A simulation study of the rapidity distributions of leptons from W
boson decays at ATLAS
Laura Gilbert
q
qW
l
l
Simulation
• Pythia: One million of each W→eνe and W→ μνμ events generated. W mass and CTEQ5 pdfs assumed.
• ATLFAST – “fast” detector simulation: uses smearing to model detector efficiency: ntuples produced and studied for two million events.
• GEANT4 – “full” detector simulation, gives accurate detector efficiencies: 100 fully simulated muon events. Hits can be seen but tracks are not reconstructed.
Why ?llWqq
• Reconstructed transverse masses can be used with MC studies to constrain real W mass
• W± rapidity asymmetry constrains pdfs and test evolution equations at low x.
• Simulations can be used to study GEANT detector efficiencies
Rapidity Distributions
z
W+
VALENCE QUARK SEA QUARK
d
W+ production:
W- production:
Balances out with max at 0 for leptons
Sea quarks less x than valence?
u
udW-
-l
l
l+υl
l+
υl
l-
l
Rapidity DistributionsGenerator-only results from Dittmar et al (1997). Plots are symmetric about 0 rapidity.
Peak: double the number of valence us
contribute to W+ than ds to W-
Central rapidities: xvalence = xsea ~10-3:
Two sea quarks, ~ even densities
xvalence>10-3; xsea<10-3:
Valence quark + sea antiquark
Preliminary ATLFAST results – lepton pseudorapidity
Muon rapidity, constructed with and without a rapidity cut:
Cuts:
•Muon transverse momentum > 20GeV
•Missing energy > 20GeV
•|recoil| > 20GeV
•|muon pseudorapidity| < 2.4
W transverse mass
• Longitudinal momentum of the neutrino cannot be measured
• Calculate transverse mass of W from transverse momenta (pT) of the neutrino and of the charged lepton
• Lepton pT is measured, whereas the transverse momentum of the neutrino is obtained from the pT of the lepton and the recoil
• The trailing edge of the spectrum is sensitive to the W mass. Compare real data to MC with difference values of MW.
)cos1(2 TlT
WT ppm
|| uppl
TT
W transverse mass
• Trailing edge smeared by W intrinsic width and the detector resolution.
• Red plot is at the generator level, blue includes energy and momentum resolution as implemented in ATLFAST.
Preliminary ATLFAST results – W transverse mass
W transverse mass, excluding (left) and including (right) cuts as before – red are W-, black are W+.
GEANT4 ResultsReconstruction currently incomplete, but visualisationpossible:
GEANT4 ResultsReconstruction currently incomplete, but visualisationpossible:
GEANT4 ResultsReconstruction currently incomplete, but visualisationpossible:
Grid at Dell
• Currently involved in investigating virtualisation of Atlas software on a Dell grid cluster in Austin, Tx.
• Should make it possible to run far larger simulations – will be able to analyse “real” data once reconstruction is complete (~1 month?)
Future Work – W asymmetry
• Reconstruct W rapidities as far as possible
• Consider how far pdfs can be constrained at rapidities allowed by the detector
• Use GEANT4 datasets to study detector efficiencies
• Background Studies
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