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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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.