In particle physics, the X-charge (or simply X) is a conserved quantum number associated with the SO(10) grand unification theory. It is thought to be conserved in strong, weak, electromagnetic and gravitational and Higgs interactions. Because the X-charge is related to the weak hypercharge, it varies depending on the helicity of a particle. For example, a left-handed quark has an X-charge of +1, whereas a right-handed quark can have either an X-charge of -1 (for up, charm and top quarks), or -3 (for down, strange and bottom quarks).
X is related to the difference between the baryon number B and the lepton number L (that is B − L), and the weak hypercharge YW via the relation:
The following table lists the X-charge values for the standard model particles and their antiparticles.