ar X iv : n uc l - ex / 0 60 10 08 v 1 4 J an 2 00 6 Single Spin Asymmetries in the BRAHMS Experiment

The BRAHMS experiment at RHIC has measured the transverse single spin asymmetries in polarized pp induced pion production at RHIC. The results from the RHIC run-5 shows a significant asymmetry for π + and π − at moderate x F. The trend of the data is in agreement with lower energy results while the absolute values are surprisingly large. The p T dependence is approximately inversely propotional to p T in agreement with the pQCD expectations. In the last decade or so, measurements of transverse single spin asymmetries in pp collisions with polarized beams have attracted much theoretical and experimental interest. Results at low beam energies[1] (√ s=20 GeV) show a sizeable asymmetry up to 30% at relative large Feynman-x (x F) and at moderate p T. It was expected, naively, from lowest order QCD estimates that the cross sections should have very little spin dependence. In order to get a non-zero asymmetry it is necessary to have a spin-flip amplitude, a phase difference in the intrinsic states, and a non zero scattering angle. This makes it a higher order effect that can occur either in the initial state or in the final state parton scattering. The asymmetry or analyzing power A N is defined as (σ + − σ −)/(σ + + σ −), where σ +(−) is the spin dependent cross section for the scattering pp → πX , and with the spin direction oriented up or down transversely to the scattering plane. The target is either un-polarized or the cross sections are averaged over target polarization states. Experiment E704 at FNAL [1] has shown that A N (π +) > A N (π 0) > 0 > A N (π −). In a short run in 2004 BRAHMS observed non-zero spin asymmetries for π + and π − at moderate x F [2]. The STAR experiment observed a positive A N for π 0 at large x F at RHIC [3]. The BRAHMS experiment at RHIC is primarily designed and operated to make measurements of semi-inclusive spectra of identified hadrons over a wide range in rapidity and p T. The PID coverage for pions up to momenta of 35 GeV/c and the option to measure at 2.3 o (η ≈ 4) makes it well suited to study transverse Single Spin Asymmetries for identified pions at moderate x F. The present contribution presents the preliminary results of A …