Paul Szczypka & Qingying Jia

Comparison of Monte Carlo event generators at LHC energies

Preview• Highlight main difference between Pythia

and Phojet• Compare Pythia and Phojet minimum bias

events• Compare Pythia and Phojet B0 events• Compare Pythia v6.205 with v6.222

PHOJET and PYTHIA use different theoretical models to deal with the low pt problems

• Phojet combine the DPM on soft

processes with the predictive power of perturbative QCD, the event generator is formulated as a two-component model (soft and hard).

The basic process is 2 → n.

• PythiaAttempt to extend perturbative (high-pt) picture down to low-pt region.

The basic processes is 2 → 2 with the usual LO matrix elements.

PHOJET• Based on the Dual Parton Model with a smooth

transition between the soft and the hard region at

tm

t

soft epd

dN ~2

cutoffp

• distribution sampled from an exponential distribution:

ATLAS investigation

Phojet v1.12 vs. Pythia 6.205 – minimum biasNotes:

•Reasonable Reasonable fitfit•Phojet ‘out Phojet ‘out of box’of box’•<n>:<n>: Phojet = Phojet = 20.3220.32 Pythia = Pythia = 21.8721.87

O(10%) O(10%) differencedifference

Pythia v6.222 vs. Pythia 6.205 – minimum biasNotes:

•6.222 latest release•No obvious difference•Same tuning

Pythia v6.222 vs. Pythia v6.205 – B0 event dataNotes:

•6.222 latest release•Good fit

B0 event distributions

1

Pythia v6.222 vs. Pythia v6.205 – B0 event dataNotes:

•6.222 latest release•Good fit

2

Pythia v6.222 vs. Pythia v6.205 – B0 dataNotes:

•6.222 latest release•Good fit

B0 distributions

1

Pythia v6.222 vs. Pythia v6.205 – B0 dataNotes:

•Forced B0•6.222 latest release•Good fit

2

Notes:

Phojet v1.12 vs. Pythia v6.205 – B0 data

•Phojet Phojet untuneduntuned•Phojet multiple interactions on•Softer PT

•B0 pseudorapidity different •Both B0 production cross sections within 5%

Notes:

Phojet v1.12 vs. Phojet v1.12 – B0 data

•Believe B0 created from soft interaction•Understand central region production•Requires investigation

Summary• Pythia v6.222 closely reproduced v6.205 data• Phojet v1.12 produces similar distributions to

Pythia v6.205 for min bias• Phojet v1.12 produces softer PT B0 particles• The soft mechanism produces B0 in Phojet –

needs further study• Need to understand pseudorapidity

distributions of B0