Q:

Prediction for graviton's decay

Why should a graviton (if it exists) decay into 2 Z bosons?
My understanding is that the graviton is supposed to be a massless spin-2 boson and that it is the conservation laws for energy and spin and the rest of quantum numbers that allow a decay.
Now I see that the above prediction makes sense, since the z boson is spin-1 and it is its own antiparticle, thus, all of its flavour quantum numbers and charges are zero. But what I find a bit more problematic or involved is that the Z boson is massive (almost 80 times as massive as the proton).
For example in the case of the observation of ttH production, they say that the Higgs can not decay into top quarks because they're too heavy...
Can someone shed some light here?

A:

This article explains the Kaluza Klein gravitational model, where there are both zero mass and massive gravitons.
For a theory of everything which would follow on the lines of the current standard model of particle physics adding gravitation ,the graviton will be the gauge boson of the gravitational interactions and will be of zero mass.  On these lines the gravitational waves observed by LIGO will be emergent from a superposition of a zillion zero mass gravitons.  
The graviton discussed in the link you give is one of the $n>0$ modes of the Kaluza Klein gravitons. In page 11 of above link,

From the equation of motion we can say that only the zero modes
  $(n=0)$ will be massless and observable at our present energy and all
  the excited  states, called as Kaluza-Klein states,will have masses .

KK theory  has only one extra dimension. If the future theory of everything is a string theory, there are even more extra dimensions , and the KK massive excitations are expected to be there and detectable given a lot of energy. The KK graviton for  decaying to these particles should have at least the mass of  the particles it decays into, and a lot more, in order to be able to "bind temporarily" heavy mesons like Z, and to be detectable in the LHC experiments.

