CERN’s Ordinary-Looking W Boson Shatters Hopes for New Physics

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 survives for now,” Josh Bendavid of the Massachusetts Institute of Technology told a packed seminar room at CERN, the European Organization for Nuclear Research, a particle physics laboratory near Geneva, Switzerland, on September 17. He presented 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 of Duke University in North Carolina, who led the scientific collaboration that produced the Tevatron results, says that while it’s great to be able to measure the W boson’s mass again, it’s difficult to compare results because the LHC and the Tevatron collider have different ways of producing particles.

“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.”