FastHelix.cc

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#include "RecoTracker/TkSeedGenerator/interface/FastHelix.h"
#include "RecoTracker/TkSeedGenerator/interface/FastLine.h"

void FastHelix::compute() {
  if (isValid() && (std::abs(tesla0) > 1e-3) && theCircle.rho() < maxRho)
    helixStateAtVertex();
  else
    straightLineStateAtVertex();
}

void FastHelix::helixStateAtVertex() {
  // given the above rho>0.
  double rho = theCircle.rho();
  //remember (radius rho in cm):
  //rho =
  //100. * pt *
  //(10./(3.*MagneticField::inTesla(GlobalPoint(0., 0., 0.)).z()));

  // pt = 0.01 * rho * (0.3*MagneticField::inTesla(GlobalPoint(0.,0.,0.)).z());
  double cm2GeV = 0.01 * 0.3 * tesla0;
  double pt = cm2GeV * rho;

  // verify that rho is not toooo large
  double dcphi = ((outerHit().x() - theCircle.x0()) * (middleHit().x() - theCircle.x0()) +
                  (outerHit().y() - theCircle.y0()) * (middleHit().y() - theCircle.y0())) /
                 (rho * rho);
  if (std::abs(dcphi) >= 1.f) {
    straightLineStateAtVertex();
    return;
  }

  GlobalPoint pMid(middleHit());
  GlobalPoint v(vertex());

  // tangent in v (or the opposite...)
  double px = -cm2GeV * (v.y() - theCircle.y0());
  double py = cm2GeV * (v.x() - theCircle.x0());
  // check sign with scalar product
  if (px * (pMid.x() - v.x()) + py * (pMid.y() - v.y()) < 0.) {
    px = -px;
    py = -py;
  }

  //calculate z0, pz
  //(z, R*phi) linear relation in a helix
  //with R, phi defined as radius and angle w.r.t. centre of circle
  //in transverse plane
  //pz = pT*(dz/d(R*phi)))

  // VI 23/01/2012
  double dzdrphi = outerHit().z() - middleHit().z();
  dzdrphi /= rho * acos(dcphi);
  double pz = pt * dzdrphi;

  TrackCharge q = 1;
  if (theCircle.x0() * py - theCircle.y0() * px < 0)
    q = -q;
  if (tesla0 < 0.)
    q = -q;

  //VI
  if (useBasisVertex) {
    atVertex = GlobalTrajectoryParameters(basisVertex, GlobalVector(px, py, pz), q, bField);
  } else {
    double z_0 = middleHit().z();
    // assume v is before middleHit (opposite to outer)
    double ds = ((v.x() - theCircle.x0()) * (middleHit().x() - theCircle.x0()) +
                 (v.y() - theCircle.y0()) * (middleHit().y() - theCircle.y0())) /
                (rho * rho);
    if (std::abs(ds) < 1.f) {
      ds = rho * acos(ds);
      z_0 -= ds * dzdrphi;
    } else {  // line????
      z_0 -= std::sqrt((middleHit() - v).perp2() / (outerHit() - middleHit()).perp2()) *
             (outerHit().z() - middleHit().z());
    }

    //double z_old = -flfit.c()/flfit.n2();
    // std::cout << "v:xyz, z,old,new " << v << "   " << z_old << " " << z_0 << std::endl;

    atVertex = GlobalTrajectoryParameters(GlobalPoint(v.x(), v.y(), z_0), GlobalVector(px, py, pz), q, bField);
  }
}

void FastHelix::straightLineStateAtVertex() {
  //calculate GlobalTrajectoryParameters assuming straight line...

  GlobalPoint pMid(middleHit());
  GlobalPoint v(vertex());

  double dydx = 0.;
  double pt = 0., px = 0., py = 0.;

  if (fabs(theCircle.n1()) > 0. || fabs(theCircle.n2()) > 0.)
    pt = maxPt;  // 10 TeV //else no pt
  if (fabs(theCircle.n2()) > 0.) {
    dydx = -theCircle.n1() / theCircle.n2();  //else px = 0
  }
  px = pt / sqrt(1. + dydx * dydx);
  py = px * dydx;
  // check sign with scalar product
  if (px * (pMid.x() - v.x()) + py * (pMid.y() - v.y()) < 0.) {
    px *= -1.;
    py *= -1.;
  }

  //calculate z_0 and pz at vertex using weighted mean
  //z = z(r) = z0 + (dz/dr)*r
  //tan(theta) = dr/dz = (dz/dr)^-1
  //theta = atan(1./dzdr)
  //p = pt/sin(theta)
  //pz = p*cos(theta) = pt/tan(theta)

  FastLine flfit(outerHit(), middleHit());
  double dzdr = -flfit.n1() / flfit.n2();
  double pz = pt * dzdr;

  TrackCharge q = 1;
  //VI

  if (useBasisVertex) {
    atVertex = GlobalTrajectoryParameters(basisVertex, GlobalVector(px, py, pz), q, bField);
  } else {
    double z_0 = -flfit.c() / flfit.n2();
    atVertex = GlobalTrajectoryParameters(GlobalPoint(v.x(), v.y(), z_0), GlobalVector(px, py, pz), q, bField);
  }
}