A new paper notes that the formulas used to convert experimental data into the mass of unstable particles like the W, Z and top quark masses, the three shortest lived particles known to physics, are approximations rather than exact conversions and uses an exact formula to revise these mass estimates.
This results in statistically significant slightly lower W and Z boson masses, but the adjustment to the top quark mass and Higgs boson mass due to these adjustments are less than the uncertainties in the experimental measurements.
The bottom line conclusions of the paper (from the body text, references omitted, bracketed bold text inserted editorially for ease of reading) are as follows:
[The Z boson]
The world-average values MZ = 91.1876 ± 0.0021 GeV and ΓZ = 2.4952 ± 0.0023 GeV can be used to derive the physical Z boson mass and width from Eqs. (23) and (24),
mZ = 91.1620 ± 0.0021 GeV (26)
ΓZ = 2.4940 ± 0.0023 GeV . (27)
The physical Z boson mass is about 26 MeV less than the parameter MZ, a result we derived long ago; this is about ten times greater than the uncertainty in the mass. The Z boson width is the same as the parameter ΓZ within the uncertainty. This yields a Z boson lifetime of τZ = 2.6391 ± 0.0024 × 10−25 s.
[The W boson]
The world-average values MW = 80.379 ± 0.012 GeV and ΓW = 2.085 ± 0.042 GeV [6] yield the physical W boson mass and width
mW = 80.359 ± 0.012 GeV (28)
ΓW = 2.084 ± 0.042 GeV . (29)
The physical W boson mass is about 20 MeV less than the parameter MW, which is nearly twice the uncertainty in the mass.
The W boson width is the same as the parameter ΓW within the uncertainty, and yields τW = 3.158 ± 0.064 × 10−25 s.
[The top quark]
The top-quark mass and width are also extracted from experiment using the parameterization of Eq. (19). The world-average values are Mt = 172.76 ± 0.30 GeV and Γt = 1.42+0.19−0.15 GeV.
The width is sufficiently narrow that these values are equal to the physical top quark mass and width well within the uncertainties. In addition, the physical top quark mass is ambiguous by an amount of order ΛQCD ∼ 200 MeV.
[The Higgs Boson]
The Higgs boson width is expected to be so narrow (∼ 4 MeV) that the difference between the physical mass and the parameter MH is negligible.
In relative terms, the downward adjustment in the Z boson mass is about 1 part per 3,500, and the downward adjustment in the W boson mass is about 1 part per 4,000, so the phenomenological consequences of the adjustment are very modest. But the W boson mass is moved closer to the amount expected in a global electroweak fit.
The paper and its abstract are:
We show that the mass and width of an unstable particle are precisely defined by the pole in the complex energy plane, μ=m−(i/2)Γ, by using the defining relationship between the width and the lifetime, Γ=1/τ. We find that the physical Z boson mass lies 26 MeV below its quoted value, while the physical W boson mass lies 20 MeV below. We also clarify the various Breit-Wigner formulae that are used to describe a resonance.
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