Friday, November 1, 2013

Refining Measurements Of Fundamental Physical Constants

A couple of new ultra-precise experimental measurements of Standard Model physical constants this released this week reduce the tensions between predictions made by the Standard Model that are calculated using these constants and the experimentally measured values of the predicted phenomena.

Tau Mean Lifetime

A new measurement of the mean lifetime of the tau charged lepton from Belle tweaks it down a bit from 290.6 +/- 1.0 * 10^-15 seconds (based mostly on the LEP experiment) to 290.17+/-0.53 stat +/- 0.33 sys * 10^-15 seconds (the combined one sigma MOE is about 0.62).  This downward adjustment helps resolve what had been a 2.6 sigma tension between the LEP data on tau decays of a particular type, and Standard Model predictions for those tau decays.

The study also measured (for the first time the measurement has been attempted) a 0.7% asymmetry between tau and anti-tau lifetimes, which are identical in the Standard Model.  This result is within 0.3 sigma of zero with the uncertainty being almost entirely statistical rather than systemic.  Thus, is confirms the Standard Model expectation.

W Boson Mass

A final report from the now closed Tevatron's D0 experiment measured the W boson mass to a precision matching that of the current world average measurement (not bad for an "obsolete" collider).  Thus result for the final measurement was:

MW = 80.367 ± 0.013 (stat) ± 0.022 (syst) GeV = 80.367 ± 0.026 GeV. When combined with our earlier measurement on 1 fb1 of data, we obtain MW = 80.375 ± 0.023 GeV

The new result is within about half of a standard deviation from the old world average measurement.  The new result closes about half of the difference between the independently measured W boson mass and the 80.362 GeV value preferred by a global fit of the W boson, top quark  and Higgs boson masses (125.7 +/- 0.4 GeV) given the relationship of these three masses to each other in the Standard Model.  The global fit expectation is less than one standard deviation from the new measurement, again confirming the Standard Model.

Footnote On Direct Dark Matter Detection

A Snowmass working group released a white paper on direct WIMP dark matter detection which is obsolete on day one for its omission of the LUX results that are more powerful than all of the previous experimental measurements to date.

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