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

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

2. Simulation • Pythia: One million of each W→eνeand W→ μνμevents 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.

3. 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

4. υl l+ υl l+ u l- -l W- d Rapidity Distributions W+ production: W+ VALENCE QUARK SEA QUARK W- production: Balances out with max at 0 for leptons Sea quarks less x than valence? z

5. 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: xvalence = xsea ~10-3: Two sea quarks, ~ even densities xvalence>10-3; xsea<10-3: Valence quark + sea antiquark

6. Preliminary ATLFAST results – lepton pseudorapidity • Cuts: • Muon transverse momentum > 20GeV • Missing energy > 20GeV • |recoil| > 20GeV • |muon pseudorapidity| < 2.4 Muon rapidity, constructed with and without a rapidity cut:

7. 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.

8. 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.

9. Preliminary ATLFAST results – W transverse mass W transverse mass, excluding (left) and including (right) cuts as before – red are W-, black are W+.

10. GEANT4 Results Reconstruction currently incomplete, but visualisation possible:

11. GEANT4 Results Reconstruction currently incomplete, but visualisation possible:

12. GEANT4 Results Reconstruction currently incomplete, but visualisation possible:

13. 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?)

14. 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