‘quark fields’. There are 25 known quantum fields in nature (although the precise number depends on how you choose to count them), twelve of which correspond to the particles of matter, twelve more for the known quantum forces and one for the Higgs field.
The anomalies seen by LHCb generally involve a beauty quark decaying into a strange quark and two other fundamental particles called muons (the muon is a heavy, unstable version of the more familiar electron). When this happens, energy flows out of the beauty quark field, destroying the beauty quark, and into the strange quark field and the muon field, creating a strange quark and two muons. However, the energy can’t go directly from the beauty quark field into the strange quark and muon fields; instead, it must go via an intermediate state made of a complicated mixture of some of the other fields in the standard model.
Since the standard model is extremely well understood, theorists can make very precise predictions of how often this process should happen based on the already known fields. However, if there is some hitherto undiscovered quantum field – say the field of a new force of nature – it could also contribute to the intermediate step, effectively providing an alternative route for the beauty quark to decay. This can lead to a subtle change in how often this decay happens, so a precise measurement of this decay process can provide evidence for brand new quantum fields.
The LHCb result announced in March compared two very similar decays, one where a beauty quark decays into a strange quark and two muons with another where it decays into a strange quark and two electrons. Now, in the standard model, the muon and the electron are near-identical copies of each other, with the only difference being that the muon is around 200 times heavier than the electron. Because of this similarity, you’d expect these two decays to occur at almost precisely the same rate. However, my colleagues found that the muon decay was only occurring around 85% as often as the electron decay, with an error margin of about 4%.
Since the standard model forces treat electrons and muons identically, this result suggests that some new force of nature, one that interacts with electrons and muons differently, is interfering in the decay and changing how often they happen. Given that the standard model has withstood every experimental test thrown at it since the 1970s, this would be a seriously big deal.
Another reason for all the hullaballoo is that this result is over ‘3-sigma’ from the standard model – in other words three error margins away from what we’d expect – meaning that there is less than a one in a thousand chance of getting a result like this just thanks to a random statistical wobble in the data. This (admittedly totally arbitrary) 3-sigma threshold holds special significance for particle physicists, as it’s the point beyond which we declare there to be ‘evidence’ that something interesting going on. However, it is some way short of the 5-sigma required to declare that you’ve conclusively discovered something, at which point the chances of it being a random fluke alone fall below around one in 3.5 million.
The reason that particle physicists generally require these high levels of statistical significance before announcing evidence or a discovery is that we perform very large numbers of measurements in typical collider experiments, and so you’d expect, just by random chance, that a few would end up deviating from your theory by quite large amounts. So, there is still a chance that this anomaly could prove to be a random wobble in the data. However, most physicists now consider this rather unlikely for a couple of reasons. The first is that hints of this anomaly were first seen back in 2014, and as more data have been added it has grown stronger, whereas you’d expect a random fluctuation to weaken. But more importantly, it’s only one of several anomalies that have been seen in beauty quark decays. In several other cases, beauty quarks are decaying to muons less often than expected, in others, the particles produced in the decays are flying out at unexpected angles. And what is really getting theorists excited is that they’ve found that it’s possible to explain all these anomalies at once by introducing a single new force of nature. If that’s right, then it’s going to be Nobel Prizes all round and an exciting new age in our understanding of the universe.
However, lurking in the background is the spectre of experimental error. A much more mundane explanation for these anomalies could be that there is some bias in the experiment that hasn’t been noticed or properly accounted for. Now, I know first-hand how much care has been taken to ensure every possible source of experimental bias has been eliminated, but you can never be absolutely sure that you’ve caught every last effect. Ultimately, we’ll only be certain once the measurements have been repeated with more data, and hopefully eventually confirmed by an independent experiment.
When it comes to the second result from Fermilab, the picture is rather different. This second anomaly was reported by an experiment called ‘Muon g–2’, whose goal is to measure how magnetic muons are. Muons behaving a bit like little bar magnets, generating their own magnetic field, and the strength of this magnetic field is interesting for the same reason that beauty quark decays are interesting – it can be affected by new quantum fields that we have never been seen before.
The magnetism of the muon is actually a mixture of the muon’s own intrinsic magnetism combined with the effect of the other quantum fields present in the vacuum. These quantum fields hang around the muon like a cloud and contribute to its overall magnetism. Again, this magnetism can be calculated using the known fields in the standard model to a precision of about one part in three million, and if you can make an equally precise measurement and the two disagree, then this too can be evidence for the existence of new quantum fields.
The muon’s magnetism was first measured in the early 2000s by an experiment at Brookhaven near New York, coming out at over 3-sigma from what the standard model predicts. This raised great excitement in the particle physics community, but the experiment wasn’t precise enough to be confident of the effect. So, the giant magnetic ring used in the experiment was shipped across the US to Fermilab near Chicago and installed in an upgraded version of the experiment. On 7th April 2021, the upgraded Muon g–2 experiment announced a new measurement that caused the deviation to grow to over 4-sigma, now meaning there is less than a 1 in 40,000 chance of it being just down to a random statistical fluke.
Again, taken at face value, this result suggests the presence of a brand-new quantum field that is altering the muon’s magnetism from what we’d expect, hence the wave of excitement that washed around the world following the announcement. In this case, an experimental error seems highly unlikely, largely because two independent versions of the experiment conducted two decades apart got more or less the same result. However, rather interestingly, what’s contested here is actually the theoretical prediction of what the muon’s magnetism ought to be in the standard model.
If that’s right, then it’s going to be Nobel Prizes all round and an exciting new age in our understanding of the universe.
Calculating the muon’s magnetism is fiendishly difficult. You have to take into account a large number of contributions from the different fields in the standard model, which is so complicated that it took an international team of over 170 theorists running calculations on supercomputers to pull it off. However, on the very same day that the experiment released their new result, a rival team published their own theoretical prediction that is much closer to the experimental measurement. If they’re right, then there’s no anomaly to explain at all and it’s all been a big fuss about nothing.
Ultimately, theorists are going to have to slug it out to decide which of the two predictions is right before we can be confident that we really are seeing signs of something beyond the standard model. In the case of the LHCb anomalies, more data will soon be coming from an upgraded version of the experiment that will record collisions at a far higher rate, meaning that we should get a conclusive answer one way or another in the next few years.
But setting all those caveats aside for one moment, what might these anomalies be telling us if they prove to be real? Well, first off, it would mean that there are new quantum fields out there waiting to be discovered. This would represent an enormous breakthrough and could potentially be connected to some of the biggest mysteries in physics such as the nature of dark matter or the reason that there is matter in the universe. However, perhaps the most likely possibility is that these anomalies could be telling us something about the structure of the standard model itself.
In the standard model, we have this unexplained fact that the matter particles come in three generations. The first generation contains the ordinary particles that make up matter: the electron, the electron neutrino, the up quark and the down quark. The next two generations contain more massive, unstable copies of these particles like the muon and the beauty quark. We have no idea why these things exist; we just observe that they do and put them in by hand.
These new quantum fields could be part of an enlarged picture of particle physics that would reveal a deeper pattern of which the standard model is only a fragment. Explaining the LHCb anomalies in particular, has generally led theorists to suggest the existence of exotic force particles known as ‘Z primes’ and ‘leptoquarks’. Unlike the forces in the standard model, these forces would interact differently with the different generations of particles – for instance, coupling more strongly to muons than to electrons – indicating that they are likely to be connected to the reason these different generations exist in the first place. If that turns out to be right, it could provide us with an ultimate explanation of why the universe is made of the basic ingredients that it is and would lead us closer to the dream of a unified theory of physics. It’s too early to say just yet, but we could well be on the brink of an exciting new age in our understanding of nature.
You can read more about particle physics and the search for the ultimate origins of matter in the author’s book, How To Make An Apple Pie From Scratch, published on the 5th August 2021 by Picador in the UK and on the 10th August by Doubleday in the USA.
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