Single inclusive particle production at next-to-leading order in proton-nucleus collisions at forward rapidities: Hybrid approach meets TMD factorization

We revisit the calculation of the cross section for forward inclusive single hadron production in pA collisions within the hybrid approach. We show that the proper framework to perform this calculation beyond leading order is not the collinear factorization, as has been assumed so far, but the transverse momentum dependent (TMD) factorized framework. Within the TMD factorized approach we show that all the large transverse logarithms appearing in the fixed order calculation are resummed into the evolution of the TMD parton distribution functions and TMD fragmentation functions with factorization scale. The resulting expressions, when written in terms of TMDs evolved to the appropriate, physically well understood factorization scale, contain no additional large logarithms. The absence of any large logarithms in the resummed result should ensure positivity of the cross section and eradicate the persistent problem that has plagued the previous attempts at calculating this observable in the hybrid approach.