Thursday, May 30, 2019

The Particle Data Group's 2019 Update Is Now Live

The Particle Data Group combines all of the experimental data on fundamental particles and hadrons to establish global averages. 

One of the most notable of the new data points in 2019 is the Higgs boson mass which is now 125.10 ± 0.14 GeV.  Previously in 2018 it was 125.18 ± 0.16 GeV. The significance of the difference between the PDG 2019 value for the Higgs boson mass and the 124.65 GeV value that would make the Higgs boson mass squared plus the W boson mass squared plus the Z boson mass squared equal to exactly one half of the Higgs vev squared is 3.21 sigma, only slightly changed from the 3.31 sigma of the 2018 value as the uncertainty is reduced by 22% relative to  last year, in short, still strongly disfavored but not completely ruled out either.

The W and Z boson masses are unchanged. The most recent LHC data for the W boson mass (from 2018 using Run-1 data) tugs down a little to 80.370 ± 0.018 GeV relative to the current fit of 80.379 ± 0.012 GeV, but not enough to pull down the global average. A global fit of the W boson mass with the Higgs boson mass and top quark mass suggests a value of 80.362 +/- 0.002 GeV. The most recent Z boson mass data is from 2001.

2018 Quark Masses v. 2019 Quark Masses v. FLAG19 (in MeV)

top quark 173,000 ± 400 v. 172,900 ± 400
bottom quark 4,180 +40-/-30  v. 4,180 + 30/-20 v. 4,198 ± 12
charm quark 1,275 +25/-35 v. 1,270 ± 20  v. 1,282 ± 17
strange quark 95+9/-3 v. 93 +11/-5 v.  93.12 ± 0.69
down quark 4.7 + 0.5/-0.4 v. 4.67 +0.48/-0.17 v. 4.88 ± 0.2
up quark 2.2 +0.5/-0.4 v. 2.16 + 0.49/-0.26 v. 2.5 ± 0.17

All of the PDG 2019 values are consistent with the FLAG19 values.

Curiously, the PDG strange quark mass uncertainty has increased even as the uncertainty in the other four non-top quark masses has declined and the uncertainty in the top quark mass has remained the same. It is also puzzling that FLAG19 reports an uncertainty in its determination of the strange quark mass (despite having a central value that has dropped to be consistent with the FLAG19 value) that is more than eleven times smaller than PDG 2019 does, even though the other error margins for FLAG19 values are smaller than the PDG values but of the same order of magnitude.

The small downward shift in the global average of direct top quark mass measurements pulls it away from the value it would need to have for the sum of the square of the fundamental fermion masses in the Standard Model to equal half of the Higgs vev squared, which is about 174,040 MeV which is 2.85 sigma from the directly measured value.  But, this is balanced out a bit by indirect measurements of the top quark pole mass of about 173,100 ± 900 MeV. The error weighed average of the direct and indirect top quark mass measurements in PDG 2019 is 172,960 ± 366 MeV, which is 2.95 sigma from 174,040 MeV. As in the case of the Higgs boson mass, this is still strongly disfavored but not completely ruled out either. But, the likelihood that both the top quark mass measurements is too low by 2.95 sigma and the Higgs boson mass measurement is high by 3.2 sigma is very low indeed.

But, since the deviations from these values are in opposite directions and similar in magnitude, the possibility the less stringent relationship, that the sum of the square of the fundamental particle masses is equal to the square of the Higgs vev, is still perfectly consistent with the data. So, the LC & P relationship lives another year.

While FLAG19 values are based purely on state of the art lattice QCD determinations, the PDG 2019 values aren't based on methodologies that are much different.

The charged lepton masses are unchanged. I didn't determine if the neutrino mass eigenstate differences, and mixing angles were precisely the same in 2019 as they were in 2018, but the neutrino mass eigenstate differences are at a minimum, very similar to at least two significant digits.

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