physics, mathematical physics, philosophy of physics

Surveys, textbooks and lecture notes

theory (physics), model (physics)

experiment, measurement, computable physics



The standard model of particle physics asserts that the fundamental quantum physical fields and particles are modeled as sections of and connections on a vector bundle that is associated to a GG-principal bundle, where the Lie group GG – called the gauge group – is the product of (special) unitary groups G=SU(3)×SU(2)×U(1)G = SU(3) \times SU(2) \times U(1) (or rather a quotient of this by the cyclic group Z/6Z/6) and where the representation of GG used to form the associated vector bundle looks fairly ad hoc on first sight.

A grand unified theory (“GUT” for short) in this context is an attempt to realize the standard model as sitting inside a conceptually simpler model, in particular one for which the gauge group is a bigger but simpler group G^\hat{G}, preferably a simple Lie group in the technical sense, which contains GG as a subgroup. Such a grand unified theory would be phenomenologically viable if a process of spontaneous symmetry breaking at some high energy scale – the “GUT scale” – would reduce the model back to the standard model of particle physics without adding spurious extra effects that would not be in agreement with existing observations in experiment.

The terminology “grand unified” here refers to the fact that such a single simple group G^\hat{G} would unify the fundamental forces of electromagnetism, the weak nuclear force and the strong nuclear force in a way that generalizes the way in which the electroweak field already unifies the weak force and electromagnetism, and electromagnetism already unifies, as the word says, electricity and magnetism.

Historically, the most studied choices of GUT-groups GG are SU(5), Spin(10) (in the physics literature often referred to as SO(10)) and E6 (review includes Witten 86, sections 1 and 2).

It so happens that, mathematically, the sequence SU(5), Spin(10), E6 naturally continues (each step by consecutively adding a node to the Dynkin diagrams) with the exceptional Lie groups E7, E8 that naturally appear in heterotic string phenomenology (exposition is in Witten 02a) and conjecturally further via the U-duality Kac-Moody groups E9, E10, E11 that are being argued to underly M-theory. In the context of F-theory model building, also properties of the observes Yukawa couplings may point to exceptional GUT groups (Zoccarato 14, slide 11, Vafa 15, slide 11).

Since no GUT model has been fully validated yet (but see for instance Fong-Meloni 14), GUTs are models “beyond the standard model”. Often quantum physics “beyond the standard model” is expected to also say something sensible about quantum gravity and hence unify not just the three gauge forces but also the fourth known fundamental force, which is gravity. Models that aim to do all of this would then “unify” the entire content of the standard model of particle physics plus the standard model of cosmology, hence “everything that is known about fundamental physics” to date. Therefore such theories are then sometimes called a theory of everything.

(Here it is important to remember the context, both “grand unified” and “of everything” refers to aspects of presently available models of fundamental physics, and not to deeper philosophical questions of ontology.)


Relation to proton decay

Many GUT models imply that the proton – which in the standard model of particle physics is a stable bound state (of quarks) – is in fact unstable, albeit with an extremely long mean liftetime, and hence may decay (e.g. KM 14). Experimental searches for such proton decay (see there for more) put strong bounds on this effect and hence heavily constrain or rule out many GUT models.

Recently it was claimed that there are in fact realistic GUT models that do not imply any proton decay (Mütter-Ratz-Vaudrvange 16, Fornal-Grinstein 17).

Relation to neutrino masses

The high energy scale required by a seesaw mechanism to produce the experimentally observer neutrino masses happens to be about the conventional GUT scale, for review see for instance (Mohapatra 06).


Original articles include

An original article with an eye towards supergravity unification is

A textbook account is in

Survey of arguments for the hypothesis of grand unification includes

Introduction to GUTs aimed more at mathematicians include

Discussion of comparison of GUTs to experiment and phenomenology includes

for non-superymmetric models:

  • L. Lavoura and Lincoln Wolfenstein, Resuscitation of minimal SO(10)SO(10) grand unification, Phys. Rev. D 48, 264 (doi:10.1103/PhysRevD.48.264)

  • Guido Altarelli, Davide Meloni, A non Supersymmetric SO(10) Grand Unified Model for All the Physics below M GUTM_{GUT} (arXiv:1305.1001)

  • Alexander Dueck, Werner Rodejohann, Fits to SO(10)SO(10) Grand Unified Models (arXiv:1306.4468)

  • Chee Sheng Fong, Davide Meloni, Aurora Meroni, Enrico Nardi, Leptogenesis in SO(10)SO(10) (arXiv:1412.4776)

for supersymmetric models:

  • Archana Anandakrishnan, B. Charles Bryant, Stuart Raby, LHC Phenomenology of SO(10)SO(10) Models with Yukawa Unification II (arXiv:1404.5628)

  • Ila Garg, New minimal supersymmetric SO(10)SO(10) GUT phenomenology and its cosmological implications (arXiv:1506.05204)

Realization of GUTs in the context of M-theory on G2-manifolds and possible resolution of the doublet-triplet splitting problem is discussed in

Discussion of GUTs in F-theory includes

Discussion of experimental bounds on proton decay in GUTs includes

  • Helena Kolešová, Michal Malinský, Proton lifetime in the minimal SO(10)SO(10) GUT and its implications for the LHC, Phys. Rev. D 90, 115001 (2014) (arXiv:1409.4961)

Claim that proton decay may be entirely avoided:

  • Andreas Mütter, Michael Ratz, Patrick K.S. Vaudrevange, Grand Unification without Proton Decay (arXiv:1606.02303)

    (claims that many string theory and supergravity models have this property)

  • Bartosz Fornal, Benjamin Grinstein, SU(5)SU(5) Unification without Proton Decay, Physics Review Letters (arXiv:1706.08535)

Last revised on July 7, 2018 at 06:16:05. See the history of this page for a list of all contributions to it.