On Hardy's $Z$-function and its derivatives associated with Selberg class

Hardy's $Z$-function $Z(t)$ is a real-valued function of the real valuable $t$, and its zeros exactly correspond to those of the Riemann zeta-function on the critical line. In 2012, K.~Matsuoka showed that for any non-negative integer $k$, there exists a $T=T(k)>0$ such that $Z^{(k+1)}(t)$ has exactly one zero between consecutive zeros of $Z^{(k)}(t)$ for $t\ge T$ under the Riemann Hypothesis. In this article, we extend Matsuoka's theorem to some $L$-functions in Selberg class.