Abstract
Within the Color Glass Condensate effective theory, we reconsider the next-to-leading order (NLO) calculation of the single inclusive particle production at forward rapidities in proton-nucleus collisions at high energy. Focusing on quark production for definiteness, we establish a new factorization scheme, perturbatively correct through NLO, in which there is no `rapidity subtraction'. That is, the NLO correction to the impact factor is not explicitly separated from the high-energy evolution. Our construction exploits the skeleton structure of the (NLO) Balitsky-Kovchegov equation, in which the first step of the evolution is explicitly singled out. The NLO impact factor is included by computing this first emission with the exact kinematics for the emitted gluon, rather than by using the eikonal approximation. This particular calculation has already been presented in the literature [1,2], but the reorganization of the perturbation theory that we propose is new. As compared to the proposal in [1,2], our scheme is free of the fine-tuning inherent in the rapidity subtraction, which might be the origin of the negativity of the NLO cross-section observed in previous studies.
Highlights
Target and acquires some transverse momentum k⊥, before eventually fragmenting into the hadrons that are measured in the final state
The above physical picture naturally lends itself to a hybrid factorization scheme [5, 11] for the calculation of the single-inclusive hadron multiplicity, which combines the collinear factorization for the parton distribution of the incoming proton and for the fragmentation of the produced quark or gluon [16], with the CGC factorization for the high-energy scattering between the collinear parton and the nucleus
That is for single-inclusive particle production at leading order, this correlator involves the trace of the product of two Wilson lines,2 which can be identified with the elastic S-matrix of a color dipole which scatters off the nuclear target
Summary
We shall briefly review the leading-order (LO) calculation of single-inclusive quark production in high-energy proton-nucleus (pA) at forward rapidities (i.e. in the proton fragmentation region). This calculation relies on a hybrid factorization scheme [11] which involves collinear factorization at the level of the proton wavefunction together with the dipole picture for the scattering between a collinear quark from the proton and the nuclear gluon distribution
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