Questions tagged [grand-unification]

A Grand Unified Theory is a model in particle physics in which at ultra-high energy, the 3 gauge interactions of the Standard Model (electromagnetic, weak, and strong) merge into a single one, characterized by one larger encompassing gauge symmetry and one unified coupling constant. After SSBreaking, force carriers differentiate themselves and are driven by the RG to their values and numbers in the lab. Use for gauge GUT and gravity unification.

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5answers
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Why do physicists believe that there exist magnetic monopoles?

One thing I've heard stated many times is that "most" or "many" physicists believe that, despite the fact that they have not been observed, there are such things as magnetic monopoles. However, I've ...
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What does it mean to say that “the fundamental forces of nature were unified”?

It is said that immediately after the Big Bang, the fundamental forces of nature were unified. It is also said that later they decoupled, becoming separate forces. Indeed, if we look at the list of ...
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Why do we need complex representations in Grand Unified Theories?

EDIT4: I think I was now able to track down where this dogma originally came from. Howard Georgi wrote in TOWARDS A GRAND UNIFIED THEORY OF FLAVOR There is a deeper reason to require ...
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How does the Super-Kamiokande experiment falsify SU(5)?

In his book "The Trouble With Physics", Lee Smolin writes that he is still stunned by the falsification of the $SU(5)$ Georgi-Glashow model by the null results of proton decay experiments. I should ...
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Are there massless bosons at scales above electroweak scale?

Spontaneous electroweak symmetry breaking (i.e. $SU(2)\times U(1)\to U(1)_{em}$ ) is at scale about 100 Gev. So, for Higgs mechanism, gauge bosons $Z$ & $W$ have masses about 100 GeV. But before ...
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Dark matter and SO(10) grand unification

$SO(10)$ grand unified theories nicely accommodate a massive $\sim 10^{14-15}\; GeV$ sterile neutrino. Would this be a viable dark matter candidate? I haven't found any specific material regarding ...
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What's the deepest reason why QCD bound states have integer charge?

What's the deepest reason why QCD bound states have integer electric charge, i.e. equal to an integer times the electron charge? Given that the quarks have the fractional electric charges they do, ...
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How do we know we've unified two interactions?

What is the precise definition of unification of fields (in classical and quantum mechanics)? In general, does unification of a field mean that we can write both of them at both sides of an equation (...
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Triality and charge

I have a few questions about triality for the representations of $SU(3)$. (I have seen the wikipedia page, but it does not make the connection with physics.) What is triality, how can you compute ...
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What's a lepto-diquark?

This questions refers to Slansky's Group theory for unified model building, page 106 of chapter 7. He assigns the weight $(1)(01)$, which is stepwise projected from $E_6$ to $SU(2)\times SU(3)$, to a ...
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status of +4/3 scalar as explanation of $t\bar t$ asymmetry

One of the early proposals for the Tevatron asymmetry on $t \bar t$ was a "fundamental diquark" with a charge (and hypercharge) +4/3, either in a triplet or a sextet colour. I am interested on the ...
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Why should proton decay into positron rather than antimuon?

Grand unification theories, such as $SU(5)$/$SO(10)$/SUSY variants, suggest proton decay. The lack of observational evidence for proton decay rules out simple GUTs. But wait a minute! The GUT's ...
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How to decompose the representation of $\rm SU(5)$?

This question comes from Srednicki's textbook "Quantum Field Theory". On pages 514-515, it states: Under the unbroken $\rm SU(3)\times SU(2) \times U(1)$ subgroup, the $5$ representation of $\rm ...
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Is the 125 GeV Higgs boson some kind of a “almost-commutative graviton” at the electroweak scale?

The clumsy "almost-commutative graviton" is provocative. I use it on purpose, to ask two questions in one : Is the observation of only one Higgs and no supersymmetric particle below 8 TeV (up to now) ...
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Why is E6 favored over E8 for GUT building?

What is the simplest explanation on why is E6 gauge group more favored as a group for Grand unified theory builders, while E8 is not? What about other exceptional groups? Which of them originate from ...
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Where does the matching condition for $U(1)$ subgroups come from in unified models?

The matching conditions for a breaking $G \rightarrow \prod_i G_i$ are $$\omega_G-\frac{C_2(G)(\mu)}{12 \pi}=\omega_{G_i}-\frac{C_2(G_i)(\mu)}{12 \pi} ,$$ where $C_2(g)$ denotes the quadratic ...
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Is the Standard Model an invariant subgroup of $SU(5)$?

It is well known that the Standard Model (SM) gauge group is a subgroup of $SU(5)$: \begin{equation} SU(3) \times SU(2)\times U(1) ~\subset~SU(5) \end{equation} This can be easily checked using the ...
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Why are both C and CP violation necessary for baryogenesis?

Sakharov condition states both C and CP violation is necessary for baryogenesis. Now consider, for example, a theory with B-number violating interaction and C-violation. Therefore, if $p\to e^+\gamma$ ...
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Why do gauge bosons/leptoquarks not mediate proton decay in the Pati-Salam model?

In the Pati-Salam $\mathrm{SU}(4)_c\times\mathrm{SU}(2)_L\times\mathrm{SU}(2)_R$ model, I see Wikipedia and some slides mention this model doesn't predict gauge mediated proton decay without giving ...
5
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657 views

Non-renormalizable corrections to GUT unification

While writing these answers: Hypercharge for $U(1)$ in $SU(2)\times U(1)$ model and Is there a concise-but-thorough statement of the Standard Model? , it occured to me that the unification prediction ...
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Introduction to Physical Content from Adjoint Representations

In particle Physics it's usual to write the physical content of a Theory in adjoint representations of the Gauge group. For example: $24\rightarrow (8,1)_0\oplus (1,3)_0\oplus (1,1)_0\oplus (3,2)_{-\...
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What could be attractive in a desert? [closed]

It is not the Sahara, but the possibility that there isn't any interesting until the GUT scale. On the Wikipedia, I've read: "The attraction of a desert is that, in such a scenario, measurements ...
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362 views

Enormous masses of $X$- and $Y$-bosons in GUTs

I was reading an article on Wikipedia about the hypothetical particles called $X$ and $Y$ Bosons. Looking at their "calculated" masses, I felt quite weirded. Their mass should be $$ m = 10^{15}\,\...
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Where does in GUT symmetry breaking $U(1)$ come from?

In GUTs one starts with some larger group, like $SU(5)$, which is then broken into smaller groups, for example $$SU(5) ~\longrightarrow~ SU(3) \times SU(2) \times U(1)$$ This can be seen, for ...
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What is the most natural definition of the weak hypercharge coupling constant if grand unification is wrong?

A tricky question. Here is the famous graph of the running of the three coupling constants in the standard model: The graph shows, in its top curve, the running of the coupling constant $\alpha_1$. ...
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Supersymmetry and grand unification

I came across this diagram in my introductory particle physics class. Given the fact that SUSY can make the plot look suspiciously nice, what can we do with it? Also, electromagnetism has been ...
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(Why) Can we simply assume a given alignment of VEVs?

In GUT theories, one often assumes that Higgs fields take VEVs in given representations, of a given magnitude at a given scale. While I am well aware that this is one of the weak spots in such ...
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Color-charge conservation in proton decay

In some extensions of the Standard Model of particle physics (Supersymmetry with R-parity violation being a prominent example), the proton is allowed to decay, e.g. via $p\to e^+\pi^0$: While this ...
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1answer
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$SU(5)$ representation and higher anti-symmetric traces

In Zee QFT book v2 p.411 eq.16-17, he shows the SU(5) gauge theory anomaly cancellation by the following: The 1st line in fundamental of SU(5) $$ tr(T^3)=3(+2)^3+2(-3)^3=30, $$ is easy to follow, ...
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What is the experimental reason to believe in a Unified Theory?

Is there any experimental findings on behalf of the belief that all forces can be unified to a single force? Or the idea has it's inspiration from kind of thought experiment or philosophical belief.
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Is there some sort of Pati-Salam model with mixed generations?

The evidence for an approximate "lepton as fourth colour" symmetry is so overhelming in the particle spectrum that Hanlon's razor does not seem to apply. Still, my own incompetence fails to recognise ...
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Adjusting the rate of proton decay in the standard $\rm SU(5)$ grand unified theory

The proton decay rate in the standard $SU(5)$ grand unified theory is given by $$ \Gamma \sim \left(\frac{g^2}{M_x^2}\right)^2 m_p^5 =\frac{g^4}{M_x^4}m_p^5 $$ Naively we could push up the bound for ...
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Is the conjectured noncommutative heavy scalar “brother” of the already detected Higgs boson is a pseudo scalar?

This is a technical (may be trivial?) question about this sigma scalar field advertised by Chamseddine and Connes to improve the electroweak vacuum stability involved by the weak mass of the already ...
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1answer
698 views

Are there any good books about Grand Unified Theories?

I'm starting to get a little desperate, because I can't find any introductory treatments of Grand Unified Theories (GUT). So far the only books I found are Mohapatra Unification and Supersymmetry ...
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Is the big desert hypothesis a wilder assumption than the see-saw mechanism to explain neutrino masses?

Sometimes I see comments about the big desert hypothesis that I don't understand. For instance in a famous blog : ...This is based on a renormalization group calculation extrapolating the Higgs ...
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Why magnetic monopole has't been shown in the particle physics's Standard Model context?

For various reasons, experts, since Dirac, believe that a unified field theory must have a magnetic monopole. For example, Polyakov said: "I am quite certain that magnetic monopoles really exist. ...
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1answer
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Symmetry breaking of $SO(10)$ or $Spin(10)$

The Spin(10) grand unification has a symmetry breaking of SO(10), or Spin(10). In Wikipedia, it says, "The symmetry breaking of SO(10) is usually done with a combination of (( a $45_H$ OR a $54_H$...
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Why are there only two linearly independet quartic Higgs terms for the adjoint $24$ in $SU(5)$ GUTs?

I've read the statement in countless papers, for example, here Eq. 4.2 or here Eq. 2.1 without any further explanation or reference, that the "most general renormalizable Higgs potential" for an ...
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1answer
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Significance of $U(1)$ extensions of SM [closed]

Let's assume $U(1)$ extensions of SM with some detalizations: 1) Fermion sector of SM is extended by adding new very massive fermions; 2) Gauge group of SM is extended by adding new spontaneously $...
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Magnetic Monopoles and the Inflationary Epoch

What leads to magnetic monopoles existing before the Inflationary Epoch and becoming infinitely diluted during inflation (if indeed that is the explanation to their absence), but the electric ...
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$\mathbb{Z}_6$ symmetry in the Standard Model

It's a well-known result that the spontaneous symmetry breaking of $SU(5)$ would lead not to the usual $G_{SM}=SU(3)\times SU(2)\times U(1)$, but to $G_{SM}/\mathbb{Z}_6$. However, it's also often ...
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Which are the obstructions for $E_8^L\times E_8^R$ unification?

After left-right symmetry extension of the standard model, $$G' \times SU(2)^L \times SU(2)^R$$ it would seem that a logical pathway to grand unification is to try to obtain it by breaking down from ...
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Doublet-triplet splitting : Meaning of some mechanism names

In order to solve the Doublet–triplet splitting problem, people have developped several mechanisms, such as the Missing Partner, the Missing VEV and the Sliding Singlet mechanism. I couldn't really ...
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1answer
56 views

Sun's energy and grand unification

According to Wikipedia, The approximate grand unification energy value is equal to $1×10^{25}$ eV or $10^{16}$ GeV This is equal to $1602176$ J. Now, The Earth receives 174 petawatts (PW) ...
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1answer
122 views

Why do we want GUT groups to be simple (rather than, say, semisimple)?

I've heard before that when looking for grand unified gauge groups, with a single gauge coupling, we want to use simple groups, in the group-theoretic sense of having no non-trivial normal subgroups. ...
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2answers
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What is the relation between extra dimensions and unification of theories?

One of the most used methods in unification of theories is the use of higher dimensions. How does it actually work? If these dimensions are extremely small curled up, how does it affect the universe. ...
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1answer
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$SU(5)$ group theory: Contracting three adjoints to make a singlet

There's probably a trivial answer to my question, but I'm having trouble finding it. In an $SU(5)$ GUT, we have a gauge field, $A_{\mu} = A^{a}_{\mu}T^{a}$ which lives in the $24 (\text{Adjoint})$ ...
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1answer
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In which representation are monopoles of grand unifying theories classified?

In the context of grand unification theories A.Zee's book states that $SU(5)$ (or $SO(10)$ if $SU(5)$ is considered as outdated as GUT candidate) as GUT and as spontaneously broken non-abelian gauge ...
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1answer
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Can $E_8 \times E_8$ contain the standard model?

I know $E_8$ by itself can't be gauge group because it has no complex representation and so would not be chiral. But assuming the existence of mirror matter which also would have $E_8$ gauge group ...
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1answer
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How to break a irreducible representation into its subgroups

In Grand Unified Theories (though I'm sure this a general group theory result) people write the irreducible representations of a group (i.e., the gauge bosons) using a sum of irreducible ...