Experiments that contradict the standard model accumulate

THAN STANDARD MODEL of Particle Physics – completed in 1973 – is the jewel in the crown of modern physics. It predicts the properties of elementary particles and forces with astonishing accuracy. Take, for example, the magnetic moment of the electron, a measure of how strongly a particle oscillates in a magnetic field. The standard model gives the correct answer with 14 decimals, the most accurate prediction in science.

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But the standard model is not perfect. It can not explain gravity, dark matter (mysterious things that can only be detected by its gravity), or where all the antibody in the early universe went. Physicists have spent a lot of time, effort and money performing increasingly complex experiments in an attempt to see where the standard model fails, hoping to find a clue to the theory that will replace it. But the standard model has fought back and stubbornly predicted the results of every experiment physicists have thrown themselves over.

But that may change. In a paper published last week in Scienceannounced a team of researchers from the Fermi National Accelerator Laboratory (Fermilab) in America that the mass of an elementary particle called W boson appears to be larger than the standard model predicts. The difference is small – only one hundredth of a percent – but the accuracy of the measurement exceeds all previous experiments combined. It places the odds that the result is false to only one in a trillion (“seven sigma”, in statistical language), well above the 3.5 m (five sigma) that physicists require to consider a found as robust.

Researchers at Fermilab analyzed historical data from Tevatron, a circular particle collider that was the most powerful in the world until the Large Hadron Collider (LHC) came online in 2009. Between 2002 and 2011 (when it ran for the last time), Tevatron produced approximately 4 million. W bosons in collisions between particles called quarks and their antibody counterparts, antiquarks. Using detailed recordings of the scattering paths of the menagerie of particles present in such collisions, the researchers were able to calculate the mass of W boson with unprecedented accuracy.

The finding has major consequences. That W boson is a force-bearing particle. Together with his siblings Z boson, it transmits the weak nuclear force that controls radioactive decay. Unlike other force-bearing particles, they are W and Z bosons have plenty – and a lot of it. That W boson is 90 times heavier than a hydrogen atom. That Z boson is even more massive. What really sets it apart W boson, however, is its ability to change the type – or “taste” – of other elemental particles it encounters. For example, it can convert the electron (and two of its cousins, muon and tau) into neutrinos. It can also turn quarks from one type to another – upside down, top to bottom, and the whimsically named “strange” quark into a “charming” one.

These protean forces mean that the mass of W boson is attached to the mass of several other elementary particles. It allows researchers to use W boson to calculate the mass of the other particles. This is how they predicted the mass of the top quark (discovered in 1995) and the mass of the Higgs boson (discovered in 2012) before any of the particles had been discovered. If W boson is more massive than the standard model predicts, it suggests that something else is also pulling at it – an as yet undiscovered particle or force. For particle physicists, it is an exciting perspective.

It’s not the only one. In March 2021 scientists from CERN-Europe’s particle physics laboratory – reported evidence that the bottom quark decays into electrons and muons in odd numbers, which contradicts the standard model. Only three weeks later, Fermilab announced that the muon’s magnetic moment appears to be greater than predicted by the standard model. Like the mass of W boson, the magnetic moment of the muon is partly determined by the properties of other particles. If it is larger than the standard model predicts, it also suggests an as yet undiscovered particle or force.

Provided the results are real. Exciting as they were, none of the 2021 results crossed the 5 sigma threshold (they hit 3.1 and 4.2 sigma, respectively). This means that further confirmation is needed. However, the recent Tevatron result contradicts the previous best measurement of W boson fair, made in 2017 by it LHC. It was in close accordance with the standard model that presented a puzzle.

On the other hand, the latest Tevatron result agrees well with previous estimates from the Large Electron-Positron Collider, LHC‘s predecessor. Consequently, it is the strongest evidence of physics to date that must be beyond the Standard Model. Anyone who prefers interesting mistakes to even more tedious confirmation will hope it holds true.

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This article appeared in the “Science and Technology” section of the print edition under the heading “A hint of excitement?”

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