We know that each elementary particle/entity must have mass, charge and spin defined. Are there any other attributes (independent) that must be defined for every elementary particle? is there any predicted attribute that can't be measured with current technology .

  • $\begingroup$ @chiralanomaly more like 'what are the most basic set of properties that EACH particle must POSSESS' . Is there any other property beside the three mentioned. Can color be considered as one. If you can give detailed answer in both cases , that would be very enlightening. $\endgroup$ – shashank shekhar singh Apr 18 '20 at 17:14
  • $\begingroup$ @chiralnomaly Fine , so those 'spectrum of particles' is what i am asking about. Their properties specifically. Even if they are predicted phenomena , they do have a set of properties. What is that common set is what i want to know. What are your thoughts on colour. Photons don't have color right? $\endgroup$ – shashank shekhar singh Apr 18 '20 at 17:39
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    $\begingroup$ Well i thought the list would be very small. Yes magnetic moment may be one and things like that. Even an inaccurate list wold be good. Yes it is similar to what i am looking for. $\endgroup$ – shashank shekhar singh Apr 18 '20 at 18:54

What property must be assigned to a fundamental particle?

That it can't be broken down into other particles.

What properties can be assigned to a fundamental particle?

Lots. Electric Charge (photon = 0), mass (gluon = 0), spin (Higgs = 0), Lepton and Baryon number, Parity, Isospin, etc.

  • $\begingroup$ Reading the comments, I think the question is a bit meaningless. What properties do they all share? If having zero mass, charge, or spin (examples for all of these exist), means that property isn't in the set, then the answer is a fundamental particle isn't necessarily defined by any of these. A fundamental particle is simply the smallest constituent currently observed. These properties alone aren't enough to describe the SM or interactions. $\endgroup$ – D. Jones Apr 18 '20 at 17:58
  • $\begingroup$ @djones But the mass, charge and spin whether zero or not have to be specified. E.g. photons don't have color charge but gluons and quarks have so it 'is' not a shared property. In that sense what are the common properties do these fundamental particles need to have. There can't be too many i think. $\endgroup$ – shashank shekhar singh Apr 18 '20 at 19:13
  • $\begingroup$ My point is this: the question doesn't mean much without context; fundamental particles aren't defined by properties, rather the idea they can't be broken down (that we know), this is the single property; claiming the fundamental particles share a set of "fundamental" parameters which can be zero-valued (as I said, all 3 attributes listed had examples in SM) means we may as well claim that all fund. particles must have defined e.g. lepton numbers, technically only associated with leptons. This leads into the realm of conservation laws and selection rules, which continues developing. $\endgroup$ – D. Jones Apr 18 '20 at 19:43
  • $\begingroup$ @djones if you can elaborate on that then it might be acceptable as the answer $\endgroup$ – shashank shekhar singh Apr 18 '20 at 19:46

For starters, the different quantum states of an elementary particle must give rise to an irreducible representation of the Poincare group, cf. Wikipedia, and this, this, this, this & this related Phys.SE posts.


What does fundamental mean? The root comes from "fundament":

Middle English (also denoting the base of a building, or the founding of a building or institution): from Old French fondement, from Latin fundamentum, from fundare ‘to found’.

Buildings, in order to be sound, were standing on the fundaments, large stone structures within the ground that helped the building to withstand floods and earthquakes.

The word "fundamental particle" says it all for particle physics theories. They are particles on which the specific theory is built, that models and predicts behavior of data , and is validated if correct or discarded if wrong.

So to answer such a question one must specify the theory. For the standard model particle theory, which has the symmetries of SU(3)xSU(2)xU(1), its fundamental particles are in the table . In this table the specific fundamental particles must obey the group structure above. The group structure does not depend on the masses, although it was first discovered by various mass arrangements. The group structure depends on the quantum numbers assigned to the particles in the table, including the charge. They are axiomatically defined as fundamental, and they keep being so whether before symmetry breaking or after . The theory fits the data up to now, and the theory is built based on the table and its specific attributes.

If new theories develop with new group symmetries , GUT for example, more fundamental particles would be needed. In string theories, which embed the standard model, the string itself is the fundamental entity , and the elementary particles of the table are string excitations.


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