Spin and flavor projection operators in the $SU(2N_f)$ spin-flavor group

TL;DR

This paper develops a method using quadratic Casimir operators to construct projection operators for decomposing representations of the $SU(2N_f)$ spin-flavor group, aiding analysis in large-$N_c$ QCD baryon studies.

Contribution

It introduces a general approach to build spin and flavor projection operators within the $SU(2N_f)$ group, specifically tailored for large-$N_c$ QCD baryon analysis, with explicit examples for $SU(2)$ and $SU(3)$.

Findings

Constructed explicit projection operators for $SU(2)$ and $SU(3)$.

Enabled effective decomposition of representations in large-$N_c$ QCD.

Facilitated analysis of the $1/N_c$ operator expansion.

Abstract

The quadratic Casimir operator of the special unitary $SU(N)$ group is used to construct projection operators, which can decompose any of its reducible finite-dimensional representation spaces contained in the tensor product of two and three adjoint spaces into irreducible components. Although the method is general enough, it is specialized to the $SU(2N_f) \to SU(2)\otimes SU(N_f)$ spin-flavor symmetry group, which emerges in the baryon sector of QCD in the large-$N_c$ limit, where $N_f$ and $N_c$ are the numbers of light quark flavors and color charges, respectively. The approach leads to the construction of spin and flavor projection operators that can be implemented in the analysis of the $1/N_c$ operator expansion. The use of projection operators allows one to successfully project out the desired components of a given operator and subtract off those that are not needed. Some explicit examples in $SU(2)$ and $SU(3)$ are detailed.