What was found
AIR TURNBACK - AFTER TAKEOFF FROM LAS EN ROUTE FOR DEN EXPERIENCED SPEED LIMIT / MACH TRIM AUTO SLAT FAIL, THERE WERE NO INDICATIONS, NO RECALL, AND NO LIGHT WHEN LIGHT WAS TESTED. TURNED BACK TO LAS AND LANDED WITHOUT FURTHER EVENT. MAINTENANCE FOUND THE MASTER DIM CIRCUIT BREAKER POPPED BEHIND FIRST OFFICERS SEAT. RESET CIRCUIT BREAKER, ALL LIGHTS AND INDICATIONS OPERATED NORMALLY.