A review of three separate experiments has turned up “remarkably similar” results, pointing to what researchers say is a strong possibility that we’ve found hits of a phenomenon that goes beyond the standard model of particle physics.
When taken together, data from experiments conducted in the US, Switzerland, and Japan, have yielded a result with 99.95 percent certainty that lepton universality – a fundamental assumption of the standard model – does not hold up.
“These studies have resulted in observations that seem to challenge lepton universality,” an international team of physicists reports.
“A confirmation of these results would point to new particles or interactions, and could have profound implications for our understanding of particle physics.”
If you’re unfamiliar with lepton universality, it’s actually a pretty simple concept – that the interactions of certain elementary particles are the same, despite those particles having different masses and lifetimes (decay rates).
The particles in question are called charged leptons, and they include electrons, muons, and the much heavier taus.
What the three experiments found is that, when considered in relation to the decay rates of electrons and muons, tau decay rates are much higher than they should be.
In other words, taus decay quicker than the standard model of particle physics predicts, based on the principle of lepton universality.
And that’s strange, because the result from one experiment could easily be a mistake, but all three experiments observed a higher-than-expected tau decay rate, despite the fact that they were all operating in different environments.
“The tau lepton is key, because the electron and the muon have been well measured. Taus are much harder because they decay very quickly,” says one of the team, Franco Sevilla from the University of California – Santa Barbara.
“Now that physicists are able to better study taus, we’re seeing that perhaps lepton universality is not satisfied as the standard model claims.”
When reviewed together, results from the LHCb experiment at CERN in Switzerland, the BaBaR detector at the SLAC National Accelerator Laboratory in California, and the Belle experiment in Japan challenged the principles of lepton universality at a level of 4 standard deviations.
That indicates a 99.95 percent certainty, the US team reports.
To be clear, that sounds like a pretty convincing case against one of the fundamental assumptions of the standard model, but this is science, so before we go dismantling anything, we’re going to need to get that result to 5 standard deviations.
That means we need more experiments for a bigger sample size, and absolute certainty that this isn’t a mistake that’s managed to appear across three continents.
We’ll also need to figure out what it could possibly mean if these results are true – but let’s not get ahead of ourselves just yet.
“We’re not sure what confirmation of these results will mean in the long term,” says Sevilla. “First, we need to make sure that they’re true, and then we’ll need ancillary experiments to determine the meaning.”
The good news is that CERN has just fast-tracked plans to build a particle collider three times larger and seven times more powerful than the LHC, so if anything is going to put this mystery to bed, something like that would be a pretty good bet.
The review has been published in Nature.