When Cern’s giant accelerator, Large Hadron Collider (LHC), launched ten years ago, there was great hope that new particles would soon be discovered that could help us solve the deepest mysteries of physics. Dark matter, microscopic black holes and hidden dimensions were just some of the possibilities.
But apart from the spectacular discovery of the Higgs boson, the project has provided no clues as to what may be outside the standard model of particle physics, our current best theory of the microcosm.
So our new paper from the LHCb, one of the four giant LHC experiments, will probably make physicists̵
But the tension is dampened by extreme caution. The standard model has withstood every experimental test that was thrown at it since it was put together in the 1970s, to claim that we finally see something it cannot explain requires extraordinary evidence.
The standard model describes nature on the smallest scale, and consists of basic particles known as leptons (such as electrons) and quarks (which can come together to form heavier particles such as protons and neutrons) and the forces they interact with.
There are many different types of quarks, some of which are unstable and can rot into other particles. The new result is related to an experimental anomaly that was first suggested in 2014, when LHCb physicists discovered “beauty” quarks that decayed in unexpected ways.
In particular, it seems that beauty quarks decayed into leptons called “muons” less frequently than they decayed into electrons. This is strange because the muonet is essentially a carbon copy of the electron, identical in every way except that it is around 200 times heavier.
You expect beauty quarks to decay to muons as often as to electrons. The only way these decay can occur at different speeds is if someone never before seen particles became involved in the decay and overturned the weight against muons.
While the result in 2014 was exciting, it was not precise enough to draw a firm conclusion. Since then, a number of other anomalies have emerged in related processes. They have all individually been too subtle for scientists to be sure that they were real signs of new physics, but teasing they all seemed to point in a similar direction.
The big question was whether these irregularities would become stronger as more data were analyzed or melted away to nothing. In 2019, the LHCb performed the same measurement of beauty quaternary again, but with additional data taken in 2015 and 2016. But things were not much clearer than they had been five years earlier.
Today’s result doubles the existing data set by adding the sample recorded in 2017 and 2018. To avoid accidental interference, the data were analyzed “blind” – the researchers could not see the result until all the procedures used in the measurement were tested and reviewed.
Mitesh Patel, a particle physicist at Imperial College London and one of the leaders of the experiment, described the excitement he felt when the moment came to look at the result. “I actually shook,” he said, “I realized that this was probably the most exciting thing I’ve done in my 20 years in particle physics.”
When the result came up on the screen, the anomaly was still there – around 85 muon decay for every 100 electron decays, but with less uncertainty than before.
What will excite many physicists is that the uncertainty in the result is now over “three sigma” – the researchers’ way of saying that there is only about one in a thousand chances that the result is a random stream of data. Conventionally, particle physicists call all over three sigma “evidence.” However, we are still far from a confirmed “discovery” or “observation” – it will require five sigma.
Theorists have shown that it is possible to explain this discrepancy (and others) by acknowledging the existence of brand new particles that affect the ways in which quarks decay. One possibility is a basic particle called a “Z prime” – essentially a carrier of a whole new force of nature. This force would be extremely weak, which is why we have not seen any signs of it until now, and would interact with electrons and muons differently.
Another alternative is the hypothetical “leptoquark” – a particle that has the unique ability to decay to quarks and leptons at the same time, and which can be part of a larger puzzle that explains why we see the particles we make in nature.
Interpret the findings
So have we finally seen evidence of new physics? Well, maybe, maybe not. We do a lot of measurements at the LHC, so you can expect at least some of them to fall that far from the standard model. And we can never completely discount the possibility that there is some bias in our experiment that we have not accounted for properly, even if this result has been checked extraordinarily thoroughly. In the end, the image will only become clearer with more data. The LHCb is currently undergoing a major upgrade to dramatically increase the speed it can detect collisions.
Even if the anomaly persists, it will probably only be fully accepted when an independent experiment confirms the results. An exciting possibility is that we may be able to discover the new particles that are responsible for the effect that is created directly in the collisions at the LHC. Meanwhile, the Belle II experiment in Japan should be able to make similar measurements.
What could this mean for the future of basic physics? If what we see is really the precursor to some new basic particles, it will finally be the breakthrough that physicists have longed for for decades.
We will finally have seen part of the bigger picture that lies outside the standard model, which can eventually allow us to solve any number of established mysteries. These include the nature of the invisible dark matter that fills the universe or the nature of the Higgs boson. It can even help theorists unite the basic particles and forces. Or, perhaps best of all, it may point to something we have never once considered.
So, should we be excited? Yes, results like this do not come very often, the hunt is definitely on. But we should be careful and humble too; extraordinary claims require extraordinary evidence. Only time and hard work will show if we have finally seen the first glimpse of what lies beyond our current understanding of particle physics.
This article originally appeared on The Conversation. It was written by Konstantinos Alexandros Petridis from the University of Bristol, and Harry Cliff and Paula Alvarez Cartelle from the University of Cambridge.