In physics, the Pati–Salam model is a Partial Unification Theory proposed in 1974 by nobel laureate Abdus Salam and Jogesh Pati. The unification is based on there being four quark color charges, dubbed red, green, blue and violet (or lilac), instead of the conventional three, with the new "violet" quark being identified with the leptons. The model also has Left–right symmetry and predicts the existence of a high energy right handed weak interaction with heavy W' and Z' bosons.
Contents
- Core theory
- Differences from the SU5 unification
- Spacetime
- Spatial symmetry
- Gauge symmetry group
- Global internal symmetry
- Vector superfields
- Chiral superfields
- Superpotential
- Left right extension
- References
Originally the fourth color was labelled "lilac" to alliterate with "lepton". Pati–Salam is a mainstream theory and a viable alternative to the Georgi–Glashow SU(5) unification. It can be embedded within an SO(10) unification model (as can SU(5)).
Core theory
The Pati–Salam model states that the gauge group is either SU(4) × SU(2)L × SU(2)R or (SU(4) × SU(2)L × SU(2)R)/Z2 and the fermions form three families, each consisting of the representations (4, 2, 1) and (4, 1, 2). This needs some explanation. The center of SU(4) × SU(2)L × SU(2)R is Z4 × Z2L × Z2R. The Z2 in the quotient refers to the two element subgroup generated by the element of the center corresponding to the two element of Z4 and the 1 elements of Z2L and Z2R. This includes the right-handed neutrino, which is now likely believed to exist. See neutrino oscillations. There is also a (4, 1, 2) and/or a (4, 1, 2) scalar field called the Higgs field which acquires a VEV. This results in a spontaneous symmetry breaking from SU(4) × SU(2)L × SU(2)R to SU(3) × SU(2) × U(1)Y)/Z3 or from (SU(4) × SU(2)L × SU(2)R)/Z2 to (SU(3) × SU(2) × U(1)Y)/Z6 and also,
(4, 2, 1) → (3, 2)1/6 ⊕ (1, 2)− 1/2 (q & l)(4, 1, 2) → (3, 1)1/3 ⊕ (3, 1)− 2/3 ⊕ (1, 1)1 ⊕ (1, 1)0 (d c, uc, ec & νc)(6, 1, 1) → (3, 1)− 1/3 ⊕ (3, 1)1/3(1, 3, 1) → (1, 3)0(1, 1, 3) → (1, 1)1 ⊕ (1, 1)0 ⊕ (1, 1)−1See restricted representation. Of course, calling the representations things like (4, 1, 2) and (6, 1, 1) is purely a physicist's convention, not a mathematician's convention, where representations are either labelled by Young tableaux or Dynkin diagrams with numbers on their vertices, but still, it is standard among GUT theorists.
The weak hypercharge, Y, is the sum of the two matrices:
Actually, it is possible to extend the Pati–Salam group so that it has two connected components. The relevant group is now the semidirect product
Since the homotopy group
this model predicts monopoles. See 't Hooft–Polyakov monopole.
This model was invented by Jogesh Pati and Abdus Salam.
This model doesn't predict gauge mediated proton decay (unless it is embedded within an even larger GUT group).
Differences from the SU(5) unification
As mentioned above, both the Pati–Salam and Georgi–Glashow SU(5) unification models can be embedded in a SO(10) unification. The difference between the two models then lies in the way that the SO(10) symmetry is broken, generating different particles that may or may not be important at low scales and accessible by current experiments. If we look at the individual models, the most important difference is in the origin of the weak hypercharge. In the SU(5) model by itself there is no left-right symmetry (although there could be one in a larger unification in which the model is embedded), and the weak hypercharge is treated separately from the color charge. In the Pati–Salam model, part of the weak hypercharge (often called U(1)B-L) starts being unified with the color charge in the SU(4)C group, while the other part of the weak hypercharge is in the SU(2)R. When those two groups break then the two parts together eventually unify into the usual weak hypercharge U(1)Y.
Spacetime
The N = 1 superspace extension of 3 + 1 Minkowski spacetime
Spatial symmetry
N=1 SUSY over 3 + 1 Minkowski spacetime with R-symmetry
Gauge symmetry group
(SU(4) × SU(2)L × SU(2)R)/Z2
Global internal symmetry
U(1)A
Vector superfields
Those associated with the SU(4) × SU(2)L × SU(2)R gauge symmetry
Chiral superfields
As complex representations:
Superpotential
A generic invariant renormalizable superpotential is a (complex) SU(4) × SU(2)L × SU(2)R and U(1)R invariant cubic polynomial in the superfields. It is a linear combination of the following terms:
Left-right extension
We can extend this model to include left-right symmetry. For that, we need the additional chiral multiplets (4, 2, 1)H and (4, 2, 1)H.