The mediating particles for the four fundamental forces form the basis of our physical standard model. But one of them – the W boson of the weak nuclear force – appeared to be more massive than it should be, according to a measurement from 2022. Physicists have now re-determined the mass of this boson using a good 100 million proton collisions in the LHC particle accelerator. The result is just as precise as the measurement made in the USA in 2022 – but provides a different mass for the W boson. Accordingly, this particle with 80,360.2 ± 9.9 megaelectron volts (MeV) fits the predictions of the Standard Model, as the team reports.
The weak nuclear force is one of the four fundamental forces. It always works when atoms decay or fuse with one another – for example in radioactive beta decay or the fusion of hydrogen nuclei in the sun. The weak interaction is mediated by two carrier particles – the W and the Z boson. The W boson, which was only discovered in 1983, is also one of the cornerstones of the Standard Model of particle physics. Because its mass and interaction are also linked to the basic electromagnetic force and to the masses of the Higgs boson and the top quark. It is therefore important to know the mass of the W boson as accurately as possible. Previous measurements had shown values that appeared to fit the predictions of the Standard Model.
How heavy is the W boson?
In 2022, however, this changed: physicists from the CDF collaboration in the USA determined the mass of the W boson more precisely than ever before using data from the Tevatron particle accelerator – but came up with surprisingly high values. With a mass of 80,433.5 megaelectronvolts ± 9.4, the W boson was significantly heavier than expected according to theory. “The strong discrepancy between this result and the Standard Model and previous measurements represents a puzzle in particle physics,” explain the physicists of the CMS collaboration at the CERN research center near Geneva. “If you consider the CDF measurement results to be correct, you have to assume that there is physics beyond the Standard Model,” adds co-author Christoph Paus from the Massachusetts Institute of Technology (MIT). The physicists have now checked whether this is really the case using data from the Large Hadron Collider (LHC) at CERN – the largest particle accelerator in the world.
For their analysis, the physicists in the CMS collaboration examined data from more than a billion proton collisions in the LHC for events in which a W boson was created and then decayed into a neutrino and a muon. “The W boson only exists for a tiny moment – only about 10-24 Seconds before it decays into the two particles,” explains co-author Kenneth Long from MIT. While the neutrino cannot be measured with the detectors, the muon reacts to the magnetic fields in the LHC’s CMS experiment and can be captured. In the data, the team identified around 117 million proton collisions in which such a muon was created and measured. The mass of the W boson can be determined from the momentum and the mass of the muon. To possible To rule out disruptive effects, the physicists also used simulations to reconstruct all potential influences and compare them with the data.
Standard model confirmed, 2022 measurement not
The analyzes resulted in a mass for the W boson that is 80,360.2 ± 9.9 megaelectron volts (MeV). “This result agrees with the predictions of the Standard Model,” state the physicists in the CMS collaboration. More importantly, however, the precision of the new measurement is just as high as that of the CDF measurement from 2022 at the Tevatron – but results in a significantly lower mass for the W boson than that one. This strengthens the assumption that the excessive mass of the W boson is a possible measurement-related outlier, as the physicists explain: “The combination of our very precise results with those of previous experiments, which also correspond to the predictions of the Standard Model, confirms this,” says Long. “Our new measurement reinforces our belief that we can trust the standard model – which is honestly a huge relief.”
According to the physicists, their measurement validates common assumptions about the W boson and the weak nuclear force. This is an important confirmation of physical theories. Nevertheless, Long and his colleagues believe it is important to continue research and measurements. “We’re not done with it yet. We want to add more data and further refine our analysis techniques,” says Paus. “Then we can say with greater certainty whether we have really understood this fundamental building block of our physical worldview.”
Source: The CMS Collaboration, Nature, doi: 10.1038/s41586-026-10168-5