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I am simply confused about the way y-axis is scaled in the figure below. So for example, if I were to read off the branching fraction value for Higgs decaying to ZZ, how would I precisely read the value?

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Photo Courtesy: University of Edinburgh Particle Physics Lecture Note

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  • $\begingroup$ You would read it exactly like you would read any other log-scale graph. A factor of two is roughly 30 percent of the way from one mark to the next, a factor of three is roughly halfway, and a factor of 5 is 70 percent of the way to the next mark. $\endgroup$
    – probably_someone
    Dec 11, 2017 at 20:07

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The "branching fraction" is the fraction of decays that occur in a particular channel; all of the branching fractions for all of the decay channels must add up to $100\%=1$. This is a graph predicting how a Standard Model Higgs would decay as a function of its mass, which was much more interesting before we managed to measure the mass of the Higgs at $\rm125\,GeV$.

So this graph tells you that a very light, $\rm80\,GeV$ Higgs would decay about 80% of the time into $b\bar b$, about 9% of the time into $\tau\tau$, and so on. There's a mass region around $\rm170\,GeV$ where the Higgs would decay into something like 98% $WW$ and 2% $ZZ$. Reality, around $M_H = \rm125\,GeV$, seems to be where the $ZZ$ and $c\bar c$ channels are equally likely at about 2.5%.

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