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Neutral pion and $η$ meson production in proton-proton collisions at $\sqrt{s}=0.9$ TeV and $\sqrt{s}=7$ TeV

ALICE Collaboration

TL;DR

This work reports the first measurements of inclusive $\pi^0$ and $\eta$ invariant differential cross sections in proton-proton collisions at $\sqrt{s}=0.9$ TeV and $7$ TeV, extending over wide $p_T$ ranges. Using two independent photon-detection methods—PHOS and the photon conversion method (PCM)—the ALICE Collaboration provides robust cross sections and tests of perturbative QCD with various PDFs and fragmentation functions. At $\sqrt{s}=0.9$ TeV, NLO pQCD describes the $\pi^0$ data well, while at $7$ TeV the predictions overestimate the cross sections and require a larger scale (e.g., $\mu=2p_T$) or different FF sets, though the $\eta/\pi^0$ ratio is better described by the calculations. The measurements constrain fragmentation functions and PDFs, offering crucial input for tuning pQCD at LHC energies and serving as a reference for heavy-ion analyses (e.g., $R_{AA}$) and background estimates for direct photons and heavy flavors.

Abstract

The first measurements of the invariant differential cross sections of inclusive $π^0$ and $η$ meson production at mid-rapidity in proton-proton collisions at $\sqrt{s}=0.9$ TeV and $\sqrt{s}=7$ TeV are reported. The $π^0$ measurement covers the ranges $0.4<p_T<7$ GeV/$c$ and $0.3<p_T<25$ GeV/$c$ for these two energies, respectively. The production of $η$ mesons was measured at $\sqrt{s}=7$ TeV in the range $0.4<p_T<15$ GeV/$c$. Next-to-Leading Order perturbative QCD calculations, which are consistent with the $π^0$ spectrum at $\sqrt{s}=0.9$ TeV, overestimate those of $π^0$ and $η$ mesons at $\sqrt{s}=7$ TeV, but agree with the measured $η/π^0$ ratio at $\sqrt{s}=7$ TeV.

Neutral pion and $η$ meson production in proton-proton collisions at $\sqrt{s}=0.9$ TeV and $\sqrt{s}=7$ TeV

TL;DR

This work reports the first measurements of inclusive and invariant differential cross sections in proton-proton collisions at TeV and TeV, extending over wide ranges. Using two independent photon-detection methods—PHOS and the photon conversion method (PCM)—the ALICE Collaboration provides robust cross sections and tests of perturbative QCD with various PDFs and fragmentation functions. At TeV, NLO pQCD describes the data well, while at TeV the predictions overestimate the cross sections and require a larger scale (e.g., ) or different FF sets, though the ratio is better described by the calculations. The measurements constrain fragmentation functions and PDFs, offering crucial input for tuning pQCD at LHC energies and serving as a reference for heavy-ion analyses (e.g., ) and background estimates for direct photons and heavy flavors.

Abstract

The first measurements of the invariant differential cross sections of inclusive and meson production at mid-rapidity in proton-proton collisions at TeV and TeV are reported. The measurement covers the ranges GeV/ and GeV/ for these two energies, respectively. The production of mesons was measured at TeV in the range GeV/. Next-to-Leading Order perturbative QCD calculations, which are consistent with the spectrum at TeV, overestimate those of and mesons at TeV, but agree with the measured ratio at TeV.

Paper Structure

This paper contains 7 sections, 2 equations, 5 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Detector description and event selection
  3. Neutral meson reconstruction
  4. Results and comparison with pQCD
  5. Conclusion
  6. Acknowledgments
  7. The ALICE Collaboration

Figures (5)

  • Figure 1: Invariant mass spectra in selected $p_{\rm t}$ slices in PCM (left) and PHOS (right) in the $\pi^0$ and $\eta$ meson mass regions. The histogram and the bullets show the data before and after background subtraction, respectively. The curve is a fit to the invariant mass spectrum after background subtraction.
  • Figure 2: Reconstructed $\pi^0$ peak width (a) and position (b) as a function of $p_{\rm t}$ in pp collisions at $\sqrt{s}=7$ TeV in PHOS and in the photon conversion method (PCM) compared to Monte Carlo simulations. The horizontal line in (b) indicates the nominal $\pi^0$ mass.
  • Figure 3: a) Differential invariant cross section of $\pi^0$ production in pp collisions at $\sqrt{s}=7$ TeV (circles) and 0.9 TeV (squares) and of $\eta$ meson production at $\sqrt{s}=7$ TeV (stars). The lines and the boxes represent the statistical and systematic error of the combined measurement respectively. The uncertainty on the pp cross section is not included. NLO pQCD calculations using the CTEQ6M5 PDF and the DSS (AESS for $\eta$ mesons) FF for three scales $\mu=0.5p_{\rm t}$, $1p_{\rm t}$ and $2p_{\rm t}$ are shown. Dotted lines in panels b) and c) correspond to the ratios using the BKK FF. Ratio of the NLO calculations to the data parametrisations are shown in panels b), c) and d). The full boxes represent the uncertainty on the pp cross sections.
  • Figure 4: Ratio of the two independent $\pi^0$ meson measurements to the fit of the combined normalized invariant production cross section of $\pi^0$ mesons in pp collisions at $\sqrt{s}=7$ TeV.
  • Figure 5: $\eta/\pi^0$ ratio measured in pp collisions at $\sqrt{s}=7$ TeV compared to NLO pQCD predictions.