Rivet analyses

title: LHCB_2021_I1913240

Measurement of nuclear modification factor and prompt charged-particle production in $p$-Pb and $pp$ collisions at $\sqrt{s_{NN}} = 5$ TeV

Experiment: LHCB (LHC)

Inspire ID: 1913240

Status: VALIDATED

Authors: - Gino Daniels - Lars Kolk - Julian Boelhauve

References: - Phys.Rev.Lett. 128 (2022) 14, 142004 - DOI:10.1103/PhysRevLett.128.142004 - arXiv: 2108.13115 - CERN-EP-2021-130 - LHCB-PAPER-2021-015

Beams: p+ p+, p+ 1000822080

Beam energies: (2510.0, 2510.0); (4000.0, 328000.0)GeV

Run details: - Minimum-bias proton-proton or proton-lead interactions at 5 TeV centre-of-mass energy.

The production of prompt charged particles in proton-lead collisions and in proton-proton collisions at the nucleon-nucleon center-of-mass energy $\sqrt{s_{NN}} = 5\,\mathrm{TeV}$ is studied at LHCb as a function of pseudorapidity ($\eta$) and transverse momentum ($p_\mathrm{T}$) with respect to the proton beam direction. The nuclear modification factor for charged particles is determined as a function of $\eta$ between $-4.8 < \eta < -2.5$ (backward region) and $2.0 < \eta < 4.8$ (forward region), and $p_\mathrm{T}$ between $0.2 < p_\mathrm{T} < 8.0\,\mathrm{GeV}/c$. The results show a suppression of charged particle production in proton-lead collisions relative to proton-proton collisions in the forward region and an enhancement in the backward region for $p_\mathrm{T}$ larger than $1.5\,\mathrm{GeV}/c$. This measurement constrains nuclear PDFs and saturation models at previously unexplored values of the parton momentum fraction down to $10^{-6}$.

Source code:LHCB_2021_I1913240.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/AliceCommon.hh"

namespace Rivet {

/// @brief nuclear modification factor and prompt charged particle production in pPb and pp at $\sqrt{s_{NN}} = 5$ TeV class LHCB_2021_I1913240 : public Analysis {

public:

RIVET_DEFAULT_ANALYSIS_CTOR(LHCB_2021_I1913240);

void init() {
  // Check collision system of input file
  const ParticlePair& beam_pair = beams();
  if (beam_pair.first.pid() == PID::PROTON && beam_pair.second.pid() == PID::PROTON) {
    _beamConfig = "pp";
  }
  else if (beam_pair.first.pid() == PID::PROTON && beam_pair.second.pid() == PID::LEAD) {
    _beamConfig = "pPb";
  }
  if (_beamConfig == "" && !merging()) {
    throw BeamError("Invalid beam configuration for " + name() + "\n");
  }

  // Link histogram to corresponding table of YODA file from HEPData
  book(_charged_part_eta_pt_hist[0], {2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 4.8},
       {"d01-x01-y01", "d01-x01-y02", "d01-x01-y03", "d01-x01-y04", "d01-x01-y05", "d01-x01-y06"});
  // forward and backward histos for pPb below
  book(_charged_part_eta_pt_hist[1], {1.6, 2.0, 2.5, 3.0, 3.5, 4.0, 4.3},
       {"d02-x01-y01", "d02-x01-y02", "d02-x01-y03", "d02-x01-y04", "d02-x01-y05", "d02-x01-y06"});
  book(_charged_part_eta_pt_hist[2], {-5.2, -4.8, -4.5, -4.0, -3.5, -3.0, -2.5},
       {"d03-x01-y01", "d03-x01-y02", "d03-x01-y03", "d03-x01-y04", "d03-x01-y05", "d03-x01-y06"});

  // Select primary (i.e. prompt long-lived) charged particles
  declare(ALICE::PrimaryParticles((Cuts::absetaIn(1.6, 5.2)) && (Cuts::abscharge == 1)), "PPs");
}

void analyze(const Event& ev) {
  // Apply PrimaryParticles projection
  const Particles prompt_charged_parts = apply<ALICE::PrimaryParticles>(ev, "PPs").particles();
  for (const Particle& part : prompt_charged_parts) {
    if (part.momentum().p() < 2 * GeV) continue;

    // Double sample size in symmetric proton-proton case by taking absolute value
    // and then scale by 0.5 due to LHCb being one-sided
    double eta;
    if (_beamConfig == "pp")
      eta = part.abseta();
    else
      eta = part.eta();
    double pt = part.pT();

    if (_beamConfig == "pp") {
      _charged_part_eta_pt_hist[0]->fill(eta, pt, 0.5);
    }
    else {
      _charged_part_eta_pt_hist[1]->fill(eta, pt);
      _charged_part_eta_pt_hist[2]->fill(eta, pt);
    }
  }
}

void finalize() {
  // Compute scale factor with inelastic cross-section from input file and sum of
  // weights (corresponds to number of events in input file)
  divByGroupWidth(_charged_part_eta_pt_hist);
  scale(_charged_part_eta_pt_hist, crossSection() / millibarn / sumOfWeights());

  // if both histo groups have filled bins then compute R_pPb
  Estimate1DPtr _ratio;
  for (size_t i = 1; i <= 5; ++i) {
    book(_ratio, 4, 1, i);
    YODA::Histo1D numer = _charged_part_eta_pt_hist[1]->bin(i + 1)->clone();
    YODA::Histo1D denom = _charged_part_eta_pt_hist[0]->bin(i)->clone();
    numer.rebinXTo(_ratio->xEdges());
    denom.rebinXTo(_ratio->xEdges());
    divide(numer, denom, _ratio);
    _ratio->scale(1. / 208.);
    book(_ratio, 5, 1, i);
    numer = _charged_part_eta_pt_hist[2]->bin(i + 1)->clone();
    denom = _charged_part_eta_pt_hist[0]->bin(7 - i)->clone();
    numer.rebinXTo(_ratio->xEdges());
    denom.rebinXTo(_ratio->xEdges());
    divide(numer, denom, _ratio);
    _ratio->scale(1. / 208.);
  }
}

string _beamConfig = "";
Histo1DGroupPtr _charged_part_eta_pt_hist[3];

};

RIVET_DECLARE_PLUGIN(LHCB_2021_I1913240); } ```