particle physics Inside

Bubbles of excitement over new experimental results that challenge the status quo are common in particle physics. Very much more rarely do such results survive the ensuing scrutiny and cross-checks and lead to a change in our basic understanding. The status quo here is the so-called `Standard Model’; a mighty beast of a theory describing the fundamental building blocks of the universe that has withstood every test thrown at it in the half century since it was established.

For all its success, the Standard Model is known to be incomplete; for example it offers no explanation for the mysterious Dark Matter and Dark Energy that are known to exist from astronomy. It is widely expected that solutions to the Standard Model’s limitations will emerge first through observations of subtle deviations of experimental measurements from theoretical predictions, indicating the presence of new, previously unknown particles or forces.

In the past couple of months, news of two experimental results that may indicate new physics has hit the mainstream media. Both are follow-up measurements that cross-check previous apparent anomalies. Both are associated with charged leptons, a class of fundamental particles that includes the electron and its heavier partner, the muon.

The first new result comes from LHCb, an experiment at the CERN Large Hadron Collider that specialises in studying the decays of short-lived heavy beauty quarks. The Standard Model predicts that beauty quarks should decay in ways that produce either muons or electrons in basically equal measure, whereas the LHCb measurements indicate deviations from this principle. The second result comes from the ‘Muon g-2’ experiment at Fermilab, Chicago, which measures the strength of the muon’s internal magnetism, a quantity that reflects the muon’s interaction with every possible force and particle - known or unknown. At the astonishing level of precision achieved, there again seem to be deviations from the Standard Model prediction.

If the Standard Model predictions are correct, the LHCb and Muon g-2 results would require freak statistical fluctuations that are expected to occur only once in about 1000 and 40000 experiments, respectively. Impressive though these numbers sound, they are still well short of the one in 3.5 million fluke probability that is conventionally required to claim a discovery.

Much more data are still to come from both LHCb and Muon g-2 and many other experiments worldwide are also looking for related anomalies. In Birmingham, we are making leading contributions to this programme through our work on LHCb and other CERN experiments (ATLAS and NA62). If the anomalies turn out to be confirmed, bulk producing and studying any associated new particles may not be possible until new colliders are built - and that is still decades away. Timescales are long in fundamental science!

If I were a betting man, my money would be on the Standard Model holding firm for the time being, but I might by now be tempted to hedge my bets if only because the two recent results might just possibly be connected as different manifestations of the same new phenomenon. Perhaps the hashtag adopted by physicists discussing the new results on social media expresses the situation best … #cautiouslyexcited.