I'm reading this webpage which introduces the idea of measuring the mass of Higgs bosons with high precision: https://cms.cern/news/cms-precisely-measures-mass-higgs-boson . They presented this diagram, it shows the result of Higgs boson decays into a pair of photons. However, I didn't find too many hints about how I can interpret this result. I can see the two 'bumps' at approximately 125 MeV, are they the signals of photons, or the signal of Higgs bosons?

enter image description here

I also found another Feynman diagram for Higgs boson decay

Is this a possible example that contribute to the measurement result above? Thanks for the help!

enter image description here

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    $\begingroup$ Your link shows the invariant mass of one of the the 2 gamma events in the plots with the invariannt mass of the two photons. The accumulation shows the resonance $\endgroup$
    – anna v
    Commented Apr 25, 2022 at 6:18

1 Answer 1


The plot here is showing events with two photons, with the invariant mass $m_{\gamma\gamma}$ of the two photons plotted. The invariant mass is given by $m_{\gamma\gamma}=\sqrt{(E_1+E_2)^2-(\vec p_1+\vec p_2)^2}$. This particular quantity is of interest because if the two photons came from a decay $\rm X\to\gamma\gamma$, then by conservation of four-momentum, $m_{X}=m_{\gamma\gamma}$.

On the top plot, the lower red line is what we expect to see from Standard Model background events. That is to say, basically everything except $\rm H\to\gamma\gamma$. The lower plot shows the same data, but with these background events removed, so it shows only the Higgs peak and some statistical noise.

These peaks show that we have more events with two photons with a specific invariant mass than we would expect without the Higgs boson. In other words, they can be interpreted as a measurement of $\rm H\to\gamma\gamma$.

The Feynman diagram you show is one contributor to the result above. The $W$ boson can be replaced with any charged Standard Model particle and that will also contribute. But the Higgs boson couples more strongly to more massive particles, so the primary contributing diagrams are the one you show and a similar one with the W boson replaced by a top quark. The next heaviest charged particle in the Standard Model (the bottom quark) is much lighter and so doesn't contribute that much.

  • $\begingroup$ Thank you so much for the answer! If we just look at the data, is there a way we can determine the peak is associated to photons, instead of other particles? $\endgroup$
    – IGY
    Commented Apr 25, 2022 at 14:40
  • $\begingroup$ @IGY The "$m_{\gamma\gamma}$" in the lower right corner and the "$\rm H\to\gamma\gamma$" in the top left corner indicate that they are measuring the invariant mass of two photons. Aside from those indicators there is no way to tell that these are diphoton events being plotted. (Well, of course you could read the paper to get all the nitty-gritty details of how this plot was produced). $\endgroup$
    – Chris
    Commented Apr 25, 2022 at 19:16

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