Predictions for p + Pb at 5.02A TeV to test initial state nuclear shadowing at the Large Hadron Collider.
Collinear factorized perturbative QCD model predictions are compared for p + Pb at 4.4A TeV to test nuclear shadowing of parton distribution at the Large Hadron Collider (LHC). The nuclear modification factor (NMF), RpPb(y = 0,pT < 20 GeV/c) = dnpPb/(Ncoll(b)dnpp), is computed with electron-nucleus (e + A) global fit with different nuclear shadow distributions and compared to fixed Q 2 shadow ansatz used in Monte Carlo Heavy Ion Jet Interacting Generator (HIJING) type models. Due to rapid DGLAP reduction of shadowing with increasing Q 2 used in e+A global fit, our results confirm that no significant initial state suppression is expected (RpPb(pT) = 1±0.1) in the pT range 5 to 20 GeV/c. In contrast, the fixed Q 2 shadowing models assumed in HIJING type models predict in the above pT range a sizable suppression, RpPb(pT) = 0.6 − 0.7 at midpseudorapidity that is similar to the color glass condensate (CGC) model predictions. For central (Ncoll = 12) p + Pb collisions and at forward pseudorapidity (� = 6) the HIJING type models predict smaller values of nuclear modification factors (RpPb(pT)) than in minimum bias events at mid-pseudorapidity (� = 0). Observation of RpPb(pT = 5 − 20 GeV/c) < ∼ 0.6 for minimum bias p+A collisions would pose a serious difficulty for separating initial from final state interactions in Pb+Pb collisions at LHC energies.