What is the matter anti-matter asymmetry in the decay of the neutral K meson? In 1964 Cronin and Fitch discovered that there is asymmetry in the weak decay of the neutral K meson.  What is this asymmetry and what causes it?
 A: I'm not an expert in HEP, but I guess what you mean by asymmetry in the decay of K meson is the $\textbf{CP violation}$ process and Cronin together with Fitch demonstrated  this in their experiment back in 1964.
CP invariance is one of the conservation laws which govern the general behavior of the physical particles (C for charge conjugation and P for parity). If you want to know more about CP Invariance (or CPT invariance) I recommend you to read this.
Getting back to the decay of Kaon, we know that strong and e.m. interactions always preserve CP invariance, but in weak decay this symmetry is slightly violated (since the name CP-violation). The two decays (also called semileptonic decays) of the Kaon into pions and electrons are
$$K_L^0\to\pi^++e^-+\bar{\nu}_e$$ and $$K_L^0\to\pi^-+e^++\nu_e$$ where L means long-lived.
The CP operation on one of the decay would be something like this: $$CP(\pi^++e^-+\bar{\nu}_e)=\pi^-+e^++\nu_e$$
so basically the CP just transforms the decay products of one decay into the other. If the Kaon decay would have CP invariance, then the probability for the two patterns would be the same. Experimentaly, it is shown that the second decay happens more often that the electron decay, so the CP invariance is violated!
If you want to know more on the decay of neutral Kaon, you can see this cool presentation, or if you prefer a more technical report see this.
I hope this helps you :)
