Double Helicity Asymmetry in Inclusive Midrapidity ${\pi }^0$ Production for Polarized $p+p$ Collisions at $\sqrt{s}=200\mathrm{G}\mathrm{e}\mathrm{V}$

We present a measurement of the double longitudinal spin asymmetry in inclusive ${\pi }^0$ production in polarized proton-proton collisions at $\sqrt{s}=200\mathrm{G}\mathrm{e}\mathrm{V}$. The data were taken at the Relativistic Heavy Ion Collider with average beam polarizations of 0.27. The measurements are the first in a program to study the longitudinal spin structure of the proton, using strongly interacting probes, at collider energies. The asymmetry is presented for transverse momenta $1–5\mathrm{G}\mathrm{e}\mathrm{V}/c$ at midrapidity, where next-to-leading-order perturbative quantum chromodynamic (NLO pQCD) calculations well describe the unpolarized cross section. The observed asymmetry is small and is compared to a NLO pQCD calculation with a range of polarized gluon distributions.

Figure 1

The raw asymmetry normalized by the beam polarization $ϵ/P$ as a function of azimuthal angle $\varphi$, for forward neutron production. The solid points and curve correspond to the spin rotators off (transverse polarization) and the open points and dashed curve correspond to the spin rotators on (longitudinal polarization). Curves are sine function fits to the data, representing possible transverse polarization. The data are for special runs used to set up the spin rotators, where the blue (yellow) polarization was 0.24 and 0.33 (0.08 and 0.28), for spin rotators off and on, correspondingly.

Figure 2

$A_{LL}^{{\pi }^0}$ versus mean $p_T$ of ${\pi }^0$'s in each bin. A scale uncertainty of $\pm 65\%$ is not included. Two theoretical calculations based on NLO pQCD are also shown for comparison with the data (see text for details).