Tuesday, May 16, 2017

Recent Experimental Measurements Of Standard Model Constants

W Boson Mass At The LHC (ATLAS) and Tevatron (CDF)
The result of the CDF analysis is MW = 80 387±12(stat.)±15(syst.) MeV; the result of the ATLAS analysis is MW = 80 370 ± 7(stat.) ± 11(exp.syst.) ± 14(mod.syst.) MeV. The total uncertainty is 19 MeV in both analyses.
From here.

My comments: The lighter ATLAS result is closer to a result expected from a global electroweak fit. The Tevatron based results will never get better because that experiment has concluded. The LHC based results will almost surely get better before the experiment concludes, if for no other reason, because the statistical error will get a bit smaller over time and because independent results from ATLAS and CMS can be combined reducing a little systemic error as well.

Heavy Quark Masses And The Strong Coupling Constant From HERA

New results on the measurements of the hadronic final state in neutral-current deep-inelastic scattering at HERA are presented. A combination of reduced charm and beauty cross sections is presented and the masses of the heavy quarks are determined to mc=1290(+7853)MeV and mb=4049(+138118)MeV. The measurement of the production of prompt photons accompanied by a jet provides a precise test of QCD predictions. Measurements of jet production cross sections are presented and compared for the first time to next-to-next-to-leading order predictions (NNLO). The strong coupling constant is determined from inclusive jet and dijet production cross sections using NNLO predictions to αs(mZ)=0.1157(6)exp(+3126)th.

My comments: The charm quark mass is a bit high, (also here and here) and bottom quark mass is a bit on the low side, as is the strong force coupling constant. But, all of the measurements have significant margins of error, so the differences aren't very meaningful.

CKM Matrix Elements Involving Bottom Quarks At Belle II

Semileptonic B meson decays involving low-mass charged leptons e or μ are expected to be free of non-Standard Model contributions and therefore play a critical role in determinations of |Vub|and |Vcb|. Of all the CKM matrix parameters, |Vub| is the least precisely measured and in most need of additional studies in order to better constrain the apex of the Unitarity Triangle. We focus on exclusive reconstruction of charmless semileptonic B meson decay Bπν, and present prospects and estimates for measuring |Vub| at Belle II with the full planned dataset of 50 ab1 of integrated luminosity.

My comments: These two element are hard to measure because Vtb = 0.99914 +/- 0.00005 is dominant, and Vub = 0.00355 ± 0.00015, in particular, is very small. The square of Vub which represents the probability is 1.26 per 100,000. The paper discusses prospects for improvements in accuracy in future measurements of this CKM matrix elements and does not actually contain any measurements.

UPDATE May 18, 2017: A survey of recent measurements of four CKM matrix elements is found here. END UPDATE.

Theoretically Expected Higgs Boson Diphoton Decays

Physicists have replicated, in a more general matter than a previous result, the conclusion that many of the terms that go into calculating the likelihood that a Higgs boson will experience a diphoton decay cancel out. In particular, they show "the cancellation of all terms among the Feynman diagrams which are to be integrated to give divergences higher than logarithmic." This conclusion which requires 30 dense pages of calculations implies that calculating the likelihood of this decay is much easier than would be naively expected.

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