A common example of $\beta$-decay is

${{}_{15}\mathrm{P}}^{32}\to {{}_{16}\mathrm{P}}^{32}+x+y$

Then, x and y stand for

In  ${\beta }^{-}$  decay, a neutron in the nucleus is transformed into a proton, an electron and an anti-neutrino.

$n\to P+\overline{e}+\overline{v}$

So, x and y stand for electron and anti-neutrino.