Everything is larger in the US: cars, homes, food portions, people. The CDF collaboration from the now defunct Tevatron collider argues that this phenomenon is rooted in fundamental physics:
The plot shows the most precise measurements of the mass of the W boson - one of the fundamental particles of the Standard Model. The lone wolf is the new CDF result. It is clear that the W mass is larger around the CDF detector than in the canton of Geneva, and the effect is significant enough to be considered as evidence. More quantitatively, the CDF result is
- 3.0 sigma above the most precise LHC measurement by the ATLAS collaboration.
- 2.4 sigma above the more recent LHC measurement by the LHCb collaboration.
- 1.7 sigma above the combined measurements from the four collaborations of the LEP collider.
All in all, the evidence that the W boson is heavier in the US than in Europe stands firm. (For the sake of the script I will not mention here that the CDF result is also 2.4 sigma larger than the other Tevatron measurement from the D0 collaboration, and 2.2 sigma larger than... the previous CDF measurement from 10 years before.)
But jokes aside, what should we make of the current confusing situation? The tension between CDF and the combination of the remaining mW measurements is whopping 4.1 sigma. What value of mW should we then use in the Standard Model fits and new physics analyses? Certainly not the CDF one, some 6.5 away from the Standard Model prediction, because that value does not take into account the input from other experiments. At the same time we cannot just ignore CDF. In the end we do not know for sure who is right and who is wrong here. While most physicists tacitly assume that CDF has made a mistake, it is also conceivable that the other experiments have been suffering from the confirmation bias. Finally, a naive combination of all the results is not a sensible option either. Indeed, at face value the Gaussian combination leads to mW = 80.410(7) GeV. This value is however not very meaningful from the statistical perspective: it's impossible to state, with 68 percent confidence, that the true value of the W mass is between 80.403 and 80.417 GeV. That range doesn't even overlap with either of the most precise measurements from CDF and ATLAS! (One should also be careful with Gaussian combinations because there can be subtle correlations between the different experimental results. Numerically, however, this should not be a big problem in the case at hand, as in the past the W mass results obtained via naive combinations were in fact very close to the more careful averages by Particle Data Group). Due to the disagreement between the experiments, our knowledge of the true value of mW is degraded, and the combination should somehow account for that.
The question of combining information from incompatible measurements is a delicate one, residing at a boundary between statistics, psychology, and arts. Contradictory results are rare in collider physics, because of a small number of experiments and a high level of scrutiny. However, they are common in other branches of physics, just to mention the neutron lifetime or the electron g-2 as recent examples. To deal with such unpleasantness, Particle Data Group developed a totally ad hoc but very useful procedure. The idea is to penalize everyone in a democratic way, assuming that all experimental errors have been underestimated. More quantitatively, one inflates the errors of all the involved results until the χ^2 per degree of freedom in the combination is equal to 1. Applying this procedure to the W mass measurements, it is necessary to inflate the errors by the factor of S=2.1, which leads mW = 80.410(15) GeV.
The inflated result make more intuitive sense, since the combined 1 sigma band overlaps with the most precise CDF measurement, and lies close enough to the error bars from other experiments. If you accept that combination, the tension with the Standard Model stands at 3 sigma. This value fairly well represents the current situation: it is large enough to warrant further interest, but not large enough to claim a discovery of new physics beyond the Standard Model.
The confusion may stay with us for long time. It will go away if CDF finds an error in their analysis, or if the future ATLAS updates shift mW significantly upwards. But the most likely scenario in my opinion is that the Europe/US divide will only grow in time. The CDF result could be eliminated from the combination when other experiments reach a significantly better precision. Unfortunately, this is unlikely to happen in the foreseeable future; new colliders and better theory calculations may be necessary to shrink the error bars well below 10 MeV. The conclusion is that particle physicists should shake hands with their nuclear colleagues and start getting used to the S-factors.
15 comments:
Thanks for the nice post! For the record, though, the PDG scale factor only works as long as the tensions are within reason, otherwise, some systematic effect, whose statistical treatment may not be warranted, is likely going to dominate. This is why in such egregious cases as the neutron life time https://pdglive.lbl.gov/DataBlock.action?node=S017T&home=BXXX005 (or, previously, the proton radius https://pdglive.lbl.gov/DataBlock.action?node=S016CR&home=BXXX005) there is usually a preamble of caveats in the PDF entries as to which data are included in the average. One could well make the case that the W-mass measurement now belongs into this category, with even the scale factor inadequate to obtain a meaningful average.
CDF tensions with other experiments are 2-3 sigma, and the methods used are similar, so one can argue that this is well within the hypothesis of underestimated errors and thus the S-factor is appropriate. I agree that for the proton radius there's been a major screwup somewhere, and thus the S factors would not be meaningful. The neutron lifetime case is more tricky. There is also a large bottle-bottle incompatibility (S factor 1.6), so the bottle measurements are not so robust either, and imo removing the beam measurements from the combination is a bit premature. My guess is that PDG did it because of theoretical arguments put forward in Czarnecki et al 1802.01804, but these arguments do not hold beyond the Standard Model. In my own fits I include the beam measurements as well and use S=2.2.
This isn't what you say in the post, though, where you present the scale factor as a remedy to deal with situations such as the beam-bottle tension for the neutron life time or the Rb-Cs tension for the electron g-2, for both of which I doubt that the scale factor captures the uncertainty in a meaningful way. For the W mass, CDF disagrees with the current PDG average at 3.6 sigma, so it may be a bit optimistic to assume that whatever causes this tension can be described by a scale factor.
They claim unprecedented precision, but they're also far away from other results. Could the first property be responsible for the second? Is there any black magic in the statistical distillation they employ?
Nice! Note that "the previous CDF measurement from 10 years before" was updated, according to their paper published in Science: "Upon incorporating the improved understanding of PDFs
and track reconstruction, our previous measurement is increased by 13.5 MeV to 80,400.5 MeV;
the consistency of the latter with the new measurement is at the percent probability level." (likely you already took it into account). Is there a CMS measurement of MW on the way?
CMS certainly has been working on it for many years, but I have no idea when the analysis will be ready. I can only guess that the current situation may provide a good motivation to publish their mW even if their precision is worse than the one in ATLAS.
Is there something to be said for incorporating a global electroweak fit that doesn't include any of the mW measurements as as one additional low uncertainty "indirect measurement" in computing a global average value, rather than using it as a set of goalposts with which to judge the other experimental measurements?
Hey, but hold on ... If "most physicists tacitly assume that CDF has made a mistake" why there are so many phenomenological papers appearing trying to explain the result with new physics?
Several reasons: tongue-in-cheek exercise, citation fishing, because we can, ...
so first find out why they disagree w the previous measurement done by cdf. then also be sure you understand the width treatment: fixed ? running ? if you use the wrong scheme this can induce apparently a shift of around 27 mev... was this taken into account ? etc. [in the discussion about newest calculations, the correct thing would be of course to now reanalyze all old data w up to date precision calculations. ....]
Apparently, systematic error from MC was underestimated, https://indico.cern.ch/event/1108518/contributions/4691380/attachments/2392473/4090175/combi_160222_EWWG.pdf
Why not just average previous W mass measurements with the Planck mass? One would get a result that is approximately as meaningful.
Off topic, but will you make a post about Microboone and Miniboone? Is the neutrino anomaly dead?
Maybe. For me it was hardly ever alive, as there are strong cosmological arguments against sterile neutrinos having large mixing with the active ones.
Surely the most important reason to doubt the CDF result is that it is in tension with measurements of taxes and egos.
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