Let's add a simple uncharged massive particle to, say, the standard model of particle physics. The question is about the physical, observable consequences of this modification. The first question is if the following considerations are correct, and if not what is wrong. From a classical point of view, this would only add some stable field, if its initial value is zero it remains zero, if nonzero it remains nonzero, once no interaction terms with anything else are introduced, thus, its only consequence would be via gravity, it would be some cold (particle is massive) dark (no interaction with anything) matter, stable und with no possibility to create it. Now let's look at this from point of QFT. If some other heavy particle meets its antiparticle, so that there is sufficient energy, then there would be a possiblility of a creation of a pair (particle and antiparticle) of this new particle type, even without any particular interaction terms. That means, different from the classical picture, in QFT the particles of this field can be created. Inversion of this in time: If this new particle meets its antiparticle, they can annihilate and create something different. Once the particle is neutral, one often hears that it is its own antiparticle. So, what is created is simply a pair of particles, and, reversely, if two particles meet each other, they can annihilate and create some particle-antiparticle pairs of other sorts of (less massive) particles. That means the particle would be stable only as long as their density is not too large, because, if two particles meet each other, they can annihilate into something else like photons. Possibility of creation of such particles in particle accelerators: If the energy is large enough, pairs of such particles could be created and fly away in different directions. Once they do not react with anything else, these outcomes would be invisible. At least in principle, these events could be nonetheless observed - by observing that, if one looks for all visible results, some large amount of energy is missed. And this effect starts to appear at a certain critical amount of energy, which would be 2m of the particle mass. If all this is fine (my first question) then, is this also realistic? If yes, it is really observed, and what are the results (I could imagine that some range of low masses for such neutral scalar particles with no other interaction at all can be excluded).