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Overview of MC Models

Overview of MC Models. Michael H Seymour University of Manchester MPI@LHC 2013, Antwerp 2/12/13. Overview of MC Models of MPI. Overview of MC Models of MPI. Herwig ++ Pythia 8 Sherpa → Korinna Zapp DIPSY → Leif Lönnblad EPOS → MPI@LHC 2012. MPI Model Basics ( Herwig & Pythia ).

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Overview of MC Models

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  1. Overview of MC Models Michael H Seymour University of Manchester MPI@LHC 2013, Antwerp 2/12/13

  2. Overview of MC Models of MPI MPI@LHC2013

  3. Overview of MC Models of MPI • Herwig++ • Pythia 8 • Sherpa → KorinnaZapp • DIPSY →Leif Lönnblad • EPOS → MPI@LHC 2012 MPI@LHC2013

  4. MPI Model Basics (Herwig& Pythia) • Matter distributions • Model of low-pt scattering • Colour connections MPI@LHC2013

  5. Matter Distributions • Usually assume x and bfactorize ( see later) • and n-parton distributions are independent ( see later) •  scatters Poissonian at fixed impact parameter MPI@LHC2013

  6. Aside 1: Inclusive Cross Sections • Defining cross section inclusively: • Reproduces partonic cross section: • see MHS & A. Siodmok: arXiv:1308.6749 MPI@LHC2013

  7. Aside 1: Inclusive Cross Sections • Analogous double-inclusive cross section is: MPI@LHC2013

  8. Aside 1: Inclusive Cross Sections • Other cross section definitions do not give process-independent A(b)! MPI@LHC2013

  9. The Herwig++ Model (formerly known as Jimmy+Ivan) • Take eikonal+partonic scattering seriously • given form of matter distribution  size and ¾inc • too restrictive  •  two free parameters Bähr, Butterworth & MHS, JHEP 0901:067, 2009 MPI@LHC2013

  10. The Herwig++ Model • see A. Siodmok’s talk → MPI@LHC2013

  11. The Pythia Model • Double Gaussian matter distribution • Replace σtot by σNSD • Consider x-dependent matter distribution MPI@LHC2013

  12. x-dependent matter distributions • Most existing models use factorization of x and b • or (Herwig++) crude separation into hard and soft components (simple hot-spot model) • R.Corke and T.Sjöstrand, arXiv:1101.5953 consider Gaussian matter distribution with width

  13. x-dependent matter distributions • Most existing models use factorization of x and b • or (Herwig++) crude separation into hard and soft components (simple hot-spot model) • R.Corke and T.Sjöstrand, arXiv:1101.5953 consider Gaussian matter distribution with width • for a10.15, matter distribution can be E-indep

  14. MPI Model Basics (Herwig& Pythia) • Matter distributions • Model of low-pt scattering • Colour connections MPI@LHC2013

  15. Low pt scattering - Pythia • Use perturbative cross sections right down to pt=0, with regulator pt0 MPI@LHC2013

  16. Pythia implementation

  17. Low pt scattering - Herwig • View pt,min as a transition scale between hard and soft scatters… MPI@LHC2013

  18. pt Final state implementation • Pure independent perturbative scatters above PTMIN • Gluonic scattering below PTMIN with total σsoft,incand Gaussian distribution in pt • dσ/dpt continuous at PTMIN • possibility that entire process could be described perturbatively?

  19. MPI Model Basics (Herwig& Pythia) • Matter distributions • Model of low-pt scattering • Colour connections MPI@LHC2013

  20. Colour correlations Can have a big influence on final states  see later

  21. Herwig - colour reconnection model • Röhr, Siodmok and Giesekeimplemented new model based on momentum structure • Refit LEP-I and LEP-II data • Conclusion: hadronization parameters correlated with reconnection probability, but good fit can be obtained for any value of preco

  22. Colour reconnection model/MPI tuning

  23. Description of Data… • Not a review, but some snapshots… MPI@LHC2013

  24. Underlying Event (Trans region) MPI@LHC2013

  25. Minimum bias events Herwig too flat for soft hadrons MPI@LHC2013

  26. Minimum bias events increasingly so for very soft hadrons MPI@LHC2013

  27. Minimum bias events and too many soft hadrons MPI@LHC2013

  28. Herwig’s dirty laundry… • Forward gaps: • Takes eikonal model seriously ⇒ should produce all inelastic events, including diffractive • but no diffractive model! → in progress • Despite this, produces too many rapidity gaps MPI@LHC2013

  29. Forward rapidity gaps distn. MPI@LHC2013

  30. Forward rapidity gaps distn. • “Bump” events are only those with no hard scatters • related to colour structure of soft scatters? • colour connection between soft and hard scatters? MPI@LHC2013

  31. Herwig 2.7.0 • released 25 Oct2013 • an interface to the Universal FeynRules Output (UFO) format allowing the simulation of a wide range of new-physics models; • developments of the Matchbox framework for next-to-leading order (NLO) simulations; • better treatment of QCD radiation in heavy particle decays in new-physics models; • a new tune of underlying event and colour connection parameters that allows a good simultaneous description of both Tevatron and LHC underlying event data and the effective cross-section parameter for double-parton scattering. MPI@LHC2013

  32. Summary • (Herwig and Pythia) MPI models well developed and describe underlying event data well • also σeff → A. Siodmok • Herwig some problems for min bias • increasingly so for very soft events, and very gappy events • Data increasingly sensitive probe of colour structures and connections MPI@LHC2013

  33. Aside 2: A(b) definitions • Recall • = convolution over two xs and two bs in each hadron • identical to convolution over two bs in the same hadron MPI@LHC2013

  34. Aside 2: A(b) definitions b2 b2 b1 b1 b B b3 b3 b4 b4 MPI@LHC2013

  35. Aside 2: A(b) definitions • A(B) is a single-hadron property • natural framework to include correlations • but extension to triple- scattering (and higher) ? MPI@LHC2013

  36. MPI@LHC2013

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