Ellie Dobson (University of Oxford) On behalf of ATLAS and the W/Z+Jets CSC note group DIS 2008. W/Z+Jets production studies in ATLAS. Why study W/Z+jet events? How do we see such events in ATLAS? Summary of current MC studies. Events. 10 fb 1. M eff. Why study W/Z+jets?.
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MeffWhy study W/Z+jets?
Beyond the standard model physics
(low pt hadronic recoil)
Example of W+1jet production
Health warning! Back of the envelope calculation
Muon trigger used to write Z→μμ and W→μν events to diskW/Z+jets reconstruction in ATLAS
Muons reconstructed as a combination of a track in the inner detector and the muon spectrometer
Met and SumPt determined by summing over calorimeter cells.
Electrons reconstructed as a combination of a track in the inner detector and the energy deposit in the EM calorimeter
Electron trigger system used to write Z→ee and W→eν events to disk
Put picture of first page of note here
W/Z+jets sample (used for main analysis)
Offline procedure (default)
→ need to correct theory with respect to fragmentation and underlying event
Underlying event adds energy to the hadron level jet
Fragmentation reduces the amount of energy in the jet cone
Negligible effect for higher energy jets
Fall with higher hadronic
activity due to the implicit
isolation cut in the level 1 trigger
Bias in reconstruction at low jet pt
No global behaviour seen
Within errors, the
Pt distributions of truth and
corrected reconstructed jets agree
Muons coming from background (particularly bbar) can be rejected using isolation requirements
QCD background dominates at a 1 jet signal. At higher jet multiplicity ttbar dominates.
Mass cuts imposed to reduce background
BG estimation and subtraction (W)
→ systematic on the cross section of up to 10%
Uncertainties increase with jet multiplicity
Uncertainties always remain <10%
Shown results for W→eν.
Similar results seen in the muons and Z analysis
PDF errors increase at central η and at low electron Pt (no y dependence seen)
Pythia predicts a softer pt spectrum
Pythia discrepancies due to tuning of leading soft radiation in parton shower
XS (Z →μμ)
Pytha predicts more low pt jets
shower predicts softer jet pt distribution
Alpgen predicts more high pt jets
btagging: cutting on the weight parameter (secondary vertex and impact parameter)
Contamination from light jets of the order 30%