Predictions for<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>+</mml:mo></mml:math>Pb at 4.4<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>TeV to test initial-state nuclear shadowing at energies available at the CERN Large Hadron Collider

Abstract

Collinear factorized perturbative QCD model predictions are compared for $p+\mathrm{Pb}$ at 4.4$A$ TeV to test nuclear shadowing of parton distribution at the CERN Large Hadron Collider (LHC). The nuclear modification factor (NMF), ${R}_{p\mathrm{Pb}}(y=0,{p}_{T}&lt;20\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}/\mathit{c})=d{n}_{p\mathrm{Pb}}/[{N}_{\mathrm{coll}}(b)d{n}_{pp}]$, 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 [${R}_{p\mathrm{Pb}}({p}_{T})=1\ifmmode\pm\else\textpm\fi{}0.1$] 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, ${R}_{p\mathrm{Pb}}({p}_{T})=0.6$--0.7, at midpseudorapidity that is similar to the color glass condensate (CGC) model predictions. For central (${N}_{\mathrm{coll}}=12$) $p+\mathrm{Pb}$ collisions and at forward pseudorapidity ($\ensuremath{\eta}=6$), the HIJING-type models predict smaller values of nuclear modification factors [${R}_{p\mathrm{Pb}}({p}_{T})$] than in minimum-bias events at midpseudorapidity ($\ensuremath{\eta}=0$). Observation of ${R}_{p\mathrm{Pb}}({p}_{T}=5\ensuremath{-}20\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}/\mathit{c})\ensuremath{\lesssim}0.6$ 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.