2
$\begingroup$

I'm not a big fan of Science Alert, but this recent piece about the so-called SMASH model, whose gory details are apparently presented in arXiv:1608.05414 seems reasonable. I'm curious about this "Standard Model Axion Seesaw Higgs portal inflation" model, which they summarize it as follows:

Now, the team led by French [sic] physicist Guillermo Ballesteros from the University of Paris-Saclay says we can add these three right-handed neutrinos to the three existing neutrinos in the standard model, plus a subatomic particle called a colour triplet fermion, to solve the first four problems listed above.

For the benefit of those of us with some rudimentary understanding of group theory and particle physics, but no in-depth understanding of quantum chromodynamics - what's a "colour triplet fermion"?

Improve this question
$\endgroup$

1 Answer 1

Reset to default
4
$\begingroup$

A colour triplet fermion is simply a fermion that behaves like a quark with respect to the strong force, i.e. it transforms in the triplet (or fundamental) representation of the $\mathrm{SU}(3)$ gauge group of the strong force, often denoted $\mathbf{3}$. That this is indeed the meaning here can be seen on the bottom of page 1 of the linked paper:

[...] two Weyl fermions $Q$ and $\bar{Q}$ in the $\mathbf{3}$ and $\bar{\mathbf{3}}$ representations of $\mathrm{SU}(3)_c$ and with charges $-1/3$ and $+1/3$ under $\mathrm{U}(1)_\text{Y}$[...]

After the Higgs-like process proposed for this model happens, the two Weyl fermions will combine into a single Dirac fermion because they develop a mass, so effectively, this "colour triplet fermion" is a very quark-like particle with the same electrical ($\pm 1/3$) and color charge as a quark and no weak force coupling in either handedness (this is different from quarks) as far as I can tell.

Improve this answer
$\endgroup$
3
  • $\begingroup$ So this would break color confinement? i.e. with enough energy you could pop one of these guys into existence and have it carry a color charge as a free particle? $\endgroup$
    – Emilio Pisanty
    Commented Oct 31, 2016 at 19:49
  • $\begingroup$ @EmilioPisanty I don't think so, I would expect these particles to be confined just like quarks below the confinement scale. I seem to recall that adding much more (iirc, around 18) quark-like particles will eventually change the sign of the running of the coupling in QCD and give a qualitatively different theory without confinement, but this single particle won't do that. $\endgroup$
    – ACuriousMind ♦
    Commented Oct 31, 2016 at 19:59
  • $\begingroup$ Hmmm. So this is more of an extra non-quark quark? $\endgroup$
    – Emilio Pisanty
    Commented Oct 31, 2016 at 20:05

Not the answer you're looking for? Browse other questions tagged or ask your own question.