SphericalShowerFront.h

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#ifndef _MdGeometryFitterAG_SphericalShowerFront_h_
#define _MdGeometryFitterAG_SphericalShowerFront_h_

#include <TVector3.h>

#include <utl/PhysicalConstants.h>

// Model of a shower front as an expanding sphere with the center located one radius away 
// from the shower core in the direction of the shower axis. The parameters adjusted during the reconstruction 
// are the axis direction, the arrival time of the shower front at the core position, and (optionally) the radius of curvature.

namespace MdGeometryFitterAG {

  class SphericalShowerFront {

    public:
      SphericalShowerFront(TVector3 axis1, double t01, double radius1, TVector3 core1) : axis(axis1), t0(t01), radius(radius1), core(core1) {}

      // Calculate the arrival time of the shower front at a given position 
      double operator()(const TVector3 &pos) const {

        const TVector3 detectorToCore = pos-core;   // desired position wrt the core
        const TVector3 centerToCore = axis*radius;  // sphere center wrt the core
        const TVector3 detectorToCenter = detectorToCore - centerToCore;  // position wrt the sphere center
        const double rDetector = detectorToCenter.Mag();   // distance from position to center
        const double delay = (rDetector-radius) / utl::kSpeedOfLight; // delay of the front arrival at the detector wrt to the arrival time at the core position
        const double arrivalTime = t0 + delay;  // correct the arrival time with the core time
        
        return arrivalTime;
      }

    private:
      TVector3 axis;  // shower axis
      double t0;     // arrival time of the sphere at the core
      double radius;  // radius of curvature
      TVector3 core;  // core position
  };

}

#endif