Rivet analyses
Measurement of energy flow, cross section and average inelasticity of forward neutrons produced in $\sqrt{s} = 13$~TeV proton-proton collisions with the LHCf Arm2 detector
Experiment: LHCF (LHC)
Inspire ID: 1783943
Status: VALIDATED
Authors: - Eugenio Berti - LHCf collaboration
References: - JHEP 07 (2020) 16 - DOI:10.1007/JHEP07(2020)016 - arXiv: 2003.02192
Beams: p+ p+
Beam energies: (6500.0, 6500.0)GeV
Run details: - Measurement of the energy flow, the cross section and the average inelasticity of forward neutrons (+ antineutrons) produced in $\sqrt{s} = 13$~TeV proton-proton collisions. These quantities are derived from the inclusive differential production cross section measured, as a function of energy, in the six pseudorapidity regions corresponding to η > 10.76, 10.06 < η < 10.75, 9.65 < η < 10.06, 8.99 < η < 9.22, 8.81 < η < 8.99 and 8.65 < η < 8.80. The measurements refer to all neutrons (and antineutrons) directly produced in the collisions or resulting from the decay of short life particles (cτ < 1 cm).
In this paper, we report the measurement of the energy flow, the cross section and the average inelasticity of forward neutrons (+ antineutrons) produced in $\sqrt{s} = 13$~TeV proton-proton collisions. These quantities are obtained from the inclusive differential production cross section, measured using the LHCf Arm2 detector at the CERN Large Hadron Collider. The measurements are performed in six pseudorapidity regions: three of them (η > 10.75, 8.99 < η < 9.21 and 8.80 < η < 8.99), albeit with smaller acceptance and larger uncertainties, were already published in a previous work, whereas the remaining three (10.06 < η < 10.75, 9.65 < η < 10.06 and 8.65 < η < 8.80) are presented here for the first time. The analysis was carried out using a data set acquired in June 2015 with a corresponding integrated luminosity of 0.194 nb−1. Comparing the experimental measurements with the expectations of several hadronic interaction models used to simulate cosmic ray air showers, none of these generators resulted to have a satisfactory agreement in all the phase space selected for the analysis. The inclusive differential production cross section for η > 10.75 is not reproduced by any model, whereas the results still indicate a significant but less serious deviation at lower pseudorapidities. Depending on the pseudorapidity region, the generators showing the best overall agreement with data are either SIBYLL 2.3 or EPOS-LHC. Furthermore, apart from the most forward region, the derived energy flow and cross section distributions are best reproduced by EPOS-LHC. Finally, even if none of the models describe the elasticity distribution in a satisfactory way, the extracted average inelasticity is consistent with the QGSJET II-04 value, while most of the other generators give values that lie just outside the experimental uncertainties.
Source
code:LHCF_2020_I1783943.cc
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/UnstableParticles.hh"
namespace Rivet {
/// @brief Measurement of forward neutron(+ antineutron) production in proton-proton Collisions at 13 TeV
class LHCF_2020_I1783943 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(LHCF_2020_I1783943);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
declare(Beam(), "Beam");
declare(FinalState(), "FS");
// Book histograms
book(_h_n_en_eta1, 1, 1, 1);
book(_h_n_en_eta2, 2, 1, 1);
book(_h_n_en_eta3, 3, 1, 1);
book(_h_n_en_eta4, 4, 1, 1);
book(_h_n_en_eta5, 5, 1, 1);
book(_h_n_en_eta6, 6, 1, 1);
book(_h_n_eflow, 7, 1, 1);
book(_h_n_sigma, 8, 1, 1);
book(_h_n_elas, 9, 1, 1);
book(_h_n_inel, 10, 1, 1);
book(_inelnorm, "_inelasticity_norm");
}
/// Perform the per-event analysis
void analyze(const Event& event) {
const FinalState &fs = apply<FinalState> (event, "FS");
Particles fs_particles = fs.particles();
bool is_fl_neutron = false;
double elasticity = 0.;
for (Particle& p: fs_particles ) {
//Artificially remove QGSJet II-04 wrong events
//if(p.pT()/GeV == 0.0) continue;
// double analysis efficiency with a two-sided LHCf
double energy = p.E()/GeV;
double eta = p.abseta();
// highest eta value must be in [10.75, 13.00]
if (eta > 13.0) eta = 11.875;
// search for forward leading particle on one side only
if (p.eta() > 0) {
if (2.*energy/sqrtS() > elasticity) {
elasticity = 2.*energy/sqrtS();
is_fl_neutron = p.abspid() == 2112;
}
}
// select neutrons above threshold
if (p.abspid() != 2112) continue; //Select neutrons and antineutrons only
_h_n_eflow->fill(eta, energy); //Energy Flow
_h_n_sigma->fill(eta); //Cross Section
// select only energy above 500 GeV
if (p.E()/GeV < 500.) continue;
// fill energy distributions
if (eta >= 10.749356) {
_h_n_en_eta1->fill( energy );
} else if(eta >= 10.056209 && eta <= 10.749356) {
_h_n_en_eta2->fill( energy );
} else if(eta >= 9.650744 && eta <= 10.056209) {
_h_n_en_eta3->fill( energy );
} else if(eta >= 8.985767 && eta <= 9.208911) {
_h_n_en_eta4->fill( energy );
} else if(eta >= 8.803446 && eta <= 8.985767) {
_h_n_en_eta5->fill( energy );
} else if(eta >= 8.649295 && eta <= 8.803446) {
_h_n_en_eta6->fill( energy );
}
}
// fill elasticity distribution if forward particle is a neutron
if (is_fl_neutron) {
_h_n_elas->fill(elasticity);
_h_n_inel->fill(1.0, 1.0-elasticity);
_inelnorm->fill();
}
}
/// Normalise histograms etc., after the run
void finalize() {
//Scale considering the LHCf Arm2 side
scale(_h_n_en_eta1, 0.5*crossSection()/millibarn/sumOfWeights()); // normalize to cross section
scale(_h_n_en_eta2, 0.5*crossSection()/millibarn/sumOfWeights()); // normalize to cross section
scale(_h_n_en_eta3, 0.5*crossSection()/millibarn/sumOfWeights()); // normalize to cross section
scale(_h_n_en_eta4, 0.5*crossSection()/millibarn/sumOfWeights()); // normalize to cross section
scale(_h_n_en_eta5, 0.5*crossSection()/millibarn/sumOfWeights()); // normalize to cross section
scale(_h_n_en_eta6, 0.5*crossSection()/millibarn/sumOfWeights()); // normalize to cross section
scale(_h_n_eflow, 0.5/sumOfWeights()); // normalize to event number
scale(_h_n_sigma, 0.5*crossSection()/millibarn/sumOfWeights()); // normalize to cross section
scale(_h_n_inel, 1. / *_inelnorm); // normalize to forward leading neutron event number
scale(_h_n_elas, crossSection()/millibarn/sumOfWeights()); // normalize to cross section
for (auto& b : _h_n_inel->bins()) {
b.scaleW(b.xWidth());
}
for (auto& b : _h_n_elas->bins()) {
b.scaleW(b.xWidth());
}
}
/// @}
private:
/// @name Histograms
/// @{
Histo1DPtr _h_n_en_eta1;
Histo1DPtr _h_n_en_eta2;
Histo1DPtr _h_n_en_eta3;
Histo1DPtr _h_n_en_eta4;
Histo1DPtr _h_n_en_eta5;
Histo1DPtr _h_n_en_eta6;
Histo1DPtr _h_n_eflow;
Histo1DPtr _h_n_sigma;
Histo1DPtr _h_n_inel;
Histo1DPtr _h_n_elas;
CounterPtr _inelnorm;
/// @}
};
RIVET_DECLARE_PLUGIN(LHCF_2020_I1783943);
}