mt2w_bisect.hpp

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/***********************************************************************/
/*                                                                     */
/*              Finding MT2W                                           */
/*              Reference:  arXiv:1203.4813 [hep-ph]                   */
/*              Authors: Yang Bai, Hsin-Chia Cheng,                    */
/*                       Jason Gallicchio, Jiayin Gu                   */
/*              Based on MT2 by: Hsin-Chia Cheng, Zhenyu Han           */ 
/*              May 8, 2012, v1.00a                                    */
/*                                                                     */  
/***********************************************************************/

#ifndef MT2W_BISECT_H
#define MT2W_BISECT_H

/*The user can change the desired precision below, the larger one of the following two definitions is used. Relative precision less than 0.00001 is not guaranteed to be achievable--use with caution*/ 

#define RELATIVE_PRECISION 0.00001 //defined as precision = RELATIVE_PRECISION * scale, where scale = max{Ea, Eb}
#define ABSOLUTE_PRECISION 0.0     //absolute precision for mt2w, unused by default

//Reserved for expert
#define MIN_MASS  0.1   //if ma<MINMASS and mb<MINMASS, use massless code
#define ZERO_MASS 0.000 //give massless particles a small mass
#define SCANSTEP 0.1
namespace mt2w_bisect
{
  class mt2w
  {  
  public:
      
    mt2w(double upper_bound=500.0, double error_value=499.0, double scan_step=0.5);
    // Constructor where:
    //    upper_bound:  the upper bound of search for MT2W, default value is 500 GeV 
    //    error_value:  if we couldn't find any compatible region below upper_bound, this value gets returned.  
    //                  -1.0 is a reasonable to indicate error, but upper_bound-1.0 allows a simple greater-than cut for signal
    //    scan_step:    if we need to scan to find the compatible region, this is the step of the scan
    void   set_momenta(double *pl0, double *pb10, double *pb20, double* pmiss0);  //b1 pairs with l
    void   set_momenta(double El,  double plx,  double ply,  double plz,
                       double Eb1, double pb1x, double pb1y, double pb1z,
                       double Eb2, double pb2x, double pb2y, double pb2z,
                       double pmissx, double pmissy);  // Same as above without pointers/arrays
    // Where the input 4-vector information represents:
    //    l is the visible lepton
    //    b1 is the bottom on the same side as the visible lepton
    //    b2 is the other bottom (paired with the invisible W)
    //    pmiss is missing momentum with only x and y components.
    double get_mt2w();  // Calculates result, which is cached until set_momenta is called.
    void   print();
      
  protected:
    void   mt2w_bisect();  // The real work is done here.
      
  private:  

    bool   solved_;
    bool   momenta_set_;
    double upper_bound_;
    double error_value_;
    double scan_step_;
    double mt2w_b_;

    int    teco(double mtop);   // test the compatibility of a given trial top mass mtop
    inline int    signchange_n( long double t1, long double t2, long double t3, long double t4, long double t5);
    inline int    signchange_p( long double t1, long double t2, long double t3, long double t4, long double t5);

    //data members
    double plx_, ply_, plz_, ml_, El_;      // l is the visible lepton
    double pb1x_, pb1y_, pb1z_, mb1_, Eb1_;   // b1 is the bottom on the same side as the visible lepton
    double pb2x_, pb2y_, pb2z_, mb2_, Eb2_;   // b2 is the other bottom
    double pmissx_, pmissy_;              // x and y component of missing p_T
    double mv_,mw_;           //mass of neutrino and W-boson
     
    //auxiliary definitions
    double mlsq_, Elsq_;
    double mb1sq_, Eb1sq_;
    double mb2sq_, Eb2sq_;

    //auxiliary coefficients
    double a1_, b1_, c1_, a2_, b2_, c2_, d1_, e1_, f1_, d2_, e2_, f2_;
    double d2o_, e2o_, f2o_;

    double precision_;
  };

}//end namespace mt2w_bisect

#endif