##SU(2)## doublets, Majorana Fermions and Higgs

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Discussion Overview

The discussion revolves around the properties and implications of ##SU(2)## doublets and Majorana fermions in the context of their coupling to the Higgs field via Yukawa interactions. Participants explore the significance of various terms in the Lagrangian, gauge invariance, and the mathematical structure of fermion fields.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants inquire about the difference between ##L## and ##L^{c}##, suggesting that the superscript ##c## signifies charge conjugation.
  • One participant claims that the term ##LL^{c}## breaks ##SU(2)## gauge invariance, prompting questions about how this occurs.
  • Another participant points out that the term ##\bar{L}L^{c}## is not a hypercharge singlet due to both fields having the same hypercharge, raising concerns about the correct formulation of the mass terms.
  • There is a discussion about the necessity of including the bar in the expression for Lorentz invariance, with one participant referencing an article that omits it.
  • Participants debate the validity of using the mass term ##M_{0}\bar{L}L## instead of ##M_{0}\bar{L}L^{c}##, with one asserting that the former cannot involve two left-handed fields and the latter violates gauge invariance.
  • Questions arise regarding the terminology of ##M_0## as a vector mass rather than simply a mass, indicating a need for clarification on this distinction.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the terms in the Lagrangian, particularly regarding gauge invariance and the necessity of certain field configurations. No consensus is reached on the correct formulation or interpretation of these terms.

Contextual Notes

There are unresolved questions regarding the assumptions made about gauge invariance and the definitions of the fermion fields. The discussion reflects a variety of perspectives on the mathematical formalism involved.

Who May Find This Useful

This discussion may be of interest to those studying particle physics, particularly in the context of electroweak interactions and the role of Higgs couplings in fermion mass generation.

spaghetti3451
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Say ##L## and ##L^{c}## are a pair of ##SU(2)## doublets (electroweak-charge fermions) and ##N_{1}## and ##N_{1}^{c}## are a pair of neutral Majorana fermions.

Say that these fermions couple to the Higgs via Yukawa coupling and have vector masses ##M_0## and ##M_1## respectively:

$$M_{0}LL^{c} + M_{1}N_{1}N_{1}^{c} + YHLN_{1}^{c} + Y^{c}H^{\dagger}L^{c}N_{1}$$What is the difference between ##L## and ##L^{c}##?

What does the superscript ##c## signify?
 
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Charge conjugation. Your ##LL^c## term breaks SU(2) gauge invariance.
 
How does the ##LL^{c}## term break ##SU(2)## gauge invariance?
 
I really meant to say SU(2)xU(1). You are missing a number of bars on your fermion fields. The term ##\bar L L^c## is not a hypercharge singlet because ##\bar L## and ##L^c## have the same hypercharge.
 
Orodruin said:
I really meant to say SU(2)xU(1). You are missing a number of bars on your fermion fields. The term ##\bar L L^c## is not a hypercharge singlet because ##\bar L## and ##L^c## have the same hypercharge.

But, ##L## is the complex conjugate of ##L^{c}##. So, isn't ##LL^{c}## a scalar?

Why then do we need to have ##\bar{L}L^{c}##?
 
Without the bar your expression is not Lorentz invariant.
 
The Lagrangian is taken from equation (1.1) in page 2 of the article in the link https://arxiv.org/abs/1609.06320.

In the article, there is no bar on ##L##. What am I getting wrong here?
 
They are being sloppy. Any specialist reading that is going to understand what they mean.
 
Okay, in the Dirac Lagrangian, it is possible to have the mass term ##m\bar{\psi}\psi##.

So, why can't we have the term ##M_{0}\bar{L}L## and not ##M_{0}\bar{L}L^{c}## here?
 
  • #10
The first one because you cannot have a mass term involving two left-handed fields. The second because it violates gauge invariance.
 
  • #11
So, let me get this right:

the correct term is ##M_{0}LL^{c}## and not ##M_{0}\bar{L}L^{c}##?
 
  • #12
Also, why is ##M_0## called the vector mass and not simply the mass?
 

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