Space and Physics
PUBLISHED
Physicists have re-measured the mass of the W boson, and are happy to report it’s an almost perfect fit with theoretical models’ predictions, unlike a previous attempt. The finding solidifies part of our knowledge of the fundamental forces of the universe, allowing researchers to chase ongoing mysteries like quantum gravity without looking over their shoulders, worried one of the things thought settled is sneaking up on them.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.The W boson is one of the two particles that carry the weak nuclear force, often shortened to the “weak force”. The structure of the universe depends on very precise relationships between the strengths of the four fundamental forces. If the weak force were a little stronger or weaker, radioactive decay would work very differently, and life would likely not exist.
Models of the universe that explain forces’ strength contain predictions about the masses of the particles involved. If these turn out to be wrong, many theories built on them would topple like a house of cards. So the Compact Muon Solenoid (CMS) team at CERN have made a lot of people happy by revealing the W boson matches expectations.
Getting there, however, was not easy. The team pored over the output of more than a billion near-light-speed collisions between protons in the Large Hadron Collider. One products of these collisions is the W boson, but it decays billions of times too rapidly to study, leaving a muon and neutrino behind. Catching neutrinos is so hard we create vast detectors through which trillions pass every second and are happy to spot one a day, but thankfully muons aren’t as shy.
By taking the measurements of the muons produced in these decays, the CMS team could calculate the vital statistics of the bosons from which they came.
Out of the billions of proton collision events, recorded in 2016, the CMS team found 117 million in which a W boson resulted, based on the muon left behind. The resulting muons’ paths had been tracked using the CMS detector, from which their momentum could be calculated.
From this, the team calculated the W boson’s mass at 80360.2 ± 9.9 megaelectron volts (MeV), or 157,261 times that of the electron. The uncertainty overlaps with the Standard Model’s predictions of 80,353 ± 6 MeV. There is much in the universe the Standard Model cannot explain, and most physicists would much prefer to be focused on those things than worrying about whether the Standard Model itself is right.
However, that worry has been nagging since 2022 when an experiment produced a value for the W boson heavy enough to conflict with the Standard Model. The previous experiment, conducted using the Collider Detector at Fermilab (CDF), reported the mass at 80,433.5 ± 9.4 (157,404 electrons), a mismatch of 80 MeV, eight times the uncertainty. It was described at the time as “the biggest observed deviation in particle physics.”
“If you take the CDF measurement at face value, you would say there must be physics beyond the Standard Model,” Professor Christoph Paus of MIT said in a statement. “And of course that was the big mystery.”
Confirmation of that finding would have forced deep reconsideration of the Standard Model, hunting for an error. Now the big question is more likely to be what was wrong with the previous experiment. “It’s just a huge relief, to be honest,” said lead author Dr Kenneth Long. “This new measurement is a strong confirmation that we can trust the Standard Model.”
Long is first author, but there are 3,000 members of the CMS collaboration who contributed to the study, with a core group composed of 30 members from 10 institutions. Anyone jumping to the idea of a cover-up doesn’t know much about large collections of physicists.
Moreover, many previous, albeit less precise, attempts had been made to measure the W boson’s mass, and most returned values much closer to this one than the Fermilab result.
The W boson measurement is so hard to get right because the muons that carry some of the W boson’s mass have momentum that is affected by the W boson’s movements before its decay, as well as its mass. Consequently, the team had to model four billion simulated collisions to get a suitably accurate relationship between muon behavior and W boson mass before looking at the experimental data.
“With the combination of our really precise result and other experiments that line up with the Standard Model’s predictions, I think that most people would place their bets on the Standard Model,” Long said. “Though I do think people should continue doing this measurement. We are not done.”
The mass of the other weak force carrier, the Z boson, is known to be 91,188 MeV, with a precision of almost one part in 50,000, and fits the Standard Model. That left the W boson as arguably the Model’s weak link.
The study is published open access in Nature.
