We are in the middle of the Baryon workshop so this is an incomplete answer, and anyhow it is official Danish vacation time. ------------------------------------------------------ Flemming Videbaek Physics Department Brookhaven National Laboratory The discussions with Djamel is really the the MRS and FS cannot be dealt with in a commen way in regarrd to the vertex. For MRS the track projection's to the vertex has (even a 30 degree) a much better resolution alkong z of the beam line than the BB. Typical numbers are ~0.2 cm at 90 at 0.2/.5 = .4 at 30 deg, while the BB is ~ .6-0.8 cm. Thus the deviation when selection tracks originating inion from the vertex is dominated by the distribution in BB. So if does not really help the selection - apart from picking a proper sig In my opion the selection of good tracks consist for MRS to project to the beam plane and use for the deviation function that will be angle dependent. (sigma(track)**2+sigma(BB)**) > > We've discussed how to select tracks from the vertex and at this > point the method is different for the MRS and the FFS. > > It's clear if one looks at the fTrackVertex member in the BrGlbTrack > class: > > fTrackVertex; // track vertex (intersection with beam line for Mrs) > // intersection with BB plane for Ffs > > (the BB plane (for Ffs) has the beam axis as normal vector) > > This means that the bbVtx-trackVtx distribution is only gaussian > for MRS at 90 deg and for FFS at 0 deg. \ I do not get this- at zero deg it does not matter where the Beam Beam vertex is , the projection is just the sigma(x) of the track projection( done to any perpendicular plane) , and I do not think can be used to reject anything based on BB (The BB-track proj on track Z). Here the deviation is dominated by the track projection (sigma(x) over 4.50 meters at a small angle. What you learn here is really if the track comes out corretly of D1 - o this is where fiducail cuts should be made. In general if a track full fills fiducial BB vertex deviation cuts, I believe the correct way to calculate the scattering angle is to pick the (theta,phi) in the D1 system at the front and transform this to the global system. Based on presision arguments the measurement is NOT improved using the Bb vertex . This is 100% correct for MRS, and and think too for the FS. At 30 deg the distribution will > be a distorted gaussian. This effect is probably not very big, but I > think we should change it so that the distribution is gaussian and the > method is the same for MRS/FFS. > > A way to do this is to define a plane with the bb vertex as origin and > the normal vector parallel with the spectrometer (z-axis of > TPM1/D1). Tracks should be projected to this plane and the track selection > cuts done in the distance from the track intersection to the origin (bb > vertex) of the plane. > This requires a few changes in BrMrsTrackingModule and BrFfsTrackingModule. > > I know that Flemming and Djamel discussed this at some point - Flemming do > you have any comments? > > Claus > > +-------------------------------------------------------------+ > | Claus Jørgensen | > | Cand. Scient. Phone : (+45) 33 32 49 49 | > | Cell : (+45) 27 28 49 49 | > | Niels Bohr Institute, Ta-2, Office : (+45) 35 32 53 07 | > | Blegdamsvej 17, DK-2100, E-mail : ekman@nbi.dk | > | University of Copenhagen Home : www.nbi.dk/~ekman/ | > +-------------------------------------------------------------+ >
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