Monday, December 5, 2016

Another Experimental Confirmation of Lepton Universality

Yet another experimental result has quashed early hints of potential beyond the Standard Model physics.

A new study of the decays of anti-B mesons to D* mesons and leptons at Belle, confirms the Standard Model prediction of lepton universality (i.e. that tau leptons, muons and electrons have weak force decays that are identical to each other except to the extent that their different masses factor into the process and then only to the extent justified by their differing masses).

Some previous experimental results for this measurement were in tension with the Standard Model prediction for this measurement, providing one of the more notable currently outstanding experimental discrepancies with the Standard Model's predictions, first widely noted in early 2015.

Previous experiments casting doubt on lepton universality violations involved different kinds of decays than those in which the tensions with the Standard Model had been noted. A 2015 paper from LHCb involving tau decays, and a 2015 paper involving pion decays at the LHC have similarly found no violation of lepton universality. Neither did a 2014 paper looking at Z boson decays. But, this paper was the first to strongly refute evidence a lepton flavor universality violation in the same decays where they had been previously hinted at by the experimental evidence. As the paper explains:
Semileptonic B decays to τ leptons (semitauonic decays) are sensitive to new physics (NP) beyond the Standard Model (SM), such as an extended Higgs sector. A prominent candidate is the Two-Higgs-Doublet Model . . . for the decay process B¯ → D(∗) τ −ν¯τ. The decays B¯ → D(∗) τ −ν¯τ have been studied by the Belle, BaBar and LHCb experiments. Most of these studies have measured ratios of branching fractions, defined as R(D(∗) ) = B(B¯ → D(∗) τ −ν¯τ )/B(B¯ → D(∗) l−ν¯l−). The denominator is the average of l− = e−, µ− for Belle and BaBar, and l− = µ− for LHCb. The ratio cancels numerous uncertainties common to the numerator and the denominator.  
The current averages of the three experiments are R(D) = 0.397 ± 0.040 ± 0.028 and R(D∗ ) = 0.316 ± 0.016 ± 0.010, which are 1.9 and 3.3 standard deviations (σ) away from the SM predictions of R(D) = 0.299 ± 0.011 or 0.300 ± 0.008 and R(D∗ ) = 0.252±0.003, respectively. The overall discrepancy with the SM is about 4σ. These tensions have been studied in the context of various NP models. . . .
Our study includes an R(D∗) measurement independent of the previous studies, in which leptonic τ decays have been used. 
This study examined the relevant decay from 772,000,000 B-anti-B meson pair decays. Based upon this data if finds that:
Our results . . . R(D∗) = 0.270 ± 0.035(stat.) +0.028 −0.025(syst.), are consistent with the theoretical predictions of the Standard Model.
In particular, the results are consistent with the Standard Model at the 1 sigma level, greatly reducing the overall tension in the experimental data from all of the experiments combined.

The study also for the first time examined the polarization of the tau leptons produced in these decays, which is sensitive to the same kinds of beyond the Standard Model physics that would lead to lepton non-universality, and found that the polarizations were also consistent with the Standard Model prediction. 
The SM predicts Pτ (D) = 0.325 ± 0.009 and Pτ (D∗ ) = −0.497 ± 0.013. . . . In this Letter, we report the first Pτ (D∗ ) measurement in the decay B¯ → D∗ τ −ν¯τ with the τ decays τ − → π −ντ and τ − → ρ −ντ . 
Belle's experimental result, Pτ (D ∗ ) = −0.38±0.51(stat.) +0.21 −0.16 (syst.), is also consistent with the Standard Model prediction, although this is somewhat less notable because the experimental measurement isn't very precise.

So, tensions in the previous studies by the Belle, BaBar and LHCb experiments with the Standard Model predictions are increasingly looking like statistical flukes.

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