Tuesday, November 27, 2018

The Latest On The Higgs Boson Mass

One of the parameters of the Standard Model which I watch very closely, because it has only been measured at all for a few years and because it is relevant for many purposes is the Higgs boson mass. Indeed, it is the only experimentally measured parameter involving the Higgs boson in the Standard Model. If you know its mass, in the Standard Model, the particle is fully described. 

An end of year paper speaking officially for both the ATLAS and CMS experiments at the Large Hadron Collider provides this summary of Higgs boson mass measurements at the LHC.
5.4 Higgs boson mass measurement 
The Higgs boson mass can be measured using the high resolution ZZ∗ and γγ final state. Combining the measurements in these two channels from 2015-2016 data and from run 1, the ATLAS collaboration reports a value of the Higgs boson mass of 124.97±0.24 GeV [32] (with ±0.19 GeV of statistical uncertainty and ±0.13 GeV of systematic uncertainty, mainly from uncertainties in the photon energy scale). With the ZZ∗ channel from 2015-2016 data, the CMS collaboration reports a mass value of 125.26 ± 0.21 GeV [33]. 
At the same time, a direct upper limit on the decay width is set at 95% confidence level at 1.1 GeV. This is still far above the predicted width in the SM which is about 4 MeV. A more model dependent constrain on the Higgs boson width can be derived comparing the rate of gg → H(∗) → ZZ(∗) events in the on-shell and off-shell Higgs mass regions. The ATLAS analysis with 2015-2016 data sets a model-dependent limit at 14.4 MeV on the decay width, at 95% confidence level [34].
All other properties of the Higgs boson measured to date are consistent with the Standard Model predictions for it, within the limits of experimental measurement uncertainty.

The most recent current combined LHC mass measurement of the Higgs boson I have see in most sources is 125.09 ± 0.24 GeV, which is based upon all measurements in all channels at ATLAS and CMS combined, in Run 1. But, the Particle Data Group reports a more precise figure of 125.18 ± 0.16 GeV, which includes one Run 2 measurement in one channel from CMS.

A Higgs boson mass of 124.65 GeV is not yet ruled out by the data and would be interesting because that mass is one for which the sum of Yukawas for all of the fundamental bosons in the Standard Model is exactly 0.5. But, the weighted global average of the Higgs boson mass is about 125.09 GeV with a MOE of 0.24 GeV, which is 0.44 GeV higher than the 124.65 GeV value that is so notable, which is a little under two sigma. So, the lower value isn't excluded experimentally, but it isn't favored either.

The gap between the ATLAS measurement and this theoretical value is 0.32 GeV, which with a MOE of 0.24 GeV is just 1.25 sigma from the expected value. But, the gap between the CMS measurement and the theoretical value is 0.61 GeV, which with a MOE of 0.21 GeV is almost three sigma.

On the other hand, the fact that two experiments using the same equipment are 0.36 GeV apart, and that the underlying measurements that went into each experiment's average value are even further apart, makes me think that the systemic and.or theoretical error is underestimated. The ATLAS and CMS individual experiments going into the global average have a swing of on the order of 1 GeV plus. Statistical error is pretty hard to get wrong (except for considering the effect of look elsewhere effects which aren't very important when there are only four measurements or so at issue), but systemic and theoretical error is inherently hard to estimate.

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