Rivet analyses
Transverse polarization of Λ0 baryons in the continuum at 10.58 GeV
Experiment: BELLE (KEKB)
Inspire ID: 1687566
Status: VALIDATED
Authors: - Peter Richardson
References: - Phys.Rev.Lett. 122 (2019) no.4, 042001
Beams: e- e+
Beam energies: (5.3, 5.3)GeV
Run details: - e+e- > hadrons in the continuum
Measurement of the transverse polarization of Λ0~(Λ̄0) in the e+e− continuum at centre-of-mass energies near 10.58 GeV. The polarization is measured as a function of the momentum fraction and transverse momentum. The polarization is also measured for events with a π± or K± in the opposite hemisphere as a function of the momentum fractions of the Λ0 and charged meson.The polarization as a function of the momentum fraction for Λ0 baryons produced in (uds) events, and for prompt Λ0 and those from the decay of Σ0 mesons is also measured.
Source
code:BELLE_2019_I1687566.cc
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/Thrust.hh"
#include "Rivet/Projections/UnstableParticles.hh"
#define I_KNOW_THE_INITIAL_QUARKS_PROJECTION_IS_DODGY_BUT_NEED_TO_USE_IT
#include "Rivet/Projections/InitialQuarks.hh"
namespace Rivet {
/// @brief Transverse Lambda polarization
class BELLE_2019_I1687566 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2019_I1687566);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
declare(Beam(), "Beams");
const FinalState fs;
declare(fs, "FS");
declare(Thrust(fs), "Thrust");
declare(UnstableParticles(), "UFS");
declare(InitialQuarks(), "IQF");
// Book histograms
for (unsigned int ix = 0; ix < 4; ++ix) {
book(_p_lam[ix], 1, ix + 1, 1);
book(_p_bar[ix], 1, ix + 1, 2);
book(_p_lam_pip[ix], 2, ix + 1, 1);
book(_p_lam_pim[ix], 2, ix + 1, 2);
book(_p_lam_Kp[ix], 2, ix + 1, 3);
book(_p_lam_Km[ix], 2, ix + 1, 4);
book(_p_bar_pip[ix], 2, ix + 1, 5);
book(_p_bar_pim[ix], 2, ix + 1, 6);
book(_p_bar_Kp[ix], 2, ix + 1, 7);
book(_p_bar_Km[ix], 2, ix + 1, 8);
}
book(_p_lam_inc, 3, 1, 1);
book(_p_lam_prompt, 3, 1, 3);
book(_p_lam_sigma, 3, 1, 5);
book(_p_bar_inc, 3, 1, 2);
book(_p_bar_prompt, 3, 1, 4);
book(_p_bar_sigma, 3, 1, 6);
}
/// Perform the per-event analysis
void analyze(const Event& event) {
static const double alpha = 0.642;
static const double norm = 3. / alpha * 100.;
// Get beams and average beam momentum
const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
const double meanBeamMom = (beams.first.p3().mod() + beams.second.p3().mod()) / 2.0;
int flavour = 0;
const InitialQuarks& iqf = apply<InitialQuarks>(event, "IQF");
// If we only have two quarks (qqbar), just take the flavour.
// If we have more than two quarks, look for the highest energetic q-qbar pair.
if (iqf.particles().size() == 2) {
flavour = iqf.particles().front().abspid();
}
else {
map<int, double> quarkmap;
for (const Particle& p : iqf.particles()) {
if (quarkmap[p.pid()] < p.E()) {
quarkmap[p.pid()] = p.E();
}
}
double maxenergy = 0.;
for (int i = 1; i <= 5; ++i) {
if (quarkmap[i] + quarkmap[-i] > maxenergy) {
flavour = i;
}
}
}
// thrust, to define an axis
const Thrust& thrust = apply<Thrust>(event, "Thrust");
const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
const FinalState& fs = apply<FinalState>(event, "FS");
if (thrust.thrust() < 0.8) vetoEvent;
for (const Particle& lambda : ufs.particles(Cuts::abspid == 3122)) {
int sign = lambda.pid() / 3122;
if (lambda.children().size() != 2) continue;
// look at the decay products
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;
double xE = lambda.momentum().t() / meanBeamMom;
if (xE < 0.2 || xE > 0.9) continue;
Vector3 axis1 = lambda.momentum().p3().unit();
// transverse polarization
Vector3 axis2;
if (lambda.momentum().p3().dot(thrust.thrustAxis()) >= 0.) {
axis2 = thrust.thrustAxis();
}
else {
axis2 = -thrust.thrustAxis();
}
Vector3 axis3 = axis2.cross(axis1).unit();
// boost to the lambda rest frame
LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(lambda.momentum().betaVec());
FourMomentum pproton = boost.transform(proton.momentum());
int ibin = -1;
if (xE <= 0.3)
ibin = 0;
else if (xE <= 0.4)
ibin = 1;
else if (xE <= 0.5)
ibin = 2;
else if (xE <= 0.9)
ibin = 3;
double cTheta = axis3.dot(pproton.p3().unit()) * norm;
double pT = sqrt(sqr(thrust.thrustMajorAxis().dot(lambda.momentum().p3()))
+ sqr(thrust.thrustMinorAxis().dot(lambda.momentum().p3())));
bool prompt = true;
for (const Particle& parent : lambda.parents()) {
if (parent.abspid() == 3212) {
prompt = false;
break;
}
}
// using thrust axis
if (sign > 0) {
_p_lam_inc->fill(xE, cTheta);
_p_lam[ibin]->fill(pT, cTheta);
if (flavour <= 3) {
if (prompt)
_p_lam_prompt->fill(xE, cTheta);
else
_p_lam_sigma->fill(xE, cTheta);
}
}
else if (sign < 0) {
_p_bar_inc->fill(xE, cTheta);
_p_bar[ibin]->fill(pT, cTheta);
if (flavour <= 3) {
if (prompt)
_p_bar_prompt->fill(xE, cTheta);
else
_p_bar_sigma->fill(xE, cTheta);
}
}
for (const Particle& p2 : fs.particles(Cuts::abspid == 211 || Cuts::abspid == 321)) {
if (axis2.dot(p2.p3()) > 0.) continue;
double xH = p2.momentum().t() / meanBeamMom;
Vector3 axisH = p2.momentum().p3();
Vector3 axis4 = axisH.cross(axis1).unit();
double cTheta2 = axis3.dot(pproton.p3().unit()) * norm;
if (sign > 0) {
if (p2.pid() == 211)
_p_lam_pip[ibin]->fill(xH, cTheta2);
else if (p2.pid() == -211)
_p_lam_pim[ibin]->fill(xH, cTheta2);
else if (p2.pid() == 321)
_p_lam_Kp[ibin]->fill(xH, cTheta2);
else if (p2.pid() == -321)
_p_lam_Km[ibin]->fill(xH, cTheta2);
}
else {
if (p2.pid() == 211)
_p_bar_pip[ibin]->fill(xH, cTheta2);
else if (p2.pid() == -211)
_p_bar_pim[ibin]->fill(xH, cTheta2);
else if (p2.pid() == 321)
_p_bar_Kp[ibin]->fill(xH, cTheta2);
else if (p2.pid() == -321)
_p_bar_Km[ibin]->fill(xH, cTheta2);
}
}
}
}
/// Normalise histograms etc., after the run
void finalize() { }
/// @}
/// @name Histograms
/// @{
Profile1DPtr _p_lam[4], _p_bar[4];
Profile1DPtr _p_lam_pip[4], _p_lam_pim[4], _p_lam_Kp[4], _p_lam_Km[4];
Profile1DPtr _p_bar_pip[4], _p_bar_pim[4], _p_bar_Kp[4], _p_bar_Km[4];
Profile1DPtr _p_lam_inc, _p_lam_prompt, _p_lam_sigma;
Profile1DPtr _p_bar_inc, _p_bar_prompt, _p_bar_sigma;
/// @}
};
RIVET_DECLARE_PLUGIN(BELLE_2019_I1687566);
}