Motivic decomposition of a generalized Severi-Brauer variety

Let A and B be two central simple algebras of a prime degree n over a field F generating the same subgroup in the Brauer group. We show that the Chow motive of a Severi-Brauer variety SB(A) is a direct summand of the motive of a generalized Severi-Brauer variety SB_d(B) if and only if [A]=d[B] or [A]=-d[B] in the Brauer group. The proof uses methods of Schubert calculus and combinatorial properties of Young tableaux, e.g., Robinson-Schensted correspondence.