Rivet analyses referenceARGUS_1992_I319105Decay asymmetries in $\Lambda^+_c \to \Lambda^0\pi^+$Experiment: ARGUS (DORIS) Inspire ID: 319105 Status: VALIDATED Authors:
Beam energies: ANY Run details:
Measurement of the decay asymmetries in $\Lambda^+_c\to\Lambda^0\pi^+$ by the ARGUS experiment. The asymmetry parameter is extracted by fitting to normalised angular distribution. N.B. the product of the asymmetry parameters for the $\Lambda_c$ and daughter baryon is implemented as this is what is measured, rather than the extracted parameter for the $\Lambda_c$ which relies on other measurements of the parameter for the daughter baryon. This analysis is useful for testing spin correlations in hadron decays. Source code: ARGUS_1992_I319105.cc 1// -*- C++ -*-
2#include "Rivet/Analysis.hh"
3#include "Rivet/Projections/UnstableParticles.hh"
4
5namespace Rivet {
6
7
8 /// @brief Lambda_c -> Lambda pi asymmetry
9 class ARGUS_1992_I319105 : public Analysis {
10 public:
11
12 /// Constructor
13 RIVET_DEFAULT_ANALYSIS_CTOR(ARGUS_1992_I319105);
14
15
16 /// @name Analysis methods
17 /// @{
18
19 /// Book histograms and initialise projections before the run
20 void init() {
21 // Initialise and register projections
22 declare(UnstableParticles(), "UFS" );
23 // Book histograms
24 book(_h_ctheta,3,1,1);
25 }
26
27
28 /// Perform the per-event analysis
29 void analyze(const Event& event) {
30 // loop over Lambda_c baryons
31 for( const Particle& Lambdac : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid==4122)) {
32 int sign = Lambdac.pid()/4122;
33 if(Lambdac.children().size()!=2) continue;
34 Particle baryon1,meson1;
35 if(Lambdac.children()[0].pid()==sign*3122 &&
36 Lambdac.children()[1].pid()==sign*211) {
37 baryon1 = Lambdac.children()[0];
38 meson1 = Lambdac.children()[1];
39 }
40 else if(Lambdac.children()[1].pid()==sign*3122 &&
41 Lambdac.children()[0].pid()==sign*211) {
42 baryon1 = Lambdac.children()[1];
43 meson1 = Lambdac.children()[0];
44 }
45 else
46 continue;
47 Particle baryon2,meson2;
48 if(baryon1.children()[0].pid()== sign*2212 &&
49 baryon1.children()[1].pid()==-sign*211) {
50 baryon2 = baryon1.children()[0];
51 meson2 = baryon1.children()[1];
52 }
53 else if(baryon1.children()[1].pid()== sign*2212 &&
54 baryon1.children()[0].pid()==-sign*211) {
55 baryon2 = baryon1.children()[1];
56 meson2 = baryon1.children()[0];
57 }
58 else
59 continue;
60 // first boost to the Lambdac rest frame
61 LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.momentum().betaVec());
62 FourMomentum pbaryon1 = boost1.transform(baryon1.momentum());
63 FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
64 // to lambda rest frame
65 LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pbaryon1.betaVec());
66 Vector3 axis = pbaryon1.p3().unit();
67 FourMomentum pp = boost2.transform(pbaryon2);
68 // calculate angle
69 double cTheta = pp.p3().unit().dot(axis);
70 _h_ctheta->fill(cTheta);
71 }
72 }
73
74 pair<double,double> calcAlpha(Histo1DPtr hist) {
75 if(hist->numEntries()==0.) return make_pair(0.,0.);
76 double sum1(0.),sum2(0.);
77 for (const auto& bin : hist->bins() ) {
78 double Oi = bin.sumW();
79 if(Oi==0.) continue;
80 double ai = 0.5*(bin.xMax()-bin.xMin());
81 double bi = 0.5*ai*(bin.xMax()+bin.xMin());
82 double Ei = bin.errW();
83 sum1 += sqr(bi/Ei);
84 sum2 += bi/sqr(Ei)*(Oi-ai);
85 }
86 return make_pair(sum2/sum1,sqrt(1./sum1));
87 }
88
89
90 /// Normalise histograms etc., after the run
91 void finalize() {
92 normalize(_h_ctheta);
93 Estimate0DPtr _h_alpha;
94 book(_h_alpha,2,1,1);
95 pair<double,double> alpha = calcAlpha(_h_ctheta);
96 _h_alpha->set(alpha.first, alpha.second);
97 }
98
99 /// @}
100
101
102 /// @name Histograms
103 /// @{
104 Histo1DPtr _h_ctheta;
105 /// @}
106
107
108 };
109
110
111 RIVET_DECLARE_PLUGIN(ARGUS_1992_I319105);
112
113}
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