Of Particular Significance

The W Boson Isn’t Behaving

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/07/2022

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/c2 out of about 80,400 MeV/c2).  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/c2
  • SM Calculation: 80,357± 4 [inputs]± 4[theory] MeV/c2
The new measurement of the W mass and its uncertainty (bottom point) versus previous ones, and the current Standard Model prediction (grey band.)

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.

Share via:


18 Responses

  1. I am sure that the researchers did their best, but the spin of the paper which is heavily oriented towards “we broke the SM” and new physics, while downplaying this result’s inconsistency with about ten prior measurements (including LHCb 2021 not mentioned in the paper) might very well be sociology/politics driven. Arguing that it is groundbreaking rather than an outlier experimental measurement looks better to the public.

    1. Frankly, I’m finding this kind of cynicism (you’re not the first to exhibit it in these comments) both ridiculous and disturbing. We live in the age of Trump and Putin, I realize, but that doesn’t mean everything is lies and spin.

  2. Assuming the result is correct, what is the most exciting, but plausible reason? A second Higgs Boson lurking around?
    And is a similar study being done for the Z Boson?

    1. One more Higgs boson probably isn’t enough, you probably would need a set of four. But there are many other possibilities, and their plausibility is being evaluated as we speak. I’ll have more insights by next week.

      As for the Z boson, see the post that follows this one for insight.

  3. “…trust the details.” Yeah… the “devil’s” domain. In the case of the muon, it’s that the R-ratio and the Lattice QCD method predictions disagree at levels significantly greater than the uncertainties between them. Hmmm… could one of them be off the mark? Perhaps the one that doesn’t match the experimental result?

    Interesting, but in a perhaps more sociological sense. Dusting off the old equipment in a shotgun approach for data to justify supersymmetry or some “new physics” is starting to sound just a little desperate. I’m not holding my breath. 😉

    1. No, don’t hold your breath. But you’re wrong about the sociology. The CDF people who did this measurement have been working on it for many years; it is not about dusting off old equipment, it is about doing an extremely careful job as best they can, which took many years, and reporting the result honestly after they finally unblinded the data. They may have made an error, but your sociological analysis is definitely missing the big picture.

      1. I’ll take your word for it; and I did catch that the data was double-blinded. But I’m starting to think that it’s become far more difficult to give up on today’s versions of “Hesperium” when so much academic capital has been invested in citations. At least no one’s working on how to use neutrinos to send messages back in time.

        1. Yeah, I think you’re overly cynical and don’t really understand either the sociology or the psychology. Senior faculty are far more interested in their scientific legacy — i.e. discovering something — than in pulling in citations, of which they have plenty. See also the post following this one for more remarks on this point.

        2. I worked at CERN for several years (hardware… this stuff is interesting but way beyond the capabilities of my think-noodle!)
          The amount of work involved in any particle physics study today is immense and the amount of checks more so. Errors can happen, but deliberate manipulations for headline grabbing? Never. The thought of having your name on a paper that has to be retracted for unethical reasons is the stuff of nightmares for physicists.
          The FTL [“Faster Than Light”] neutrino study you alluded to is a good example. The lead scientist involved didn’t make a claim of faster than light particles (although he probably hoped it was true). He purely asked for help and ideas from the physics community as the group had exhausted their searches for errors. Unfortunately, them main stream media twisted the story somewhat and wrote some sensationalist headlines.

          1. I think others are implying that this is a search for fame; and it’s not the dedication or expertise that I question. My own experience moving through the academics was merely of immoveable investment in particular approaches to interpretation. To quote Matt, “…there are an especially large number of zealots who love supersymmetry and an equal number of zealots who hate it.” So it’s the zealotry that I wonder about.

            Humor aside, and the “FTL neutrino” case indeed working out as good science should dictate, the academic landscape has become increasingly littered with some rather convoluted explanations that keep doubters at bay. So at what point does a non-disprovable idea merge with the dogma of mere faith in something? What happens when an idea such as “dark energy” is awarded a Nobel, and then swarms of other researchers cite that work in their own? It’s not that these ideas are necessarily wrong; rather, it’s that it’s become almost impossible to back-track when they aren’t right.

            1. I profoundly disagree that back-tracking is almost impossible. These ideas *are* disprovable, and that’s the whole point. The particular question of the W mass will be checked, independently, by researchers at the LHC. The implications of the W mass, if indeed it is discrepant, will eventually show up in other measurements: you can’t shift the W mass without there being new fields and particles, and we *will*, eventually, find them. The same goes for “dark energy”, whose implications show up in several places; it is disprovable, and disproving it is exactly what Subir Sarkar is trying to do (though in my personal opinion he is likely to fail, and thereby provide, in the long run, stronger evidence in its favor.) Science involves a web of interelated facts, and you can’t just add convoluted ideas in to the mix unless they can be integrated in with all the others. If indeed these were “non-disprovable” ideas, then indeed it would be a matter of faith; but the whole point of all the blog posts on astronomy that I’ve been writing this year is that science is never based on non-disprovable ideas. Sometimes disproving them takes a while, but there is always a path to doing so. Maybe you can’t always see that path yourself, but professionals spend a lot of time discussing, designing and pursuing those paths.

  4. Assuming that the data are correctly analyzed. will exchange of very high mass supersymmetric particles help(say more than 5-10 TeV)?

  5. Not a physicist, but after all the disappointments over the last 20 years with seemingly exciting BSM results, I don’t like that plot. Result looks consistent with many experiments but seems troubling that the two others with the tightest error bars are in good agreement but quite inconsistent with this one. Either they had some significant error or this one did, I would think.

    1. One has to be aware that biases can creep in on all sides. There is at least one measurement here that you should not trust, I agree. But which one is it?

Leave a Reply


Buy The Book

A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC


The particle physics community is mourning the passing of Peter Higgs, the influential theoretical physicist and 2013 Nobel Prize laureate. Higgs actually wrote very few

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/12/2024

I recently pointed out that there are unfamiliar types of standing waves that violate the rules of the standing waves that we most often encounter

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 03/25/2024