Rivet analyses

X(3872) mass and angular distributions using B → KX(3872)

Experiment: BELLE (KEKB)

Inspire ID: 916712

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 84 (2011) 052004

Beams: * *

Beam energies: ANY

Run details: - Any process producting B+ and V0 mesons, origniallyu Upsilon(4S) decay

Measurement of mass and angular distribution for the production of X(3872) in the decay B → KX(3872). There is no consensus as to the nature of the X(3872) cc̄ state and therefore we taken its PDG code to be 9030443, i.e. the first unused code for an undetermined spin one cc̄ state. This can be changed using the PID option if a different code is used by the event generator performing the simulation.

Source code:BELLE_2011_I916712.cc

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

namespace Rivet {


  /// @brief B -> K X(3872)
  class BELLE_2011_I916712 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2011_I916712);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // set the PDG code
      _pid = getOption<double>("PID", 9030443);
      // projections
      declare(UnstableParticles(Cuts::abspid == 511 || Cuts::abspid == 521), "UFS");
      // histograms
      for (unsigned int ix = 0; ix < 4; ++ix) book(_h[ix], 1 + ix, 1, 1);
    }

    void findChildren(const Particle& p,
                      Particles& pim,
                      Particles& pip,
                      Particles& Jpsi,
                      unsigned int& ncount) {
      for (const Particle& child : p.children()) {
        if (child.pid() == PID::PIPLUS) {
          pip.push_back(child);
          ncount += 1;
        }
        else if (child.pid() == PID::PIMINUS) {
          pim.push_back(child);
          ncount += 1;
        }
        else if (child.pid() == PID::JPSI) {
          Jpsi.push_back(child);
          ncount += 1;
        }
        else if (child.children().empty()) {
          ncount += 1;
        }
        else
          findChildren(child, pim, pip, Jpsi, ncount);
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles()) {
        if (p.children().empty()) continue;
        if (p.children().size() == 1) continue;
        if (p.children().size() != 2) continue;
        Particle K, X;
        if (p.children()[0].pid() == _pid) {
          X = p.children()[0];
          K = p.children()[1];
        }
        else if (p.children()[1].pid() == _pid) {
          X = p.children()[1];
          K = p.children()[0];
        }
        else
          continue;
        if (K.abspid() != 311 && K.abspid() != 321 && K.abspid() != 310 && K.abspid() != 130) continue;
        // X(3872) decay
        unsigned int ncount = 0;
        Particles pip, pim, Jpsi;
        findChildren(X, pim, pip, Jpsi, ncount);
        if (ncount != 3 || !(pim.size() == 1 && pip.size() == 1 && Jpsi.size() == 1)) continue;
        _h[3]->fill((pip[0].momentum() + pim[0].momentum()).mass());
        LorentzTransform boostB = LorentzTransform::mkFrameTransformFromBeta(p.momentum().betaVec());
        Vector3 axisX = -boostB.transform(K.momentum()).p3().unit();
        FourMomentum pX = boostB.transform(X.momentum());
        LorentzTransform boostX = LorentzTransform::mkFrameTransformFromBeta(pX.betaVec());
        FourMomentum pPsi = boostX.transform(boostB.transform(Jpsi[0].momentum()));
        double cTheta = axisX.dot(pPsi.p3().unit());
        _h[0]->fill(cTheta);
        // finally the leptons from J/psi decay
        if (Jpsi[0].children().size() != 2) vetoEvent;
        if (Jpsi[0].children()[0].pid() != -Jpsi[0].children()[1].pid()) vetoEvent;
        if (Jpsi[0].children()[0].abspid() != PID::EMINUS && Jpsi[0].children()[0].abspid() != PID::MUON)
          vetoEvent;
        Particle lm = Jpsi[0].children()[0];
        Particle lp = Jpsi[0].children()[1];
        Vector3 axispi = boostX.transform(boostB(pip[0].momentum())).p3().unit();
        Vector3 axisZ = axispi.cross(axisX).unit();
        Vector3 axisL = boostX.transform(boostB(lp.momentum())).p3().unit();
        _h[2]->fill(abs(axisZ.dot(axisL)));
        _h[1]->fill(abs(axisX.dot(axisL)));
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      for (unsigned int ix = 0; ix < 4; ++ix) normalize(_h[ix], 1., false);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[4];
    int _pid;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2011_I916712);

}