Air carrier First Officer reported a NMAC and RA during arrival phase. Crew advised ATC RA after escape maneuver and continued approach to SMF airport.

Narrative

While on a DESCEND VIA clearance on the SUUTR4 arrival into SMF; we were flying level on an intermediate level-off at 7;000 feet after waypoint NGGET. ATC told us to maintain 7;000 feet. Received TCAS Traffic Advisory and identified the intruder 400 feet below and climbing on the Navigation Display. We were instructed to turn left heading 270 but received TCAS Resolution Advisory in the turn. We performed the RA maneuver and reported it to ATC. Intruder was visually identified as Aircraft X only 300 feet below. Aircraft callsign or registration unknown.