 | This articlerelies largely or entirely on asingle source. Relevant discussion may be found on thetalk page. Please helpimprove this article byintroducing citations to additional sources. Find sources: "X" charge – news ·newspapers ·books ·scholar ·JSTOR(September 2025) |
| Flavour in particle physics |
|---|
| Flavourquantum numbers |
|
| Related quantum numbers |
| Combinations |
|
| Flavour mixing |
Inparticle physics, theX charge (or simplyX) is aconservedquantum number associated with theSO(10)grand unification theory. It is thought to be conserved instrong,weak,electromagnetic,gravitational, andHiggs interactions. Because the X charge is related to theweak hypercharge, it varies depending on thehelicity of a particle. For example, a left-handedquark 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 thebaryon numberB and thelepton numberL (that is,B –L), and the weak hyperchargeYW via the relation:[1]
Proton decay is a hypothetical form ofradioactive decay, predicted by manygrand unification theories. During proton decay, the common baryonicproton decays into lighter subatomic particles. However, proton decay has never been experimentally observed and is predicted to be mediated by hypotheticalX and Y bosons. Manyprotonic decay modes have been predicted, one of which is shown below:
This form of decay violates the conservation of bothbaryon number andlepton number, however the X charge is conserved. Similarly, all experimentally confirmed forms of decay also conserve the X charge value.
The following table lists the X charge values for theStandard Modelfermions and theirantiparticles. Note that the CP conjugate of a fermion has the opposite X charge (e.g.e
L vs.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 bosons also have zero X charge.
 | Thisparticle physics–related article is astub. You can help Wikipedia byadding missing information. |