ONE in three people reading these words will do so on a device powered by electrons. To make that possible, we go to incredible lengths: generating electrons in vast power stations, stringing cables across the countryside to bear them to us, and installing sockets in the walls of every room. In short, we depend on them – which makes it a shade embarrassing that we don’t fully grasp what they are.
It’s not just that our best theories paint a strange picture of their nature, although that is true. As far as we can tell, electrons are points with precisely zero size that obey the strange rules of the quantum world. One of these particles can influence another through a spooky property called entanglement, for example. Electrons can also tunnel from one place to another without existing in between.
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But the truly inexplicable thing about the electron is that it has two heavier siblings. The universe would tick along fine without these particles, which never hang around long anyway. So why do they exist? And why are there three siblings, not four or 104?
For decades, physicists have had no answer to these questions, but at last we may be edging towards an understanding. If anomalies at particle accelerators across the globe are to be believed, there may be a hidden world buzzing beneath the surface of the electron – one that would force us to rethink all the fundamental building blocks of matter.
The electron was our first subatomic particle, discovered in 1897 when J. J. Thompson observed rays that would bend under the influence of an electric field. It turned out that those rays were electrons.
By the mid-1930s, we had found the other components of atoms too (see “Make it up, break it down”). But just as things began to look tidy, confusion descended again. In 1936, a pair of physicists in California who were studying radiation from deep space noticed a particle unlike anything they had seen before. It was negatively charged, like the electron, but its path bent less sharply in an electric field, suggesting it was heavier. It came to be called the muon, a particle like the electron in every way – only with 207 times the mass.
Physicists had four decades to scratch their heads before the plot thickened again. Starting in 1974, a series of experiments at the Stanford Linear Accelerator Center (SLAC) in California uncovered another sibling of the electron, this one 3400 times as massive and named the tau.
“It’s as if someone has put an electron through a Xerox machine and set the number of copies to three”
Today, physicists refer to the electron, the muon and the tau – collectively known as the charged leptons – as being different “flavours” of the same particle. “It is as if someone has put an electron through a Xerox machine and set the number of copies to three,” says John Ellis, a particle physicist at King’s College London.
Actually, it’s as if that mysterious photocopying has been carried out right across the standard model, our best description of the fundamental particles and forces of nature. All the fundamental particles of matter seem to come in three successively heavy generations (see diagram).
Physicists accept this picture, but that doesn’t mean they are comfortable with it. “It is an absolute puzzle,” says , a theoretical physicist at Heidelberg University in Germany. “Everything would be consistent with just one generation; why exactly the others are there is just not clear.”
It could be chance, but there are hints that it is not (see “Freaky flavours“). In any case, particle physicists are desperate to prise open the slightest cracks in the standard model. After all, we know it is not complete, because, among other things, it has nothing to say about how gravity works. Probing flavour physics might help us see further.
One way to do that probing is to test a principle called lepton universality, which roughly states that all leptons behave alike. It means, for instance, that when larger particles decay, any leptons in the fallout should be produced in equal proportions when their mass is accounted for.
Particle shrapnel
In 2014, scientists at the Large Hadron Collider’s beauty (LHCb) experiment were studying the decays of B mesons. These particles are made of two quarks and can decay in many different ways, making them a pet test bed for physicists. This time, something odd came to light: there were 25 per cent fewer muons than electrons produced, .
That sounds like a big difference, but CERN experiments create so many particle decays that we expect anomalies to crop up by chance now and then. Last year, for instance, some suspected that a bump in the data from the LHC’s two largest detectors would reveal new particles. It proved to be a quirk that faded away on closer examination.
To avoid getting over excited at such quirks, particle physicists use a statistical threshold called 5 sigma to define when a result can be classified as a discovery. A 5-sigma result signifies that the odds are only 1 in 3.5 million of a result like that cropping up by chance without any new physics. The 2014 LHCb result was around 2.5 sigma, so at first it did not garner much attention.
The following May, the LHCb picked up another odd signal. This time researchers were looking at a different particle decay that generated either a muon or a tau. Again, the tau particles were produced more often than the muons. And on 18 April this year, LHCb researchers announced a third strike: a separate measurement of B meson decays, which again showed about a 25 per cent departure from standard-model predictions. “The significance of this measurement is not extremely large, but it has become very interesting because it goes in the same direction as other similar measurements,” says of the University of Zurich, who works at LHCb.
What makes it even more interesting is that two now-defunct experiments, BaBar at SLAC and Belle at the KEK laboratory in Japan, looked at similar decays. The Heavy Flavour Averaging research group at SLAC added those results to the ones from LHCb, took an average, and arrived at a significance of 4 sigma. Tantalisingly close, but that still doesn’t count as a discovery. All the same, the potential find “would be so revolutionary, it is important we must take the time to be sure of what we are seeing”, says Ellis.
If lepton universality is broken, the most popular explanation is that an unknown particle appears fleetingly during the meson decay and interacts with the various pieces of particle shrapnel, pushing their ratios away from what would be predicted. This particle would appear and disappear in a flash – too quickly to be directly detected itself.
“It’s presumptuous to think we know all the fundamental building blocks of nature”
As to its identity, one possibility emerged in 2009, when theorist at the University of Cambridge was preoccupied with the Higgs boson, the famous manifestation of the eponymous field that gives particles mass. Gripaios was exploring the idea that the Higgs, whose existence was only confirmed in 2012, might not be a single, fundamental particle, but rather a composite made of smaller pieces. This might help solve a long-standing niggle in physics called the hierarchy problem; put simply, the observation that all the fundamental particles we know are lighter than the standard model predicts. In the course of his thinking, Gripaios worked out that if the Higgs is indeed a composite, that means it is likely that a hypothetical particle called a leptoquark must also be one.
At first blush, that seemed uninteresting. Leptoquarks have been theoretical curiosities for decades, cropping up in ambitious, unconfirmed theories that paint the three forces in the standard model as ultimately the same thing. at the Institute of Theoretical Physics in Warsaw, Poland, has called it the Nickelback of particle physics: like a band everyone has heard of but no one seems to particularly like.
The LHCb findings, however, thrust the leptoquark into the spotlight. Its properties allow it to bridge the gap between quarks and leptons, making it possible for one to turn into the other. If the leptoquark were also to fix things such that certain leptons change to quarks more readily than others, it would explain the unequal proportions observed at the LHCb. And here’s the best part: calculations Gripaios made in 2012 predicted a discrepancy of about 25 per cent.
There are alternative explanations, however, including particles carrying exotic new forces. Worse, a search for leptoquarks in the 2000s at an accelerator called HERA in Germany found zilch.
That doesn’t rule them out entirely, because HERA operated only at a certain energy. But some physicists think wondering about compositeness in particles that are not yet discovered is whimsy. “I see that as speculation on top of speculation,” says Ellis.
Others see that speculation as valuable. “Compositeness is a very appealing idea,” says , a theorist at Northwestern University in Illinois. “It is, I think, always presumptuous to believe we have identified the strictly fundamental building blocks of nature, even if all evidence we have collected so far points that way.”
De Gouvêa is alluding to the possibility that leptons could be composites too. A composite leptoquark in itself does not require them to be, but once you start down this road, you can come up with all sorts of scenarios. In one, called partial compositeness, the charged leptons we see – electrons, muons and taus – are composites. That means the electrons whizzing through our electrical wires are far more complex beasts than we’ve assumed. And the chunks they are made from might conceivably combine in other ways. “This could mean that for every lepton we currently see there exists a heavier partner, which should be discoverable,” says Gripaios.
Under that scenario the glib answer to “why are there three charged leptons?” is simply: there aren’t. That might seem unhelpful. But Gripaios thinks leptoquarks provide a fresh way to think about flavour physics and might just present novel ways to grapple with its puzzles. “It would open a completely new window through which we could probe such questions, both theoretically and experimentally,” he says.
Direct experimental confirmation of compositeness in what we now assume to be a fundamental particle will be a tall order. If the electron, for example, is a composite, it must be held together by an extremely strong, unknown force. In all experiments so far, no matter how high energy, electrons have behaved like dimensionless points. So we may have no hope of breaking them apart.
Compositeness might be marginally easier to prove in another supposedly fundamental type of lepton: the neutrino. There are already hints that these ghostly particles, which barely interact with normal matter, might come in more than three flavours.
The standard model predicts that neutrinos are massless, but we know they aren’t. In the 1990s, the Super-Kamiokande detector in Japan observed them changing from one flavour to another, a feat that requires they have a tiny mass at least, according to quantum mechanics. The most popular explanation is called the see-saw mechanism, which would entail each of the three neutrinos having a heavier partner.
Spot one of those and we would add weight to Gripaios’s ideas. But although we have reason to think heavy neutrinos might be out there, it will still be an incredible challenge to find one. If they do exist, they must be at least 45 billion times as massive as the neutrinos we are familiar with.
Before we go to that trouble, it is probably worth checking on the leptoquark itself first by trying to produce it directly. If it isn’t a composite, it’s too heavy for us to make right now, but it should be doable at a bigger, better particle collider of the future. With the mass of the composite leptoquark predicted to be under 1TeV, however, it should be possible to make that at the LHC, Gripaios says.
Even before that, it would be as well to see if those indirect sightings at the LHCb are real. Here at least, we don’t have long to wait. We could see evidence for it in the analysis of the LHC’s second run, due by spring 2018. “By that time,” says Bauer, “we will know whether we are dealing with a true sign of new physics.”
Make it up, break it down
The history of physics often involves finding that fundamental particles are nothing of the sort
1897
J. J. Thompson discovers electrons by deflecting cathode rays in an electric field. It is the first subatomic particle ever discovered
1908
Hans Geiger and Ernest Marsden find that some positively charged particles can pass through gold foil, suggesting the gold atoms are mostly empty space with a small nucleus
1917
Geiger and Marsden’s boss, Ernest Rutherford, finds that the nitrogen nucleus contains smaller particles called protons
1932
James Chadwick discovers the neutron. Now all three components of the atom – electron, proton and neutron – are unmasked
1936
Carl Anderson and Seth Neddermeyer find that particles in cosmic rays curve less sharply in an electric field than electrons. These heavier cousins of the electron are called muons
1968
Scattering experiments at SLAC reveal that protons contain smaller particles, later confirmed as quarks
1974
Martin Lewis Perl and colleagues discover the tau, an even heavier version of the electron
1980s
The standard model attains roughly its present form, with quarks and leptons as the indivisible particles of matter
2012
The Higgs boson is discovered at CERN
2014
Particles called B mesons decay to leptons in ways that appear to contradict the standard model. One explanation is that leptons, too, may contain smaller particles that we haven’t yet seen
Freaky flavours
Vanilla, chocolate or strawberry? Just as ice cream comes in many flavours, so too do fundamental particles. It’s just a shame that they leave a slightly bitter taste in the mouth.
All the basic particles of matter, leptons and quarks, come in similar versions that differ subtly, for example in their mass. Those versions are called flavours.
But flavour physics contains a labyrinth of questions that leave physicists stumped. For one, why do they exist at all? For another, why are there six flavours of quark and six flavours of lepton? What prevents quarks from turning into leptons and vice versa?
There are tantalising hints that the answers is not just chance. One arises from the fact that quarks can change flavours. Physicists use a measure called “mixing” to define how easily this happens, and if you take the square root of the ratio of the quark masses, the results are roughly proportional to their mixings. Some see that as more than a coincidence; a clue that there’s something about flavour physics that we haven’t yet tasted.
This article appeared in print under the headline “Hidden world”
