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Event Generation with HERWIG

Event Generation with HERWIG. Nick Brook University of Bristol. Introduction Multiple Interactions in HERWIG Parameter Tuning B-production. Herwig vs Pythia Different hadronisation mechanism – clusters as opposed to strings

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Event Generation with HERWIG

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  1. Event Generation with HERWIG Nick Brook University of Bristol • Introduction • Multiple Interactions in HERWIG • Parameter Tuning • B-production

  2. Herwig vs Pythia • Different hadronisation mechanism – clusters as opposed to strings • Different implementation of parton showers – pT ordering compared to angular ordering • HERWIG known from e+e- to give larger contribution of gluon splitting to heavy quarks gbb • No implementation of diffraction in HERWIG • (Use CTEQ4 LO parton densities for both generators)

  3. Multiple Interactions in HERWIG • In principle MI not available within HERWIG • In practice, interface program (JIMMY – Butterworth, Forshaw & Walker) allows MI • Also available, ad-hoc modelling of the “soft underlying” event (SUE - based on UA5 model) • Parameter available for tuning in both JIMMY and SUE options.

  4. UA5 Minimum Bias Model (SUE) • Mean event charged multiplicity chosen according to • 1/k in negative binomial given by • The mass spectrum of soft clusters derived from • Soft cluster pT spectra

  5. Comparison of JIMMY and UA5 Data - essentially one “free” parameter which is the pTmin of the hard scatt. As pTmin the # of scatters decrease & predictions approach UA5 data. Failed to find a setting that could describe the data. No further study presented here.

  6. HERWIG & UA5 Data • comparison of HERWIG min. bias option with UA5 • should be reasonable as it’s implementation of expt’s model !! First glance suggests (not too suprisingly !) a good description of the data. BUT….

  7. More HERWIG & UA5 data Comparison with UA5 pseudorapidity distributions at 3 CoM energies (200, 546 & 900 GeV) The <nch> may look fine – but for the distribution of η, room for improvement.

  8. Parameter Scan for SUE • the cluster mass distribution is going to effect the η distribution •  perform scan over m1 and m2phase space at s½= 546 GeV. Favoured values of parameters: m1= 0.1 m2= 9.0 m1  m2 

  9. Comparison with “tuned” HERWIG Still not perfect but a large improvement

  10. Comparisons with PYTHIA Models straddle the η distributions HERWIG slightly better description of <nch> data at s½=546 GeV

  11. Generator Comparison at LHC Energies look at non-single diffractive events at LHC events in LHCb expt. acceptance Note – double diffractive peak in PYTHIA NOT present in HERWIG

  12. Generator Comparison Double diffraction turned off in PYTHIA PYTHIA has a greater mean charged multiplicity with larger tails

  13. A Quick Look at b-production • No B meson production in the HERWIG underlying event implementation • b production possible in PYTHIA low pT processes Generate “hard” QCD processes in PYTHIA and HERWIG with pTmin = 5 GeV The choice of scale chosen to be same in both generators (MSTP(31)=1 in PYTHIA) No SUE in HERWIG and no MI in PYTHIA

  14. b-Production Greater B meson production in HERWIG than PYTHIA

  15. Summary • PYTHIA is more “versatile” – greater implementation of physics processes • Improved parameter settings for HERWIG • HERWIG & PYTHIA reasonable description of UA5 data • HERWIG has a lower mean charged multiplicity than PYTHIA at LHC energies in the forward region • B-meson production greater in HERWIG than PYTHIA at LHC

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