# Neutral pseudo meson decay into neutrino antineutrino

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1. Mar 15, 2013

### shakeel

what are standards model limits on decay of K0, Do, B0 meson into a pair of neutrino and anti neutrino. I know that these highly suppressed due to involve FCNC. but if someone can tell me about theoretical limits on these reactions.

2. Mar 15, 2013

### Staff: Mentor

I'm not sure if there are any calculations of that process - I would expect a branching fraction similar to the decay in two muons or less*, but experimentally it is probably the worst thing you can study.*
$B^0 \to \nu \nu$ has an experimental upper limit of 2.2*10-4 from BaBar.

*there, the experimental limits are still above theory, with the Bs measurement from LHCb and KL as exceptions

3. Mar 15, 2013

### Hepth

Should be helicity suppressed as well, correct?

Any Pseudoscalar (spin 0) going to two spin 1/2 particles requires a spin-flip on one of the outgoing fermions to conserve angular momentum. This spin flip introduces a proportionality to the mass of the outgoing fermion, in this case a neutrino, which is very very small in relationship to the decaying particle, thus highly suppressed.

A naive estimate, I would take the predicted rate for {K,D,B}_0 -> e+e-, and multiply it by the mass of the neutrino over the mass of the electron squared.

So

$$\frac{m_{\nu_e}^2}{m_{e}^2} = \left(\frac{2.2\,eV}{511\,keV}\right)^2 = 2 \times 10^{-11}$$

so much smaller than the already-small branching ratios.