This note is mainly to inform for those who may not already be aware of the different convention that pieces of BRAHMS software uses in regard to FS. 1) The official coordinate system has Z along the beam axis pointing 'north' along the FS. Y point towards the heavens and X toward the MRS spectrometer. This is what is used by BRAG (GEANT) and thus for FS it implies that for track y> 0 -> phi is in range 180-190 y< 0 -> phi is in range 170-190 2) The tracking code calculates phi,theta from the vector connecting the vertex and the entrance to D1, but has implemented this using atan.. rather than atan2 and thus resulting y>0 -> phi in range of -10-0 (negative) y<0 -> phi ~ -10-0 thus phi(reco) = pi-phi(geant) 3) The acceptance code uses yet another convention phi ~ -90,90 but y> 0 phi > 0 y <0 phi<0 In general as long as we look at single (semi-inclusive) spectra and average over symmetric acceptances in the tracking i.e. no y-dependence this is not a problem. In case of correlation studies (HBT, v2) this IS an issue that has to be dealt with properly. Flemming ---------------------------------------------------------------- Flemming Videbaek Physics Department Brookhaven National Laboratory e-mail: videbaek@bnl.gov phone: 631-344-4106 _______________________________________________ Brahms-dev-l mailing list Brahms-dev-l@lists.bnl.gov http://lists.bnl.gov/mailman/listinfo/brahms-dev-lReceived on Wed Dec 22 19:41:14 2004
This archive was generated by hypermail 2.1.8 : Wed Dec 22 2004 - 19:41:37 EST