# 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 $U_{V}(1)$ group with vector meson $V$ as mediator of $U_{V}(1)$ interactions

What's role play massive fermions and $V$ boson in explanation of unanswered phenomenas of modern physics (like baryogenesis, dark matter, neutrino oscillations problem)? Is the value of $V$ boson mass important for given explanation?

• You are referring to Z-prime extensions of the SM. See PDG (pdg.lbl.gov/2012/reviews/rpp2012-rev-zprime-searches.pdf) and references therein. Your question is rather broad May 18, 2015 at 8:51

That is quite a general question, and I can't give you a definite answer if you don't give me some other information. For example, are the SM fermions charged under this interaction? If so, with what charges? What are exactly the gauge representations of the new fermions? How do they appear in the Lagrangian?

You see, there are MANY ways of answering these questions, and each lead to a different model. I could give a good example, for example $U(1)$ being a $B-L$ symmetry that arises naturally in $SO(10)$ or large group GUTs. In this case the extra heavy fermions would be the conjugated right-handed neutrinos, with a heavy Majorana mass. This would lead to an anomaly free theory (as the right-handed neutrinos have the desired charges for that) with a sufficiently heavy right-handed SM singlet to produce a see-saw mechanism to generate low physical masses for the neutrinos.

Other than that, the options are literally infinite, even if the interesting ones are significantly less this means that you can have $U(1)$ symmetries with very different phenomena at low energies.

Also, if you are thinking in supersymmetric model building, some $U(1)$ can be added to enfore an R-symmetry, which has many appealing low energy consequences.

http://en.wikipedia.org/wiki/B_%E2%88%92_L

http://en.wikipedia.org/wiki/Seesaw_mechanism

http://en.wikipedia.org/wiki/R-symmetry