Using data from over one billion proton-colliding events collected at CERN’s Large Hadron Collider (LHC), physicists have measured the mass of the W boson with record accuracy. The value matches the Standard Model’s prediction, giving the researchers confidence that no unexpected force is hiding in the measurement.
CMS candidate collision event for a W boson decaying into a muon (red line) and a neutrino that escapes detection (pink arrow). Image credit: CMS / CERN.
First discovered in 1983, the W boson is one of two elementary particles that embody the weak force, which is one of the four fundamental forces of nature.
The weak force enables certain particles to change identities, such as from protons to neutrons and vice versa. This morphing is what drives radioactive decay, as well as nuclear fusion, which powers the Sun.
Catching a W boson is nearly impossible, as it decays almost immediately into two types of particles, one of which, a neutrino, is so elusive that it cannot be detected.
Physicists are left to measure the other particle, known as a muon, and model how it might add up to the total mass of its parent, the W boson.
In the new study, the physicists used the Compact Muon Solenoid (CMS) experiment, a particle detector at the LHC that precisely tracks muons and other particles produced in the aftermath of proton collisions.
From billions of proton-proton collisions, they identified 100 million events that produced a W boson decaying to a muon and a neutrino.
For each of these events, they carried out detailed analyses to narrow in on a precise mass measurement.
In the end, they determined that the W boson has a mass of 80360.2 ± 9.9 megaelectron volts (MeV).
This new mass is in line with predictions of the Standard Model, which is physicists’ best rulebook for describing the fundamental particles and forces of nature.
The precision of the new measurement is on par with a previous measurement made in 2022 by the Collider Detector at Fermilab (CDF).
That measurement took physicists by surprise, as it was significantly heavier than what the Standard Model predicted, and therefore raised the possibility of new physics, such as particles and forces that have yet to be discovered.
Because the new CMS measurement is just as precise as the CDF result and agrees with the Standard Model along with a number of other experiments, it is more likely that physicists are on solid ground in terms of how they understand the W boson.
“It’s just a huge relief, to be honest,” said Dr. Kenneth Long, a physicist at MIT.
“This new measurement is a strong confirmation that we can trust the Standard Model.”
The team’s work was published this month in the journal Nature.
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CMS Collaboration. 2026. High-precision measurement of the W boson mass with the CMS experiment. Nature 652, 321-327; doi: 10.1038/s41586-026-10168-5
