JGeane.hh

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#ifndef __JPHYSICS__JGEANE__
#define __JPHYSICS__JGEANE__

#include <cmath>

/**
 * \file
 * Energy loss of muon.
 * \author mdejong
 */

namespace JPHYSICS {}
namespace JPP { using namespace JPHYSICS; }

namespace JPHYSICS {

  
  /**
   * Equivalent muon track length per unit shower energy.
   *
   * \return        equivalent muon track length [m/Gev]
   */
  inline double geanc()
  {
    return 4.7;                   // dx/dE [m/GeV]
  }


  /**
   * Function object for the Energy loss of the muon.
   * The energy loss can be formulated as:
   *
   *     \f[ -\frac{dE}{dx}  =  a + bE\f]
   *
   * N.B:
   * \f$a\f$ and \f$b\f$ are assumed constant (internal units m and GeV, respectively).
   */
  class JGeane {
  public:
    /**
     * constructor
     * \param  __a     Energy loss due to ionisation [GeV/m]
     * \param  __b     Energy loss due to pair production and Bremstrahlung [m^-1]
     */ 
    JGeane(const double __a,
           const double __b) :
      a(__a),
      b(__b)
    {}
    

    /**
     * Get energy loss constant.
     *
     * \return         Energy loss due to ionisation [GeV/m]
     */
    double getA() const
    {
      return a;
    }


    /**
     * Get energy loss constant.
     *
     * \return         Energy loss due to pair production and Bremstrahlung [m^-1]
     */
    double getB() const
    {
      return b;
    }


    /**
     * Get energy of muon after specified distance.
     *
     * \param  E       Energy of muon [GeV]
     * \param  dx      distance traveled [m]
     * \return         Energy of muon [GeV]
     */
    double getE(const double E, const double dx) const
    {
      const double y = (a/b + E) * exp(-b*dx)  -  a/b;

      if (y > 0.0)
        return y;
      else
        return 0.0;
    }


    /**
     * Get derivative of energy of muon after specified distance.
     *
     * \param  dx      distance traveled [m]
     * \return         derivative of energy of muon 
     */
    double getDE(const double dx) const
    {
      return exp(-b*dx);
    }


    /**
     * Energy of muon after specified distance.
     *
     * \param  E       Energy of muon [GeV]
     * \param  dx      distance traveled [m]
     * \return         Energy of muon [GeV]
     */
    double operator()(const double E, const double dx) const
    {
      return getE(E, dx);
    }
    

    /**
     * Range of muon.
     *
     * \param  E       Energy of muon [GeV]
     * \return         range [m]
     */
    double operator()(const double E) const
    {
      const double x = b*E/a;

      if (x > -1.0)
        return (1/b) * log(1.0 + x);
      else
        return 0.0;
    }


    /**
     * Equivalent unit track length per unit shower energy and per unit track length.
     *
     * \return        equivalent unit track length [Gev^-1]
     */
    double operator()() const
    {
      return b * geanc();
    }

  protected:
    const double a;
    const double b;
  };


  /**
   * Function object for Energy loss of muon in sea water.
   *
   * Approximate values of energy loss parameters taken from reference:
   * Proceedings of ICRC 2001, "Precise parametrizations of muon energy losses in water",
   * S. Klimushin, E. Bugaev and I. Sokalski.
   */
  static const JGeane gWater(2.67e-1 * JTOOLS::DENSITY_SEA_WATER, 
                             3.4e-4  * JTOOLS::DENSITY_SEA_WATER);


  /**
   * Function object for Energy loss of muon in rock.
   */
  static const JGeane gRock(2.67e-1 * 0.9 * JTOOLS::DENSITY_ROCK, 
                            3.4e-4  * 1.2 * JTOOLS::DENSITY_ROCK);

}

#endif