I've done some investigating on the matter of what happens if there is a hung jury. It varies by state. See below for the relevant California Penal Code 190.4 (b).
Hung jury - Wikipedia
Hung jury in sentencing phase of death penalty trials[edit]
Of the 30 U.S. states with the
death penalty, 28 provide the sentence to be decided by a
jury, and 27 require a unanimous sentence.
However, the states differ on what happens if the penalty phase results in a hung jury:
[11][12]
- In 4 states (Arizona, California, Kentucky and Nevada), a retrial of the penalty phase will be conducted before a different jury (the common-law rule for mistrial).[13]
- In 2 states (Indiana and Missouri), the judge will decide the sentence.
- In the 22 other states, a hung jury results in a life sentence, even if only one juror opposed death. Federal law also provides that outcome.
The first outcome is referred as the "true unanimity" rule, while the third has been criticized as the "single-juror veto" rule.
[14]
In Alabama, the sentence is decided by the jury and at least 10 jurors must concur. A retrial happens if the jury deadlock.
[15]
Nebraska is the only state in which the sentence is decided by a three-judge panel. If one of the judges on the panel opposes death, the defendant is sentenced to life imprisonment.
[16] Montana is the only state where the trial judge decides the sentence alone.
[17]
In all states in which the jury is involved, only
death-qualified veniremen can be selected in such a jury, to exclude both people who will always vote for the death sentence and those who are categorically opposed to it.
California Code, Penal Code - PEN § 190.4 | FindLaw
(b) If defendant was convicted by the court sitting without a jury the trier of fact at the penalty hearing shall be a jury unless a jury is waived by the defendant and the people, in which case the trier of fact shall be the court. If the defendant was convicted by a plea of guilty, the trier of fact shall be a jury unless a jury is waived by the defendant and the people.
If the trier of fact is a jury and has been unable to reach a unanimous verdict as to what the penalty shall be, the court shall dismiss the jury and shall order a new jury impaneled to try the issue as to what the penalty shall be. If such new jury is unable to reach a unanimous verdict as to what the penalty shall be, the court in its discretion shall either order a new jury or impose a punishment of confinement in state prison for a term of life without the possibility of parole.