Stable conjugacy and epipelagic L-packets for Brylinski-Deligne covers of Sp(2n)

Let $F$ be a local field of characteristic not $2$. We propose a definition of stable conjugacy for all the covering groups of $\mathrm{S}\mathrm{p}(2n,F)$ constructed by Brylinski and Deligne, whose degree we denote by $m$. To support this notion, we follow Kaletha’s approach to construct genuine epipelagic $L$-packets for such covers in the non-archimedean case with $p\nmid 2m$, or some weaker variant when $4\mid m$; we also prove the stability of packets when $F\supset {\mathbb{Q}}_p$ with $p$ large. When $m=2$, the stable conjugacy reduces to that defined by J. Adams, and the epipelagic $L$-packets coincide with those obtained by $\mathrm{\Theta }$-correspondence. This fits within Weissman’s formalism of L-groups. For $n=1$ and $m$ even, it is also compatible with the transfer factors proposed by K. Hiraga and T. Ikeda.