Involution matrix loci and orbit harmonics

Abstract: Let $\mathrm{Mat}{n \times n}(\mathbb{C})$ be the affine space of $n \times n$ complex matrices with coordinate ring $\mathbb{C}[\mathbf{x}{n \times n}]$. We define graded quotients of $\mathbb{C}[\mathbf{x}{n \times n}]$ which carry an action of the symmetric group $\mathfrak{S}_n$ by simultaneous permutation of rows and columns. These quotient rings are obtained by applying the orbit harmonics method to matrix loci corresponding to all involutions in $\mathfrak{S}_n$ and the conjugacy classes of involutions in $\mathfrak{S}_n$ with a given number of fixed points. In the case of perfect matchings on ${1, \dots, n}$ with $n$ even, the Hilbert series of our quotient ring is related to Tracy-Widom distributions and its graded Frobenius image gives a refinement of the plethysm $s{n/2}[s_2]$.