When the gargantuan of Cern accelerates, the Large Hadron Collider (LHC), fired a decade ago, hopes to discover new grains that could help us unravel the deepest mysteries in physics. Dark subject, microscopic black holes and hidden dimensions were just some of the possibilities.
But other than the amazing discovery of the Higgs boson, the project failed to give any idea of what might be beyond the conventional model of grains physics, the best theory of which is we currently have about the micro-cosmos.
So our new paper from LHCb, one of the big four LHC tests, tends to put down physics hearts beating just a little faster. After examining trillions of accidents that have taken place over the last decade, we may be seeing evidence of something completely new – perhaps carrying a new force of nature.
But the excitement is ruined by great care. The standard model has withstood all the experimental tests thrown at it since it was collected in the 1970s, so to claim that we are finally seeing something that cannot be explained requires remarkable evidence.
The standard model describes the nature of the smallest number of scales, including basic particles called leptons (such as electrons) and quarries (which come together to form heavier particles such as proteins and neutrons ) and the forces with which they interact.
There are many types of quarries, some of which are unstable and may go down to other fragments. The new result is related to experimental anomalies that were first introduced in 2014, when LHCb physicists saw “beauty” quarries decay in unexpected ways.
In particular, beauty quarries appeared to rot into leptons called “muons” as often as they decomposed into electricity. This is strange because the muon is actually a carbon copy of the electron, identical in every way except that it is about 200 times heavier.
You would expect beauty quarries to rot into pigs just as often as they do on electricity. The only way these decay could occur at different stages is if some previously unseen grains were involved in the decay and sharpened the blades against muons.
While the 2014 result was interesting, it was not detailed enough to reach a strong conclusion. Since then, several other anomalies have emerged in related processes. They’ve all been too exiled for researchers to be confident that they were real signs of new physics, but interestingly, they all seemed to point to the same side.
The big question was whether these anomalies would become stronger as more data was analyzed or melted away to zero. In 2019, LHCb achieved the same measure of beauty quark decline again but with additional data introduced in 2015 and 2016. But things were not much clearer than they were five years earlier.
Today’s result duplicates the existing database by adding the sample recorded in 2017 and 2018. To prevent accidental submission, the data were analyzed “Blind” – the scientists could not see the result until all the methods used in the measurement had been tested and revised.
Mitesh Patel, a physicist at Imperial College London and one of the leaders of the experiment, described the excitement he felt when the time came to look at the result. “I was shaken,” he said, “I realized that this was probably the most exciting thing I’ve done in my 20 years in particle physics.”
When the product came up on the screen, the anomaly still existed – about 85 muons rot for every 100 electrons decaying, but with less uncertainty than before.
What will wake up many physicists is that the uncertainty of the product is now above “three sigma” – a way scientists say there is only about one in a thousand chances that flu is -more random random data. Consistently, physicists call anything more than three sigma “evidence”. However, we are still a long way from a definite “find” or “look” – which required five sigmas.
Theorists have shown that it is possible to explain this anomaly (and others) by recognizing that there are new grains that affect the ways in which the quarries decompose. One key element is the so-called “Z prime” – in fact a carrier of the force of modern nature. This force would be very weak, which is why we have not seen any signs of it so far, and we would interact with electricity and muons differently.
Another option is the hypothetical “leptoquark” – a label that has the unique ability to decompose into quarries and leptons at the same time and which could be part of a larger puzzle that explain why we see the grains we make in nature.
Explaining the conclusions
So did we finally see evidence of new physics? Well, maybe, maybe not. We do a lot of measurement at the LHC, so you would expect at least some of them to fall so far from the standard model. And we can never underestimate the possibility that there is some bias in our test that we have not properly accounted for, even though this result has been examined in great detail. Eventually, the picture will only become clearer with more data. LHCb is currently undergoing a major overhaul to significantly increase the rate at which accidents can be recorded.
Even if the anomaly persists, it may only be fully accepted once an independent test confirms the results. One exciting opportunity is that we will be able to find the new items that are responsible for the impact being directly generated in the crashes at the LHC. At the same time, the Belle II test in Japan should be able to make similar measurements.
What, then, might this mean for basic physics in the future? If what we’re seeing is really the basis of some new fundamental grains, this is the last thing physics has been craving for decades.
We finally saw part of the larger picture that is outside the standard model, which could allow us to solve any number of fixed mysteries. These include the nature of the invisible dark matter that fills the universe or the nature of the Higgs boson. It could even help theorists unify the elements and the underlying forces. Or, perhaps best of all, it could be pointing to something we never thought possible.
So, should we be excited? Yes, results like this don’t come around very often, the hunt is definitely going on. But we should also be careful and humble; remarkable applications require remarkable evidence. It is only time and hard work that will tell whether we have finally seen the first glimpse of what is beyond our current understanding of the physics of grains.
This article originally appeared on An Còmhradh. It was written by Konstantinos Alexandros Petridis of Bristol University, and Harry Cliff and Paula Alvarez Cartelle of Cambridge University.