I'm trying to understand this picture of the branching factors of the Higgs as a function of it's mass. I understand that of course as the Higgs' mass increases more decay branches become available. However I don't understand a few things in particular:

  1. Why does $\Gamma(H\rightarrow ZZ)$ dip so strongly at 160 GeV and then why does it recover afterwards again.
  2. Why is $\Gamma(H\rightarrow WW)>\Gamma(H\rightarrow ZZ)$ at very large $M_H$? Is this because there's two W's? but why is not twice as big then?
  3. Why does $\Gamma(H\rightarrow b\bar b)$ drop so significantly. My understanding is that the higher the mass of the final product, the more likely the decay channel (which would explain why $b \bar b$ is more likely that $c\bar c$ for example) but why does it drop so heavily even though there's no other channels being opened up till 160 GeV I believe

I have tried looking this up but no luck so maybe anyone here has an idea why?


1 Answer 1


You might first appreciate this is an extreme hypothetical, by now, with the actual Higgs Mass at 125 GeV, near the origin of the masses, far below any of the interesting structure you are asking about.

The controlling feature of these BR plots is the fixed-sum nature of the modes contrasted: since the ratios must all together sum to 1, much of the action you observe is normally the consequence of another, competing, mode fluctuating wildly at that scale, and not necessarily the mode you are focussing on! So, then,

  1. The ZZ mode is competing with the WW mode, which is rushing to the WW threshold at 160 GeV, where the Higgs decay is real Ws, unsuppressed by virtual W propagators. So, at 160, the WW mode triumphs and sucks the probability out of the entire picture, and the virtual ZZ mode has to dip, in response, like the fermion ones. But, then, at the ZZ threshold, 182 GeV, also real Zs become a possibility, so the ZZ mode option becomes competitive, again, and it "recovers", grabbing its share of total probability.

  2. Yes, this is merely counting the interactions of the Higgs to the eaten goldstons. There must be a Bose symmetry enhancement of the factor of 2 to its square, a factor of 4, which I have not found a simple illustration of, yet. The apparent ratio there is 3:1, which jibes with the ratio of couplings squared, $\cos^4\theta_W$...

  3. Back to 1. The Yukawa couplings of the Higgs to fermions stay put, while the effective couplings to the above competing gauge bosons grow bigger and bigger, sucking probability out of the total. The bigger competing participant modes such as $b\bar b$ are the biggest losers when the gauge bosons hit the jackpot.

Admittedly, a hand-wavey picture, but that's just it: it is a fixed-sum picture, and, unless you can naturally sum graphs in your head...

Here is a superior version of your picture, from the horse's mouth, with detail informing my point, enter image description here


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