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Computation of DCAxyz for forward tracks #14460

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Merged
shahor02 merged 2 commits into from
Jul 4, 2025
Merged

Computation of DCAxyz for forward tracks #14460

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1e2a52e
Computation of 3D DCA for forward tracks
Jul 1, 2025
8e2011a
clang format
Jul 1, 2025
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1 change: 1 addition & 0 deletions DataFormats/Reconstruction/include/ReconstructionDataFormats/TrackFwd.h
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Original file line number Diff line number Diff line change
Expand Up @@ -161,6 +161,7 @@ class TrackParCovFwd : public TrackParFwd
void propagateToZquadratic(double zEnd, double zField);
void propagateToZhelix(double zEnd, double zField);
void propagateToZ(double zEnd, double zField); // Parameters: helix; errors: quadratic
void propagateToDCAhelix(double zField, const std::array<double, 3>& p, std::array<double, 3>& dca);

// Add Multiple Coulomb Scattering effects
void addMCSEffect(double x2X0);
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69 changes: 69 additions & 0 deletions DataFormats/Reconstruction/src/TrackFwd.cxx
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Expand Up @@ -11,6 +11,7 @@

#include "ReconstructionDataFormats/TrackFwd.h"
#include "Math/MatrixFunctions.h"
#include <GPUCommonLogger.h>

namespace o2
{
Expand Down Expand Up @@ -503,5 +504,73 @@ bool TrackParCovFwd::getCovXYZPxPyPzGlo(std::array<float, 21>& cv) const
return true;
}

//________________________________________________________________

void TrackParCovFwd::propagateToDCAhelix(double zField, const std::array<double, 3>& p, std::array<double, 3>& dca)
{
// Computing DCA of fwd track w.r.t vertex in helix track model, using Newton-Raphson minimization

auto x0 = mParameters(0);
auto y0 = mParameters(1);
auto z0 = mZ;
auto phi0 = mParameters(2);
auto tanl = mParameters(3);
auto qOverPt = mParameters(4);
auto k = TMath::Abs(o2::constants::math::B2C * zField);
auto qpt = 1.0 / qOverPt;
auto qR = qpt / std::fabs(k);
auto invtanl = 1.0 / tanl;
auto Hz = std::copysign(1, zField);

auto xPV = p[0];
auto yPV = p[1];
auto zPV = p[2];

auto qRtanl = qR * tanl;
auto invqRtanl = 1.0 / qRtanl;
auto [sinp, cosp] = o2::math_utils::sincosd(phi0);

auto z = zPV;
double tol = 1e-4;
int max_iter = 10;
int iter = 0;

while (iter++ < max_iter) {
double theta = (z0 - z) * invqRtanl;
double phi_theta = phi0 + Hz * theta;
double sin_phi_theta = sin(phi_theta);
double cos_phi_theta = cos(phi_theta);

double DX = x0 - Hz * qR * (sin_phi_theta - sinp) - xPV;
double DY = y0 + Hz * qR * (cos_phi_theta - cosp) - yPV;
double DZ = z - zPV;

double dD2_dZ =
2 * DX * cos_phi_theta * invtanl +
2 * DY * sin_phi_theta * invtanl +
2 * DZ;

double d2D2_dZ2 =
2 * invtanl * invtanl +
2 * invtanl * (DX * Hz * sin_phi_theta - DY * Hz * cos_phi_theta) * invqRtanl +
2;

double z_new = z - dD2_dZ / d2D2_dZ2;

if (std::abs(z_new - z) < tol) {
z = z_new;
this->propagateToZhelix(z, zField);
dca[0] = this->getX() - xPV;
dca[1] = this->getY() - yPV;
dca[2] = this->getZ() - zPV;
LOG(debug) << "Converged after " << iter << " iterations for vertex X=" << p[0] << ", Y=" << p[1] << ", Z = " << p[2];
return;
}
z = z_new;
}
LOG(debug) << "Failed to converge after " << iter << " iterations for vertex X=" << p[0] << ", Y=" << p[1] << ", Z = " << p[2];
return;
}

} // namespace track
} // namespace o2