We haven’t yet seen any direct signs of new particles at the LHC or in any other experiments. However, what has been happening over the last few years is that we’ve started to see what we call anomalies at a range of experiments.
Anomalies are measurements of some property of a particle or some way that it decays that seem to disagree with what our theories predict. One of these anomalies is a result at LHCb that’s been getting stronger and stronger for about six or seven years now.
What we’ve done is measure how often a particle called a beauty quark decays into electrons and anti-electrons versus how often it decays into muons and anti-muons. A muon is a heavy version of the electron. In our current theory of particle physics, electrons and muons are basically identical copies of each other. They have all the same properties. The only difference between them is their masses. The muon is about 200 times heavier than the electron, but otherwise, it’s the same electric charge and same interactions with all the other forces. This means you’d expect beauty quarks to decay into muons equally as often as they decay into electrons, but what we’ve found is that beauty quarks appear to be decaying into muons less often than into electrons. You cannot explain this with our current theory of particle physics unless you introduce some new force, some new fundamental particle that interacts with muons and electrons differently.
This anomaly is not yet at the position where we can say for sure that we’re seeing something new. If you did a thousand of these measurements, you would expect around one of them just to randomly end up this far away from our theoretical prediction. Because we do many experiments at the LHC, there’s a chance this result could be a statistical fluctuation, a sort of cruel quirk of the data, but we’ll see in the coming years, as we gather more data, whether this anomaly gets stronger.
This is not the only anomaly. We’ve seen them in other experiments, and they could all be pointing towards some single new ingredient of our universe that we’ve never seen before. So, we’re in this uncertain position of perhaps being on the verge of something big and exciting, but we can’t yet say for sure.