1. Sarah Allwood-Spiers ATLAS UK Top Meeting 17/09/08
Alpgen 10TeV ttbar+jets
Sarah Allwood-Spiers
ATLAS UK Top Meeting
17/09/08

2. Sarah Allwood-Spiers, UK Top meeting, 17/09/08
Introduction
Several MC samples will be available for 10TeV ttbar studies:
dataset , fullsim and Atlfast2, ~2500pb-1
Alpgen:
datasets – (leptonic+semileptonic, different jet multiplicities) ~1300 pb-1 with Atlfast2.
AcerMC:
~500 pb-1 with Atlfast2
Dataset (AcerMC through Pythia),
Dataset (AcerMC through Herwig).
Sarah Allwood-Spiers, UK Top meeting, 17/09/08

3. Cross Sections (leptonic+semileptonic)
AcerMC (through Pythia or Herwig): 109 pb
Alpgen Q2 = Σ(m2 +pT2) (through Herwig):
= 127 pb
0j j j ≥3j j j j ≥3j
~ 191 pb (old number, with mt=175GeV)
Reference cross section :
419pb for all ttbar ( S Moch, P Uwer)
→ 225pb for semileptonic+leptonic
Following plots are normalised to unity for comparison of shapes…
lnqq
lnln
Sarah Allwood-Spiers, UK Top meeting, 17/09/08

4. Sarah Allwood-Spiers, UK Top meeting, 17/09/08
(Thanks to Joe Foster and Paul Miyagawa for some plotting code)
Alpgen
Alpgen versus
b quark
electron W+
tops
Fairly similar distributions
mc T1_McAtNlo_Jimmy.evgen.EVNT.e325
through atlfast1
Sarah Allwood-Spiers, UK Top meeting, 17/09/08
Alpgen input files through evgen and Atlfast1 in

5. Alpgen versus MC@NLO, Cone 0.4 Jets
5th jet
Highest pT jet
Alpgen has higher number of reconstructed jets.
Alpgen jets slightly more central
Pt distributions fairly similar
Sarah Allwood-Spiers, UK Top meeting, 17/09/08

6. Alpgen versus AcerMC (Herwig)
t, W, b distributions for Alpgen and AcerMC(Herwig) similar
pT(t+tbar) slightly harder for Alpgen
Sarah Allwood-Spiers, UK Top meeting, 17/09/08

7. Alpgen versus AcerMC, Cone 0.4 Jets
Through
Herwig
AcerMC
Through
Pythia
Sarah Allwood-Spiers, UK Top meeting, 17/09/08

8. Sarah Allwood-Spiers, UK Top meeting, 17/09/08
Summary
Alpgen ttbar MC sample gives distributions for leptons, tops, W, quarks that have similar shapes to AcerMC and
Alpgen predicts higher number of jets than and AcerMC.
Pt distributions for each jet, ordered in pt, are similar for Alpgen and AcerMC (through Herwig)
AcerMC with Pythia parton shower gives harder jet pt distribution and this is a greater effect than the difference between underlying generators.
Sarah Allwood-Spiers, UK Top meeting, 17/09/08