Do we understand the single-spin asymmetry for $pi^0$ inclusive production in pp collisions?

Abstract

The cross section data for $π^0$ inclusive production in $pp$ collisions is considered in a rather broad kinematic region in energy $\sqrt{s}$, Feynman variable $x_F$ and transverse momentum $p_T$. The analysis of these data is done in the perturbative QCD framework at the next-to-leading order. We find that they cannot be correctly described in the entire kinematic domain and this leads us to conclude that the single-spin asymmetry, $A_N$ for this process, observed several years ago at FNAL by the experiment E704 and the recent result obtained at BNL-RHIC by STAR, are two different phenomena. This suggests that STAR data probes a genuine leading-twist QCD single-spin asymmetry for the first time and finds a large effect.

Figures (5)

• Figure 1: The single-spin asymmetry $A_N$ as a function of $x_F$, at two different energies. The data are from Refs. E704aSTAR.
• Figure 2: $Ed^3\sigma/d^3p$ at 90$^o$ and various energies, as a function of $p_T$. Data are from Refs. E706E704bPHEISRBCL and the curves are the corresponding NLO pQCD calculations with $\mu = p_T$ (solid lines) and $\mu = p_T/2$ (dotted- dashed lines).
• Figure 3: $Ed^3\sigma/d^3p$ at $\sqrt{s}=23.3 \hbox{GeV}$, as a function of $x_F$ for two different scattering angles. The data are from Ref. ISR and the curves, solid $\theta$= 22$^o$ and dashed $\theta$= 15$^o$, are the corresponding NLO pQCD calculations with $\mu = p_T$. The dotted-dashed curves are for $\mu = p_T/2$.
• Figure 4: $Ed^3\sigma/d^3p$ at $\sqrt{s}=52.8 \hbox{GeV}$, as a function of $x_F$ for three different scattering angles. The data are from Ref. ISR and the curves are the corresponding NLO pQCD calculations with $\mu = p_T$. The dotted-dashed curves are for $\mu = p_T/2$.
• Figure 5: $Ed^3\sigma/d^3p$ at $\sqrt{s}=200 \hbox{GeV}$, as a function of $x_F$. The solid and dashed curves are the NLO pQCD calculations, with $\mu = p_T$, at two different angles and the data points are from Ref. STAR. The dotted-dashed curves are for $\mu = p_T/2$.