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 Different implementation of parton showers – p T 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)

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.

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 p T spectra

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

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….

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

Parameter Scan for SUE -the cluster mass distribution is going to effect the η distribution  perform scan over m 1 and m 2 phase space at s ½ = 546 GeV. Favoured values of parameters: m 1 = 0.1 m 2 = 9.0 m2  m1 

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

Comparisons with PYTHIA Models straddle the η distributions HERWIG slightly better description of data at s ½ =546 GeV

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

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

A Quick Look at b-production No B meson production in the HERWIG underlying event implementation b production possible in PYTHIA low p T processes Generate “hard” QCD processes in PYTHIA and HERWIG with p T min = 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

b-Production HERWIGPYTHIA Cross-section (mb) 62.9± % of events with B-meson Greater B meson production in HERWIG than PYTHIA

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