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Inclusive cross section and double helicity asymmetry for pi^0 production in p+p collisions at sqrt(s) = 62.4 GeV

PHENIX Collaboration, A. Adare

TL;DR

The paper reports inclusive mid-rapidity $\pi^0$ production in polarized $p+p$ collisions at $\sqrt{s}=62.4$ GeV, providing a cross-section benchmark for pQCD and measurements of the double helicity asymmetry $A_{LL}^{\pi^0}$ to probe the proton's gluon polarization. The cross section is compared to NLO pQCD with $\mu=p_T$ and to NLL calculations, with threshold logarithms improving agreement at this energy. The $A_{LL}$ results extend sensitivity to the polarized gluon distribution $\Delta G$ in the gluon momentum range $0.06< x_g<0.4$, complementing higher-energy measurements and informing global fits. Taken together, these results establish a baseline for heavy-ion studies at RHIC and highlight the potential importance of threshold-resummed pQCD in describing cross sections at moderate energies.

Abstract

The PHENIX experiment presents results from the RHIC 2006 run with polarized proton collisions at sqrt(s) = 62.4 GeV for inclusive pi^0 production at mid-rapidity. Unpolarized cross section results are measured for transverse momenta p_T = 0.5 to 7 GeV/c. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries A_LL are presented for p_T = 1 to 4 GeV/c and probe the higher range of Bjorken_x of the gluon (x_g) with better statistical precision than our previous measurements at sqrt(s)=200 GeV. These measurements are sensitive to the gluon polarization in the proton for 0.06 < x_g < 0.4.

Inclusive cross section and double helicity asymmetry for pi^0 production in p+p collisions at sqrt(s) = 62.4 GeV

TL;DR

The paper reports inclusive mid-rapidity production in polarized collisions at GeV, providing a cross-section benchmark for pQCD and measurements of the double helicity asymmetry to probe the proton's gluon polarization. The cross section is compared to NLO pQCD with and to NLL calculations, with threshold logarithms improving agreement at this energy. The results extend sensitivity to the polarized gluon distribution in the gluon momentum range , complementing higher-energy measurements and informing global fits. Taken together, these results establish a baseline for heavy-ion studies at RHIC and highlight the potential importance of threshold-resummed pQCD in describing cross sections at moderate energies.

Abstract

The PHENIX experiment presents results from the RHIC 2006 run with polarized proton collisions at sqrt(s) = 62.4 GeV for inclusive pi^0 production at mid-rapidity. Unpolarized cross section results are measured for transverse momenta p_T = 0.5 to 7 GeV/c. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries A_LL are presented for p_T = 1 to 4 GeV/c and probe the higher range of Bjorken_x of the gluon (x_g) with better statistical precision than our previous measurements at sqrt(s)=200 GeV. These measurements are sensitive to the gluon polarization in the proton for 0.06 < x_g < 0.4.

Paper Structure

This paper contains 10 sections, 7 equations, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. EXPERIMENT
  3. The $pp\rightarrow\pi^0 X$ CROSS SECTION
  4. $\pi^0$ analysis
  5. Vernier scan analysis
  6. $\pi^0$ cross section results and discussion
  7. Inclusive $\pi^0$ Double Helicity Asymmetry
  8. $\pi^0$ $A_{LL}$ analysis
  9. $\pi^0$ $A_{LL}$ results and discussion
  10. SUMMARY

Figures (6)

  • Figure 1: (color online) The fraction of the inclusive $\pi^0$ yield which satisfied the BBC trigger condition.
  • Figure 2: (color online) Collision $z$-vertex distribution in the PHENIX IR measured by ZDCs in a Vernier scan at $\sqrt{s}=200$ GeV (points) and calculations from convolution of colliding bunch intensity profiles along $z$-axis and including the hour-glass effect for $\beta^*=1$ m, for bunches with typical length of 1 m and transverse size of 0.3 mm (histograms); (a) beams are head-on; (b) one beam is 0.9 mm displaced relative to the other beam in the horizontal direction (illustrates the hour-glass effect) and (c) one beam is 0.9 mm displaced relative to the other beam in the vertical direction. The calculations include the bunch crossing angle with a vertical projection of 0.15 mrad.
  • Figure 3: (color online) (a) The neutral pion production cross section at $\sqrt{s}=62.4$ GeV as a function of $p_{T}$ (circles) and the results of NLO (solid) and NLL (dashed) pQCD calculations for the theory scale $\mu=p_T$. (b) The relative difference between the data and NLO pQCD calculations for the three theory scales $\mu=p_T/2$ (upper line), $p_T$ (middle line) and $2p_T$ (lower line); experimental uncertainties (excluding the 11% normalization uncertainty) are shown for the $\mu=p_T$ curve. (c) The same as b) but for NLL pQCD calculations.
  • Figure 4: (color online) (a) The neutral pion production cross section at $\sqrt{s}=62.4$ GeV and $\sqrt{s}=200$ GeV as a function of $x_{T}$, scaled by $(\sqrt{s}/{\rm GeV})^n$ with n=6.38; the solid line is a parameterization of $\sqrt{s}=200$ GeV data. (b) The parameter $n$ in (\ref{['eq:xt_scale']}) obtained from the ratio of invariant cross section at $\sqrt{s}=62.4$ GeV and $\sqrt{s}=200$ GeV, at each $x_T$ of $\sqrt{s}=62.4$ GeV data; error bars show the statistical and systematic uncertainties of the $\sqrt{s}=62.4$ GeV and $\sqrt{s}=200$ GeV data. The shaded band reflects the $11\% \oplus 9.7\%$ normalization uncertainty in the $\sqrt{s}=62.4$ and 200 GeV cross section measurements, correspondingly.
  • Figure 5: (color online) The double helicity asymmetry for neutral pion production at $\sqrt{s}=62.4$ GeV as a function of $p_T$ (GeV/$c$). Error bars are statistical uncertainties, with the 14% overall polarization uncertainty not shown; other experimental systematic uncertainties are negligible. Four GRSV theoretical calculations based on NLO pQCD (solid curves) and on NLL pQCD (dashed curves) are also shown for comparison with the data (see text for details). Note that the $\Delta G=0$ curves for NLO and NLL overlap.
  • ...and 1 more figures