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The $W$ and $Z$ particles are noted in Lagrangian in the form of $W^{\mu}$ and $Z^{\mu}$, in order to construct quantities such as $W_{\mu}W^{\mu}$ and $Z_{\mu}Z^{\mu}$.

Could the Higgs (that appears in $\frac{v+H}{\sqrt{2}}$) also be noted $H^{\mu}$?

If so, what about quantities such as: $W_{\mu}W^{\mu}H$? Should a $\mu$ be put on the $H$? If not, why?

Qmechanic
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  • I know that Higgs has a spin of 0, and I know that it means scalar. But I don't know if there is a relationship with that. I don't know if a quadrivector (which expresses the contribution of time and space) is possible only for a vector particle. Do you mean that quadrivector is only for vector particles ? – Mathieu Krisztian Jul 06 '22 at 18:38
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    Are you familiar with the GSW theory of electroweak symmetry breaking? There are plenty of free notes available online. A simple answer is, that W and Z are vector bosons, hence you have on the component form a mu index. Higgs is a scalar boson, thus it does not have indices – Kregnach Jul 06 '22 at 18:39
  • yes, I know about GSW theory. ok. But from your explanation, if Higgs does not depends on x, y, z, then, what does mean terms as $\partial^{\mu} H$ ? Derivate of Higgs with x, y and z should be 0 as well ? So $\partial^{\mu} H$ should be 0, isn't it ? – Mathieu Krisztian Jul 06 '22 at 18:40
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    @MathieuKrisztian no, if you put h and Z, W into the Lagrangian, you gave a kibetic term for "h", the Higgs field. Think about temperature, T, is it a vector? No... Does it depend on x, y, z, t ? Yes, thus you could technically take a derivative of it and would get a nonzero quantity (in case it's not a constant). Being a scalar doesn't mean it's a constant. – Kregnach Jul 06 '22 at 19:02

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After reading your comment below your question: a field is a function of space-time, no matter its spin: roughly speaking, a field with spin 0

  • has scalar values, because its spin is 0
  • depends on space and time variables, because it's a field

while a spin 1 field

  • has vector values, because its spin is 1
  • depends on space and time variables, because it's a field

So the Higgs field doesn't have any Lorentz index ($H$), while the $W$ field has a Lorentz index ($W^\mu$).

A term like $W^\mu W_\mu H$ is an acceptable interaction term because it's a Lorentz scalar (no Lorentz index left). It describes any interaction between a Higgs and two $W$, for example, two $W$ interacting to produce a Higgs.

Edit:

A vector field is a function of space and time that takes vector values.

  • In classical physics, they're traditionally written $\vec{F}(x,y,z,t)$, the arrow meaning that the field has vector values in $\mathbb{R}^3$ or $\mathbb{C}^3$.
  • In (non quantum) relativity, they're traditionally written $F^\mu(x,y,z,t)$. It's exactly the same thing, except that the vector values are in $\mathbb{R}^4$ or $\mathbb{C}^4$.

It's a bit more complicated than that in quantum field theory because the fields don't have numerical values, but this idea is the same (they're representations of the Lorentz group in 4 dimensions).

Miyase
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  • sorry, where is the explanation for your comment : "So the Higgs field doesn't have any Lorentz index" ? The lorentz indexes express : for index 0 : the time, for 1, 2, 3 : the space. So time index always exist, isn't it ? Do the space index not exist for a scalar ? – Mathieu Krisztian Jul 06 '22 at 19:00
  • @MathieuKrisztian It's the fact that it's spin 0. As I said in my answer, spin 0 = scalar = no index, spin 1 = vector = index. – Miyase Jul 06 '22 at 19:01
  • but scalar is meaning no depending on the space ? So why do you skip the time component ? – Mathieu Krisztian Jul 06 '22 at 19:02
  • No it doesn't mean that. Scalar only means that its value is a scalar, but that scalar can still depend on position and time (like a potential in classical physics). – Miyase Jul 06 '22 at 19:03
  • ok, thanks. So what do the Lorentz indexes express ? – Mathieu Krisztian Jul 06 '22 at 19:04
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    I'll edit my answer in a minute, tell me if it's clearer. – Miyase Jul 06 '22 at 19:05
  • It only means that it has components, concretely as many as space-time coordinates that is why they are called vector-fields (each coordinate can depend on all of the space-time coordinates, not just the one with the same index). And then the scalar field only has 1 component (scalar), so it doesn't have index, but that component can depend on everything again. [Sumarizing, the index tells you which component of the field you are talking about, some field have more components some less, but all of those depend on all coordinates of space-time] – Guillermo Abad Lopéz Jul 06 '22 at 19:14
  • Thanks a lot for your kind explanations. – Mathieu Krisztian Jul 06 '22 at 19:17