On 7 April, researchers announced results from the now-defunct Tevatron collider in Illinois finding that the mass of the W boson is higher than the standard model of particle physics predicts – a shockingly solid measurement that could demand new physics. Ideas to explain it have already begun flowing in earnest.
The W boson is one of the particles that carries the weak nuclear force in the same way that a photon carries electromagnetic force. It is also important to interactions between neutrinos and other subatomic particles. Before the Tevatron measurement, all of the modern observations of its mass clustered around 80.379 gigaelectronvolts.
The new result puts it at 80.4335 gigaelectronvolts, which is different from the previously observed value with a statistical significance of 5 sigma – meaning that there is about a 1 in 3.5 million chance of finding this new value by chance if the old one is actually correct. A decade of data analysis by the Tevatron team has made it apparently one of the most secure particle physics measurements in modern memory.
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The W boson mass anomaly indicates that something about the standard model is wrong or incomplete. This is shocking in one sense because the standard model has been extraordinarily accurate in its predictions thus far, but it isn’t necessarily surprising in that we already knew it was incomplete – the model contains no explanation of dark matter, the prevalence of matter of antimatter in the known universe or even gravity.
Since the Tevatron results were announced, particle physicists have produced numerous papers explaining how the standard model could be adapted or expanded to account for the higher W boson mass. “What we are finding is that it is very easy to accommodate this anomaly – it’s almost a bit surprising that this is so easy to do,” says at the University of Padua in Italy. “In the past, with particle anomalies, it has been far more difficult to accommodate them.”
Many of those explanations involve a strange or additional Higgs boson, the fundamental particle coupled to the Higgs field, which provides other particles – including the W boson – with mass. “The most obvious mechanism to justify a larger mass of the W is either a non-standard Higgs or a different number of matter fields [similar to the Higgs field], or a combination of both,” says at the University of Southern Denmark. “There are a lot of variations on the theme, but these are the basic mechanisms.”
“Even such a simple change as adding a second Higgs boson is able to accommodate the value that we observed,” says at Duke University in North Carolina, who was part of the Tevatron team. “There are grand schemes and there are simple schemes, and the ones I find the most intriguing are the ones which are the simplest.”
The various additional Higgs bosons that have been suggested have a range of properties different from the known Higgs. Some , while the regular Higgs boson is neutral. Some are made up of other, smaller particles – in some models, these are known particles, such as gluons making up what is called a Glueball Higgs – and in others they are , such as so-called techniquarks making up the possible . In all of these models, the new Higgs particles come with additional fields that lend extra mass to the W boson.
Many of them also aim to solve other open questions in particle physics. For example, a popular model that accounts for the W boson’s extra heft would add a new type of particle called a that could explain another major mystery in particle physics – the muon g-2 anomaly, which arises from the fact that muons seem to rotate faster than the standard model predicts.
One set of possible new particles, called supersymmetric particles or “sparticles”, would accommodate a more massive W boson, the muon g-2 anomaly, and the mystery of why fundamental particles in general have the masses that they do, in one fell swoop.
“We expect the new particles this supersymmetry predicts to have masses that are not much heavier than the Higgs”, which is at the upper mass limit that the LHC can probe, says Peter Athron at Nanjing Normal University in China. This would explain why we haven’t spotted them yet.
Some of the could also be candidates for dark matter, potentially another reason why they haven’t been detected yet.
Combing through these many ideas to find the correct one would require time, tests and, in some cases, a new generation of particle colliders. “By no means will a new mass of the W be enough to select a new theory of nature,” says Sannino. However, before particle physicists gallivant off in search of new particles and a new theory of nature, they will need to thoroughly check the Tevatron measurement.
“It is premature to think of any new physics at this point, before we figure out why this measurement is in discrepancy with the other measurements we have,” says at CERN near Geneva, Switzerland, who worked on a previous W boson measurement using the Large Hadron Collider (LHC). “Obviously, new particles is a much cooler explanation, but all the other measurements of the W boson mass just fit together perfectly, and there’s this one that doesn’t fit.”
Schott and his colleagues at CERN have already begun the process of checking the new measurement against data from the LHC, and they should have results in the next 6 months to 2 years, he says. Depending on what those results say, the search for new physics may be on.
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Article amended on 3 May 2022
We corrected the university affiliation of Peter Athron
