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I was studying particle physics and I saw the word "scalar particle" many times, also in books. On the internet there wasn't any thing about it.

So can anyone say what is a scalar particle ?

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    $\begingroup$ -1 Sorry, but are you serious about "on the internet there wasn't anything about it"? I find that highly improbable. There is literally a wikipedia page about it: en.wikipedia.org/wiki/Scalar_boson $\endgroup$ – infinitezero Oct 18 '19 at 21:54
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In short (and not very accurately) – a scalar particle is a particle that has no spin and no ‘inner structure’ that requires its field operator to have representation in terms of different components.

More accurately – the particles are excitations of fields, and when we construct these fields we have to ask ourselves how do they transform under rotations and boosts, which together form the group of Lorentz transformations. When analyzing the different ways that fields can transform under these transformations, the most basic and ‘simple’ type is the scalar, which is invariant and remains the same regardless of rotations and boosts. The particles that behave like this under the Lorentz group are then scalar particles. Similarly, we have spinors (such as electrons) and vectors (such as photons), which transform differently when we rotate the coordinate system or preform a boost operation.

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    $\begingroup$ The Higgs is an example of a scalar field, also called a “spin 0 boson”. $\endgroup$ – Andrea Oct 18 '19 at 14:21
  • $\begingroup$ Welcome to Physics StackExchange. Thank you for the great answer. Can you please give an example of a scalar particle? $\endgroup$ – Amey Joshi Oct 18 '19 at 14:21
  • $\begingroup$ @Andrea thanks. Your comment and my request for an example of a scalar particle came simultaneously!! $\endgroup$ – Amey Joshi Oct 18 '19 at 14:22
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    $\begingroup$ @AmeyJoshi I think that for the fundamental particles (that are in the standard model) only the Higgs boson is a scalar particle. However, I think that there are composite particles (Mesons) that are scalars. For example the Pions (I think?) $\endgroup$ – yu-v Oct 18 '19 at 14:25
  • $\begingroup$ I think it's a little awkward to call photon a vector. Since there is a distinction between a gauge spin 1 boson and a true vector boson. Or am I just wrongly informed about the naming convention? $\endgroup$ – Dvij Mankad Oct 18 '19 at 14:28
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Scalar particles are those which have total angular momentum is 0 and parity even (+).

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