Physics:X and Y bosons

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X± and Y± bosons
CompositionElementary particle
Mass≈ 1015 GeV/c2
Decays intoX: two quarks, or one antiquark and one charged antilepton
Y: two quarks, or one antiquark and one charged antilepton, or one antiquark and one antineutrino
electric chargeX: ±4/3 e
Y: ±1/3 e
|u}}r chargetriplet or antitriplet
Spin states3
Weak isospin projectionX: ±1/2
Y: ∓1/2
Weak hypercharge±5/6

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.


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

X+   →   uL   +   uR
X+   →   e+L   +   dR

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   +   uR
Y+   →   dL   +   uR
Y+   →   dL   +   νeR

where u is an up antiquark and νe 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+uL + uR ,
XdL + eR ;

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


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:

[math]\displaystyle{ \mathbf{24}\rightarrow (8,1)_0\oplus (1,3)_0\oplus (1,1)_0\oplus (3,2)_{-\frac{5}{6}}\oplus (\bar{3},2)_{\frac{5}{6}} }[/math].

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 [math]\displaystyle{ \ \mathbb{C}^5\ , }[/math] 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