Rivet analyses

title: LHCB_2022_I1971920

Decay asymmetry in $\Lambda_b^0\to\Lambda^0\gamma$

Experiment: LHCB (LHC)

Inspire ID: 1971920

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 105 (2022) 5, L051104

Beams: * *

Beam energies: ANY

Run details: - Any process producing Lambda_b, original pp

Measurement of the decay asymmetry in $\Lambda_b^0\to\Lambda^0\gamma$ by LHCb

Source code:LHCB_2022_I1971920.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Lambdab0 -> Lambda0 gamma class LHCB_2022_I1971920 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(LHCB_2022_I1971920);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(Cuts::pid == 5122), "UFS");
  // histos
  book(_h_ctheta_gamma, "TMP/ctheta_gamma", 20, -1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // loop over Lambda_b baryons
  for (const Particle& lamB : apply<UnstableParticles>(event, "UFS").particles()) {
    int sign = lamB.pid() / 5122;
    // Lambda_b -> Lambda0 gamma
    if (lamB.children().size() != 2) continue;
    Particle lambda, gamma;
    if (lamB.children()[1].pid() == sign * 3122 && lamB.children()[0].pid() == 22) {
      lambda = lamB.children()[1];
      gamma = lamB.children()[0];
    }
    else if (lamB.children()[0].pid() == sign * 3122 && lamB.children()[1].pid() == 22) {
      lambda = lamB.children()[0];
      gamma = lamB.children()[1];
    }
    else
      continue;
    // Lambda0 -> p pi+
    if (lambda.children().size() != 2) continue;
    Particle proton, pion;
    if (lambda.children()[0].pid() == sign * 2212 && lambda.children()[1].pid() == -sign * 211) {
      proton = lambda.children()[0];
      pion = lambda.children()[1];
    }
    else if (lambda.children()[1].pid() == sign * 2212 && lambda.children()[0].pid() == -sign * 211) {
      proton = lambda.children()[1];
      pion = lambda.children()[0];
    }
    else
      continue;
    // first boost to the lamB rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(lamB.momentum().betaVec());
    FourMomentum plambda = boost1.transform(lambda.mom());
    FourMomentum pproton = boost1.transform(proton.mom());
    // to lambda rest frame
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(plambda.betaVec());
    Vector3 axis = plambda.p3().unit();
    FourMomentum pp = boost2.transform(pproton);
    // calculate angle
    const double cTheta = pp.p3().unit().dot(axis);
    _h_ctheta_gamma->fill(cTheta);
  }
}

pair<double, double> calcAlpha(Histo1DPtr hist) {
  if (hist->numEntries() == 0.) return make_pair(0., 0.);
  double sum1(0.), sum2(0.);
  for (const auto& bin : hist->bins()) {
    double Oi = bin.sumW();
    if (Oi == 0.) continue;
    double ai = 0.5 * (bin.xMax() - bin.xMin());
    double bi = 0.5 * ai * (bin.xMax() + bin.xMin());
    double Ei = bin.errW();
    sum1 += sqr(bi / Ei);
    sum2 += bi / sqr(Ei) * (Oi - ai);
  }
  return make_pair(sum2 / sum1, sqrt(1. / sum1));
}

/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_ctheta_gamma);
  pair<double, double> alpha = calcAlpha(_h_ctheta_gamma);
  const double aLam = 0.754;
  alpha.first /= aLam;
  alpha.second /= aLam;
  Estimate0DPtr tmp;
  book(tmp, 1, 1, 1);
  tmp->set(alpha.first, alpha.second);
}

/// @}


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

};

RIVET_DECLARE_PLUGIN(LHCB_2022_I1971920);

} ```