## Monday, June 22, 2015

### New Experiment Tends To Confirm Electron-Muon Universality

In the Standard Model, electrons and muons are assumed to have all of the same properties except mass.  This is called lepton universality.

Some recent experimental results from the LHC involving B meson decays have shown seemingly statistically significant deviations of electron-muon universality, with decays from the same source particle producing statistically significant differences in the number of decay products with electrons, from the number of decay products with muons, when they are predicted to be the same subject to adjustments based upon the relative masses of the particles (at roughly a 2.6 standard deviation level before considering look elsewhere effects).

Those results, while not very interesting in isolation, seemed notable because of two other anomalies of similar significance, one involving lepton flavor violation a somewhat similar phenomena, that could have origins in a single theory.

But, a new experimental results, looking at the decays of positively charged pions to electrons and muons respectively, confirms Standard Model prediction of muon-electron universality to within 0.1%.

The electron to muon decay product ratio from charged pions predicted in the Standard Model is (1.2352 ± 0.0002) × 10−4. The experimentally measured value is (1.2344 ± 0.0023(stat) ± 0.0019(syst)) × 10−4, which is 0.0008 × 10−4 less than the Standard Model prediction.  The combined uncertainty of the comparison of the measurement to the prediction (theoretical, statistical and systemic) is about 0.0030 × 10−4. Thus, the difference between the measured result and the predicted one is within a quarter of a standard deviation of the theoretical prediction (suggesting that the true systemic error in the experiment is probably much smaller than reported and is actually nearly zero).

This result makes it appear more likely that the apparent deviations from lepton universality seen at the LHC are actually just statistical flukes or the product of systemic error, rather than actual evidence of beyond the Standard Model physics.

The new result also makes a common source for three modest anomalies seem less likely.

andrew said...

On the other hand, a measurement of decays of neutral anti-B mesons to taus and muons respectively shows a 2.1 sigma deviation from the expectation of lepton universality.

Of course, as long as the deviations from lepton universality are not decisively in one direction or another, the likelihood that flukes like this are just experimental error is high.

andrew said...

The relative proportions of decays of tau leptons to muons and electrons respectively, closely matches the standard model prediction, thereby supporting lepton universality.