The Underlying-Event Model in PYTHIA (6&8) - Peter Skands - Cern

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Northwest Terascale Research Projects: Modeling the underlying event and minimum bias events Energy Scaling The Underlying-Event Model in PYTHIA (6&8)
P. Skands Multiple Parton Interactions (MPI) Energy Scaling Multiparton interactions Asecondhardprocess Regularise cross section with p ⊥0 as free parameter Multiple interactions key aspect dˆσ of PYTHIA since > 20 years. dp Central to obtain agreement with data: Tune A, Professor, Perugia, . . . 2 ∝ ⊥ α2s (p2 ⊥ ) p4 ⊥ IR Regularization with energy dependence Energy Scaling p ⊥0(E CM) =p ref ⊥0 × Before 8.1 no chance to select character of second interaction. Now free choice of first process (including LHA/LHEF) and second process combined from list: • TwoJets (with TwoBJets as subsample) Tevatron • PhotonAndJet, tunes appear TwoPhotons to be “low” • Charmonium, on LHC data Bottomonium (colour octet framework) • SingleGmZ, SingleW, GmZAndJet, WAndJet Problem • TopPair, for “global” SingleTop tunes. Poor Can man’s be short-term expanded among solution: existing processes as need arises. dedicated LHC tunes By default same phase space cuts as for “first” hard process =⇒ second can be harder than first. However, possible to set ˆm and pˆ ⊥ range separately. From Tevatron to LHC → α2 s (p 2 ⊥0 + p2 ⊥ ) (p 2 ⊥0 + p2 ⊥ )2 ɛ ECM E ref CM Matter profile in impact-parameter space gives time-integrated overlap which determines level of activity: simple Gaussian or more peaked variants See, e.g., new MCnet Review: “General-purpose event generators for LHC physics”, arXiv:1101.2599 ISR and MPI compete for beam momentum → PDF rescaling + flavour effects (valence, qq pair companions, . . . ) + correlated primordial k⊥ and colour in beam remnant Many partons produced close in space–time ⇒ colour rearrangement; reduction of total string length ⇒ steeper 〈p⊥〉(nch) E.g., Rick Field 33
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Page 8 and 9: The Pedestal Effect pT = 160 (HP) p
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Page 37 and 38: P. Skands PYTHIA 6 @ 1.8 TeV PYTHIA
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The Underlying-Event Model in PYTHIA (6&8) - Peter Skands - Cern

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