Monday, February 26, 2018

Recent Direct Top Quark Width Measurement

According to this paper (recently updated from an earlier September version) containing the latest top quark width measurement (which is a function of its mean lifetime, a large decay width translates to a short mean lifetime with a top quark having a mean lifetime a bit more than 10^-25 seconds), the Standard Model prediction for the top quark width is:
The top quark is the heaviest particle in the Standard Model (SM) of elementary particle physics, discovered more than 20 years ago in 1995. Due to its large mass of around 173 GeV, the lifetime of the top quark is extremely short. Hence, its decay width is the largest of all SM fermions. A next-to-leading-order (NLO) calculation predicts a decay width of Γt = 1.33 GeV for a top-quark mass (mt) of 172.5 GeV. Variations of the parameters entering the NLO calculation, the W-boson mass, the strong coupling constant αS, the Fermi coupling constant GF and the Cabibbo–Kobayashi–Maskawa (CKM) matrix element Vtb, within experimental uncertainties yield an uncertainty of 6%. The recent next-to-next-to-leading-order (NNLO) calculation predicts Γt = 1.322 GeV for mt = 172.5 GeV and αS = 0.1181.
A 6% uncertainty is +/- 0.08 GeV, although the error from NLO to NNLO differences is surprisingly small, which suggests that this is a sufficient number of loops for this calculation. This implies that the top quark width at a mass of 172.5 GeV is in a one sigma range of 1.25 to 1.41 GeV if the Standard Model is correct.

This compares to the following direct measurement of the top quark width from the Run-1 LHC data from the ATLAS experiment:
This paper presents a direct measurement of the decay width of the top quark using tt¯ events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb^−1. The decay width of the top quark is measured using a template fit to distributions of kinemat ic observables associated with the hadronically and semileptonically decaying top quarks. The result, 
for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.
The value with the combined error on the latest combined width measurement of the top quark is 1.76+0.86-0.76, which expressed as a one sigma range is 1.00 to 2.62.

Of course, experimental consistency with the predictions of the Standard Model isn't very impressive when the error bars on your experimental measurement are huge (at least on a percentage basis).

Incidentally, a somewhat higher than predicted width tends to favor a somewhat higher than currently estimated mass for the top quark. But, given that the precision of the top quark mass measurement is so much greater than the precision of the top quark width measurement, it doesn't make a lot of sense to make determinations in that direction.

By the end of the LHC experiment's run, the statistical error will decrease by quite a bit, and the systemic error will decrease by a little, and averaging this result with measurements by other means that do not have correlated systemic errors will improve a global estimate further. But, the error will still remain quite significant, so this won't necessarily provide any great insights.

No comments: