Higher moments for symmetric powers of modular forms

Let $f$ be a cuspidal eigenform of weight $k$ on ${\text{SL}}_2(\text{BZ})$ and let ${\lambda }_{{\text{Sym}}^{d}f}(n)$ be the normalized Fourier coefficients of its $d$-th symmetric power lift. This paper establishes asymptotic formulas for the moments ${\sum }_{n\le x}{\lambda }_{{\text{Sym}}^{d}f}^l(n)$ for all positive integers $d$ and $l$. We also prove an asymptotic formula for the corresponding sum over the values of any positive definite binary quadratic form $Q$. Our results generalize and improve upon previous work, which was limited to small values of $d$ or $l$. The proofs rely on the decomposition of $\mathrm{\ell }$-adic Galois representations and the analytic properties of the associated $L$-functions.