Rivet analyses

title: ATLAS_2016_I1468167

Measurement of the inelastic proton--proton cross-section at $\sqrt{s} = 13\,\TeV$

Experiment: ATLAS (LHC)

Inspire ID: 1468167

Status: VALIDATED

Authors: - Zachary Marshall - Christian Gutschow

References: - Expt page: ATLAS-STDM-2015-05 - arXiv: 1606.02625

Beams: p+ p+

Beam energies: (6500.0, 6500.0)GeV

Run details: - Inelastic events (non-diffractive and inelastic diffractive).

Inelastic cross-section is measured for $\xi > 1 \times 10^{-6}$, where $\xi=M_X^2/s$ is calculated from the invariant mass, $M_X$, of hadrons selected using the largest rapidity gap in the event.

Source code:ATLAS_2016_I1468167.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// Measurement of the inelastic proton-proton cross-section at sqrt(s) = 13 TeV class ATLAS_2016_I1468167 : public Analysis { public:

RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2016_I1468167);


/// Initialisation
void init() {
  declare(FinalState(), "FS");
  book(_h_sigma, 1, 1, 1);
}


/// Per-event analysis
void analyze(const Event& event) {

  // Get all particles, sorted from minus to plus in eta
  const FinalState& fs = apply<FinalState>(event, "FS");
  if (fs.size() < 2) vetoEvent; // need at least two particles to calculate gaps
  const Particles particles = fs.particles(cmpMomByEta);

  // Find this event's largest gap size and center
  double etapre = particles.front().eta();
  double gapcenter = 0.;
  double gapsize = -1;
  for (const Particle& p : particles) {
    const double gap = fabs(p.eta() - etapre);
    if (gap > gapsize) { // new largest gap
      gapsize = gap;
      gapcenter = (p.eta() + etapre) / 2.;
    }
    etapre = p.eta();
  }

  // Calculate xi variable of the more massive side of the event, and apply xi cut
  FourMomentum mxFourVector, myFourVector;
  for (const Particle& p : particles) {
    ((p.eta() > gapcenter) ? mxFourVector : myFourVector) += p;
  }
  const double M2 = max(mxFourVector.mass2(), myFourVector.mass2());
  const double xi = M2 / sqr(sqrtS()); // sqrt(s)=7000 GeV, note that units cancel
  if (xi < 1e-6) vetoEvent;

  // Fill the histogram
  _h_sigma->fill(sqrtS() / GeV);
}


/// Scale the acceptance histogram to inelastic cross-section
void finalize() {
  scale(_h_sigma, crossSection() / millibarn / sumOfWeights());
}


/// Histogram
Histo1DPtr _h_sigma;

};

RIVET_DECLARE_PLUGIN(ATLAS_2016_I1468167);

} ```