Predictions for p + Pb at 4.4A TeV to test initial-state nuclear shadowing at energies available at the CERN 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 CERN Large Hadron Collider (LHC). The nuclear modification factor (NMF), R-pPb(y = 0, p(T) < 20 GeV/c) = dn(pPb)/[N-coll(b)dn(pp)], is computed with an electron-nucleus (e + A) global fit with different nuclear shadow distributions and compared to the fixed Q(2) shadow ansatz used in Monte Carlo Heavy Ion Jet Interacting Generator (HIJING)-type models. Because of the rapid Dokshitzer Gribov Lipatov Altarelli Parisi (DGLAP) reduction of shadowing with increasing Q(2) used in the e + A global fit, our results confirm that no significant initial state suppression is expected [R-pPb(p(T)) = 1 +/- 0.1] in the p(T) range 5 to 20 GeV/c. In contrast, the fixed Q(2) shadowing models assumed in HIJING-type models predict in the range above p(T) a sizable suppression, R-pPb(p(T)) = 0.6-0.7, at midpseudorapidity that is similar to the color glass condensate (CGC) model predictions. For central (N-coll = 12) p + Pb collisions and at forward pseudorapidity (eta = 6), the HIJING-type models predict smaller values of nuclear modification factors [R-pPb(p(T))] than in minimum-bias events at midpseudorapidity (eta = 0). Observation of R-pPb(p(T) = 5 - 20 GeV/c) less than or similar to 0.6 for minimum bias p + A collisions would pose a serious difficulty for separating initial-state from final-state interactions in Pb + Pb collisions at LHC energies.