fourvec.cc

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//
//
// File: src/fourvec.cc
// Purpose: Define 3- and 4-vector types for the hitfit package, and
//          supply a few additional operations.
// Created: Jul, 2000, sss, based on run 1 mass analysis code.
//
// CMSSW File      : src/fourvec.cc
// Original Author : Scott Stuart Snyder <snyder@bnl.gov> for D0
// Imported to CMSSW by Haryo Sumowidagdo <Suharyo.Sumowidagdo@cern.ch>
//

#include "TopQuarkAnalysis/TopHitFit/interface/fourvec.h"
#include <cmath>

using std::atan;
using std::atan2;
using std::cos;
using std::exp;
using std::fabs;
using std::log;
using std::sin;
using std::sqrt;
using std::tan;

namespace {  // unnamed namespace

  double cal_th(double theta, double z)
  //
  // Purpose: Helper for deteta.  Find `calorimeter theta' given
  //          physics theta and z-vertex.  Copied from run 1 code.
  //
  // Inputs:
  //   theta -       Physics theta.
  //   z -           Z-vertex.
  //
  // Returns:
  //   Calorimeter theta.
  //
  {
    const double R_CC = 91.6;
    const double Z_EC = 178.9;
    const double BIGG = 1e8;

    double tanth;
    if (fabs(cos(theta)) < 1 / BIGG)
      tanth = BIGG * sin(theta);
    else
      tanth = tan(theta);

    double z_cc = R_CC / tanth + z;

    if (fabs(z_cc) < Z_EC)
      theta = atan2(R_CC, z_cc);

    else {
      double zz = Z_EC;
      if (tanth < 0)
        zz = -zz;
      double r_ec = (zz - z) * tanth;
      theta = atan2(r_ec, zz);
    }

    if (theta < 0)
      theta += 2 * M_PI;
    return theta;
  }

}  // unnamed namespace

namespace hitfit {

  void adjust_p_for_mass(Fourvec& v, double mass)
  //
  // Purpose: Adjust the 3-vector part of V (leaving the energy unchanged)
  //          so that it has mass MASS.  (Provided that is possible.)
  //
  // Inputs:
  //   v -           The 4-vector to scale.
  //   mass -        The desired mass of the 4-vector.
  //
  // Outputs:
  //   v -           The scaled 4-vector.
  //
  {
    CLHEP::Hep3Vector vect = v.vect();
    double old_p2 = vect.mag2();
    if (old_p2 == 0)
      return;
    double new_p2 = v.e() * v.e() - mass * mass;
    if (new_p2 < 0)
      new_p2 = 0;
    vect *= sqrt(new_p2 / old_p2);
    v.setVect(vect);
  }

  void adjust_e_for_mass(Fourvec& v, double mass)
  //
  // Purpose: Adjust the energy component of V (leaving the 3-vector part
  //          unchanged) so that it has mass MASS.
  //
  // Inputs:
  //   v -           The 4-vector to scale.
  //   mass -        The desired mass of the 4-vector.
  //
  // Outputs:
  //   v -           The scaled 4-vector.
  //
  {
    v.setE(sqrt(v.vect().mag2() + mass * mass));
  }

  void rottheta(Fourvec& v, double theta)
  //
  // Purpose: Rotate V through polar angle THETA.
  //
  // Inputs:
  //   v -           The 4-vector to rotate.
  //   theta -       The rotation angle.
  //
  // Outputs:
  //   v -           The rotated 4-vector.
  //
  {
    double s = sin(theta), c = cos(theta);
    double old_pt = v.perp();
    double new_pt = old_pt * c - v.z() * s;
    v.setZ(old_pt * s + v.z() * c);

    v.setX(v.x() * new_pt / old_pt);
    v.setY(v.y() * new_pt / old_pt);
  }

  void roteta(Fourvec& v, double eta)
  //
  // Purpose: Rotate a Fourvec through a polar angle such that
  //          its pseudorapidity changes by ETA.
  //
  // Inputs:
  //   v -           The 4-vector to rotate.
  //   eta -         The rotation angle.
  //
  // Outputs:
  //   v -           The rotated 4-vector.
  //
  {
    double theta1 = v.theta();
    double eta1 = theta_to_eta(theta1);
    double eta2 = eta1 + eta;
    double theta2 = eta_to_theta(eta2);

    rottheta(v, theta1 - theta2);
  }

  double eta_to_theta(double eta)
  //
  // Purpose: Convert psuedorapidity to polar angle.
  //
  // Inputs:
  //   eta -         Pseudorapidity.
  //
  // Returns:
  //   Polar angle.
  //
  {
    return 2 * atan(exp(-eta));
  }

  double theta_to_eta(double theta)
  //
  // Purpose: Convert polar angle to psuedorapidity.
  //
  // Inputs:
  //   theta -       Polar angle.
  //
  // Returns:
  //   Pseudorapidity.
  //
  {
    return -log(tan(theta / 2));
  }

  double deteta(const Fourvec& v, double zvert)
  //
  // Purpose: Get the detector eta (D0-specific).
  //
  // Inputs:
  //   v -           Vector on which to operate.
  //   zvert -       Z-vertex.
  //
  // Returns:
  //   Detector eta of V.
  //
  {
    return theta_to_eta(cal_th(v.theta(), zvert));
  }

  double phidiff(double phi)
  //
  // Purpose: Handle wraparound for a difference in azimuthal angles.
  //
  // Inputs:
  //   phi -         Azimuthal angle.
  //
  // Returns:
  //   PHI normalized to the range -pi .. pi.
  //
  {
    while (phi < -M_PI)
      phi += 2 * M_PI;
    while (phi > M_PI)
      phi -= 2 * M_PI;
    return phi;
  }

  double delta_r(const Fourvec& a, const Fourvec& b)
  //
  // Purpose: Find the distance in R between two four-vectors.
  //
  // Inputs:
  //   a -           First four-vector.
  //   b -           Second four-vector.
  //
  // Returns:
  //   the distance in R between A and B.
  //
  {
    double deta = a.pseudoRapidity() - b.pseudoRapidity();
    double dphi = phidiff(a.phi() - b.phi());
    return sqrt(deta * deta + dphi * dphi);
  }

}  // namespace hitfit