Rivet analyses

title: LHCB_2012_I1114753

$\pi^+\pi^-$ mass distribution in $\Lambda_b^{*0}(5920)\to\Lambda_b^0\pi^+\pi^-$ 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 $\Lambda_b$ baryons by LHCb. The only interesting distribution from a simulation perspective is the mass disutribution of $\pi^+\pi^-$ in $\Lambda_b^{*0}(5920)\to\Lambda_b^0\pi^+\pi^-$ decays which is sensitive to the decay model used for this decay mode.

Source code:LHCB_2012_I1114753.cc

```c++ // -- 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);

} ```