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’s data to what we could expect from the Standard Model caused a curious discrepancy of about 3 standard deviations, but we needed a lot more information to make sure it was really something new. reflected in physics.
Newly released data has now brought us closer to that confidence, bringing the results to 3.1 sigma; there’s still a 1 in 1,000 chance that what we’re seeing is the result of physics being just messy, and not of a new law or particle. Read our original coverage below to learn all the details.
Original (August 31, 2018): Previous experiments with CERN’s super-sized particle destroyer, the Large Hadron Collider (LHC), pointed to something unexpected. A particle called a beauty meson disintegrated in ways that just weren’t lined up with predictions.
That means one of two things: our predictions are wrong, or the numbers are wrong. And a new approach makes it less likely that the observations are purely coincidental, making it almost enough for scientists to get excited.
A small group of physicists took the collider’s data on disintegration of beauty meson (or b meson for short) and explored what could happen if they exchanged one assumption regarding its decay for another that assumed interactions were still taking place after they were transformed.
The results were more than a little surprising. The alternative approach doubles the assumption that something really strange is going on.
In physics, deviations are usually considered good things. Great stuff. Unexpected numbers may be the window to a whole new way of seeing physics, but physicists are conservative too – you to have to be when the fundamental laws of the universe are at stake.
So when experimental results don’t quite agree with theory, it is first assumed to be a random error in the statistical chaos of a complicated test. If a follow-up experiment shows the same thing, it’s still supposed to be “one of those things.”
But after sufficient experimentation, enough data can be collected to compare the probability of error with the probability of an interesting new discovery. If an unexpected result differs from the predicted result by at least three standard deviations, it is called a 3 sigma, and physicists are allowed to watch the results while nodding enthusiastically with their eyebrows raised. It becomes an observation.
To really grab the attention, if there’s enough data to drive that difference up to five standard deviations, the anomaly must persist: a 5-sigma event is cause to break out the champagne.
Over the years, the LHC has been used to create particles called mesons with the aim of seeing what happens in the moments after they are born.
Mesons are a kind of hadron, a bit like the proton. Only instead of being made up of three quarks in a stable formation under strong interactions, are they made of just two – a quark and an antiquark.
Even the most stable mesons disintegrate after hundredths of a second. The framework we use to describe the construction and decay of particles – the Standard Model – describes what we should see when different mesons split up.
The beauty meson is a down quark that is connected to a bottom anti-quark. When the properties of the particle are plugged into the Standard Model, b-meson decay should produce pairs of electrons and positrons, or electron-like muons and their opposites, anti-muons.
This result of an electron or muon should be 50-50. But that’s not what we see. The results show many more electron positron products than muon anti-muons.
This is worth paying attention to. But when the sum of the results is held up next to the Standard Model prediction, they show a few standard deviations. If we factor in other effects, it could be even further away – a real break from our models.
But how sure can we be that these results reflect reality and are not just part of the sound of experiments? Significance is far behind that sigma of 5, which means that there is a risk that the gap with the Standard Model is nothing interesting after all.
The standard model is a fine piece of work. It was built for decades on the foundations of the field theories first drafted by the brilliant Scottish theorist James Clerk Maxwell, and served as a map to the invisible realms of many new particles.
But it is not perfect. There are things we’ve seen in nature – from dark matter to the mass of neutrinos – that seem out of reach of the Standard Model right now.
At times like these, physicists adjust the model’s basic assumptions and see if they can better explain what we see.
“In previous calculations, it was assumed that when the meson breaks up, there are no more interactions between its products,” said physicist Danny van Dyk of the University of Zurich in 2018.
“In our latest calculations, we included the additional effect: long-range effects that we call the charm loop.”
The details of this effect are not for the amateur, and are not really Standard Model material.
Basically, it’s about complex interactions of virtual particles – particles that don’t last long enough to go anywhere, but basically arise in the fluctuations of quantum uncertainty – and an interaction between the decay products after they split up.
What’s interesting is that by explaining the disintegration of the meson by explaining this speculative charm loop, the meaning of the anomaly jumps to a compelling 6.1 sigma.
Despite the jump, it’s still not a champagne affair. More needs to be done, including stacking the observations in light of this new process.
“We will probably have enough within two or three years to confirm the existence of an anomaly with a credibility that gives us the right 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 getting closer.)
If confirmed, it would show enough flexibility in the Standard Model to push the boundaries, potentially revealing paths to new areas of physics.
It’s a tiny crack and still nothing can appear. But no one said it would be easy to solve the greatest mysteries in the universe.
The 2018 study is published in European Physical Journal CThe results for 2021 await peer review, but are available for researchers to check out on arXiv.