What is the burden of proof for the prosecution in a murder case?

The federal Constitution does not impose on the prosecution a burden of proof as to penalty, and the state need not prove beyond a reasonable doubt whether aggravating circumstances exist, that the aggravating circumstances outweigh the mitigating circumstances, or that death is the appropriate penalty. [Citation.] Nor does the federal Constitution require that the jury be unanimous on which aggravating factors apply. (People v. Duenas (2012) 55 Cal.4th 1, 2728, 144 Cal.Rptr.3d 820, 281 P.3d 887.)