Interaction vertex reconstruction in pp collisions at LHC with ALICE

The knowledge of the position of the interaction vertex is used not only to reconstruct primary tracks in the Inner Tracking System (ITS), but it is also crucial for the measurement of the physics signals characterized by the presence of a secondary vertex with a small displacement. This is, in particular, the case for the detection of particles with open charm and open beauty, namely D^0 (c\tau\simeq 123 micron), D^+ (c\tau\simeq 315 micron) and B mesons (c\tau\sim micron). We discuss how, for the proton-proton runs, the position of the interaction vertex will have to be measured for each event, not only along the beam direction (as in Pb-Pb), but also in the transverse plane. We present a complete strategy to accomplish this task.