1. 2004 Xmas MeetingSarah Allwood WW Scattering at ATLAS

2. 2004 Xmas MeetingSarah Allwood Introduction An important goal of LHC is to investigate electroweak symmetry breaking. Without some new physics W L W L → W L W L violates perturbative unitarity at CoM E~1.2 TeV. Possibilities are: additional particle(s) with m ≤ 1TeV, and/or W and Z interactions become strong at E ~TeV. W L W L → W L W L is described at low energy by an effective Lagrangian: the EWChL. a 4 and a 5 parameterise the “new physics”. EWChL made valid up to higher energies by unitarity constraints: this can predict resonances ~1 TeV in WW scattering. Map of a 4 -a 5 space obtained using the Padé unitarisation protocol. Taken from hep-ph/0201098 J.M. Butterworth, B.E. Cox, J.R. Forshaw.

3. 2004 Xmas MeetingSarah Allwood Signal Scenarios Five representative scenarios for the “new physics” were chosen: a scalar resonance of 0.9 TeV, a vector resonance of 1.4 TeV, a vector resonance of 1.9 TeV, a double resonance of a scalar at 800 GeV and a vector at 1.4 TeV, a scenario with no resonances (the continuum). How sensitive is ATLAS to these resonances in WW→WW→l qq ? All investigated using k T and cone algorithms.

4. 2004 Xmas MeetingSarah Allwood Signal and Backgrounds The main backgrounds are from W+jets (where W  l ) and production, with cross sections ~60,000 fb and ~16,000 fb compared to signal cross sections ~100 fb. high p T lepton high E T miss and high p T of the leptonic W reconstructed from these. Jet(s) with high p T and m ~ m W. Little hadronic activity in the central region (|η|<2.5) apart from the hadronic W. Tag jets at large η (|η|>2), from the quarks that produced the W’s.

5. 2004 Xmas MeetingSarah Allwood ATLFAST The fast detector simulation and reconstruction program for ATLAS.  Includes magnetic field and the η coverage and size of detectors.  Constructs 3 simple calorimeters – barrel (0.1×0.1 in η×φ) and forward (0.2×0.2). Part of ATHENA, the ATLAS software framework, and linked to other ATHENA packages (event generators and jet algorithms). Changes made to  add the modified version of Pythia,  output ntuple with extra information – the 4-vectors of the W’s, and the calorimeter cells,  add pile-up for low luminosity running and other detector smearing to cells before clustering – important if we want to change the cone radius or use the k T algorithm. Underlying event included.

6. 2004 Xmas MeetingSarah Allwood Jet Finding Cone algorithm: Constructs cones of a fixed radius ΔR=√(Δη 2 + Δφ 2 ) around seed cells. Defines these as jets. Kt algorithm: For each object, calculate  d kl (~p T 2 of k with respect to l)  d kB (~p T 2 of k with respect to the beam) Scale d kB by the R-parameter d k =d kB R 2 If d k < d kl, k is a jet. If d kl < d k, merge k and l (add their 4-momenta) and define this as a new object. Repeat until all objects are in jets.

7. 2004 Xmas MeetingSarah Allwood Reconstructing the hadronic W Mass of the highest p T jet in the event: For the cone, a better procedure than 1 jet approach:  Use cones of ΔR=0.2 to find 2 jet centres.  Sum 4–momenta of all calorimeter cells within ΔR=0.4 of the jet centres to define the hadronic W. kTkT cone Best resolution for k T : R=0.5 Best resolution for cone: ΔR=0.7

8. 2004 Xmas MeetingSarah Allwood Reconstructing the hadronic W, k T For the k T, use an R-parameter of 0.5 and get an extra cut from “subjet analysis”: Rerun k T algorithm in subjet mode on the cells in the highest p T jet. Clustering is stopped at a scale y cut p T 2 → clusters remaining are subjets. Scale at which jet is resolved into two subjets is ~m W 2 for a true W. Make a cut at 1.55<log(p T √y)<2.0. R=0.5 used for all other jet finding in the event.

9. 2004 Xmas MeetingSarah Allwood Summary of analysis Select highest p T isolated lepton in event. Reconstruct leptonic W from lepton and missing energy. Reject events with p T W < 320 GeV. Reconstruct hadronic W  From two jets for the cone,  From one jet and a subjet cut for the k T. Reject events with p T W < 320 GeV. Reject events outside the range m W ±2σ kTkT cone s/b, cone Efficiency, cone, % s/b, ktEfficiency, kt, % 0.00086.920.00086.92 0.0024.50.00095.55 0.0063.80.0073.46

10. 2004 Xmas MeetingSarah Allwood Further cuts Top mass cut – reject events where m (W+jet) ~m top Tag jet veto – require forward and backward jets with E > 300 GeV and |η| > 2. p T cut – reject events with p T (WW+tag jets) > 50 GeV Minijet veto – reject events that have more than one jet (p T > 15 GeV) in the central region k T s/bK T efficiency Cone s/b Cone efficiency 0.0153.090.0133.28 1.041.470.931.46 1.311.081.381.06 1.451.061.551.01

11. 2004 Xmas MeetingSarah Allwood Low luminosity results conekTkT Kt s/bCone s/b Kt efficCone effic After all cuts A:3.28 B:2.18 C:1.87 D:4.17 E:1.45 A:3.65 B:2.47 C:2.07 D:4.52 E:1.55 A:1.40 B:1.33 C:1.25 D:1.13 E:1.06 A:1.40 B:1.36 C:1.24 D:1.10 E:1.01 For 30 fb -1 :

12. 2004 Xmas MeetingSarah Allwood Full simulation ATLAS is preparing samples for the Rome physics workshop (June 2005), where each working group will present results from full simulation. The full chain is  generation → outputs 4-momentum of particles.  simulation → tracks particles through detector, outputs hits in the detector.  pile-up → merging hits that came from the same (or close) bunch crossings.  digitisation → simulates the response of the detector. Output should look like raw data.  mixing → mix different physics events.  reconstruction → output reconstructed particles and jets. 15 million events overall, of which 10 million are backgrounds that are common between several working groups (mine fall into this category):  4 million W+jets, where W→l. A high p T subsample will be generated.  1.5 million, including a subset with p T (t) > 500 GeV Generate 10000 events for each of the signals. The emphasis is on the first year of running – i.e. low luminosity.

13. 2004 Xmas MeetingSarah Allwood Conclusions and further work The results depend on the cone radius and k T R-parameter used. For k T, can reconstruct hadronic W using one jet (due to the useful subjet analysis cut) and the optimum R-parameter to use is 0.5. k T and cone results are similar. Final signal/background > 1 in all cases. Will get much more information (spin of resonance) from one year of high luminosity running (100 fb -1 ):  Pile-up is much worse – perform a similar analysis, but: Use a cell threshold E > 2 GeV (was 1 GeV for low luminosity), Minijet veto on p T > 25GeV jets (was 15 GeV for low luminosity). But this is just fast simulation – next step is to look at full simulation.