Ex-Chicagoan Roscoe Mitchell–the alternately lyrical, intense, melodic, minimaliost, humorous, and brutal composer-saxophonist–has brought about major events in American music in his recurring visits to the University of Chicago’s Mandel Hall. As long ago as 1966 he and pianist Jodie Christian led one of the very first A.A.C.M. concerts there; Mitchell then discovered new worlds of form and sound when he led his early Art Ensemble in weekly jam sessions at the hall’s student lounge. The church across the street was the site of the Art Ensemble’s last concert before leaving America in 1969, but the group was back at Mandel Hall for the first concert upon its return in 1972 after becoming famous as the Art Ensemble of Chicago (preserved on Delmark’s Art Ensemble of Chicago Live); 13 years later Mitchell and pianist Cecil Taylor played together in an already historic Mandell Hall concert. Since Mitchell’s Chicago appearances have been relatively infrequent in recent years, it’s a pleasure to report that Saturday’s concert will be his second visit in five months. Last September he brought a unique septet with Christian, two bassists, and three drummers to the Hothouse, and the marvelous event was distinguished by his long, burning solos on saxophone (especially alto) over rhythm-section thunder; this visit’s Note Factory is that same group plus fine trumpeter Hugh Ragin. The show could be the high point of the winter jazz season. Saturday, 8 PM, Mandel Hall, University of Chicago, 1131 E. 57th; 702-8068.