Kinematic Fitter used for TagAndProbe lines of B+->(Jpsi->l+l-)K+ (l=mu/e).

The file Phys/DecayTreeFitter/src/component/VeloToLongTrackEffFilter.cpp is a kinematic fitter (and filter) and is described in this presentation: https://indico.cern.ch/event/1195028/contributions/5130840/attachments/2543241/4379164/TagAndProbe_November2022.pdf We are currently finishing tag-and-probe lines using B+->(Jpsi->l+l-)K+ (l=mu/e) decays. In these lines, one of the two leptons is velo track (the probe). Therefore, its momentum is not known. In about 7% of the events, the three-body combination does not point to the "correct" best PV, which can be very dangerous. This kinematic fitter allows to perform a Jpsi constraint on the three-body combination, recomputing the associated probe momentum, allowing for the three-body combination to point to the correct best PV. Related kinematic quantities are computed (such as the Jpsi constrained IP of the B+), and used to remove a large fraction of the combinatorial background through a series of cuts. Additionally, a PV constraint is computed, constraining the B+ momentum to align with the B+ flight distance.

Goes with Moore!1893 (merged)