Update (March 24, 2021): The Lemon Collider (LHCb) experiment of great beauty still insists that there is a flaw in our best model of particle physics.
As explained below, previous results comparing the collider data with what we might expect from the standard model generated a curious discrepancy around 3 standard deviations, but we needed a lot more information to be sure it really reflected some new thing in physics.
Recently published data have brought us closer to this confidence, placing the results at 3.1 sigma; there is still the possibility of 1 in 1,000 that what we are seeing is the result of physics being only disordered and not of a new law or particle. Read our original coverage below for full details.
Original (August 31, 2018): Past experiments with CERN’s large particle crusher, the Large Hadron Collider (LHC), hinted at something unexpected. A particle called a beauty meson decomposed in ways that simply did not align with the predictions.
This means one of two things: our predictions are incorrect or the numbers are out. And a new approach makes observations less likely to be a mere coincidence, so it’s almost enough for scientists to start getting excited.
A small group of physicists took the collider’s data on the disintegration of the beauty inn (or meson b to summarize) and investigated what might happen if they changed one assumption about its decay to another that assumed the interactions were still they occurred after they were transformed.
The results were more than a little surprising. The alternative approach is duplicated at the sight of something strange really happening.
In physics, anomalies are often seen as good things. Fantastic things. Unexpected numbers could be the window to a new way of looking at physics, but physicists are also conservative: you have be when the fundamental laws of the Universe are at stake.
Therefore, when the experimental results do not coincide much with the theory, it is first presumed to be a random error in the statistical chaos of a complicated test. If a follow-up experiment shows the same, it is still presumed to be “one of those things.”
But after enough experiments, enough data can be collected to compare the possibilities of errors with the probability of an interesting new discovery. If an unexpected result differs from the result predicted by at least three standard deviations, it is called 3 sigma and physicists can see the results as they enthusiastically nod with their eyebrows raised. It becomes an observation.
To really attract attention, the anomaly should persist when there is enough data to move this difference to five standard deviations: a 5 sigma event is the reason for the champagne breaking.
Over the years, the LHC has been used to create particles called mesons, for the purpose of seeing what happens in the moments after birth.
Mesons are a type of hadron, similar to the proton. Just instead of consisting of three quarks in a stable formation under strong interactions, they are made up of only two: a quark and an antiquark.
Even the most stable inns fall apart after hundredths of a second. The framework we use to describe the construction and disintegration of particles (the standard model) describes what we should see when different mesons split.
The beauty inn is a low quark connected to a lower anti-quark. When the properties of the particle are connected to the standard model, the decay of meson b must produce pairs of electrons and positrons, or electrons similar to electrons and their opposites, anti-muons.
This result of electrons or muons should be 50 to 50. But that is not what we are seeing. The results show much more of electron-positron products than muon-anti-muon products.
This is worth paying attention to. But when the sum of the results stays next to the prediction of the standard model, a couple of standard deviations occur. If we consider other effects, it could be even further: a real break with our models.
But what confidence can we have that these results reflect reality and are not just part of the noise of experimentation? The importance is much lower than the sigma of 5, which means that there is a risk that the difference in the standard model is nothing interesting after all.
The standard model is a good work. Built for decades on the basis of field theories first exposed by the brilliant Scottish theorist James Clerk Maxwell, it has served as a map for the invisible realms of many new particles.
But it is not perfect. There are things we have seen in nature, from dark matter to neutrino masses, that currently seem to be beyond the scope of the standard model framework.
At times like this, physicists adjust basic assumptions about the model and see if they do a better job of explaining what we are seeing.
“In previous calculations, it was assumed that when the inn disintegrates, 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 have included the additional effect: long-distance effects called charm-loop.”
The details of this effect are not for the amateur nor are they materials of the standard model.
In short, they involve complicated interactions of virtual particles (particles that do not persist long enough to go anywhere, but that arise in principle in fluctuations in quantum uncertainty) and an interaction between decay products after splitting.
What is interesting is that in explaining the breakdown of the inn through this loop of speculative charm the importance of the anomaly jumps to a compelling 6.1 sigma.
Despite the jump, it’s still not a champagne affair. More work needs to be done, which includes accumulating observations in light of this new process.
“We will probably have a sufficient amount in two or three years to confirm the existence of an anomaly with a credibility that allows us to talk about a discovery,” said Marcin Chrzaszcz of the University of Zurich in 2018. (How you know, it’s 2021 and we’re not quite there yet, but we’re getting closer and closer).
If confirmed, it would show enough flexibility in the standard model to extend its limits, possibly revealing paths to new areas of physics.
It’s a small crack and yet nothing can show up. But no one said that solving the greatest mysteries of the Universe was easy.
The 2018 study was published in European Physical Journal C; the 2021 results are pending peer review, but are available to researchers at the archive.