1// -*- C++ -*-
 2#include "Rivet/Analysis.hh"
 3#include "Rivet/Projections/FinalState.hh"
 4
 5namespace Rivet {
 6
 7
 8  class ATLAS_2011_I894867 : public Analysis {
 9  public:
10
11    RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2011_I894867);
12
13    void init() {
14      declare(FinalState(), "FS");
15      book(_h_sigma ,1, 1, 1);
16    }
17
18
19    void analyze(const Event& event) {
20
21      const FinalState& fs = apply<FinalState>(event, "FS");
22      if (fs.size() < 2) vetoEvent; // need at least two particles to calculate gaps
23
24      const Particles particles = fs.particles(cmpMomByEta);
25      double etaprev = particles.front().eta();
26      double gapcenter = etaprev;
27      double detamax = -1;
28      for (const Particle& p : particles) { // sorted from minus to plus
29        const double deta = p.eta() - etaprev; // guaranteed positive
30        if (deta > detamax) { // largest gap so far
31          detamax = deta;
32          gapcenter = (p.eta() + etaprev)/2.; // find the center of the gap to separate the X and Y systems.
33        }
34        etaprev = p.eta();
35      }
36
37      FourMomentum mxFourVector, myFourVector;
38      for (const Particle& p : particles)
39        (p.eta() > gapcenter ? mxFourVector : myFourVector) += p.momentum();
40      const double m2 = max(mxFourVector.mass2(), myFourVector.mass2());
41      const double xi = m2/sqr(sqrtS()); // sqrt(s) = 7000 GeV
42      if (xi < 5e-6) vetoEvent;
43
44      _h_sigma->fill(sqrtS()/GeV);
45    }
46
47
48    void finalize() {
49      scale(_h_sigma, crossSection()/millibarn/sumOfWeights());
50    }
51
52
53    Histo1DPtr _h_sigma;
54
55  };
56
57
58  RIVET_DECLARE_PLUGIN(ATLAS_2011_I894867);
59
60}