Underlying event settings This is a quick look at how the underlying event multiplicity changes with pt-min (PARP(82)) in PYTHIA to see what to how to change UE settings to look at how robust jet reconstruction is in di-jet and gamma-jet events.

The events were generated with PYTHIA 6.323 using the underlying event settings given on the web:

* pythia tune for current ATLAS pythia MSTP(68)=1 MSTP(70)=2 MSTP(82)=4 *def PARP(82)=2.6 PARP(82)=2.0 PARP(84)=0.3 PARP(90)=0.24 MSTJ(11)=3 MSTJ(22)=2 PARJ(54)=-0.07 PARJ(55)=-0.006 MSTP(128)=1 MSTP(52)=2 MSTP(54)=2 MSTP(56)=2 MSTP(51)=10042 MSTP(53)=10042 MSTP(55)=10042

10k events were generated for MB jet settings: * setup mb events MSEL=0 MSUB(11)=1 MSUB(12)=1 MSUB(13)=1 MSUB(28)=1 MSUB(53)=1 MSUB(68)=1 MSUB(94)=1 MSUB(95)=1 MSUB(96)=1

Some particles were made stable and this will introduce a small change in the multiplicity.

* make some particles stable: pi0, Ks, Lambda0 MDCY(PYCOMP(111),1)=0 MDCY(PYCOMP(310),1)=0 MDCY(PYCOMP(3122),1)=0

The underlying event was defined based on the CDF analysis (ref CDF). The particle distribution relative to the leading jet axis (this was defined using the simple PYTHIA cone algorithm-PYCELL) was plotted for rapidity<1. The multiplicity in the underlying event region was then taken as the multiplicity in the region 60-120degrees relative to the jet axis.

The results are given in:

* Name: Craig Buttar

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