Deduce that X must be an electron neutrino.

Distinguish between hadrons and leptons.

it has a lepton number of 1 «as lepton number is conserved»

it has a charge of zero/is neutral «as charge is conserved»

OR

it has a baryon number of 0 «as baryon number is conserved»

Do not credit answers referring to energy

hadrons experience strong force

OR

leptons do not experience the strong force

hadrons made of quarks/not fundamental

OR

leptons are not made of quarks/are fundamental

hadrons decay «eventually» into protons

OR

leptons do not decay into protons

Accept leptons experience the weak force

Allow “interaction” for “force”