LHCb unveils new particles
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^1 A hadron is is made of quarks and gluons, which can only exist as bound states, such that they have no net strong charge. Contrast this with a lepton like the electron, that has no strong charge and can consequently exist as a free particle.
Even though we presume to know the dynamical equations of QCD exactly, I am pretty sure it was never shown rigorously that tetraquarks are possible under those equations.
Maybe if you edited your original post to better explain the answer you're looking for, someone could give it to you.
My own guess, as not-a-physicist, is that physicists expect the predictions made by existing science and their hypotheses to eventually be observed through experimentation. Given what we know, we can predict that some particles/types of particles should exist; projects like the LHC are created to test these predictions.
Or to put it simply: physicists might expect an upper bound on particle types because the available science doesn't suggest there should be an unlimited number of types of particles.
I was completely serious that insects and particles don't really constitute an apt comparison, but if I was a jerk for pointing it out then my apologies.
So that explains why nobody can figure out how the Universe works!
According to Don Page[1], that time (for our universe) is approximately 10^10^10^10^2.08 "Plank times, millenia, or whatever"[2].
[1] http://arxiv.org/abs/hep-th/9411193
[2] The units given in the paper (see [1], page 8)
Numberphile video about this paper: https://www.youtube.com/watch?v=1GCf29FPM4k
A.) It was clearly a joke. B.) That number was based off the Wikipedia heat death of the universe page reference for this: https://arxiv.org/abs/hep-th/0410270 C.) Regardless of the correct amount of time, no one will be there to verify it. We don't even understand enough yet to say with absolute confidence that heat death is the definitive final outcome of the universe.
But thanks for the reply :)
Of course, it's still very interesting, particularly that these are apparently composed of quarks in a manner than previously particles had not been.
(Edit: slightly recorded comment to make it sound less sarcastic, since I wasn't trying to be. I actually do think it's completely possible that "particle" doesn't have a definition; I just didn't think it was likely that that was what was being said.)
I'd also be interested in knowing whether particle physicists expect a finite number of quark-based particles and, if so, why.
Note that a tetraquark is different from two mesons, and a pentaquark is different from a baryon+meson.
As you increase the number of fundamental particles in a particle, modelling the exact mechanism of the bound state becomes harder. So for larger particles, we aren't even sure they should exist.
IIRC pentaquarks were something that theoretically were expected to exist, but were overall experimentally elusive.
All I see is that (again) models don't predict things, and they managed to make a slightly odd bit of dust. Every single other academic field has uses...this one seems to be filled with expensive errors.