Hopes for new physics dashed by ordinary-looking W bosons at CERN

Possible cracks in the Standard Model of particle physics appear to be narrowing as new data from the Large Hadron Collider (LHC) at CERN contradict previous puzzling results that had physicists excited about the possibility of new and unconventional physics, but some mysteries remain.

“The Standard Model has survived for now.” Josh Ben-David “It’s a big deal,” MIT professor Jerry Brown said in a packed seminar room at CERN, the European Organization for Nuclear Research, a particle physics laboratory near Geneva, Switzerland, on September 17. Brown was presenting new data on the mass of W, a fundamental particle crucial to processes such as nuclear decay and determining the mass of the Higgs boson.

The question about the W boson’s mass began when physicists working on data from the Tevatron Collider at Fermi National Accelerator Laboratory in Illinois shocked the particle physics community in 2022. The value of the W boson’s mass was significantly different from that predicted by the Standard Model, which best describes how particles and forces in the universe interact, suggesting that physicists may have missed something.

However, in 2023, CERN researchers questioned this discrepancy after reanalysing old data obtained with the ATLAS detector at the LHC: they found that the value of the W boson’s mass again coincides with the predictions of the Standard Model, weakening expectations of any deviations from known physics.

Now, using new data from another detector at the LHC, the Compact Muon Solenoid (CMS), Bendavid and his colleagues have produced a new value for the W boson’s mass, finding a value of 80,353 million electronvolts (MeV), which matches the Standard Model with an uncertainty of 6 MeV. The small uncertainty makes this the most precise measurement produced at the LHC, Bendavid said.

Ashutosh Kotwal Researchers at Duke University in North Carolina, who led the scientific collaboration that produced the Tevatron results, say it’s great to be able to measure the W boson’s mass again, but that it’s difficult to compare the results because the LHC and the Tevatron collider have different ways of producing particles. Martin Mulders CERN researchers working in the CMS collaboration say this difference will be taken into account within the overall uncertainty.

“In fundamental terms, the beams of ATLAS and CMS are identical,” Kotwal says. “Ideally, the Tevatron will provide additional and independent data.” Unfortunately, the Tevatron closed in 2011, so it will no longer provide new data.

All this means that it’s too early to tell which W boson mass measurement is correct, and the differences still have to be explained. “Having two numbers on the table isn’t the end, it’s the beginning,” Kotwal says. “It’s time to start discussing the scientific and technical details about the procedure. The truth is out there: there is a W boson mass in the universe, and we’re all trying to find it.”

It said the model behind the CMS calculations differs from models used to obtain other results. Matthias Schott Schott, who is part of the ATLAS collaboration at CERN, says the results are consistent with results using other models, meaning they’re likely correct. “We now have a very high level of confidence,” Schott says. “All of the measurements agree very well with each other, except for one that’s slightly off, so we can say the problem is solved.”

Mulders agrees that the Fermilab group is responsible for explaining the results. “Most of my colleagues would believe that these W boson mass measurements agree with each other and with the Standard Model predictions, and that that’s probably the real answer, and that the CDF result is an outlier,” Mulders says. Schott and his ATLAS colleagues are now preparing an entirely new measurement of the W boson from the new data, which he says should help solve the problem.