Wednesday, November 15, 2017

LHC Measures Electroweak Mixing Angle

One of the physical constants in the Standard Model of Particle Physics is called the electroweak mixing angle also known as the Weinberg angle.  This is a function of two more fundamental physical constants in the Standard Model. Crudely speaking, the coupling constants of the electromagnetic force and the weak force (g and g'), which more precisely speaking are the

  and  couplings (weak isospin g and weak hypercharge g', respectively).

The cosine of the weak mixing angle is also equal to the mass of the W boson divided by the mass of the Z boson.

Measuring it is complicated somewhat because one has to defined the weak mixing angle at a particular energy scale to produce a numerical value for it since it runs with energy scale, in order to make a consistent measurement.

The CMS experiment at the Large Hadron Collider (LHC) has measured the electroweak mixing angle with twice the precision of any previous LHC measurement. The new measurement is as follows (combining all sources of error into a single uncertainty figure):


Put another way, the plus or minus one standard deviation range that flows from this measurement is 0.23049 to 0.23153.

The central value of the previous state of the art LHC measurement (via the first link in this post) was 0.23142, which is consistent with, but at the high end, of the new measurement and only have a margin of error of about ± 0.00100.

A 2004 global average from a variety of experimental sources (from the same link) was 0.23120 ± 0.00015 which is also consistent with this result.

The CODATA 2014 determination based upon W and Z boson mass measurements (from the same link) was 0.2223(21) implying a one sigma range of 0.22010 to 0.22440, which is below the value of the new measurement and below the 2004 global average at the one standard deviation level, and is just barely consistent at a two sigma level with the new LHC measurement.

The new paper notes that:
The most precise previous measurements of sin2 θ lept eff are reported by LEP and SLD experiments However, the two most precise measurements differ by more than 3 standard deviations. Measurements of sin2 θ lept eff are also reported by LHC and Tevatron experiments. . . . The results are consistent with the most precise LEP and SLD measurements [ed. when they are combined]. 
Meta Footnote

This post brings the number of posts at this blog for the year to an all time record high for a yearly number of posts at the blog, of 223, with 46 days left to go in 2017.

No comments: