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CMS_2018_I1653948

Measurement of the inelastic proton-proton cross section at 13 TeV
Experiment: CMS (LHC)
Inspire ID: 1653948
Status: VALIDATED
Authors:
  • cms-pag-conveners-smp@cern.ch
  • Sercan Sen
  • Pierre Van Mechelen
  • Hans Van Haevermaet
References:
  • JHEP 07 (2018) 161
  • DOI 10.1007/JHEP07(2018)161
  • CMS-FSQ-15-005
  • arXiv: 1802.02613
  • CERN-EP-2018-004
  • http://cms-results.web.cern.ch/cms-results/public-results/publications/FSQ-15-005/
Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • Inelastic events (non-diffractive and inelastic diffractive).

A measurement of the inelastic proton-proton cross section at $\sqrt{s} = 13$ TeV with the CMS detector at the LHC has been presented. An inelastic cross section of $67.5 \pm 0.8 \text{(syst)} \pm 1.6 \text{(lumi)}$ mb is obtained for $\xi = M^2/s > 10^{-6}$ (corresponding to $M > 13$ GeV), with $M$ the larger of $M_{\mathrm{X}}$ and $M_{\mathrm{Y}}$, where $M_{\mathrm{X}}$ and $M_{\mathrm{Y}}$ are the masses of the diffractive dissociation systems with negative and positive pseudorapidities, respectively. In addition, an inelastic cross section of $68.6 \pm 0.5 \text{(syst)} \pm 1.6 \text{(lumi)}$ mb is obtained in the enlarged phase space $\xi_{\mathrm{X}} > 10^{-7}$ and/or $\xi_{\mathrm{Y}} > 10^{-6}$ (corresponding to $M_{\mathrm{X}} > 4.1$ GeV and/or $M_{\mathrm{Y}} > 13$ GeV).

Source code: CMS_2018_I1653948.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"

namespace Rivet {


  class CMS_2018_I1653948 : public Analysis {
  public:

    CMS_2018_I1653948()
      : Analysis("CMS_2018_I1653948"), _xi_hf_cut(1E-6), _xi_castor_cut(1E-7)
    {    }


    /// Book projections and histograms
    void init() {
      addProjection(FinalState(),"FS");
      _h_xsec = bookHisto1D(1, 1, 1);
    }


    /// Analyze each event
    void analyze(const Event& event) {

      const double weight = event.weight();

      const FinalState& fs = applyProjection<FinalState>(event, "FS");
      if (fs.size() < 3) vetoEvent; // veto on elastic events
      const ParticleVector particlesByRapidity = fs.particles(cmpMomByRap);
      const size_t num_particles = particlesByRapidity.size();

      vector<double> gaps;
      vector<double> midpoints;

      for (size_t ip = 1; ip < num_particles; ++ip) {
        const Particle& p1 = particlesByRapidity[ip-1];
        const Particle& p2 = particlesByRapidity[ip];
        const double gap = p2.momentum().rapidity() - p1.momentum().rapidity();
        const double mid = (p2.momentum().rapidity() + p1.momentum().rapidity()) / 2.;
        gaps.push_back(gap);
        midpoints.push_back(mid);
      }

      int imid = std::distance(gaps.begin(), max_element(gaps.begin(), gaps.end()));
      double gapcenter = midpoints[imid];

      FourMomentum MxFourVector(0.,0.,0.,0.);
      FourMomentum MyFourVector(0.,0.,0.,0.);

      for (const Particle& p : fs.particles(cmpMomByEta)) {
        if (p.momentum().rapidity() < gapcenter) {
          MxFourVector += p.momentum();
        } else {
          MyFourVector += p.momentum();
        }
      }

      double Mx = MxFourVector.mass();
      double My = MyFourVector.mass();

      double xix = (Mx * Mx) / (sqrtS()/GeV * sqrtS()/GeV);
      double xiy = (My * My) / (sqrtS()/GeV * sqrtS()/GeV);
      double xi  = max(xix, xiy);

      if (xi > _xi_hf_cut) _h_xsec->fill(0.5, weight);
      if (xix > _xi_castor_cut || xiy > _xi_hf_cut) _h_xsec->fill(1.5, weight);
    }


    /// Normalizations, etc.
    void finalize() {
      scale(_h_xsec, crossSection()/millibarn/sumOfWeights());
    }


  private:

    Histo1DPtr _h_xsec;
    double _xi_hf_cut;
    double _xi_castor_cut;

  };


  DECLARE_RIVET_PLUGIN(CMS_2018_I1653948);

}