Monday, February 27, 2017

ATLAS Measures The Top Quark Mass With Hadrons

The most precise ways to measure the top quark mass are with leptonic decays. But, while hadronic decay measurements are harder to do and less precise, they make the overall measurement more robust, because the systemic errors involved in hadronic decay measurements are largely independent of the systemic errors involved in leptonic decay measurements. 
The top-quark mass is measured in the all-hadronic top-antitop quark decay channel using proton-proton collisions at a centre-of-mass energy of s√ = 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The data set used in the analysis corresponds to an integrated luminosity of 20.2 fb−1. The large multi-jet background is modelled using a data-driven method. The top-quark mass is obtained from template fits to the ratio of the three-jet to the dijet mass. The three-jet mass is obtained from the three jets assigned to the top quark decay. From these three jets the dijet mass is obtained using the two jets assigned to the W boson decay. The top-quark mass is measured to be 173.72 ± 0.55 (stat.) ± 1.01 (syst.) GeV.
ATLAS Collaboration "Top-quark mass measurement in the all-hadronic tt¯ decay channel at s√ = 8 TeV with the ATLAS detector" (February 24, 2017).

The combined error of the ATLAS hadronic top quark mass measurement is about ± 1.15 GeV. This is higher than the leptonic measurements, but not outrageously so and is 37% of the error for the CMS hadronic measurement, for which the measured value was quite similar. Honestly, the latest CMS leptonic top quark mass error estimate is suspiciously low.

My latest summary of top quark mass measurements can be found in a December 16, 2016 post. The highlights of that post were as follows:
The combined ATLAS result from the √ s = 7 TeV dileptonic and semileptonic tt¯ measurements yields a value of mtop = 172.99 ± 0.48 (stat) ± 0.78 (syst) GeV. 
Additional precision is to be expected with the inclusion of √ s = 8 TeV measurements. The combined CMS result of mtop = 172.44 ± 0.13 (stat) ±0 .47 (syst) GeV is based on all √ s =7 and 8 TeV measurements performed in the three standard tt¯ decay channels. The results of both experiments are in good agreement. . . .
A previous low precision CMS report using a hadronic channel not included in the CMS average above is here. It reported a top quark mass of 173.5 ± 3.13 GeV. 
The final Tevatron mass measurement for the top quark was 174.30 ± 0.65 GeV.

The combined ATLAS error is ± 0.92 GeV. The combined CMS error is ± 0.49 GeV. The range of the independent average top quark mass measurements is 1.86 GeV. . . .
The inverse error weighted average of the three main results plus the CMS hadronic channel is 173.20 GeV.

Excluding the CMS hadronic channel (which there is no good reason to do in an error weighted measurement), the inverse error weighted average is 173.18 GeV. . . .

If the the sum of the square of the boson masses equals the sum of the square of the fermion masses equals one half of the Higgs vacuum expectation value, the implied top quark mass is 174.03 GeV if pole masses of the quarks are used, and 174.05 GeV if MS masses at typical scales are used. . . .
The expected value of the top mass from the formula that the sum of the square of each of the fundamental particle masses equals the square of the Higgs vaccum expectation value (a less stringent condition because the fermion and boson masses don't have to balance), given the global average Higgs boson mass measurement (and using a global fit value of 80.376 GeV for the W boson rather than the PDG value) is 173.73 GeV. The top quark mass can be a little lighter in this scenario because the global average measured value of the Higgs boson mass is a bit heavier than under the more stringent condition.
The latest hadronic measurement of the top quark mass from ATLAS is in the high end, and is very close to the much less precise CMS hadronic measurement, and to the theoretically predicted value that I generated. Because of the significant margin of error, the new ATLAS hadronic measurement is consistent with all of the other recent measurements of the top quark mass. 

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