Speaker
Urs Wiedemann
(CERN)
Description
The experimental finding that in pA collisions at the LHC, $R_{pA}^{\rm jet}$ is consistent with unity at high transverse energies, while $R_{pA}^{\rm hadron}$ exceeds unity above $p_T \sim 30$ GeV poses a fundamental problem for our understanding of high-$p_T$ processes in nuclear collisions. It can neither be accounted for in a collinear factorized formalism with process-independent fragmentation functions, nor can it be understood as a final state modification of the final state parton shower (on which standard jet quenching models in nucleus-nucleus collisions are based). We argue that these findings, point to a process-dependent element in the high-$p_T$ fragmentation mechanism. We note that the color recombination mechanisms implemented in general purpose event generators like SHERPA provide for such a process-dependent mechanism, since they imply that single inclusive hadron production depends not only on the factorized hard partonic interaction,
but also on the density of the underlying event within which this hard process color recombines during hadronization. We demonstrate both on the level of qualitative arguments and with Monte Carlo simulations that by simply accounting for the factor 3 higher underlying event activity in pA collisions as compared to pp collisions, this mechanism can account for
the observed nuclear modification factors of jet and hadron spectra in pA collisions. We emphasize that if the proposed mechanism is confirmed, it has interesting consequences for the understanding of high-pT hadron spectra in proton-proton collisions, since it allows one to quantify the size of a process-dependent correction to the standard collinearly factorized framework.
Author
Korinna Christine Zapp
(CERN)
Co-authors
Guilherme Teixeira De Almeida Milhano
(Instituto Superior Tecnico (PT))
Urs Wiedemann
(CERN)
