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Georgi-Glashow SU(5) vs Pati-Salam Model

Grand Unified Theories · within family

	Georgi-Glashow SU(5) Rejected	Pati-Salam Model Frontier
Proposed	1974	1974
Key figures	Howard Georgi, Sheldon Glashow	Jogesh Pati, Abdus Salam
In one sentence	Georgi and Glashow proposed in 1974 that three of nature's forces come from one SU(5) force; the minimal SU(5) model predicts a proton decay rate that experiments have ruled out, while larger SU(5) models are still studied.	Pati and Salam proposed in 1974 that lepton number is a fourth color charge, which puts quarks and leptons in shared families linked by new force carriers.
Predictions	The proton decays, most visibly into a positron and a neutral pion, with a lifetime the minimal model puts near 10^31 years The three Standard Model coupling strengths become equal at a single, very high unification energy The weak mixing angle, which sets how the weak and electromagnetic forces blend, is fixed by the symmetry and can be checked against measurement Stable superheavy magnetic monopoles can form when the unified symmetry breaks in the early universe, in numbers set by the cosmological history	Vector leptoquark bosons drive rare decays that the Standard Model forbids, such as a long-lived neutral kaon decaying into a muon and an electron A heavy right-handed W boson carries weak interactions of right-handed particles In the minimal seesaw realization, neutrino masses are tied to the right-handed W mass: the heavier that boson, the lighter the neutrinos
Where it breaks	Super-Kamiokande found no proton decaying into a positron and a neutral pion and set a lifetime limit above 2.4 x 10^34 years, far beyond the minimal prediction. Precise LEP data show that the three couplings of the non-supersymmetric model miss a single meeting point by more than seven standard deviations. The model offers no reason why the light Higgs doublet stays light while its colored partner, which also drives proton decay, must be superheavy. A hot early universe would fill with monopoles. Removing them needs an extra ingredient such as [[inflation]], which Guth linked to monopole suppression in 1981.	In the conventional model, rare kaon decays force the symmetry to break above the PeV scale, far out of collider reach. The exact bound depends on how quarks pair with leptons, and general fermion mixing still leaves leptoquarks above 86 TeV. Pati-Salam combines three groups, each with its own coupling, so the forces stay partly separate unless the model sits inside a larger group. Bringing leptoquarks down to the TeV scale needs extra vectorlike fermions that the original model did not have.
Key unresolved problem	The minimal model is ruled out, so the open question is which extension of SU(5), if any, nature uses: more Higgs fields, supersymmetry, or a larger group such as SO(10).	The breaking scale: rare kaon decays push the conventional model above the PeV range, so its quark-lepton force carriers may lie far beyond any collider.
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Georgi-Glashow SU(5)

1974 · Rejected

Pati-Salam Model

1974 · Frontier

Proposed

1974

Key figures

Howard Georgi, Sheldon Glashow

Jogesh Pati, Abdus Salam

In one sentence

Georgi and Glashow proposed in 1974 that three of nature's forces come from one SU(5) force; the minimal SU(5) model predicts a proton decay rate that experiments have ruled out, while larger SU(5) models are still studied.

Pati and Salam proposed in 1974 that lepton number is a fourth color charge, which puts quarks and leptons in shared families linked by new force carriers.

Predictions

The proton decays, most visibly into a positron and a neutral pion, with a lifetime the minimal model puts near 10^31 years
The three Standard Model coupling strengths become equal at a single, very high unification energy
The weak mixing angle, which sets how the weak and electromagnetic forces blend, is fixed by the symmetry and can be checked against measurement
Stable superheavy magnetic monopoles can form when the unified symmetry breaks in the early universe, in numbers set by the cosmological history

Vector leptoquark bosons drive rare decays that the Standard Model forbids, such as a long-lived neutral kaon decaying into a muon and an electron
A heavy right-handed W boson carries weak interactions of right-handed particles
In the minimal seesaw realization, neutrino masses are tied to the right-handed W mass: the heavier that boson, the lighter the neutrinos

Where it breaks

Super-Kamiokande found no proton decaying into a positron and a neutral pion and set a lifetime limit above 2.4 x 10^34 years, far beyond the minimal prediction.
Precise LEP data show that the three couplings of the non-supersymmetric model miss a single meeting point by more than seven standard deviations.
The model offers no reason why the light Higgs doublet stays light while its colored partner, which also drives proton decay, must be superheavy.
A hot early universe would fill with monopoles. Removing them needs an extra ingredient such as [[inflation]], which Guth linked to monopole suppression in 1981.

In the conventional model, rare kaon decays force the symmetry to break above the PeV scale, far out of collider reach. The exact bound depends on how quarks pair with leptons, and general fermion mixing still leaves leptoquarks above 86 TeV.
Pati-Salam combines three groups, each with its own coupling, so the forces stay partly separate unless the model sits inside a larger group.
Bringing leptoquarks down to the TeV scale needs extra vectorlike fermions that the original model did not have.

Key unresolved problem

The minimal model is ruled out, so the open question is which extension of SU(5), if any, nature uses: more Higgs fields, supersymmetry, or a larger group such as SO(10).

The breaking scale: rare kaon decays push the conventional model above the PeV range, so its quark-lepton force carriers may lie far beyond any collider.

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