This article is about bosons of a hypothetical new interaction. For mesons with the same names, see
XYZ particle.
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).
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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.
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
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.
