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I'm reading an essay discussing the measurement of Higgs boson mass: https://cms.cern/news/cms-precisely-measures-mass-higgs-boson, which says that the latest and more accurate measurement results came from two procedures. One of them looked at decays to two Z bosons, which subsequently decay into electron or muon pairs, and the other focused on decays to two photons in the Figure below.

enter image description here

I also found similar graphs which feature other Higgs decay mechanisms, such as

enter image description here

I can see for both decays, the products are around 125 GeV, which is also the mass of the Higgs boson. My question is why don't experimentalists determine the mass through the second (or other) decay processes? What else information can we learn from the graphs like this?

Many thanks for the help!

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In principle, all of the different major Higgs channels, from both the CMS and ATLAS groups, can be combined to determine the mass. For an intro into the sort of things that go into this process (which is used for many Higgs parameters), you can read this talk.

$\rm H\to\gamma\gamma$ is a relatively rare decay, but it is very clean and has low backgrounds. By comparison something like $\rm H\to b\bar{b}$ is much more common but has a huge background and is much more complicated to analyze.

It turns out that if you only care about the mass, it tends to be much better to go with the "rare but clean" decay as it gives a very clear indication of the mass without the larger systematic errors that can build up with more complicated decays.

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