nuclearhead said:
Any theory today that predicts only the Standard Model particles will almost certainly be wrong as more particles and forces are discovered in the future. . . . So if you think that there will always be new particles at higher energies then you have to either believe in string theory or kalulza-klein theories as they are the ones that predict this. But this is impossible to prove experimentally, so you can only prove it with math and show that the infinities go away if you accept these heavy particles exist.
FWIW, I am not at all certain that more particles and forces will be discovered in the future. The SM particles plus a graviton plus a better understanding of the fabric of space-time may be it. If I were a Baysean, I might assign a prior probability of 33% to a scenario like that.
* Even if there are more particles, the number may be quite few. We may need new physics, and with it, new particles, to explain dark matter, dark energy, inflation, the strong CP problem, neutrino mass and oscillation, and baryogenesis and leptogenesis. But, we might not.
If there are dark matter fermions, there could be a set as complex as the SM fermions, there could be a triplet of sterile neutrino-like particles, or there could be a spin-1/2 or spin-3/2 singlet gravitino.
There might be in the dark sector, a set of bosons as complex as the SM bosons, there could be a DM self-interaction boson (perhaps a MeV scale mass dark photon), dark matter itself could be an axion-like boson (which might also give rise to dark energy), there might be a dark energy scalar boson, or there might be a couple of extra graviton-like bosons (the spin-2 graviton we know and love, a spin-1 vector graviton to give rise to DM effects, and a spin-0 graviton to give rise to dark energy).
There might be a separate inflation boson (probably a scalar or a tensor), or inflation might arise from some source that also explains something else (e.g. the Higgs field or a unified GUT boson or the fabric of space-time's properties or gravitational potential energy in a near singularity regime).
There might be a light Z boson-like particle that facilitates neutrino oscillation (perhaps also interacting with dark matter to generate neutrino mass in a see-saw mechanism) or there might not.
There might be an axion that addresses the strong CP problem and has no role in the dark sector or inflation or neutrino oscillation, but I really doubt it.
There might be a heavy W/Z boson-like particle that violates B and L number conservation or the universe's non-zero B and L number might have no discernible particle or force mechanism to explain it any more than we have a particle or force mechanism to explain why the universe has precisely the amount of mass-energy that it does.
* Honestly, I really doubt that there are even that many. My money would be on 5-6 more at most, and probably less. Dark energy, inflation and B and L violation, are probably more likely to be manifestations of already known particles and forces that behave in unexpected ways under certain circumstances.
For example, an inflation may be the form the SM gauge bosons (i.e. the photon, gluon, W and Z bosons) take at the GUT scale when there is a gauge unification, or might be a high energy manifestation of the Higgs boson and field, or it might arise from quantum gravity effects. Dark energy might very well be a manifestation of a baseline of graviton or photon or both kinds of radiation or a property of space-time itself. B and L might not be violated at all and have had their values since the Big Bang and not be broken symmetries
The DM sector might very well not have its own boson and only have a DM fermion. A singlet DM particle is as likely as a triplet in my mind. Then again, DM may not exist at all with its phenomena actually due to quantum gravity effects, or the exclusion of gravitational waves with lengths approaching the length of the universe, or ill appreciated non-Newtonian aspects of GR, or due to one or two more gravitational bosons in addition to the tensor graviton (e.g. perhaps a vector graviton giving rise to DM phenomena and a scalar graviton giving rise to dark energy).
At this point, I think that the likelihood of a zoo full of new fundamental particles a la the extra Higgs bosons and superpartners of SUSY is remote (probably less than 5%).
* I could imagine, and at some level expect, that all "fundamental" particles are actually made of some smaller subset of preons bound together by a single preon binding force with a boson to carry it, or even from just one or two kinds of strings. While, I suppose that this kind of substructure would qualify as new particles or forces, substructure like this might very well manifest only in the existing particle set.
* Also, I could imagine that we actually have too many particles already.
For example, I could imagine that the Higgs boson is really some manner of composite of the photon and the W+, W-, and Z bosons (e.g. if the photon and Z have opposite spin, and the W+ and W- have opposite spin, these four have a combined spin-0 even like the SM Higgs, and have the same combined charge, further the Higgs boson mass is very close to the masses of these four electro-weak bosons combined divided by the square root of four (the number of bosons being combined), and all particles that have a Higgs boson Yukawa also interact with the W and Z bosons, unlike the gluon which does not).
* I could also imagine that the existing SM forces unify and become indistinguishable from each other at a GUT scale, giving rise to a unified GUT boson that might be critical in understanding inflation, baryongenesis, leptogenesis and DM creation, but I wouldn't necessarily call that a new particle or force.
* I could also imagine that there are composite particles that exist in some circumstances but have not yet been discovered (e.g. various kinds of lepton-lepton atoms like muonium, unstable baryons that are stable in esoteric high energy conditions and form atoms in those circumstances, rare and short lived top quark hadrons, tetra/penta/hexa/septaquarks, glueballs and glue-quark hybrid particles).
* Similarly, I could imagine new effective "spillover forces" like the nuclear binding force between baryons which is a spillover of the strong force and carried by pions.