Changeset 248 in svn for trunk/src/BFieldProp.cc

Timestamp:

Feb 5, 2009, 6:10:28 PM (16 years ago)

Author:

Xavier Rouby

Message:

new TrackPropagation::bfield function

File:

• trunk/src/BFieldProp.cc (modified) (2 diffs)

trunk/src/BFieldProp.cc

@@ -72 +72 @@
 }
 
-
-
 void TrackPropagation::init() {
  MAXITERATION = 10000; 
@@ -293 +291 @@
  } // if b_x or b_y non zero
 }
+
+
+
+void TrackPropagation::bfield(const TRootGenParticle *Part, float& etacalo, float& phicalo) {
+
+  // initialisation, valid for z_max==0, R_max==0 and q==0
+  etacalo = Part->Eta;
+  phicalo = -atan2(Part->Px,Part->Py);
+
+  q  = Charge(Part->PID);
+  if(q==0) return;
+  if(R_max ==0) { cout << "ERROR: magnetic field has no lateral extention\n"; return;}
+  if(z_max==0) { cout << "ERROR: magnetic field has no longitudinal extention\n"; return;}
+
+  if (B_x== 0 && B_y== 0) { // faster if only B_z
+    if (B_z==0) return; // nothing to do
+
+    // initial conditions:
+      // p_X0 = Part->Px, p_Y0 = Part->Py, p_Z0 = Part->Pz, p_T0 = Part->PT;
+      // X_0 = Part->X, Y_0 = Part->Y, Z_0 = Part->Z;
+
+    // 1.  initial transverse momentum p_{T0} : Part->PT
+    //     initial transverse momentum direction \phi_0 = -atan(p_X0/p_Y0) 
+    //     relativistic gamma : gamma = E/mc² ; gammam = gamma \times m
+    //     giration frequency \omega = q/(gamma m) B_z
+    //     helix radius r = p_T0 / (omega gamma m)
+      phi_0 = -atan2(Part->Px,Part->Py);
+      gammam = Part->E; // here c==1
+      //cout << "gammam" << gammam << "\t gamma" << gammam/Part->M << endl;
+      omega = q * B_z /gammam;
+      r = Part->PT / (omega * gammam);
+
+    // 2.  Helix parameters : center coordinates in transverse plane
+    //     x_c = x_0 - r*cos(phi_0) and y_c = y_0 - r*sin(phi_0)
+    //     R_c = \sqrt{x_c² + y_c²} and \Phi_c = atan{y_c/x_c}
+      x_c = Part->X - r*cos(phi_0);  /// TEST !!
+      y_c = Part->Y - r*sin(phi_0); 
+      R_c = sqrt(pow(x_c,2.) + pow(y_c,2.) );
+      Phi_c = atan2(y_c,x_c);
+
+    // 3. time evaluation t = min(t_T, t_z)
+    //    t_T : time to exit from the sides
+    //    t_T= [ Phi_c - phi_0 + atan( (R_max^2 - (R_c^2 + r^2))/(2rR_c) )  ]/omega
+    //    t_z : time to exit from the front or the back
+    //    t_z = gamma * m /p_z0 \times (-z_0 + z_max * sign(p_z0))
+      rr = sqrt( pow(R_c,2.) + pow(r,2.) ); // temp variable
+      t_T=0;
+      int sign_pz= (Part->Pz >0) ? 1 : -1;
+      t_z = gammam / Part->Pz * (-Part->Z + z_max*sign_pz ) ;
+      if ( fabs(R_c - r) > R_max || R_c + r  < R_max ) t = t_z;
+      else {
+         t_T = (Phi_c - phi_0 + atan2( (R_max + rr)*(R_max - rr) , 2*r*R_c ) ) / omega; 
+         t = min(t_T,t_z);
+      }
+
+    // 4. position in terms of x(t), y(t), z(t)
+    //    x(t) = x_c + r cos (omega t + phi_0)
+    //    y(t) = y_c + r sin (omega t + phi_0)
+    //    z(t) = z_0 + (p_Z0/gammam) t
+      x_t = x_c + r * cos(omega * t + phi_0); 
+      y_t = y_c + r * sin(omega * t + phi_0); 
+      z_t = Part->Z + Part->Pz / gammam * t;
+
+    // 5. position in terms of Theta(t), Phi(t), R(t), Eta(t)
+    //    R(t) = sqrt(x(t)² + y(t)²)
+    //    Phi(t) = atan(y(t)/x(t))
+    //    Theta(t) = atan(R(t)/z(t))
+    //    Eta(t) = -ln tan (Theta(t)/2)
+      R_t   = sqrt( pow(x_t,2.) + pow(y_t,2.)  );
+      Phi_t = atan2( y_t, x_t);
+      if(R_t>0) {
+              Theta_t = acos( z_t / sqrt(z_t*z_t+ R_t*R_t));
+              Eta_t = - log(tan(Theta_t/2.));
+      } else{
+                Theta_t=0; Eta_t = 9999;
+      }
+/*      Not needed here. but these formulae are correct -------
+        Px_t = - Part->PT * sin(omega*t + phi_0);
+        Py_t =   Part->PT * cos(omega*t + phi_0);
+        Pz_t =   Part->Pz;
+        PT_t =   sqrt(Px_t*Px_t + Py_t*Py_t);
+        p_t = sqrt(PT_t*PT_t + Pz_t*Pz_t);
+        E_t=sqrt(Part->M*Part->M +p_t);
+        //if(p_t != fabs(Pz_t) ) Eta_t = log( (p_t+Pz_t)/(p_t-Pz_t) )/2.;
+        //if(p_t>0) Theta_t = acos(Pz_t/p_t);
+        momentum.SetPxPyPzE(Px_t,Py_t,Pz_t,E_t);
+*/
+        etacalo = Eta_t;
+        phicalo = Phi_t;
+        return;
+// test zone ---
+/*
+        cout << cos(atan(R_t/z_t)) << "\t" << cos(Theta_t) << "\t" << cos(momentum.Theta()) << "\t" << Pz_t/temp_p << endl;
+        double Eta_t1 = log( (E+Pz_t)/(E-Pz_t) )/2.;
+        double Eta_t2 = log( (temp_p+Pz_t)/(temp_p-Pz_t) )/2.;
+        if(0 &&  fabs(Eta_t -Eta_t2)>1e-310) {
+                cout << "ERROR-BUG: Eta_t != Eta_t2\n";
+                cout << "Eta_t= " << Eta_t << "\t Eta_t1= " << Eta_t1 << "\t Eta_t2= " << Eta_t2 << endl;
+        }
+
+        double R_t2  = sqrt( pow(R_c,2.) + pow(r,2.) + 2*r*R_c*cos(phi_0 + omega*t - Phi_c) ); // cross-check
+        if(fabs(R_t - R_t2) > 1e-7) 
+                cout << "ERROR-BUG: R_t != R_t2:  R_t=" << R_t << " R_t2=" << R_t2 << " R_t - R_t2 =" << R_t - R_t2 << endl;
+        if( fabs(E - gammam) > 1e-3 ) {
+                cout << "ERROR-BUG: energy is not conserved in src/BFieldProp.cc\n";
+                cout << "E - momentum.E() = " <<  fabs(E - momentum.E()) <<  " gammam - E " << fabs(gammam -E) << endl; }
+        if( fabs(PT_t - Part->PT) > 1e-10 ) {
+                cout << "ERROR-BUG: PT is not conversed in src/BFieldProp.cc. ";
+                cout << "(at " << 100*(PT_t - Part->PT) << "%)\n";
+                }
+        if(momentum.Pz() != Pz_t)
+                cout << "ERROR-BUG: Pz is not conserved in src/BFieldProp.cc\n";
+
+        double temp_p0=sqrt(Part->PT*Part->PT + Part->Pz*Part->Pz);
+        if(fabs( (temp_p-temp_p0)*(temp_p+temp_p0) )>1e-10  ) {
+                cout << "ERROR-BUG: momentum |vec{p}| is not conserved in src/BFieldProp.cc\n";
+                cout << temp_p << "\t" << temp_p0 << endl;
+        }
+
+   // if x_c == y_c ==0 (set it by hand!), easy cross-check
+   //cout << "tan(phi_p)= " << momentum.Py()/momentum.Px() << "\t -1/tan(phi_x)= " << -x_t/y_t << endl;
+*/
+
+  } else { // if B_x or B_y are non zero: longer computation
+
+  float Xvertex1 = Part->X;
+  float Yvertex1 = Part->Y;
+  float Zvertex1 = Part->Z;
+  
+  //out of tracking coverage?
+  if(sqrt(Xvertex1*Xvertex1+Yvertex1*Yvertex1) > R_max){return;}
+  if(fabs(Zvertex1) > z_max){return;}
+  
+  double px = Part->Px / 0.003;
+  double py = Part->Py / 0.003;
+  double pz = Part->Pz / 0.003;
+  double pt = Part->PT / 0.003; // sqrt(px*px+py*py);
+  double p  = sqrt(pz*pz + pt*pt); //sqrt(px*px+py*py+pz*pz);
+ 
+  double M  = Part->M;
+  double vx = px/M;
+  double vy = py/M;
+  double vz = pz/M;
+  double qm = q/M;
+
+  double ax =  qm*(B_z*vy - B_y*vz);
+  double ay =  qm*(B_x*vz - B_z*vx);
+  double az =  qm*(B_y*vx - B_x*vy);
+  double dt = 1/p;
+  if(pt<266 && vz < 0.0012)       dt = fabs(0.001/vz); // ?????
+ 
+  double xold=Xvertex1;         double x=xold;
+  double yold=Yvertex1;         double y=yold;
+  double zold=Zvertex1;         double z=zold;
+
+  double VTold = pt/M; //=sqrt(vx*vx+vy*vy);
+ 
+  unsigned int k = 0;
+  double VTratio=0;
+  double R_max2 = R_max*R_max;
+  double r2=0; // will be x*x+y*y
+
+  while(k < MAXITERATION){
+        k++;
+
+        vx += ax*dt;
+        vy += ay*dt;
+        vz += az*dt;
+
+        VTratio = VTold/sqrt(vx*vx+vy*vy);
+        vx *= VTratio;
+        vy *= VTratio;
+
+        ax = qm*(B_z*vy - B_y*vz);
+        ay = qm*(B_x*vz - B_z*vx);
+        az = qm*(B_y*vx - B_x*vy);
+
+        x  += vx*dt;
+        y  += vy*dt;
+        z  += vz*dt;
+        r2  = x*x + y*y;
+
+       if( r2 > R_max2 ){ 
+                x /= r2/R_max2; 
+                y /= r2/R_max2; 
+                break;
+       }
+       if( fabs(z)>z_max)break;
+
+       xold = x;
+       yold = y;
+       zold = z;
+  } // while loop
+
+  if(k == MAXITERATION) loop_overflow_counter++;
+  //cout << "too short loop in " << loop_overflow_counter << " cases" << endl;
+  float Theta=0; 
+  if(x!=0 && y!=0 && z!=0) {
+          Theta = atan2(sqrt(r2),z);
+          etacalo  = -log(tan(Theta/2.));
+          phicalo = atan2(y,x);
+          //momentum.SetPtEtaPhiE(Part->PT,eta,phi,Part->E);
+  }
+ } // if b_x or b_y non zero
+}