Wednesday, September 10, 2014

More Higgs Boson Measurements From CMS (with corrections and updates 091114)

The CMS experiment at the Large Hadron Collider (LHC) has released its latest sets of experimental Higgs boson measurements today, from H-->ZZ-->four lepton decay channels.

The key points:

* The Higgs boson discovery is confirmed with a 6.8 sigma significance.

* The latest measurement by CMS of the Higgs boson mass in this channel is 125.6 GeV +/- 0.4 +/- 0.2.

Before this preprint was released the most recent CMS four lepton mass measurement was 125.8 +/- 0.5 +/- 0.2.  The new CMS four lepton mass estimate is 0.2 GeV lighter and is 8% more precise.

Thus, the relevant Higgs boson mass data points are now:

* The ATLAS diphoton mass is 125.98 +/- 0.42 +/- 0.28 (June 15, 2014)
* The ATLAS four lepton mass is 124.51 +0.52 +/- 0.06 (June 15, 2014)
* The ATLAS combined mass value is 125.36 +/- 0.37 +/- 0.18 (June 15, 2014)
* The CMS diphoton number is 124.7 +/- 0.31 +/- 0.15 (July 2, 2014)
* The CMS four lepton mass is 125.6 +/- 0.4 +/- 0.2 (September 10, 2014).  This is substantially similar to a June 10, 2014 release from CMS updating one of its December 2013 papers.
* The combined CMS number is 125.03 +0.26/-0.25+0.13/-0.15 (July 3, 2014).

The naive global average Higgs boson mass is now about 125.15 GeV.  The range of value for measurements by the same experiments and by different experiments of the same decay channel, however, cast doubt on the accuracy of the error bars claimed.

* Another Standard Mode-like Higgs boson was excluded with 95% confidence in the mass range from 129.5-832.0 GeV.

* The strength of the ZZ decay channel of the Higgs boson relative to the Standard Model expectation is mu=0.93 +0.26/-0.23 +0.13/-0.09.  This is closer to the expected value of 1.0 if the Standard Model is correct, than the previous measurement, and is less than a quarter of a Standard Deviation from the expected value.  The PDG value of mu in the ZZ channel was 0.80 +0.35/-0.28 at CMS and was 1.2 +/- 0.6 at ATLAS.

* The Higgs boson width is less than 33 MeV at the 95% confidence level (the SM expectation is 4.15 MeV).  A limit of about 17 MeV has already been established by other means, but this is still a significant improvement at measuring this property of the Higgs boson in this decay channel alone.

* CMS confirms the results of other experiments in ruling out non-Standard Model spin (actually total angular momentum J) or parity (the SM Higgs is + aka "even").


andrew said...

A new ATLAS results overview here. Also of note is a paper on WW and VV backgrounds from CMS.

andrew said...

Another major width measurement that is not quite as tight but makes the measurement more robust because it uses different methods, was announced today. The best fit value for CMS is very nearly the same as the SM prediction.