Rivet analyses

Measurement of the energy spectrum for B̄ → Xsγ

Experiment: BABAR (PEP-II)

Inspire ID: 1123662

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 86 (2012) 112008

Beams: * *

Beam energies: ANY

Run details: - any process making B− and B̄0 mesons, eg. particle gun or $\Upslion(4S)$ decay.

Photon energy spectrum in B → sγ decays measured by BELLE. Useful for testing the implementation of these decays

Source code:BABAR_2012_I1123662.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief B -> X_s gamma spectrum
  class BABAR_2012_I1123662 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2012_I1123662);


    /// @name Analysis methods
    /// @{

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");

      // Book histograms
      book(_h_spectrum, 1, 1, 1);
      book(_nBottom, "TMP/BottomCounter");
    }

    void findDecayProducts(const Particle& mother, unsigned int& nK0, unsigned int& nKp, unsigned int& nKm) {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == PID::KPLUS)
          ++nKp;
        else if (id == PID::KMINUS)
          ++nKm;
        else if (id == PID::K0S)
          ++nK0;
        else if (id == PID::PI0 || id == PID::PIPLUS || id == PID::PIMINUS) {
          continue;
        }
        else if (!p.children().empty()) {
          findDecayProducts(p, nK0, nKp, nKm);
        }
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // Loop over bottoms
      for (const Particle& bottom :
           apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == 521 || Cuts::abspid == 511)) {
        // remove mixing entries etc
        for (const Particle& child : bottom.children())
          if (child.abspid() == 511 || child.pid() == bottom.pid()) continue;
        _nBottom->fill();
        FourMomentum pgamma(0., 0., 0., 0.);
        unsigned int ngamma = 0;
        for (const Particle& child : bottom.children()) {
          if (child.pid() == PID::PHOTON) {
            ngamma += 1;
            pgamma += child.momentum();
          }
        }
        if (ngamma != 1) continue;
        unsigned int nK0(0), nKp(0), nKm(0);
        FourMomentum p_tot(0, 0, 0, 0);
        findDecayProducts(bottom, nK0, nKp, nKm);
        unsigned int nk = nKp - nKm + nK0;
        if (nk % 2 == 1) {
          const LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(
              bottom.momentum().betaVec());
          double eGamma = boost.transform(pgamma).E();
          _h_spectrum->fill(eGamma);
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_h_spectrum, 1e5 / _nBottom->sumW());
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_spectrum;
    CounterPtr _nBottom;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BABAR_2012_I1123662);

}