Compare · A Theory of Everything

SO(10) Unification vs Georgi-Glashow SU(5)

Grand Unified Theories · within family

	SO(10) Unification Frontier	Georgi-Glashow SU(5) Rejected
Proposed	1975	1974
Key figures	Harald Fritzsch, Peter Minkowski, Howard Georgi	Howard Georgi, Sheldon Glashow
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.	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.
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)	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
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.	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.
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 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).
Reader vote	No votes yet	No votes yet

SO(10) Unification

1975 · Frontier

Georgi-Glashow SU(5)

1974 · Rejected

Proposed

1975

1974

Key figures

Harald Fritzsch, Peter Minkowski, Howard Georgi

Howard Georgi, Sheldon Glashow

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.

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.

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)

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

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.

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.

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 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).

Reader vote

No votes yet