A simulation study of the rapidity distributions of leptons from W boson decays at ATLAS Laura Gilbert

Simulation Pythia: One million of each W→eν e and W→ μν μ 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? 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+W+ VALENCE QUARK SEA QUARK W + production: W - production: Balances out with max at 0 for leptons Sea quarks less x than valence? u d W-W- -l l+l+ υlυl l+l+ υlυl l-l-

Rapidity Distributions Generator-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: x valence = x sea ~10 -3 : Two sea quarks, ~ even densities x valence >10 -3 ; x sea <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 (p T ) of the neutrino and of the charged lepton Lepton p T is measured, whereas the transverse momentum of the neutrino is obtained from the p T 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 M W.

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 Results Reconstruction currently incomplete, but visualisation possible:

GEANT4 Results Reconstruction currently incomplete, but visualisation possible:

GEANT4 Results Reconstruction currently incomplete, but visualisation possible:

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