(1) When a person has been found guilty of murder in the first degree and (a) a jury renders a verdict finding the existence of one or more aggravating circumstances as provided in section 29-2520 or (b)(i) the information contains a notice of aggravation as provided in section 29-1603 and (ii) such person waives his or her right to a jury determination of the alleged aggravating circumstances, the sentence of such person shall be determined by:

(a) A panel of three judges, including the judge who presided at the trial of guilt or who accepted the plea and two additional active district court judges named at random by the Chief Justice of the Supreme Court. The judge who presided at the trial of guilt or who accepted the plea shall act as the presiding judge for the sentencing determination proceeding under this section; or

(b) If the Chief Justice of the Supreme Court has determined that the judge who presided at the trial of guilt or who accepted the plea is disabled or disqualified after receiving a suggestion of such disability or disqualification from the clerk of the court in which the finding of guilty was entered, a panel of three active district court judges named at random by the Chief Justice of the Supreme Court. The Chief Justice of the Supreme Court shall name one member of the panel at random to act as the presiding judge for the sentencing determination proceeding under this section.

(2) In the sentencing determination proceeding before a panel of judges when the right to a jury determination of the alleged aggravating circumstances has been waived, the panel shall, as soon as practicable after receipt of the written report resulting from the presentence investigation ordered as provided in section 29-2261, hold a hearing. At such hearing, evidence may be presented as to any matter that the presiding judge deems relevant to sentence and shall include matters relating to the aggravating circumstances alleged in the information, to any of the mitigating circumstances set forth in section 29-2523, and to sentence excessiveness or disproportionality. The Nebraska Evidence Rules shall apply to evidence relating to aggravating circumstances. Each aggravating circumstance shall be proved beyond a reasonable doubt. Any evidence at the sentencing determination proceeding which the presiding judge deems to have probative value may be received. The state and the defendant or his or her counsel shall be permitted to present argument for or against sentence of death. The presiding judge shall set forth the general order of procedure at the outset of the sentencing determination proceeding. The panel shall make written findings of fact based upon the trial of guilt and the sentencing determination proceeding, identifying which, if any, of the alleged aggravating circumstances have been proven to exist beyond a reasonable doubt. Each finding of fact with respect to each alleged aggravating circumstance shall be unanimous. If the panel is unable to reach a unanimous finding of fact with respect to an aggravating circumstance, such aggravating circumstance shall not be weighed in the sentencing determination proceeding. After the presentation and receipt of evidence and argument, the panel shall determine an appropriate sentence as provided in section 29-2522.

(3) When a jury renders a verdict finding the existence of one or more aggravating circumstances as provided in section 29-2520, the panel of judges shall, as soon as practicable after receipt of the written report resulting from the presentence investigation ordered as provided in section 29-2261, hold a hearing to receive evidence of mitigation and sentence excessiveness or disproportionality. Evidence may be presented as to any matter that the presiding judge deems relevant to (a) mitigation, including, but not limited to, the mitigating circumstances set forth in section 29-2523, and (b) sentence excessiveness or disproportionality as provided in subdivision (3) of section 29-2522. Any such evidence which the presiding judge deems to have probative value may be received. The state and the defendant and his or her counsel shall be permitted to present argument for or against sentence of death. The presiding judge shall set forth the general order of procedure at the outset of the sentencing determination proceeding. After the presentation and receipt of evidence and argument, the panel shall determine an appropriate sentence as provided in section 29-2522.