The mass of the W boson, one of the fundamental particles within the Standard Model of particle physics, is apparently not what the Higgs boson, top quark, and the rest of the Standard Model say it should be. Such is the claim from the CDF experiment, from the long-ago-closed but not forgotten Tevatron. Analysis of their old data, carried out with extreme care, and including both more data and improved techniques, calibrations, and modeling, has led to the conclusion that the W boson mass is off by 1/10 of one percent (by about 80 MeV/c^{2} out of about 80,400 MeV/c^{2}). That may not sound like much, but it’s seven times larger than what is believed to be the accuracy of the theoretical calculation.

- New CDF Result: 80,443.5 ± 9.4 MeV/c
^{2} - SM Calculation: 80,357± 4 [inputs]± 4[theory] MeV/c
^{2}

What could cause this discrepancy of 7 standard deviations (7 “sigma”), far above the criteria for a discovery? Unfortunately we must always consider the possibility of an error. But let’s set that aside for today. (And we should expect the experiments at the Large Hadron Collider to weigh in over time with their own better measurements, not quite as good as this one but still good enough to test its plausibility.)

A shift in the W boson mass could occur through a wide variety of possible effects. If you add new fields (and their particles) to the Standard Model, the interactions between the Standard Model particles and the new fields will induce small indirect effects, including tiny shifts in the various masses. That, in turn, will cause the relation between the W boson mass, top quark mass, and Higgs boson mass to come into conflict with what the Standard Model predicts. So there are lots of possibilities. Many of these possible new particles would have been seen already at the Large Hadron Collider, or affected other experiments, and so are ruled out. But this is clearly not true in all cases, especially if one is conservative in interpreting the new result. Theorists will be busy even now trying to figure out which possibilities are still allowed.

It will be quite some time before the experimental and theoretical dust settles. The implications are not yet obvious and they depend on the degree to which we trust the details. Even if this discrepancy is real, it still might be quite a bit smaller than CDF’s result implies, due to statistical flukes or small errors. *[After all, if someone tells you they find a 7 sigma deviation from expectation, that would be statistically compatible with the truth being only a 4 or 5 sigma deviation.] *I expect many papers over the coming days and weeks trying to make sense of not only this deviation but one or more of the other ones that are hanging about (such as this one.)

Clearly this will require follow-up posts.

Note added: To give you a sense of just how difficult this measurement is, please see this discussion by someone who knows much more about the nitty-gritty than a theorist like me ever could.