Monday, October 24, 2016

New ATLAS top quark mass measurements

ATLAS has released new top quark mass measurements using the dilepton and all hadronic channels.

The measurement in the all hadronic channel is:
mtop = 173.80 ± 0.55(stat.) ± 1.01(syst.) GeV is measured, where the systematic uncertainty is dominated by the hadronization modelling (0.64 GeV) and the jet energy scale (0.60 GeV).
The measurement in the dilepton channel is:
mtop = 172.99 ± 0.41(stat.) ± 0.74(syst.) GeV is obtained, where the systematic uncertainty is dominated by the jet energy scale (0.54 GeV). 
This result is combined with the ATLAS top-quark mass measurements in the single-lepton and dilepton channels performed at √ s = 7 TeV [4] using the Best Linear Unbiased Estimate method [5]. The combined measurement gives a combined top-quark mass value of: mtop = 172.84 ± 0.34(stat.) ± 0.61(syst.) GeV. 
A new CMS measurement of the top quark mass in a new channel with low precision was released in August.  A new CMS measurement of the top quark mass in an old, more precise channel was released in March.

Sunday, October 23, 2016

About Time Scale In The Standard Model

The linked video is a powerful illustration of the notion of different orders of magnitude of scale from the human scale on up.

It doesn't go the other direction and only looks at distance, however.

Since I am often guilty of lumping all small time intervals into the tiny "ephemeral" category, I'll do penance by touching on the remarkable orders of magnitude differences is decay rates in particle physics.

There are actually huge disparities of scale between the mean lifetimes of various fundamental particles and hadrons (27 orders of magnitude from the shortest lived to the longest lived unstable particle to be exact). It is hard to get you head around numbers like that. It is particularly hard to do when humans have no ability to consciously distinguish between all but the two or three longest time periods involved.

To help you do so, let's look at all of mean lifetimes for fundamental particles and hadrons that have been measured experimentally, from longest to shortest in mean lifetimes (all data via Wikipedia) in terms of a scale where the mean lifetime of a W boson (the shortest lived particle, tied with the Z boson) is set arbitrarily at 1 W second, which can make this easier to understand (below the break).