Update (March 24, 2021): The Large Hadron Collider Beauty (LHCb) experiment still maintains that there is a flaw in our best model of particle physics.
As explained below, previous results comparing the collider data with what we would expect from the standard model caused a strange difference with about 3 standard deviations, but we needed a lot more information to trust that it was really something new reflected in physics.
Recently released data has now pushed us closer to that confidence and put the results at 3.1 sigma; there is still a 1 in every 1000 possibility that what we see is the result of the fact that physics is just messy, and not of a new law or particle. Read our original cover below to learn all the details.
Original (31 August 2018): Past experiments with CERN’s super-large particle breaker, the Large Hadron Collider (LHC), suggest something unexpected. A particle called a beauty meson was breaking down in ways that just did not line up with predictions.
This means one of two things – our predictions are wrong, or the numbers are not. And a new approach makes it less likely that the observations are purely coincidental, making it nearly enough scientists to get excited.
A small group of physicists disassembled the collision data on beauty meson (or b meson for short) and investigated what might happen if they exchanged one assumption regarding its decay with another who assumed that interactions still occur after they have transformed.
The results were more than just a little surprising. The alternative approach doubles the idea that something strange is really going on.
In physics, deviations are usually considered good things. Fantastic stuff. Unexpected numbers can be a new way of looking at physics, but physicists are also conservative – you has to be when the fundamental laws of the universe are at stake.
Thus, if experimental results do not completely agree with the theory, it is assumed to be a random blip in the statistical chaos of a complex test. If a follow-up experiment shows the same, it is still called ‘one of those things’.
But after enough experiments, sufficient data can be collected to compare the chances of errors with the probability of an interesting new discovery. If an unexpected result differs by at least three standard deviations from the predicted result, it is called a 3 sigma, and physicists may view the results while nodding enthusiastically with their eyebrows. It becomes an observation.
To really draw attention, the anomaly should continue if there is enough data to place the difference on five standard deviations: a 5 sigma event causes the champagne to erupt.
Over the years, the LHC has been used to create particles called mesons, with the goal of looking at what happens in the moments after they are born.
Mesone is a type of hadron, somewhat like the proton. Only instead of three quarks in a stable formation under strong interaction, it consists of only two – a quark and an antiquark.
Even the most stable mesons fall apart after hundreds of seconds. The framework we use to describe the construction and decay of particles – the Standard Model – describes what we need to see when different mesons tear apart.
The beauty mason is a down quark attached to a lower anti-quark. When the properties of the particle are plugged into the standard model, b-meson decay pairs must produce electrons and positrons, or electron-like moons and their opposite, anti-moons.
This electron or muon outcome should be 50-50. But that’s not what we’re seeing. Results show much more of the electron-positron products than muon anti-muons.
It’s worth paying attention to. But if the sum of the results is maintained following the prediction of the standard model, these are some standard deviations. If we consider other effects, it could be even further – a real disruption of our models.
But how confident can we be that these results reflect reality and not just be part of the noise of experimentation? The significance is less than the sigma of 5, which means that there is a risk that the gap in the standard model is not interesting at all.
The standard model is a fine piece of work. Built over decades on the foundations of the field theories first set forth by the brilliant Scottish theorist James Clerk Maxwell, it serves as a map for the invisible realms of many new particles.
But it’s not perfect. There are things we’ve seen in nature – from dark matter to the mass of neutrinos – that currently seem beyond the scope of the Standard Model.
In such moments, physicists adjust basic assumptions to the model and see if they can better explain what we see.
“In previous calculations, it was assumed that when the meson disintegrates, there are no more interactions between the products,” said physicist Danny van Dyk of the University of Zurich in 2018.
“In our latest calculations, we have included the additional effect: long-distance effects called the charm loop.”
The details of this effect are not for the amateur and are not entirely standard model material.
In short, it involves complex interactions of virtual particles – particles that do not last long enough to go anywhere, but which in principle arise in the fluctuations of quantum uncertainty – and an interaction between the decay products after they are divided.
What is interesting is that, by exposing the meson through this speculative charm loop, the meaning of the anomaly jumps to a convincing 6.1 sigma.
Despite the leap, it’s still not a champagne affair. More work needs to be done, which includes drafting the observations in light of this new process.
“We will probably have a sufficient amount within two or three years to confirm the existence of a disorder that is credible to talk about a discovery,” said Marcin Chrzaszcz of the University of Zurich in 2018. (As you know, it’s 2021 and we’re still not quite there, but we’re getting closer.)
If confirmed, it will show enough flexibility in the standard model to stretch its boundaries, possibly revealing the path to new areas of physics.
It’s a small tear and can still yield nothing. But no one said it would be easy to solve the biggest puzzles in the universe.
The 2018 study was published in European Physical Journal C; the 2021 results are awaiting peer review but are available for researchers to look at arXiv.