Inclusive prompt photon production in polarized pp collisions at HERA-

Abstract

We present a NLO study of inclusive polarized prompt photon production in a conceivable fixed target pp mode of HERA with longitudinally polarized protons at √ s = 39 GeV.We analyze the sensitivity of the corresponding double spin asymmetry to the proton's polarized gluon distribution ∆g and estimate the expected statistical precision in its determination. The main theoretical uncertainties in the predictions are examined.It has recently become possible to perform a complete and consistent study of longitudinally polarized deep-inelastic scattering (DIS) in next-to-leading order (NLO) of QCD, since the spin-dependent two-loop splitting functions, needed for the NLO evolution of the polarized parton distributions, have become available [1,2]. A first such phenomenological NLO analysis, taking into account all available experimental data on polarized DIS [3] has been presented in [4], followed by the analyses [5,6]. The studies of [4,6] have shown that present polarized DIS data are still quite far from providing accurate knowledge about the nucleon's spin-dependent sea quark and gluon distributions. This holds true, in particular, for the polarized gluon density ∆g, the x-shape of which seems to be hardly constrained at all [4,6] by the DIS data, even though a tendency towards a sizeable positive total gluon polarization, 1 0 ∆g(x, Q 2 = 4 GeV 2 )dx 1, was found [4,5,6]. Thus, there is clearly some need for independent information on ∆g. For this purpose, it seems expedient to look at processes for which ∆g enters in leading order (LO) already, rather than as a NLO correction as for the spin-dependent DIS structure function g 1 . One of such processes is inclusive large-p T prompt photon production in collisions of longitudinally polarized protons, p p → γX [7,8,9,10]. In the unpolarized case where this process has been studied in a huge number of experiments it has been an invaluable tool for pinning down the proton's unpolarized gluon distribution g(x, Q 2 ) [11,12,13,14]. Hence prompt photon production with polarized beams seems a promising source for obtaining information on ∆g.It is being discussed as one future option for HERA to polarize its 820 GeV proton beam. If this can be achieved, one could use the beam in a fixed target experiment, scattering it off an internal polarized nucleon target. This conceivable constellation, dubbed , would yield √ s ≈ 39 GeV and thus could provide information complementary to that obtained from planned similar spin physics experiments at much higher energies at the RHIC collider [16]. Theoretical predictions for polarized prompt photon production at √ s ≈ 40 GeV have been made in the past [8,10], taking into account the spin-dependent 'direct' subprocess cross sections for ab → γX (a, b = q,q, g) including their full NLO QCD corrections as calculated in [8,9]. From the experience in the unpolarized case, the inclusion of NLO corrections is expected to be quite important 1 in order to make reliable predictions. The main shortcoming of the studies [8,10] was, however...

Spin physics and polarized structure functions

“…[301] the 't Hooft-Veltman scheme was used and the dependence of the cross section on isolation cuts has been examined carefully. Isolation cuts are needed in order to single out isolated direct hard photons from the background [302,300].…”

Spin structure of the proton and largepTprocesses in polarizedppcollisions

“…The sensitivity of HERA-N to ∆G for direct-γ production is sufficient to distinguish between the various gluon models. [20] Coupling this with the other planned experiments at HERA-N would span a sufficient kinematic range to give a very good indication as to both the size and shape of ∆G. [21] There are significant enough differences in the models for the gluon distributions [2,7,15,17] to warrant careful study of these processes.…”

Probing nucleon spin structure

“…(3) Direct photon and double photon production: A clean signature for ∆G, but one that is harder to measure, is that of direct photon production, ( p p → γ + X). [29,113] The NLO corrections have been calculated for HERA-N [114,115] and RHIC. [116] Although the asymmetries are only very large for the largest gluon models, the signal is clean and could distinguish if ∆G is large.…”