A320 PIC BECAME DISTRACTED PROGRAMMING FMS AND DID NOT NOTICE THE FO DSND IN VERT SPD MODE THROUGH A XING ALT ON THE MITTS ARR.

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WE WERE ON THE CIVET 4 ARR TO LAX RWY 25L. AT THE LAST MIN; WE WERE ISSUED CLRNC FOR THE MITTS 1 ARR TO RWY 24R. ATC ISSUED CLRNC TO DSND VIA THE ARR. THE ACFT WAS DSNDING IN VERT SPD MODE INSTEAD OF 'MANAGED DSCNT;' WHICH WOULD HAVE MADE ALL THE XING ALTS. I HAD BEEN BUSY PROGRAMMING THE NEW ARR AND VERIFYING THE ALT RESTRS. WE RECEIVED A FREQ CHANGE AND THEN I NOTICED WE WERE LEVEL AT 8000 FT AT MITTS INTXN. MITTS HAS A PUBLISHED XING ALT OF 10000-11000 FT MSL. ATC TOLD US TO STAY AT 8000 FT. WE SHOULD NOT HAVE LET THE INCREASED WORKLOAD DISTRACT FROM WORKING AS A TEAM. AS PNF I SHOULD HAVE DIVIDED MY ATTN BTWN THE BASICS OF MONITORING THE PF AND NAV DATA ENTRY. SUPPLEMENTAL INFO FROM ACN 581457: THINKING WE WERE IN MANAGED NAV; WE WERE REALLY IN VERT SPD MODE AND ENDED UP XING MITTS BELOW 10000 FT WHICH IS WHERE WE SHOULD HAVE BEEN. WE CONTINUED THE APCH WITH A NORMAL LNDG RWY 24R. WE BOTH LEARNED A LESSON FROM THIS AND CAN ASSURE YOU THAT WE WILL BE MORE AWARE OF FMA DISPLAYS AND XING RESTRS IN THE FUTURE.