StRoot: StHbtMaker/Infrastructure/StHbtSmearPair.cxx Source File

00001 /***************************************
00002  * $Id: StHbtSmearPair.cxx,v 1.2 2003/09/02 17:58:32 perev Exp $
00003  *
00004  * Author: Mike Lisa, Ohio State lisa@mps.ohio-state.edu
00005  ****************************************
00006  *
00007  * Description: given a StHbtPair, this class provides a corresponding
00008  *     StHbtPair whose constituent StHbtParticles' momenta have been
00009  *     smeared according to a parameterization of the STAR momentum
00010  *     resolution.
00011  *     >> The input StHbtPair is not altered in anyway <<
00012  *
00013  ******************************************
00014  *
00015  * $Log: StHbtSmearPair.cxx,v $
00016  * Revision 1.2  2003/09/02 17:58:32  perev
00017  * gcc 3.2 updates + WarnOff
00018  *
00019  * Revision 1.1  2001/05/23 00:19:05  lisa
00020  * Add in Smearing classes and methods needed for momentum resolution studies and correction
00021  *
00022  *
00023  ******************************************/
00024 
00025 #include "StHbtSmearPair.h"
00026 #include "StRandom.hh"
00027 
00028 #ifdef __ROOT__
00029 ClassImp(StHbtSmearPair)
00030 #endif
00031 
00032 
00033 //______________________________________
00034 StHbtSmearPair::StHbtSmearPair(){
00035   setup();
00036 }
00037 //______________________________________
00038 StHbtSmearPair::StHbtSmearPair(const StHbtPair* unSmearedPair){
00039   setup();
00040   SetUnsmearedPair(unSmearedPair);
00041 }
00042 //______________________________________
00043 void StHbtSmearPair::setup(){
00044   mSmearedPair.SetTrack1(&mParticle1);
00045   mSmearedPair.SetTrack2(&mParticle2);
00046 }
00047 //______________________________________
00048 void StHbtSmearPair::SetUnsmearedPair(const StHbtPair* unSmearedPair){
00049   mParticle1.ResetFourMomentum(SmearedMomentum(unSmearedPair->track1()->FourMomentum()));
00050   mParticle2.ResetFourMomentum(SmearedMomentum(unSmearedPair->track2()->FourMomentum()));
00051 }
00052 //______________________________________
00053 // philosophy behind this smearing is as follows:
00054 // what we *measure* is pT (via curvature), and the angles phi and theta
00055 // momentum is related to this via
00056 // px = pT*cos(phi)   --> Dpx = DpT*cos(phi) - pT*sin(phi)*Dphi = px*DpT/pT - py*Dphi
00057 // py = pT*sin(phi)   --> Dpy = DpT*sin(phi) + pT*cos(phi)*Dphi = py*DpT/pT + px*Dphi
00058 // pz = pT/tan(theta) --> Dpz = DpT/tan(theta) - pT*Dtheta/(sin(theta))^2
00059 //                            = pT*(DpT/pT)/tan(theta) - pT*Dtheta/(sin(theta))^2
00060 //                            = pz*DpT/pT - pT*Dtheta/(sin(theta))^2
00061 //______________________________________
00062 StHbtLorentzVector StHbtSmearPair::SmearedMomentum(StHbtLorentzVector fourmom){
00063   double pT = fourmom.perp();
00064   double mass2 = fourmom.m2();
00065   double px = fourmom.x();
00066   double py = fourmom.y();
00067   double pz = fourmom.z();
00068   double sin2theta = sin(fourmom.theta());
00069   sin2theta = sin2theta*sin2theta;
00070   //
00071   double DpT_div_pT = StRandom::gauss(0.0,mFracPtRes);
00072   double Dphi = StRandom::gauss(0.0,mPhi_a+mPhi_b*::pow(pT,mPhi_alpha));
00073   double Dtheta = StRandom::gauss(0.0,mTheta_a+mTheta_b*::pow(pT,mTheta_alpha));
00074   //
00075   fourmom.setX(px*(1.0+DpT_div_pT) - py*Dphi);
00076   fourmom.setY(py*(1.0+DpT_div_pT) + px*Dphi);
00077   fourmom.setZ(pz*(1.0+DpT_div_pT) - pT*Dtheta/sin2theta);
00078   fourmom.setE(::sqrt(mass2 + fourmom.x()*fourmom.x()+fourmom.y()*fourmom.y()+fourmom.z()*fourmom.z()));
00079   //
00080   return fourmom;
00081 }
00082