Event Generation with HERWIGEvent Generation with HERWIG

Nick BrookNick Brook

University of BristolUniversity of Bristol

• Introduction

• Multiple Interactions in HERWIG

• Parameter Tuning

• B-production

Herwig vs PythiaHerwig 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 gbb

• No implementation of diffraction in HERWIG

(Use CTEQ4 LO parton densities for both generators)

Multiple Interactions in HERWIGMultiple 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)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

312 nsnn n

ch

21 )ln( ksk

)exp()( 21 MmmMMM ba

22exp Mpbp TT

Comparison of JIMMY and UA5 DataComparison 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.

HERWIG & UA5 DataHERWIG & 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 dataMore 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.

Parameter Scan for SUEParameter Scan for SUE-the cluster mass distribution is going to effect the η distribution

perform scan over m1 and m2 phase space at s½= 546 GeV.

Favoured values of parameters:

m1= 0.1

m2= 9.0

m2

m1

Comparison with “tuned” HERWIGComparison with “tuned” HERWIG

Still not perfect but a large improvement

Comparisons with PYTHIAComparisons with PYTHIA

Models straddle the η distributions

HERWIG slightly better description of <nch> data at s½=546 GeV

Generator Comparison at LHC EnergiesGenerator 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 ComparisonGenerator Comparison

Double diffraction turned off in PYTHIA

PYTHIA has a greater mean charged

multiplicity with larger tails

A Quick Look at b-productionA 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 pT

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

222 ˆˆˆ

ˆˆˆ2

uts

uts

b-Productionb-Production

HERWIG PYTHIA

Cross-section (mb)

62.9±0.2 65.1

% of events with

B-meson

1.7 1.1

Greater B meson production in

HERWIG than PYTHIA

SummarySummary

• 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