In particle physics, the X and Y bosons (sometimes collectively called "X bosons"^[1]: 437) are hypothetical elementary particles analogous to the W and Z bosons, but corresponding to a unified force predicted by the Georgi–Glashow model, a grand unified theory (GUT).

Since the X and Y boson mediate the grand unified force, they would have unusual high mass, which requires more energy to create than the reach of any current particle collider experiment. Significantly, the X and Y bosons couple quarks (constituents of protons and others) to leptons (such as positrons), allowing violation of the conservation of baryon number thus permitting proton decay.

However, the Hyper-Kamiokande has put a lower bound on the proton's half-life as around 10³⁴ years.^[2] Since some grand unified theories such as the Georgi–Glashow model predict a half-life less than this, then the existence of X and Y bosons, as formulated by this particular model, remain hypothetical.

Details[edit]

An X boson would have the following two decay modes:^[1]: 442

X
⁺   →  
u
_L   +  
u
_R

X
⁺   →  
e⁺
_L   +  
d
_R

where the two decay products in each process have opposite chirality,
u
is an up quark,
d
is a down antiquark, and
e⁺
is a positron.

A Y boson would have the following three decay modes:^[1]: 442

Y
⁺   →  
e⁺
_L   +  
u
_R

Y
⁺   →  
d
_L   +  
u
_R

Y
⁺   →  
d
_L   +  
ν
_eR

where
u
is an up antiquark and
ν
_e is an electron antineutrino.

The first product of each decay has left-handed chirality and the second has right-handed chirality, which always produces one fermion with the same handedness that would be produced by the decay of a W boson, and one fermion with contrary handedness ("wrong handed").

Similar decay products exist for the other quark-lepton generations.

In these reactions, neither the lepton number (L) nor the baryon number (B) is separately conserved, but the combination B − L is. Different branching ratios between the X boson and its antiparticle (as is the case with the K-meson) would explain baryogenesis. For instance, if an
X
⁺ /
X
⁻ pair is created out of energy, and they follow the two branches described above:

X
⁺ →
u
_L +
u
_R ,

X
⁻ →
d
_L +
e⁻
_R ;

re-grouping the result   (
u
+
u
+
d
) +
e⁻
 = 
p
+
e⁻
  shows it to be a hydrogen atom.

Origin[edit]

The X^± and Y^± bosons are defined respectively as the six Q = ± 4/3 and the six Q = ± 1/3 components of the final two terms of the adjoint 24 representation of SU(5) as it transforms under the standard model's group:

.

The positively-charged X and Y carry anti-color charges (equivalent to having two different normal color charges), while the negatively-charged X and Y carry normal color charges, and the signs of the Y bosons' weak isospins are always opposite the signs of their electric charges. In terms of their action on X bosons rotate between a color index and the weak isospin-up index, while Y bosons rotate between a color index and the weak isospin-down index.

See also[edit]

• B − L
• Grand unification theory
• Leptoquark
• Proton decay
• W' and Z' bosons

References[edit]