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Nuclear Modification of Neutral Pion Production at Low x in √s=200 GeV d+Au and p+p Collisions


Nuclear modification factors quantify suppression in particle production due to nuclear effects. They are defined as a ratio of invariant yields, with a numerator derived from a given species of nuclear collision and a denominator derived from a hypothetically equivalent ensemble of independent proton-proton collisions. At large momentum transfer Q^2 and low momentum fraction x, the neutral pion nuclear modification factor R_d+Au for d+Au collisions is useful for investigating initial state gluon saturation. The large initial state gluon multiplicity of the Au nucleus causes saturation effects to occur at lower energies in d+Au collisions, as compared to p+p collisions, resulting in a relative suppression. Measuring the relative suppression R_d+Au can therefore test the validity of competing models describing saturation, including the framework of a color glass condensate (CGC).

Measurements at low x are of particular interest because in this region linear pQCD evolution equations begin to break down. The Froissart theorem places a robust theoretical upper limit on the behavior of hadronic cross sections: a cross section can increase at most like ln^2(E). Equivalently, an hadronic structure function can increase at most like ln^2(1/x). Adherence to this theorem is necessary to preserve S-matrix unitarity; no physical system should exhibit behavior to the contrary. However linear evolution equations, which dictate structure function behavior, predict an unchecked growth of low-x gluons, in violation of the theorem. For this reason, it is expected that gluon saturation, via non-linear evolution, will take place at low x to steer the gluon distribution function back within the limitations of the Froissart bound.

Greater suppression is expected at lower Q^2; however, at low x, regions of high Q^2 are more difficult to access experimentally. Pushing out to higher Q^2 is important for discriminating between competing theoretical models.

In practice, regions of low x and high Q^2 translate to measurements at, respectively, high rapidity η and high transverse momentum p⊥. The high rapidity 3.1 < η < 3.9 Muon Piston Calorimeter (MPC) detector at PHENIX is ideally suited for measurements of neutral pion R_d+Au probing regions of low x. At √s = 200 GeV, a combinatoric analysis of neutral pion decay products in the MPC can obtain measurements of R_d+Au up to a transverse momentum of p⊥ = 2 GeV/c. However, at p⊥ greater than 2 GeV/c, photons from neutral pion decay have insufficient spatial separation to be independently resolved in the detector. In this analysis the transverse momentum range of the detector, measuring R_d+Au at √s = 200 GeV, is extended to p⊥ = 3.5 GeV/c by studying photon pairs from neutral pions that resolve in the MPC as a single cluster. Increased suppression is reproduced at low p⊥, in agreement with previous data. For p⊥ > 2 GeV/c Cronin enhancement is not observed, as anticipated by the CGC framework. However, the data can not rule out the possibility that the observed suppression is the result of extreme nuclear shadowing. Also presented are invariant neutral pion yields for p+p and d+Au collisions and the invariant neutral pion cross section for p+p collisions at √s = 200 GeV.

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