${ }_1{\mathrm{H}}^1{+}_1{\mathrm{H}}^1{\to }_1{\mathrm{H}}^2+\mathrm{X}{+}_1{\mathrm{e}}^0+$ energy. The emitted particle is

${ }_1{\mathrm{H}}^1+{ }_1{\mathrm{H}}^1{\to }_1{\mathrm{H}}^2+{}_{\mathrm{Z}}\mathrm{X}^{\mathrm{A}}{+}_1{\mathrm{e}}^0+ \mathrm{energy}$
$\Rightarrow \mathrm{Z}=1+1-1-1=0 ; \mathrm{A}=1+1-2=0$

given particle has zero mass and zero charge. 

Hence, It is neutrino.