Measurement of the double longitudinal spin asymmetry in inclusive jet production in polarized p + p collisions at √ s = 200 GeV

Measurement of the double longitudinal spin asymmetry in inclusive jet production in polarized p+p collisions at √ s = 200 GeV Abstract We present preliminary results for the first measurements of the double longitudinal spin asymmetry A LL in inclusive jet production at mid-rapidity in polarized proton-proton collisions at √ s = 200 GeV. The data amount to ∼ 0.5 pb −1 collected at RHIC in 2003 and 2004 with beam polarizations up to 45%. The jet transverse energies are in the range of 5 < p T < 17 GeV/c. The data are consistent with theoretical evaluations using deep-inelastic scattering parametrizations for gluon polarization in the nucleon, and tend to disfavor large positive values of gluon polarization. One of the main objectives of the STAR (Solenoid Tracker At RHIC) spin physics program is the precise determination of the polarized gluon distribution in the nucleon by measurements of double longitudinal spin cross section asymmetries A LL = ∆σ/¯ σ = (σ ++ − σ +−)/(σ ++ + σ +−) in collisions of polarized protons at √ s = 200 and √ s = 500 GeV [1]. The STAR setup provides charged particle tracking, particle identification, and electromagnetic calorimetry covering large acceptance [2]. These allow A LL measurements for various processes, including the production of inclusive jets and pions, of di-jets and di-hadrons, and of prompt-photon and jet coincidences. The golden channel at STAR for a direct gluon polarization determination is the production of a prompt-photon in coincidence with a jet, which is dominated by the gluon Compton process qg → γq. The polarized gluon distribution function ∆g(x) can be extracted from the measured asymmetry A LL over a wide and resolved kinematic range 0.01 < x < 0.3. The gluon spin contribution to the proton spin is expected to be determined to a precision better than 0.5 [3] for projected future beam polarizations and luminosities at √ s = 200 and √ s = 500 GeV at RHIC [1]. While luminosity and polarization are being developed at RHIC, STAR has collected about 0.5 pb −1 of data in 2003 and 2004 at √ s = 200 GeV. The longitudinal average beam polarizations are 30 − 40% for these data. The data allow an exploratory measurement of the asymmetry A LL in inclusive jet production, a process with a sizable cross section that is sensitive to the size of the gluon …