RooFlatte.cc

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/*****************************************************************************
* Project: BaBar detector at the SLAC PEP-II B-factory
* Package: RooRarFit
 *    File: $Id: RooFlatte.cc,v 1.3 2011/08/26 17:54:16 fwilson Exp $
 * Authors: Fergus Wilson
 * History:
 * 
 * Copyright (C) 2005-2012, RAL
 *****************************************************************************/

// -- CLASS DESCRIPTION [PDF] --
// This is an implementation of the BES parameterisation of the Flatte
// distribution
//
// Reference : S.M.Flatte Phys. Rev. B63, 224 (1976); 
// B.S.Zou and D.V.Bugg Phys Rev. D48, R3948 (1993);
// M.Ablikim wt al (BES collaboration), Phys. Rev. D70, 092002, (2004)
//
//
// BEGIN_HTML
// This is an implementation of the BES parameterisation of the Flatte
// distribution
//
// Reference : S.M.Flatte Phys. Rev. B63, 224 (1976); 
// B.S.Zou and D.V.Bugg Phys Rev. D48, R3948 (1993);
// M.Ablikim wt al (BES collaboration), Phys. Rev. D70, 092002, (2004)
//
// END_HTML
//
#include "RooRarFit/rarVersion.hh"

#include "Riostream.h"

#include "RooRarFit/RooFlatte.hh"
#include "RooFitCore/RooAbsReal.hh"
#include "RooFitCore/RooRealVar.hh"
#include "RooFitCore/RooComplex.hh"

ClassImp(RooFlatte)

//------------------------------------------------------------------
RooFlatte::RooFlatte(const char *name, const char *title,
                     RooAbsReal& _x, RooAbsReal& _mean,
                     RooAbsReal& _g0, RooAbsReal& _m0a, RooAbsReal& _m0b, 
                     RooAbsReal& _g1, RooAbsReal& _m1a, RooAbsReal& _m1b) :
  RooAbsPdf(name,title),
  x("x","Dependent",this,_x),
  mean("mean","Mean",this,_mean),
  g0("g0","Channel 1 coupling",this,_g0),
  m0a("m0a","Mass of particle 1 in channel 1",this,_m0a),
  m0b("m0b","Mass of particle 2 in channel 1",this,_m0b),
  g1("g1","Channel 2 coupling",this,_g1),
  m1a("m1a","Mass of particle 1 in channel 2",this,_m1a),
  m1b("m1b","Mass of particle 2 in channel 2",this,_m1b)
{
}

//------------------------------------------------------------------
RooFlatte::RooFlatte(const RooFlatte& other, 
                     const char* name) : 
  RooAbsPdf(other,name), 
  x("x",this,other.x), 
  mean("mean",this,other.mean),
  g0("g0",this,other.g0),
  m0a("m0a",this,other.m0a),
  m0b("m0b",this,other.m0b),
  g1("g1",this,other.g1),
  m1a("m1a",this,other.m1a),
  m1b("m1b",this,other.m1b)
{
}

//------------------------------------------------------------------
Double_t RooFlatte::evaluate() const
{
  // calculate Flatte amplitude

  if (g0<0 || g1<0) {return(0);}

  Double_t s = x*x;

  // Energy, centre of mass p^2 of first channel
  Double_t E0a = 0.5 * (s + m0a*m0a - m0b*m0b) / x;
  Double_t qSq0 = E0a*E0a - m0a*m0a; 

  // Energy, centre of mass p^2 of second channel
  Double_t E1a = 0.5 * (s + m1a*m1a - m1b*m1b) / x;
  Double_t qSq1 = E1a*E1a - m1a*m1a; 

  RooComplex gamma0 = (qSq0 > 0) ? RooComplex(g0*sqrt(qSq0),0) : 
    RooComplex(0, g0*sqrt(-qSq0));

  RooComplex gamma1 = (qSq1 > 0) ? RooComplex(g1*sqrt(qSq1),0) : 
    RooComplex(0, g1*sqrt(-qSq1));

  RooComplex gamma = gamma0 + gamma1;

  RooComplex partB = RooComplex(0.0, 2*mean/x) * gamma;
  RooComplex partA(mean*mean - s, 0);

  RooComplex denom = partA - partB;

  //RooComplex T(mean*sqrt(g0*g1),0);
  RooComplex T(1,0);
  T = T / denom;

  return(T.abs2()); // Amplitude (arbitrary scale)

}

---