Friday Flyer Supplement - September 20, 2024

QuarkNet staff talked with three University of Notre Dame physicists about the new CMS result on the mass of the W boson. Here are their immediate reactions:

Christopher Kolda:

The 2022 measurement of the W mass by the CDF collaboration threw a wrench into our field, both because of its disagreement with previous experimental measurements, but more importantly its strong disagreement with the Standard Model prediction. Attempts to fit the CDF measurement into a global fit of the Standard Model were mostly unsuccessful, leading to hundreds of papers that sought to explain the anomaly with new physics. This new CMS measurement, with uncertainties similar to those obtained by CDF, is back in agreement with pre-2022 measurements and with the Standard Model, but about 7 sigma from the CDF value. Some theorists will take this as confirmation that the Standard Model was right all along and that the CDF measurement is mistaken in some yet unknown way. But others will investigate whether there are types of new physics effects that could shift the W mass measurement at CDF while leaving the CMS measurement unaffected. Whether this will be successful is yet to be seen!

Kevin Lannon:

In terms of the significance of the result, I would say that being able to achieve this level of precision in the measurement of a quantity like the W mass represents a real tour de force of experimental ingenuity and creative problem solving.  I don’t think anyone thought that the LHC would be able to do precision physics in a way that would be competitive with past electron-positron colliders, like LEP, let alone surpass that precision.  But here we are!

In terms of the comparisons to other results, I’m sure you’re aware of the tension between all previous precision W mass measurements, including measurements by D0 and ATLAS, and the most recent CDF results.  The CDF result was previously the most precise, but it was significantly different from the others.  That difference fueled a lot of speculation that what we were seeing was signs of new physics.  Having the CMS result land in line with past results, but with a comparable precision to the CDF one, really underscores that CDF is the outlier.  Probably that means that there is some flaw in the CDF measurement that needs to be identified and corrected before we have a consistent picture, but with results like this, you never know.  Maybe there’s something special about the CDF approach that bypasses some pitfalls that all the other analyses have fallen prey to.  That’s the great thing about science.  There are always mysteries like this, but you don’t know if the answer to the mystery is going to be a breakthrough that leads to a scientific discovery, or just finally figuring out some mistake you were making that skewed your result.

In terms of why we care so much about the W mass, I think the best answer I can give there is that the W boson, along with the Z boson and the photon, makes up the electroweak sector of physics.  The weak force, where the W and Z have their most significant influences, is a pretty odd part of the standard model.  There’s also a strong connection to the Higgs boson, which is the thing that “breaks electroweak symmetry” which is why the Z boson is so different in properties from the photon.  Taken together, the W, Z, and Higgs are a weird little corner of physics and one that we’ve only recently started understanding in recent decades.  With our increasingly precise ability to study W, Z, and Higgs bosons, we open the possibility that some small deviation from what we expect will point the way to some new physics at a much higher energy scale.  So, the more we can pin down the W mass, the more chances we have to spot something new but unexpected!

Marc Osheron:

Indeed, this is a huge result, especially given the previous CDF measurement. It's tough to condense all of it into one sentence... I'm a bit depressed that it ended up being so close to the SM prediction, since it's yet another avenue for moving past the SM that seems to be cut off. Maybe keeping it somewhat neutral about CDF /the future of our field is best:

People may well remember the Higgs Boson discovery as the most "important" measurement by the CMS collaboration, but the recent W mass measurement far outshines it in terms of difficulty, requiring enormous technical prowess and a near perfect understanding of the detector to pull off.

Unfortunately it's probably just the case that CDF "messed up" their measurement. The size of their disagreement is probably not just statistical fluctuation (though in theory it could be). I'm sure we'll see some scholarship on where they may have gone astray. The implication of the actual result is that whatever new physics is out there doesn't significantly perturb the SM, though that doesn't help us at all to figure out what it is (conversely, had CMS confirmed CDF, it would be a huge hint about what kind of processes exist outside the SM).