The Latest Higgs Boson Mass Measurements

[ohwilleke]

It is summer conference season for physicists and one of the highlights this time around is the first set of new Large Hadron Collider Run-2 measurements of the Higgs boson mass.

The Status Quo

At the end of Run-1 at the Large Hadron Collider (LHC), the combined estimate of the Higgs boson mass from all sources was 125.09 +/- 0.24 GeV.

The New Results

New measurements from ATLAS and CMS (the two main experimental groups at the LHC) of the Higgs mass were announced this week, and the error weighted average of those mass measurements is now 125.14 +/- 0.17 GeV, a global best fit value that is about 29% more accurate than the previous state of the art measurement after LHC Run 1. The error in the new combined Higgs boson mass measurements is less than 0.14%.

The one sigma margin of error was +/- 0.28 GeV for the ATLAS result and +/- 0.22 GeV for the CMS result. The latest ATLAS measurement of the Higgs boson mass is 124.98 GeV and the latest CMS measurement is 125.26 GeV.

(This is based upon slides from a conference presentation. It is not clear to me if the pre-print papers are available at this point, but I haven't seen them anywhere yet. Often pre-prints are withheld for long enough to make the conference presentation the grand reveal of new experimental results.)

The slides also report on the experimental measures of the Higgs boson couplings with various other kinds of fundamental particles.

 

[Vanadium 50]

29% more accurate

You mean 29% more precise, I think.

 

[arivero]

You mean 29% more precise, I think.

Is there some rumour/hint of systematic bias?

 

[Vanadium 50]

The quoted error is a statement about precision. No need to read anything else into it.

 

[mfb]

and the error weighted average of those mass measurements

Be careful with these combinations. It should give a reasonable value here as the CMS systematic uncertainty is small (only H->4 lepton channel), but in general systematic uncertainties can be correlated and then you underestimate the uncertainty.

CMS mass measurement at arXiv
ATLAS mass measurement at CDS

 

[ohwilleke]

You mean 29% more precise, I think.

Pedantic but fair.

 

[ohwilleke]

Be careful with these combinations. It should give a reasonable value here as the CMS systematic uncertainty is small (only H->4 lepton channel), but in general systematic uncertainties can be correlated and then you underestimate the uncertainty.

CMS mass measurement at arXiv
ATLAS mass measurement at CDS

They can be the case, as I actually noted that in a recent discussion about the mathematics of doing combinations at my blog, and is particularly a potential concern at the LHC where a lot of the apparatus that would be a source of systemic error is shared.

But, if you look at the Particle Data Group global combinations of a variety of measurements (or past LHC combinations), it turns out that this potential theoretical concern is quantitatively immaterial.

In part, this is because when you combined statistical and systemic errors, the combination is highly sensitive to the biggest error category and only tugged up a little by the other smaller error type. So, any adjustment that affects only one of two or more kinds of uncertainty is mathematically damped.

 

[mfb]

There are measurements where you absolutely have to take it into account. Top mass measurements, for example, where model uncertainties are important and common to both ATLAS and CMS.
As I said, here it works, but in general it can be problematic and underestimate the uncertainty.