The W boson might be extra hefty. If so, it could hint at new physics

There’s something amiss with a mass.

A new measurement of the mass of an elementary particle, the W boson, has defied expectations. The result hints at a possible flaw in physicists’ otherwise stalwart theory of the fundamental bits and bobs of our world, known as the standard model.

That theory predicts a W boson with a mass of about 80,357 million electron volts, or MeV. But the new measured mass is larger, at 80,433.5 MeV, physicists with the Collider Detector at Fermilab, or CDF, collaboration report in the April 8 Science.

The finding could hint at new particles or other mysteries of physics yet to be discovered. “If confirmed, this would clearly mean very interesting new physics that we can explore,” says theoretical physicist Sven Heinemeyer of the Institute for Theoretical Physics in Madrid.

Still, several earlier, less precise measurements found W boson masses more closely aligned with the standard model, including one from the ATLAS experiment at the Large Hadron Collider at CERN near Geneva. So physicists are awaiting further confirmation before declaring their prized theory incorrect.

“CDF’s new result seems barely compatible with the previous ones, including its own previous result, which prompts questions,” says ATLAS physicist Maarten Boonekamp of the Institute of Research into the Fundamental Laws of the Universe at Université Paris-Saclay. 

Discovered in 1983, the W boson plays an important role in the standard model (SN: 2/5/83). The particle comes in two varieties, with either positive or negative electric charge. Together with their uncharged partner, the Z boson, the particles carry the weak nuclear force, which is responsible for certain types of radioactive decay and plays an important role in the nuclear reactions that power the sun.

Using data that CDF collected from 2002 to 2011, the team looked for W bosons produced in collisions of protons and their antimatter counterparts, antiprotons, in the now-shuttered Tevatron particle collider at Fermilab in Batavia, Ill. (SN: 9/9/11). The analysis was designed so that researchers couldn’t tell what the end result was until they were done.

The moment of the unveiling was striking, says experimental particle physicist Ashutosh Kotwal of Duke University. “When the answer popped up … we were awestruck about what we might have just learned.”

With a precision of 0.01 percent, the new W boson mass measurement is about twice as precise as the previous record. “This is a very special measurement; this is a true legacy,” says experimental particle physicist Rafael Coelho Lopes de Sá of the University of Massachusetts Amherst, who worked on measuring the W boson mass for another Tevatron experiment. “The level of dedication and care and detail … is amazing.”

The new measurement disagrees with the standard model expectation by 7 sigma, a measure of the statistical significance of a result. That’s well above the 5 sigma that physicists usually require to claim a discovery.

Still, “before getting too excited,” says ATLAS physicist Guillaume Unal of CERN, “I would like to see an independent measurement that confirms the CDF measurement.” In addition to the ATLAS measurement, described in 2018 in the European Physical Journal C, another measurement of the W boson’s mass from the CERN experiment LHCb was also in line with the standard model prediction, researchers reported in the January Journal of High Energy Physics.

“The W boson mass is notoriously difficult to measure,” says LHCb physicist Mika Vesterinen of the University of Warwick in Coventry, England. That explains why it took CDF so long to wrap up this analysis, published more than 10 years after the experiment ended.

Hopefully, scientists won’t have to wait that long for another measurement. The ATLAS and LHCb collaborations are already working on improved W boson mass analyses. CMS, another experiment at CERN, could also size up the particle.

If the new measurement holds up, it’s not yet clear what secrets of physics might be at play. New particles — such as those predicted by the theory of supersymmetry, which posits that each known particle has a heavier partner — could help shift the W boson mass upward (SN: 9/6/16). Intriguingly, Heinemeyer points out, those same particles might also help explain another recent physics mystery — the magnetic gyrations of muons reported by the Muon g−2 experiment (SN: 4/7/21).

Whatever physicists uncover, they’ll gain a new grasp on the particulars of this crucial particle, says theoretical physicist Nathaniel Craig of the University of California, Santa Barbara. “At the end of the day, the added energy and attention devoted to the W mass measurement … will be an immensely positive thing.”