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Muons have particle physicists in a spin as two hotly anticipated experimental results deviate from theory. Paul Jackson explains why it matters.
Physicist PAUL JACKSON explains why whispers about the Standard Model’s fidelity are gaining volume.
We find ourselves on the cusp of what could be a turning point in fundamental physics. Within weeks of each other, the Muon g-2 experiment at Fermilab in the US and the LHCB experiment at CERN, in Switzerland, reported eagerly anticipated results, which have particle physicists wobbling with excitement.
Both experiments are testing the Standard Model of Particle Physics in different but complementary ways. The results indicate that it’s time to strap ourselves in for a bumpy ride along what might become an increasingly broken Standard Model highway.
For starters, what is the “Standard Model”? Confusingly, it’s neither standard nor a model, but in fact an amazing theory: a set of mathematical laws that describe how the universe operates, specifically the fundamental interactions between elementary particles and the forces that bind them. The theory, beautiful as it is, has some missing pieces – it doesn’t explain itself and is somewhat ad-hoc. It’s like the frustrating genius in school, who always gets the right answer but never explains how or why they worked it out. The Standard Model has withstood a barrage of intense scrutiny for decades, and whatever we do – collide particles, stare into space, poke around in atoms – it seems to work. Or does it?
The muon causing all this fuss is really just a fat, skittish electron: 200 times more massive with a lifetime of about one millionth of a second. Like all fundamental particles, muons have “spin”: a quantum mechanical property, similar to how we think of spin in a classical sense. Like electrons, muons also have electric charge; when they move they produce a current and subsequently a magnetic field. When placed in an external magnetic field, muons “precess” (the axis starts to tilt like a spinning top slowing down) at a rate that we can calculate and predict precisely. This value is known as the gyromagnetic ratio “g” and was initially predicted to equal 2 (in appropriate units), but quantum fluctuations cause the actual value to differ ever so slightly – by about 0.1% – from 2, giving the “Muon g-2 experiment” its name. This is known as the anomalous magnetic moment of the muon.
While a minor deviation is okay, the really big deal is in testing how significantly
experimental results agree with theoretical predictions. The result of the Fermilab experiment has been anticipated for about 20 years. An experiment named E821 – initially run at Brookhaven National Laboratory in Upton, New York, between 1997 and 2001 – created a stir when it produced a measurement of the muon’s anomalous magnetic moment that didn’t agree with the Standard Model. The physics community was curious, but treated the result with a certain scepticism: without any corroboration, and with theoretical physicists divided on how best to calculate the expected value, it was hard to get too excited. In 2013 that experiment was literally picked up and moved to Fermilab in Illinois, a journey spanning 35 days and some 5000 kilometres under extraordinary conditions. Then – after three more years of data-taking, and after the international team had checked every part of their experiment – the “unblinding” was performed using two separate values in sealed envelopes, opened simultaneously to ensure they were consistent. This provided the number to input into the calculation, yielding a breathtaking result. The Fermilab result agreed with the earlier measurement, with a slightly reduced uncertainty. The anomaly persists and (see figure opposite) the significance has grown to the extent that it’s unlikely that this is simply a chance event. There’s some heated debate in the physics community about the exact theoretical prediction (it’s a tricky calculation) but a team of more than 100 theoretical particle physicists have worked on the current estimate for many years. The result is truly amazing.
But there’s more! The LHCB experiment at the CERN Large Hadron Collider has recently published results that also disagree with the Standard Model, and also involve muons. LHCB focuses on collisions of high-energy protons to study the production and decay of particles containing beauty (or bottom) quarks. These quarks combine with the lighter quarks that make up the proton and neutron to form mesons. When a meson containing a b-quark decays, it can follow a huge variety of possible decay chains. One rare method involves the decay of the b-quark to a meson containing a strange quark and either an electron and positron, or the equivalent with the muon (see figure below). The Standard Model predicts that in this rare decay, electrons and muons should be produced equally often, which can be calculated by the theory with very high precision. But recent results show there seem to be more muons than expected! The measurement disagrees with the Standard Model with a significance of 3.1 standard deviations, or about a 1 in 400 chance of it being a fluke.
This suggests quite astounding possibilities. A fifth force? One or more new particles? The likelihood that these experiments are just “wrong” is starting to seem far-fetched, and further anomalous measurements are currently under investigation. At some point, the Standard Model could start to resemble a relationship that you know isn’t working but you patch up and cling to anyway, because moving on feels scary and a little painful. There may be a whole suite of discoveries on the horizon that can excite the next wave of scientists, with new technologies and ideas to design experiments that get to the bottom of how to revamp the Standard Model.