ftsFromVertexToPoint.cc

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// -*- C++ -*-
//
//
//
// Original Author:  Ursula Berthon, Claude Charlot
//         Created:  Mon Mar 27 13:22:06 CEST 2006
//
//
#include "TrackingTools/TrajectoryState/interface/ftsFromVertexToPoint.h"
#include "MagneticField/Engine/interface/MagneticField.h"

FreeTrajectoryState trackingTools::ftsFromVertexToPoint(MagneticField const& magField,
                                                        GlobalPoint const& xmeas,
                                                        GlobalPoint const& xvert,
                                                        float momentum,
                                                        TrackCharge charge) {
  auto magFieldAtPoint = magField.inTesla(xmeas);
  auto BInTesla = magFieldAtPoint.z();
  GlobalVector xdiff = xmeas - xvert;
  auto mom = momentum * xdiff.unit();
  auto pt = mom.perp();
  auto pz = mom.z();
  auto pxOld = mom.x();
  auto pyOld = mom.y();

  auto curv = (BInTesla * 0.29979f * 0.01f) / pt;

  // stays as doc...
  // auto alpha = std::asin(0.5f*xdiff.perp()*curv);
  // auto ca = std::cos(float(charge)*alpha);
  // auto sa = std::sin(float(charge)*alpha);

  auto sa = 0.5f * xdiff.perp() * curv * float(charge);
  auto ca = sqrt(1.f - sa * sa);

  auto pxNew = ca * pxOld + sa * pyOld;
  auto pyNew = -sa * pxOld + ca * pyOld;
  GlobalVector pNew(pxNew, pyNew, pz);

  GlobalTrajectoryParameters gp(xmeas, pNew, charge, &magField, std::move(magFieldAtPoint));

  return FreeTrajectoryState(gp);
}