Layout of a cross-shaped inner tracker

The LHCb Technical Proposal described a tracking system in which the Inner Tracker covered a rectangular surface of 60\,cm$\times$40\,cm around the beampipe. In the meantime, simulation studies have shown that the borderline between Inner and Outer Trackers has to be shifted further away from the beampipe in order to obtain acceptably low occupancies in the innermost regions of the Outer Tracker. In an effort to keep the surface covered by the relatively expensive Inner Tracker technology as small as possible, a new layout of the tracking stations has been proposed, in which the Inner Tracker covers a cross-shaped area around the beampipe. The layout described in LHCb note 2000-107, in which each Inner Tracker station consisted of two "L"-shaped detector boxes, would not be well suited to cover such a cross-shaped area. A more elegant solution is proposed here, in which each Inner Tracker station consists of four rectangular detector boxes, one above, one below and one on each side of the beampipe.