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Pati-Salam Model vs Supersymmetric SU(5)

Grand Unified Theories · within family

	Pati-Salam Model Frontier	Supersymmetric SU(5) Frontier
Proposed	1974	1981
Key figures	Jogesh Pati, Abdus Salam	Savas Dimopoulos, Howard Georgi
In one sentence	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.	Dimopoulos and Georgi built an SU(5) theory with softly broken supersymmetry in 1981; LEP data in 1991 suggested its couplings could meet at one energy, but the minimal version is now excluded by proton decay searches.
Predictions	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	Superpartners of the known particles, which the 1991 coupling fit expected around 1 TeV, an expectation LHC searches have since pushed higher The three couplings meet at a single point near 10^16 GeV Proton decay mainly into a positive kaon and an antineutrino, driven by dimension-five operators Colored Higgs triplets far heavier than the Higgs doublets they are paired with
Where it breaks	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.	Murayama and Pierce (2002) showed that the minimal version is excluded by the Super-Kamiokande search for proton decay into a kaon and an antineutrino. Non-minimal supersymmetric SU(5) models remain under study. LHC searches at 13 TeV have found no gluinos or squarks, which puts pressure on simple scenarios with superpartners near the 1 TeV the 1991 fit assumed. Keeping the Higgs doublets light while their colored triplet partners stay superheavy needs an extremely precise adjustment of parameters, as Dimopoulos and Georgi noted.
Key unresolved problem	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.	The doublet-triplet problem: the Higgs doublets must stay light while their colored partners sit near the unification scale, and the minimal model manages this only by fine-tuning.
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Pati-Salam Model

1974 · Frontier

Supersymmetric SU(5)

1981 · Frontier

Proposed

1974

1981

Key figures

Jogesh Pati, Abdus Salam

Savas Dimopoulos, Howard Georgi

In one sentence

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.

Dimopoulos and Georgi built an SU(5) theory with softly broken supersymmetry in 1981; LEP data in 1991 suggested its couplings could meet at one energy, but the minimal version is now excluded by proton decay searches.

Predictions

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

Superpartners of the known particles, which the 1991 coupling fit expected around 1 TeV, an expectation LHC searches have since pushed higher
The three couplings meet at a single point near 10^16 GeV
Proton decay mainly into a positive kaon and an antineutrino, driven by dimension-five operators
Colored Higgs triplets far heavier than the Higgs doublets they are paired with

Where it breaks

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.

Murayama and Pierce (2002) showed that the minimal version is excluded by the Super-Kamiokande search for proton decay into a kaon and an antineutrino. Non-minimal supersymmetric SU(5) models remain under study.
LHC searches at 13 TeV have found no gluinos or squarks, which puts pressure on simple scenarios with superpartners near the 1 TeV the 1991 fit assumed.
Keeping the Higgs doublets light while their colored triplet partners stay superheavy needs an extremely precise adjustment of parameters, as Dimopoulos and Georgi noted.

Key unresolved problem

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.

The doublet-triplet problem: the Higgs doublets must stay light while their colored partners sit near the unification scale, and the minimal model manages this only by fine-tuning.

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