Monday, May 11, 2020

W Boson Polarizations At The LHC Again Confirm The Standard Model

The nuclear weak force is not indifferent to parity (it interacts with left parity particles, but not right parity particles), so it isn't surprising that the W boson polarizations aren't left-right balanced either.

The Standard Model theoretical prediction for W boson polarizations produced in the experimental setup tested involving top quark decays, for which the larger quantities have about one percent relative accuracy (about the same as the accuracy of the QCD coupling constant measurement) is that:
Theoretical calculations at next-to-next to-leading order (NNLO) in perturbative quantum chromodynamics (QCD) predict the fractions to be F0 = 0.687 ± 0.005, FL = 0.311 ± 0.005, and FR = 0.0017 ± 0.0001, assuming a top quark mass of 172.8 ± 1.3 GeV.
The experimental results are consistent at a better than half a standard deviation level for the two quantities actually measured and about 1.3 standard deviations for a third inferred quantity. Treating the three quantities as independent, which they are not, a Chi-square statistic would actually be a better tool to compare the quantities but would still show a very good fit. The experimental results are as follows:
The results are quoted as fractions of W bosons with longitudinal (F0), left-handed (FL), or right-handed (FR) polarizations. The resulting combined measurements of the polarization fractions are F0= 0.693 ± 0.014 and FL= 0.315 ± 0.011. The fraction FR is calculated from the unitarity constraint to be FR=0.008 ± 0.007. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for F0 (FL) with respect to the most precise single measurement. A limit on anomalous right-handed vector (VR), and left- and right-handed tensor (gL,gR) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [0.11, 0.16] for VR, [0.08, 0.05] for gL, and [0.04, 0.02] for gR, at 95% confidence level. 
The preprint is here.

A combined result so close to the expected value suggests that some of the systemic errors relevant to this measurement are probably overestimated somewhat. It also seriously limits the parameter space of any beyond the Standard Model physics that could be consistent with experimentally observed outcomes. Constraints like these mean that any deviations from the Standard Model must be very subtle if they exist at all.

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