Rivet analyses referenceE756_2000_I530367Measurement of the asymmetry in $\Xi^-\to\Lambda^0\pi^-$Experiment: E756 () Inspire ID: 530367 Status: VALIDATED Authors:
Beam energies: ANY Run details:
The Hyper CP experiment measured the asymmetry parameter in the decay $\Xi^-\to\Lambda^0\pi^-$ and the charge conjugate mode, in practice this is a fit to a normalised distribution $\frac12(1+\alpha\cos\theta)$. The paper only gives the number for the $\alpha$ parameter and not the distribution, so the distribution is calculated. The $\alpha$ parameter is then extracted using a $\chi^2$ fit. This analysis is useful for testing spin correlations in hadron decays. Source code: E756_2000_I530367.cc 1// -*- C++ -*-
2#include "Rivet/Analysis.hh"
3#include "Rivet/Projections/UnstableParticles.hh"
4
5namespace Rivet {
6
7
8 /// @brief Xi-> Lambda pi asymmetry
9 class E756_2000_I530367 : public Analysis {
10 public:
11
12 /// Constructor
13 RIVET_DEFAULT_ANALYSIS_CTOR(E756_2000_I530367);
14
15
16 /// @name Analysis methods
17 /// @{
18
19 /// Book histograms and initialise projections before the run
20 void init() {
21
22 // Initialise and register projections
23 declare(UnstableParticles(), "UFS" );
24
25 // Book histograms
26 book(_h_cthetaP, "cthetaP",20,-1,1);
27 book(_h_cthetaM, "cthetaM",20,-1,1);
28
29 }
30
31
32 /// Perform the per-event analysis
33 void analyze(const Event& event) {
34 // loop over Xi- baryons
35 for (const Particle& Xi : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid==3312)) {
36 int sign = Xi.pid()/3312;
37 if(Xi.children().size()!=2) continue;
38 Particle Lambda;
39 if(Xi.children()[0].pid()==sign*3122 &&
40 Xi.children()[1].pid()==-sign*211) {
41 Lambda = Xi.children()[0];
42 }
43 else if(Xi.children()[1].pid()==sign*3122 &&
44 Xi.children()[0].pid()==-sign*211) {
45 Lambda = Xi.children()[1];
46 }
47 else
48 continue;
49 if(Lambda.children().size()!=2) continue;
50 Particle proton;
51 if(Lambda.children()[0].pid()==sign*2212 &&
52 Lambda.children()[1].pid()==-sign*211) {
53 proton = Lambda.children()[0];
54 }
55 else if(Lambda.children()[1].pid()==sign*2212 &&
56 Lambda.children()[0].pid()==-sign*211) {
57 proton = Lambda.children()[1];
58 }
59 else
60 continue;
61 // boost to xi rest frame first
62 LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xi.momentum().betaVec());
63 FourMomentum pLambda = boost1.transform(Lambda.momentum());
64 FourMomentum pproton = boost1.transform(proton.momentum());
65 // to lambda rest frame
66 LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pLambda.betaVec());
67 Vector3 axis = pLambda.p3().unit();
68 FourMomentum pp = boost2.transform(pproton);
69 // calculate angle
70 double cTheta = pp.p3().unit().dot(axis);
71 if(sign==1) {
72 _h_cthetaM->fill(cTheta);
73 }
74 else {
75 _h_cthetaP->fill(cTheta);
76 }
77 }
78 }
79
80 pair<double,double> calcAlpha(Histo1DPtr hist) {
81 if(hist->numEntries()==0.) return make_pair(0.,0.);
82 double sum1(0.),sum2(0.);
83 for (const auto& bin : hist->bins() ) {
84 double Oi = bin.sumW();
85 if(Oi==0.) continue;
86 double ai = 0.5*(bin.xMax()-bin.xMin());
87 double bi = 0.5*ai*(bin.xMax()+bin.xMin());
88 double Ei = bin.errW();
89 sum1 += sqr(bi/Ei);
90 sum2 += bi/sqr(Ei)*(Oi-ai);
91 }
92 return make_pair(sum2/sum1,sqrt(1./sum1));
93 }
94
95 /// Normalise histograms etc., after the run
96 void finalize() {
97 normalize(_h_cthetaP);
98 normalize(_h_cthetaM);
99 // calculate the values of alpha
100 // xibar+
101 Estimate0DPtr _h_alphaP;
102 book(_h_alphaP, 1,1,2);
103 pair<double,double> alpha = calcAlpha(_h_cthetaP);
104 _h_alphaP->set(alpha.first, alpha.second);
105 // xi-
106 Estimate0DPtr _h_alphaM;
107 book(_h_alphaM, 1,1,1);
108 alpha = calcAlpha(_h_cthetaM);
109 _h_alphaM->set(alpha.first, alpha.second);
110 }
111
112 /// @}
113
114
115 /// @name Histograms
116 /// @{
117 Histo1DPtr _h_cthetaP,_h_cthetaM;
118 /// @}
119
120
121 };
122
123
124 RIVET_DECLARE_PLUGIN(E756_2000_I530367);
125
126
127}
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