What is the isospin of the Higgs particle

In summary: The physical Higgs boson is its own antiparticle, so by CPT invariance it cannot carry a nonzero U(1) charge.
  • #1
johanw
24
1
I had always assumed that the simplest possible standard model Higgs particle had isospin 0, but when I tried to verify this assumption I read some conflicting opinions (not to mention discussions about more complicated Higgs mechanisms).

I also read conflicting views about the conservation of isospin, I know it is conserved under CPT but which processes violate it?
 
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  • #2
johanw said:
I had always assumed that the simplest possible standard model Higgs particle had isospin 0, but when I tried to verify this assumption I read some conflicting opinions (not to mention discussions about more complicated Higgs mechanisms).

I also read conflicting views about the conservation of isospin, I know it is conserved under CPT but which processes violate it?

There are two types of isospin commonly used in particle physics. What is usually called just "isospin" is the approximate SU(2) flavor symmetry of the up and down quarks that would be exact in the absence the mass difference between the quarks. The corresponding charge is the eigenvalue of the diagonal (##I_3##) generator on the individual particle states. Only the hadrons have nonzero isospin, so it is associated with the strong interaction.

The other type is "weak isospin," which is the eigenvalue of diagonal generator (##T_3##)for the SU(2) subgroup of the electroweak gauge group ##SU(2)\times U(1)_Y##. In the case of the up and down quarks, this coincides with the (strong) isospin, but this charge is carried by all left-handed fermions, including leptons.

The Higgs field is a doublet under weak isospin, with the electrically neutral component usually assigned ##T_3 = -1/2##. This includes the physical Higgs boson, but this excitation is defined around the vacuum state, for which ##T_3## is no longer a symmetry. The conserved charge remaining from the electroweak symmetry is the electric charge ##Q = T_3 + Y/2##, where ##Y## is the weak hypercharge of ##U(1)_Y##. Weak isospin and hypercharge are not conserved below the EW symmetry breaking scale.

Several facts correlate with nonconservation of the weak charges in the vacuum. The mass eigenstates of a Dirac fermion are combinations of left and right-handed components, so the mass eigenstate of, say, the electron, is not in a definite weak eigenstate. The Dirac mass terms explicitly break weak isospin.

Futhermore, the physical Higgs boson is its own antiparticle, so by CPT invariance cannot carry a nonzero U(1) charge. This would suggest the assignment ##T_3=0##, but the above discussion of symmetry breaking should already convince you that it doesn't make sense to assign any particular number to a broken symmetry.
 
  • #3
fzero said:
Futhermore, the physical Higgs boson is its own antiparticle, so by CPT invariance cannot carry a nonzero U(1) charge. This would suggest the assignment ##T_3=0##,

Sorry, but I don't understand that. If ##Y\neq0## and ##Q = 0##, how can then be ##T_3=0## if ##Q=T_3+Y/2##?

but the above discussion of symmetry breaking should already convince you that it doesn't make sense to assign any particular number to a broken symmetry.

That makes sense of course.
 
Last edited:

What is the isospin of the Higgs particle?

The isospin of the Higgs particle is a quantum number that describes its intrinsic angular momentum. It is denoted by I and can have a value of either 0 or 1. This quantum number is related to the strong nuclear force and is used to classify particles into different groups.

Why is the isospin of the Higgs particle important in particle physics?

The isospin of the Higgs particle is important because it helps us understand the fundamental forces and interactions between particles. It also plays a crucial role in the Standard Model of particle physics, which is the current theory that describes the behavior of all known particles and their interactions.

How is the isospin of the Higgs particle determined?

The isospin of the Higgs particle is determined through experiments and calculations based on its interactions with other particles. The Higgs particle is a boson, meaning it has integer spin, and its isospin can be measured through its interactions with other particles of different isospin values.

Does the isospin of the Higgs particle change in different environments?

Yes, the isospin of the Higgs particle can change in different environments. This is because its interactions with other particles can be affected by the environment, such as temperature and energy levels. However, the overall value of its isospin remains constant.

Is the isospin of the Higgs particle related to its mass?

No, the isospin of the Higgs particle is not related to its mass. Mass is a separate property of particles, while isospin is a quantum number that describes its intrinsic angular momentum. The Higgs particle has a fixed mass, while its isospin can vary depending on its interactions.

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