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SO(10) Unification vs Pati-Salam Model

Grand Unified Theories · within family

	SO(10) Unification Frontier	Pati-Salam Model Frontier
Proposed	1975	1974
Key figures	Harald Fritzsch, Peter Minkowski, Howard Georgi	Jogesh Pati, Abdus Salam
In one sentence	Fritzsch and Minkowski proposed in 1975 that SO(10) can unify the forces and place every particle of a generation, a right-handed neutrino included, in a single multiplet.	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	Every generation contains a right-handed neutrino, and ordinary neutrino masses come from the seesaw mechanism A gauged B-L symmetry, the difference between baryon and lepton number, breaks at a scale tied to the heavy neutrino masses Most versions predict proton decay, with a lifetime and dominant channel set by the breaking route; routes that push unification toward 10^18 GeV put it beyond current searches Some supersymmetric SO(10) constructions can suppress the Higgsino-driven proton decay that troubles supersymmetric SU(5)	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	Minimal models need large Higgs multiplets, such as a 126, to give neutrinos their seesaw masses, and the group does not fix the Higgs content. In most versions the heavy right-handed neutrinos sit far above collider energies, so the seesaw is tested only indirectly. Non-supersymmetric versions rely on intermediate breaking stages to keep the proton stable for long enough, which adds new scales chosen by hand.	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 breaking route: SO(10) can descend to the Standard Model by several paths, and each path changes the unification energy, the proton lifetime and the neutrino masses.	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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SO(10) Unification

1975 · Frontier

Pati-Salam Model

1974 · Frontier

Proposed

1975

1974

Key figures

Harald Fritzsch, Peter Minkowski, Howard Georgi

Jogesh Pati, Abdus Salam

In one sentence

Fritzsch and Minkowski proposed in 1975 that SO(10) can unify the forces and place every particle of a generation, a right-handed neutrino included, in a single multiplet.

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

Every generation contains a right-handed neutrino, and ordinary neutrino masses come from the seesaw mechanism
A gauged B-L symmetry, the difference between baryon and lepton number, breaks at a scale tied to the heavy neutrino masses
Most versions predict proton decay, with a lifetime and dominant channel set by the breaking route; routes that push unification toward 10^18 GeV put it beyond current searches
Some supersymmetric SO(10) constructions can suppress the Higgsino-driven proton decay that troubles supersymmetric SU(5)

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

Minimal models need large Higgs multiplets, such as a 126, to give neutrinos their seesaw masses, and the group does not fix the Higgs content.
In most versions the heavy right-handed neutrinos sit far above collider energies, so the seesaw is tested only indirectly.
Non-supersymmetric versions rely on intermediate breaking stages to keep the proton stable for long enough, which adds new scales chosen by hand.

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 breaking route: SO(10) can descend to the Standard Model by several paths, and each path changes the unification energy, the proton lifetime and the neutrino masses.

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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