## Thursday, April 11, 2019

### New CMS Measurements Of Top Quark Mass And Strong Force Coupling Constant Are Low

The money sentence is on page 30 of the 113 of a new paper from CMS regarding their latest LHC measurement of the top quark mass and the strong force coupling constant.
The resulting values of αS(mZ) and m pole t extracted using NLO calculations are: αS(mZ) = 0.1135 ± 0.0016(fit) +0.0002 −0.0004(model) +0.0008 −0.0001(param) +0.0011 −0.0005(scale) = 0.1135+0.0021 −0.0017(total), m pole t = 170.5 ± 0.7(fit) ± 0.1(model) +0.0 −0.1(param) ± 0.3(scale) GeV = 170.5 ± 0.8(total) GeV. (8)
These values utilize Run-2 date from the Large Hadron Collider as well as a methodology that increases the amount of data considered:
Our present paper provides a new measurement, following the procedures of Ref. [20]. It is based on data taken by the CMS experiment in 2016 at √ s = 13 TeV, corresponding to an integrated luminosity of 35.9±0.9 fb−1 . In addition to eµ, the decay modes e +e − (ee) and µ +µ − (µµ) are also selected, roughly doubling thereby the total number of expected tt signal events. Our latest measurement complements the analyses [21, 22], based on data taken at √ s = 13 TeV, but using tt decays in the +jets ( = e, µ) final state.
These results are both quite a bit lower than the current global average of previous measurements published by the Particle Data Group.

For the top quark: "OUR AVERAGE of 173.0 ±0.4 (GeV) is an average of top mass measurements from LHC and Tevatron Runs."

The new CMS measurement is in a 2.8 sigma tension with prior measurements. The paper it complements ([22] from 2018 of which [21] was a 2017 version) found that:
The measured value of mMCt=172.33 ± 0.14 (stat) +0.660.72 (syst) GeV
The combined downward uncertainty in the previous CMS measurement is 0.73 GeV.

For the strong force coupling constant:  0.1181(11).

The new CMS measurement is consistent with the PDG value at the 1.94 sigma level. The paper it complements ([22]) found that:
Table 5: Values of αS(mZ) with their uncertainties obtained from a comparison of the measured σtt value to the NNLO prediction in the MS scheme using different PDF sets. The first uncertainty is the combination of the experimental and PDF uncertainties, and the second is from the variation of the renormalization and factorization scales.
PDF set αS(mZ)
ABMP16 0.1139 ± 0.0023 (fit + PDF) +0.0014 −0.0001 (scale)
NNPDF3.1 0.1140 ± 0.0033 (fit + PDF) +0.0021 −0.0002 (scale)
CT14 0.1148 ± 0.0032 (fit + PDF) +0.0018 −0.0002 (scale)
MMHT14 0.1151 ± 0.0035 (fit + PDF) +0.0020 −0.0002 (scale)
The values of αS(mZ) obtained using different PDF sets are consistent among each other and are in agreement with the world-average value [29] within the uncertainties, although suggesting a smaller value of αS(mZ). The value of αS(mZ) is also in good agreement with the recent result of the analysis in Ref. [84] of jet production in deep-inelastic scattering using the NNLO calculation by the H1 experiment, and is of comparable precision.
It isn't immediately obvious why this CMS measurement of these two quantities would be so low compared to prior measurements by both CMS and other experiments. My gut instinct is that the adjustment from the energy levels observed to the Z boson mass scale may be flawed, perhaps by implicitly underestimating the energy scale of the measurement, since the strong force coupling constant gets smaller at higher energies and the thing that is most exceptional about the latest measurement compared to earlier measurements is the higher energies of the collisions. It could also flow from using NLO rather than NNLO calculations.