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.