Belle II Makes A New Tau Lepton Mass Measurement

We present a measurement of the τ-lepton mass using a sample of about 175 million e+e−→τ+τ− events collected with the Belle II detector at the SuperKEKB e+e− collider at a center-of-mass energy of 10.579GeV. This sample corresponds to an integrated luminosity of 190fb^−1.

We use the kinematic edge of the τ pseudomass distribution in the decay τ−→π−π+π−ντ and measure the τ mass to be 1777.09±0.08±0.11MeV/c^2, where the first uncertainty is statistical and the second systematic. This result is the most precise to date.

Belle II Collaboration, "Measurement of the τ-lepton mass with the Belle~II experiment" arXiv:2305.19116 (May 30, 2023).

Combining the uncertainties in quadrature, the newly measured tau quark mass is 1777.09 ± 0.136 MeV/c^2. 

This is a big improvement over the previous Belle II tau lepton mass measurement of 1777.28 ± 0.82 MeV/c^2 from August of 2020.

This will eventually pull up the Particle Data Group value which is currently 1776.86 ± 0.12 MeV/c^2. The new measurement is consistent with the Particle Data Group value at the 1.62 sigma level. The Particle Data Group value is considering only measurements in 2014 and earlier. The new inverse error weighted PDG value should be about 1776.97 ± 0.11.

The Koide's rule prediction is 1776.968 94 ± 0.000 07 MeV/c^2. 

The Koide's rule prediction is consistent with the new measurement at the 0.89 sigma level and is consistent with the PDG value at the 0.91 sigma level. The Belle II result pulls the global average closer to the Koide's rule prediction based upon a formula stated in 1981 and also is closer to the Koide's rule prediction than its previous less precise measurement from August of 2020.

Before the new Belle II measurement, the most precise single measurement of the tau lepton mass was the BESIII measurement from 2014 which was 1776.91 ± 0.12 + 0.10/− 0.13 MeV/c^2 (the combined error is ± 0.17).