1. The role of hadronization on jet and hadron nuclear modification factors Urs Achim Wiedemann CERN PH-TH QM15, Kobe, Japan, 28 March 2015 Based on: K.C. Zapp, G. Milhano and UAW, work in progress In modern pp event generators (PHYTHIA, HERWIG, SHERPA), hard processes “color recombine” with underlying event (UE). This may be regarded as rudimentary implementation of “medium effects”. Here we ask: how do such color recombination effects extrapolate from pp -> pA?

2. Measured R pPb ‘s for hadrons and jets CMS Charged hadronsCMS Inclusive jets CMS PAS HIN-14-001CMS, EPJ C75 (2015) 5, 237 pQCD factorized ansatz cannot account for this difference

3. Measured R pPb ‘s for hadrons and jets CMS Charged hadronsCMS Inclusive jets CMS PAS HIN-14-001CMS, EPJ C75 (2015) 5, 237 pQCD factorized ansatz cannot account for this difference (jet quenching has opposite effect)

4. ATLAS reports similar trends ATLAS-CONF-2014-029 ATLAS Charged hadrons ATLAS PLB748 (2015) 392 ATLAS jets Enhancement seen only in the R pPb of hadrons.

5. Caveat If the pp baseline would be different, then the effect would be different CMS PAS HIN-15-004 The absence of distinct features in the pPb jet fragmentation functions at large x gives support to such a resolution of the R pPb -anomaly.

6. Hadronization: color bleaching in event generators The pQCD factorized approach to jet/hadron production: Hadronization prescription neglects color correlations with underlying event. ( formally ok since power-suppressed ) For standard parton showers (as implemented e.g. in PHYTHIA, HERWIG, SHERPA ), evolving hard quark up to hadronization scale partonic fragments are much harder than MLLA. Leading partons x ~ O(1) form color singlet strings / clusters with much softer ones. Hadronizing these objects of large invariant mass leads to a much softer hadronic distribution. S. Sapeta & UAW, JHEP 0905 (2009) 023 Color correlations matter!

7. Hadronization: color bleaching in event generators Leading color singlet clusterLund string In jet quenching models, perturbative mechanism of color reconnection exists. Can lead to additional softening (!) of hadronic spectra Beraudo, Milhano, Wiedemann, JHEP 1207 (2012) 144

8. Color reconnection – an idea with history Effect discussed since long, e.g. - in hadronic W pairs at LEP - in rapidity gap events Minimizing length of color strings helps accounting for in pp minbias All modern event generators - assign color flow to MPI’s by some minimization of color flow - include at least one color reconnection model In the SHERPA cluster hadronization model, color is swapped and singlets (12) and (34) are reconnected to (14) and (23) with probability If color reconnection plays a role in pp, then it must play an enhanced role in pPb. ( number of components to color reconnect with increases with multiplicity ) Sjostrand, Khoze ZPhysC62 (1994) 281 Buchmüller, Hebecker PLB355 (1995) 573 Sjostrand, van Zijl PRD36 (1987) 2019 Invariant mass of clusters no. of clusters between clusters under consideration

9. Cluster Hadronization recombined with UE (schematic) Leading hadrons come from small invariant mass where clusters do not undergo resonance-like decays. If singlets can form exogamously with components of underlying event, then there are more hard singlet clusters of low invariant mass. => hadron spectrum hardens => calorimetric measurements unchanged => jet FF affected at large x Effects would persists as long as UE-partons are found in - area of radius This dies out slowly with. If in area, parton multiplicity in UE is smaller one, then color recombination increases linearly with multiplicity => enhancement in pA compared to pp

10. Results of simulations No fitting or tuning - take SHERPA out of the box - mimic pPb minbias by increasing UE multiplicity of pp by factor 3 Results of a SHERPA 2.1 developer version ( not an official release! ) Code is more sophisticated than the schematic model on previous slide: Even if color reconnection is switched off, modified color flow amongst initial state MPIs leads to some enhancement. Small but (for p T <60 GeV) statistically significant differences between jet and hadron nuclear modification factors. hadrons 0 20 40 60 80 p T (GeV) jets 50 100 150 200 250 p T (GeV)

11. Results of simulation No fitting or tuning - take SHERPA out of the box - mimic pPb minbias by increasing UE multiplicity by factor 3 Results of a SHERPA 2.2 ( official release ) jetshadrons No significant effects from color recombination in this version.

12. Conclusions Within the present study, we were unable to reproduce quantitatively the high-p T features of the jet and hadron nuclear modification factors reported by CMS and ATLAS. We observed, however, characteristic differences between the pPb modification factors for hadrons and jets in one implementation of a cluster hadronization model with color reconnection. We emphasize our main motivation: To understand the physics origin of possible characteristics of collectivity in small systems, it is interesting to study how effects implemented in modern simulations of the underlying event in proton-proton collisions scale if extrapolated to pPb? (Clearly, this question is not only relevant for the high-p T sector studied here.) We have seen that depending on details of the UE simulation in pp, the scaling to pPb can be different: => data on pA can contribute to constraining the soft physics implemented in standard pp event generators.