Rivet analyses

π+π− mass distribution in Λb*0(5920) → Λb0π+π− decays

Experiment: LHCB (LHC)

Inspire ID: 1114753

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 109 (2012) 172003

Beams: * *

Beam energies: ANY

Run details: - any process producing lambda_b(5920), original pp

Measurement of the production of excited Λb baryons by LHCb. The only interesting distribution from a simulation perspective is the mass disutribution of π+π− in Λb*0(5920) → Λb0π+π− decays which is sensitive to the decay model used for this decay mode.

Source code:LHCB_2012_I1114753.cc

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

namespace Rivet {


  /// @brief pi+pi- mass in lambdab* decays
  class LHCB_2012_I1114753 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(LHCB_2012_I1114753);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(Cuts::abspid == 102154), "UFS");
      book(_h_mpipi, 1, 1, 1);
    }

    void findDecayProducts(const Particle& mother,
                           unsigned int& nstable,
                           Particles& pip,
                           Particles& pim,
                           Particles& lambda) {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == PID::PIMINUS) {
          pim.push_back(p);
          ++nstable;
        }
        else if (id == PID::PIPLUS) {
          pip.push_back(p);
          ++nstable;
        }
        else if (abs(id) == 5122) {
          lambda.push_back(p);
          ++nstable;
        }
        else if (!p.children().empty()) {
          findDecayProducts(p, nstable, pip, pim, lambda);
        }
        else
          ++nstable;
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over unstable particles
      for (const Particle& lb : apply<UnstableParticles>(event, "UFS").particles()) {
        unsigned int nstable(0);
        Particles pip, pim, lambda;
        findDecayProducts(lb, nstable, pip, pim, lambda);
        // check for lambda
        if (lambda.size() != 1 || nstable != 3 || pip.size() != 1 || pim.size() != 1) continue;
        FourMomentum q = pip[0].momentum() + pim[0].momentum();
        _h_mpipi->fill(q.mass() / MeV);
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h_mpipi);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_mpipi;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(LHCB_2012_I1114753);

}