Improving bounds on penguin pollution in B -> pi pi

In the presence of penguin contributions, the indirect CP asymmetry in B-0( t) --> pi (+)pi (-) measures sin(2 alpha + 2 theta), where 2 theta parametr izes the size of the penguin "pollution". We derive a new upper bound on \2 theta \, requiring the measurement of BR(B+ --> pi (+)pi (0)) and an upper bound on B-00 = 1/2[BR(B-0 --> pi (0)pi (0)) + BR((B-0) over bar --> pi (0 )pi (0))]. The new bound is stronger than those previously discussed in the literature. We also present a lower bound on B-00. Current data may sugges t that it is not very small, in which case theta can be determined using a complete isospin analysis. (C) 2001 Published by Elsevier Science B.V.