X (charge)

Inparticle physics,theX charge(or simplyX) is aconservedquantum numberassociated with theSO(10)grand unification theory.It is thought to be conserved instrong,weak,electromagnetic,gravitational,andHiggsinteractions. Because the X charge is related to theweak hypercharge,it varies depending on thehelicityof a particle. For example, a left-handedquarkhas 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).

Xis related to the difference between thebaryon numberBand thelepton numberL(that is,B–L), and the weak hyperchargeY_Wvia the relation: $$X=5(B-L)-2\,Y_{\text{W}}.$$

X charge in proton decay

Proton decayis a hypothetical form ofradioactive decay,predicted by manygrand unification theories.During proton decay, the common baryonicprotondecays into lighter subatomic particles. However, proton decay has never been experimentally observed and is predicted to be mediated by hypotheticalX and Y bosons.Manyprotonic decaymodes have been predicted, one of which is shown below: $$\mathrm {p} ^{+}\longrightarrow \mathrm {e} ^{+}+\pi ^{0}$$

This form of decay violates the conservation of bothbaryon numberandlepton 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 thestandard modelfermionsand theirantiparticles.Note that the CP conjugate of a fermion has the opposite X charge (e.g.
e
_Lvs.
e
_R,X= −3 vs. +3).

Particle name	Symbol	Left-handed X charge	Right-handed X charge
Up quark	u	+1	−1
Charm quark	c	+1	−1
Top quark	t	+1	−1
Down quark	d	+1	+3
Strange quark	s	+1	+3
Bottom quark	b	+1	+3
Electron	e−	−3	−1
Muon	μ−	−3	−1
Tauon	τ−	−3	−1
Electron neutrino	ν e	−3	−5
Muon neutrino	ν μ	−3	−5
Tau neutrino	ν τ	−3	−5
Up antiquark	u	+1	−1
Charm antiquark	c	+1	−1
Top antiquark	t	+1	−1
Down antiquark	d	−3	−1
Strange antiquark	s	−3	−1
Bottom antiquark	b	−3	−1
Positron	e+	+1	+3
Antimuon	μ+	+1	+3
Antitau	τ+	+1	+3
Electron antineutrino	ν e	+5	+3
Muon antineutrino	ν μ	+5	+3
Tau antineutrino	ν τ	+5	+3

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

Particle name	Symbol	X-charge
Photon	γ	0
W boson	W	0
Z boson	Z	0
Gluon	g	0
Higgs boson	H0	−2

Although not part of the Standard Model, the GUTX and Y bosonsalso have zero X charge.

See also

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