NLO QCD and SMC Algorithms Richard Corke Outline Parton shower - PowerPoint PPT Presentation

Nov 28, 2022 •167 likes •287 views

NLO QCD and SMC Algorithms Richard Corke Outline Parton shower matching POWHEG to Pythia interface Hardest emission: Pythia vs NLO QCD Conclusion Parton shower matching SMC best in soft/collinear regions Match NLO matrix

NLO QCD and SMC Algorithms Richard Corke
Outline ● Parton shower matching ● POWHEG to Pythia interface ● Hardest emission: Pythia vs NLO QCD ● Conclusion
Parton shower matching ● SMC best in soft/collinear regions ● Match NLO matrix elements to existing parton shower generators ● MC@NLO (Frixione, Webber) – Subtract approximate NLO SMC terms from exact calculation – Generator specific – Possibility of negative weighted events
Parton shower matching (contd.) ● POWHEG (Nason; Frixione, Oleari, Ridolfi) – Positive Weight Hardest Emission Generator – Generate hardest emission (p T -ordered) using a Sudakov – Sudakov contains only Born and real terms
POWHEG to Pythia Interface ● Pythia shower evolution in lightcone p T ● Start shower at kinematical limit with veto on p T ● Small correction
Pythia vs NLO QCD ● Shower behaviour (and single W/Z): ● t-channel g exchange (for p T > p Thard ): ● Something in between? Guess: – No colour in final state ● Shower behaviour to kinematical limit – Colour final state ● Coherence between ISR and FSR ● Damp shower emissions by factor:
Pythia vs NLO QCD (contd.)
Pythia vs NLO QCD (contd.) ● Use Madgraph/Madevent to get rough idea of corrections for other processes ● Generate probability of emission as: ● Not differential in rapidity and no NLO pre-factor ● Watch out for: – Unwanted graphs – α S and PDFs
Pythia vs NLO QCD (contd.)
Pythia vs NLO QCD (contd.)
Conclusion ● When no matching to higher orders available - possible set of processes for which an appropriate dampening factor may improve high p T tail of parton shower ● Initial W + W - /Z 0 Z 0 results appear to follow original guess – dampening not needed ● SUSY processes still to come (squark/gluino production)

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