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,pion1;
39 if(Xi.children()[0].pid()==sign*3122 &&
40 Xi.children()[1].pid()==-sign*211) {
41 Lambda = Xi.children()[0];
42 pion1 = Xi.children()[1];
43 }
44 else if(Xi.children()[1].pid()==sign*3122 &&
45 Xi.children()[0].pid()==-sign*211) {
46 Lambda = Xi.children()[1];
47 pion1 = Xi.children()[0];
48 }
49 else
50 continue;
51 if(Lambda.children().size()!=2) continue;
52 Particle proton,pion2;
53 if(Lambda.children()[0].pid()==sign*2212 &&
54 Lambda.children()[1].pid()==-sign*211) {
55 proton = Lambda.children()[0];
56 pion2 = Lambda.children()[1];
57 }
58 else if(Lambda.children()[1].pid()==sign*2212 &&
59 Lambda.children()[0].pid()==-sign*211) {
60 proton = Lambda.children()[1];
61 pion2 = Lambda.children()[0];
62 }
63 else
64 continue;
65 // boost to xi rest frame first
66 LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xi.momentum().betaVec());
67 FourMomentum pLambda = boost1.transform(Lambda.momentum());
68 FourMomentum pproton = boost1.transform(proton.momentum());
69 // to lambda rest frame
70 LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pLambda.betaVec());
71 Vector3 axis = pLambda.p3().unit();
72 FourMomentum pp = boost2.transform(pproton);
73 // calculate angle
74 double cTheta = pp.p3().unit().dot(axis);
75 if(sign==1) {
76 _h_cthetaM->fill(cTheta);
77 }
78 else {
79 _h_cthetaP->fill(cTheta);
80 }
81 }
82 }
83
84 pair<double,double> calcAlpha(Histo1DPtr hist) {
85 if(hist->numEntries()==0.) return make_pair(0.,0.);
86 double sum1(0.),sum2(0.);
87 for (auto bin : hist->bins() ) {
88 double Oi = bin.area();
89 if(Oi==0.) continue;
90 double ai = 0.5*(bin.xMax()-bin.xMin());
91 double bi = 0.5*ai*(bin.xMax()+bin.xMin());
92 double Ei = bin.areaErr();
93 sum1 += sqr(bi/Ei);
94 sum2 += bi/sqr(Ei)*(Oi-ai);
95 }
96 return make_pair(sum2/sum1,sqrt(1./sum1));
97 }
98
99 /// Normalise histograms etc., after the run
100 void finalize() {
101 normalize(_h_cthetaP);
102 normalize(_h_cthetaM);
103 // calculate the values of alpha
104 // xibar+
105 Scatter2DPtr _h_alphaP;
106 book(_h_alphaP, 1,1,2);
107 pair<double,double> alpha = calcAlpha(_h_cthetaP);
108 _h_alphaP->addPoint(0.5, alpha.first, make_pair(0.5,0.5),
109 make_pair(alpha.second,alpha.second) );
110 // xi-
111 Scatter2DPtr _h_alphaM;
112 book(_h_alphaM, 1,1,1);
113 alpha = calcAlpha(_h_cthetaM);
114 _h_alphaM->addPoint(0.5, alpha.first, make_pair(0.5,0.5),
115 make_pair(alpha.second,alpha.second) );
116 }
117
118 //@}
119
120
121 /// @name Histograms
122 //@{
123 Histo1DPtr _h_cthetaP,_h_cthetaM;
124 //@}
125
126
127 };
128
129
130 // The hook for the plugin system
131 RIVET_DECLARE_PLUGIN(E756_2000_I530367);
132
133
134}
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