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Supersymmetric SU(5) vs Georgi-Glashow SU(5)

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	Supersymmetric SU(5) Frontier	Georgi-Glashow SU(5) Rejected
Proposed	1981	1974
Key figures	Savas Dimopoulos, Howard Georgi	Howard Georgi, Sheldon Glashow
In one sentence	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.	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	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	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	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.	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 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.	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).
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Supersymmetric SU(5)

1981 · Frontier

Georgi-Glashow SU(5)

1974 · Rejected

Proposed

1981

1974

Key figures

Savas Dimopoulos, Howard Georgi

Howard Georgi, Sheldon Glashow

In one sentence

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.

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

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

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

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.

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 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.

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).

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