Mass. Gen. Laws ch. 278 § 33E
In a capital case as hereinafter defined the entry in the supreme judicial court shall transfer to that court the whole case for its consideration of the law and the evidence. Upon such consideration the court may, if satisfied that the verdict was against the law or the weight of the evidence, or because of newly discovered evidence, or for any other reason that justice may require (a) order a new trial or (b) direct the entry of a verdict of a lesser degree of guilt, and remand the case to the superior court for the imposition of sentence. For the purpose of such review a capital case shall mean: (i) a case in which the defendant was tried on an indictment for murder in the first degree and was convicted of murder in the first degree; or (ii) the third conviction of a habitual offender under subsection (b) of section 25 of chapter 279. After the entry of the appeal in a capital case and until the filing of the rescript by the supreme judicial court motions for a new trial shall be presented to that court and shall be dealt with by the full court, which may itself hear and determine such motions or remit the same to the trial judge for hearing and determination. If any motion is filed in the superior court after rescript, no appeal shall lie from the decision of that court upon such motion unless the appeal is allowed by a single justice of the supreme judicial court on the ground that it presents a new and substantial question which ought to be determined by the full court.
Mass. Gen. Laws ch. 278, § 33E