X (charge)

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, 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).

Template:Mvar is related to the difference between the baryon number Template:Mvar and the lepton number Template:Mvar (that is, Template:Math), and the weak hypercharge Template:Math via the relation: $$X=5(B-L)-2Y_{\text{W}}.$$

X charge in proton decay

Proton decay is a hypothetical form of radioactive decay, predicted by many grand unification theories. During proton decay, the common baryonic proton decays into lighter subatomic particles. However, proton decay has never been experimentally observed and is predicted to be mediated by hypothetical X and Y bosons. Many protonic decay modes have been predicted, one of which is shown below: $${\mathrm{p}}^{+}⟶{\mathrm{e}}^{+}+{\pi }^0$$

This form of decay violates the conservation of both baryon number and lepton number, however the X charge is conserved. Similarly, all experimentally confirmed forms of decay also conserve the X charge value.

Values of X charge for known elementary particles

The following table lists the X charge values for the standard model fermions and their antiparticles. Note that the CP conjugate of a fermion has the opposite X charge (e.g. Template:Physics particle vs. Template:Physics particle, Template:Mvar = −3 vs. +3).

Particle name	Symbol	Left-handed X charge	Right-handed X charge
Up quark	Template:Subatomic particle	+1	−1
Charm quark	Template:Subatomic particle	+1	−1
Top quark	Template:Subatomic particle	+1	−1
Down quark	Template:Subatomic particle	+1	+3
Strange quark	Template:Subatomic particle	+1	+3
Bottom quark	Template:Subatomic particle	+1	+3
Electron	Template:Subatomic particle	−3	−1
Muon	Template:Subatomic particle	−3	−1
Tauon	Template:Subatomic particle	−3	−1
Electron neutrino	Template:Subatomic particle	−3	−5
Muon neutrino	Template:Subatomic particle	−3	−5
Tau neutrino	Template:Subatomic particle	−3	−5
Up antiquark	Template:Subatomic particle	+1	−1
Charm antiquark	Template:Subatomic particle	+1	−1
Top antiquark	Template:Subatomic particle	+1	−1
Down antiquark	Template:Subatomic particle	−3	−1
Strange antiquark	Template:Subatomic particle	−3	−1
Bottom antiquark	Template:Subatomic particle	−3	−1
Positron	Template:Subatomic particle	+1	+3
Antimuon	Template:Subatomic particle	+1	+3
Antitau	Template:Subatomic particle	+1	+3
Electron antineutrino	Template:Subatomic particle	+5	+3
Muon antineutrino	Template:Subatomic particle	+5	+3
Tau antineutrino	Template:Subatomic particle	+5	+3

The next table gives the X charge of the standard model bosons.

Particle name	Symbol	X-charge
Photon	Template:Subatomic particle	0
W boson	Template:Subatomic particle	0
Z boson	Template:Subatomic particle	0
Gluon	Template:Subatomic particle	0
Higgs boson	Template:Subatomic particle	−2

Although not part of the Standard Model, the GUT X and Y bosons also have zero X charge.

See also

• Standard Model (mathematical formulation)
• Noether's theorem
• X and Y bosons

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