Smallest $\mathrm{SU}\mathbf{(}N\mathbf{)}$ hadrons

If new physics contains new, heavy strongly interacting particles belonging to irreducible representations of SU(3) different from the adjoint or the (anti)fundamental, it is a nontrivial question to calculate what is the minimum number of quarks/antiquarks/gluons needed to form a color-singlet bound state (“hadron”), or, perturbatively, to form a gauge-invariant operator, with the new particle. Here, I prove that for an SU(3) irreducible representation with Dynkin label $(p,q)$, the minimal number of quarks needed to form a product that includes the (0,0) representation is $2p+q$. I generalize this result to $\mathrm{SU}(N)$, with $N>3$. I also calculate the minimal total number of quarks/antiquarks/gluons that, bound to a new particle in the $(p,q)$ representation, gives a color-singlet state, or, equivalently, the smallest-dimensional gauge-invariant operator that includes quark/antiquark/gluon fields and the new strongly interacting matter field. Finally, I list all possible values of the electric charge of the smallest hadrons containing the new exotic particles and discuss constraints from asymptotic freedom both for QCD and for grand unification embeddings thereof.

Figure 1

A schematic representation of how the minimal number of direct products of the fundamental representation [here for $\mathbf{27}\sim (2,2)$ that number is $2p+q=6$] produces a Young tableau containing the trivial representation.