The study of particle production in proton-nucleus ($pA$) collisions provides essential
information about high-density effects (like gluon saturation) in the nuclear wavefunction and offers a benchmark for the corresponding studies in nucleus-nucleus collisions. The cross-sections for particle production in $pA$ can in principle be computed within perturbative QCD, using the framework of the Color Glass Condensate (CGC). However, recent efforts trying to extend such calculations beyond the leading-order (LO)
approximation met with an unexpected difficulty: the next-to-leading order (NLO) prediction for the hadron multiplicity suddenly turns negative at transverse momenta of the order of a few GeV, in a range where perturbation theory was expected to be reliable.
This problem triggered much interest and several studies over the last 5 years, but not satisfactory solution has emerged.
In a recent publication , we have revisited the previous proposals for the CGC
factorization at NLO and identified the source of the negativity problem: this is related
to the subtraction method used to separate LO from NLO contributions. To overcome this difficulty, we proposed a new factorization scheme which involves no such a subtraction: the relevant, LO or NLO, perturbative contributions are included once and only once. We have thus obtained a manifestly positive expression for the cross-section for hadron multiplicities in $pA$. On this occasion, we have also extended the resummation program that we recently proposed  for the BK and JIMWLK evolution equations to the calculation of cross-sections. Besides its phenomenological implications, this new factorization scheme should provide a better framework for computing particle production in QCD at high energy.
 ``CGC factorization for forward particle production in proton-nucleus collisions at next-to-leading order,'' E. Iancu, A. Mueller, and D. Triantafyllopoulos, e-Print: arXiv:1608.05293 [hep-ph].
 E. Iancu et al, Phys.Lett. B744 (2015) 293; Phys.Lett. B750 (2015) 643; JHEP 1608 (2016) 083.
|Preferred Track||Initial State Physics and Approach to Equilibrium|