Short-interval sector problems for CM elliptic curves

Abstract

Let $E/\mathbb{Q}$ be an elliptic curve that has complex multiplication (CM) by an imaginary quadratic field $K$. For a prime $p$, there exists $\theta_p \in [0, \pi]$ such that $p+1-\#E(\mathbb{F}_p) = 2\sqrt{p} \cos \theta_p$. Let $x>0$ be large, and let $I\subseteq[0,\pi]$ be a subinterval. We prove that if $\delta>0$ and $\theta>0$ are fixed numbers such that $\delta+\theta<\frac{5}{24}$, $x^{1-\delta}\leq h\leq x$, and $|I|\geq x^{-\theta}$, then \[ \frac{1}{h}\sum_{\substack{x < p \le x+h \\ \theta_p \in I}}\log{p}\sim \frac{1}{2}\mathbf{1}_{\frac{\pi}{2}\in I}+\frac{|I|}{2\pi}, \] where $\mathbf{1}_{\frac{\pi}{2}\in I}$ equals 1 if $\frac{\pi}{2}\in I$ and $0$ otherwise. We also discuss an extension of this result to the distribution of the Fourier coefficients of holomorphic cuspidal CM newforms.