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WW Scattering at ATLAS

WW Scattering at ATLAS. Sarah Allwood University of Manchester IOP HEPP conference 2005, Dublin. Introduction. Without some new physics W L W L → W L W L violates perturbative unitarity at E~1.2 TeV. W L W L → W L W L is described at low energy by an effective Lagrangian: the EWChL.

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WW Scattering at ATLAS

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  1. WW Scattering at ATLAS Sarah Allwood University of Manchester IOP HEPP conference 2005, Dublin Sarah Allwood

  2. Introduction Without some new physics WLWL → WLWL violates perturbative unitarity at E~1.2 TeV. • WL WL → WL WL is described at low energy by an effective Lagrangian: the EWChL. • a4 and a5 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 a4-a5 space obtained using the Padé unitarisation protocol. Taken from hep-ph/0201098 J.M. Butterworth, B.E. Cox, J.R. Forshaw. Sarah Allwood

  3. Signal Scenarios Five representative signal scenarios were chosen: • A: scalar resonance of 1.0 TeV, • B: vector resonance of 1.4 TeV, • C: vector resonance of 1.9 TeV, • D: double resonance of a scalar at 800 GeV and a vector at 1.4 TeV, • E: scenario with no resonances (the continuum). This follows on from the hadron level analysis in hep-ph/0201098 • How sensitive is ATLAS to these resonances in WW→WW→lqq ? • Optimum jet parameters to use? Sarah Allwood

  4. Signal and Backgrounds • high pT lepton • high ETmiss • Jet(s) with high pT and m ~ mW. • Little hadronic activity in the central region (|η|<2.5) apart from the hadronic W. • Tag jets at large η (|η|>2). Backgrounds: W+jets (W  l), σ~60,000 fb, and , σ~16,000 fb (cf signal σ<100 fb). high pT W Sarah Allwood

  5. Overview of Analysis • Leptonic: • Reconstruct W from high pT lepton and ETmiss. • Reject events with pTW < 320 GeV. • Hadronic: • Reject events with pTW < 320 GeV. • Reject events outside the range mW± 2σ Environment cuts • Events generated in Pythia (modified to include EWChL), simulated/reconstructed in ATLFAST. • Cells smeared before clustering. • Pile-up for high/low luminosity added to cells. • Cell threshold of 1 GeV for low luminosity, 2 GeV for high luminosity. • Underlying event included. Sarah Allwood

  6. l k beam Jet Finding KT algorithm (inclusive mode): For each object, calculate • dkl (~pT2 of k with respect to l) • dkB (~pT2 of k with respect to the beam direction) • Scale dkB by the R-parameter: dk=dkBR2 • If dk < dkl, k is a jet. • If dkl < dk, merge k and l (add their 4-momenta) and define this as a new object. • Repeat until all objects are in jets. Decide which R-parameter to use based on resolution of hadronic W… Sarah Allwood

  7. Hadronic W Mass of the highest pT jet in the event for different R-parameters and cone radii: • From a Gaussian fit to the peak: • Best resolution for cone: ΔR=0.7 • Best resolution for kT: R=0.5 (hep-ph/0201098 used R=1.0) cone kT High mass tail from underlying event and pile-up W resolved into 2 jets Sarah Allwood

  8. Hadronic W Reconstruct hadronic W as 1 kT jet, but follow this with a “subjet” cut. • Run kT algorithm in subjet mode on the cells in the highest pT jet. • Clustering is stopped at a scale ycutpT2→ clusters remaining are subjets. • Scale at which jet is resolved into two subjets is ~mW2 for a true W. Make a cut at 1.55 < log(pT√y) <2.0 Sarah Allwood

  9. kT cone Hadronic W kTgives higher s/b and better hadronic W resolution than cone 1 jet approach. • Use cones of ΔR = 0.2 to find 2 jet centres • Sum 4-momenta of all cells within ΔR = 0.4 of jet centres. But a fairer comparison is given by reconstructing hadronic W as two overlapping jets… (similar to 1 TeV Higgs study in ATLAS TDR) Similar resolutions achieved for kT and cone (7.2 GeV). Sarah Allwood

  10. Environment Cuts Top mass cut: Reject events where m(W+jet) ~ mtop 130 GeV < m(W+jet)< 240 GeV Reduces top background by a factor of 10 s/b = 0.015 (kT), 0.013 (cone) efficiency = 3.1% (kT), 3.3% (cone) Tag jet veto: Require forward and backward jets with E > 300 GeV and |η| > 2. Reduces W+jets by a factor of 200, top by factor of 100 s/b = 1.0 (kT), 0.9 (cone) efficiency = 1.1% (kT), 1.1% (cone) Sarah Allwood

  11. Environment Cuts pT cut: Expect pT(WW+tag jets)~ 0. Reject events with pT(WW+tag jets) > 50 GeV. s/b = 1.3 (kT), 1.4 (cone) efficiency = 1.1% (kT), 1.1% (cone) Minijet veto: Reject events that have more than one jet (pT > 15 GeV) in the central region. s/b = 1.5 (kT), 1.6 (cone) efficiency = 1.1% (kT), 1.0% (cone) Sarah Allwood

  12. Low Luminosity Results For 30 fb -1: kT cone Sarah Allwood

  13. Low Luminosity Results KT Cone Sarah Allwood

  14. High Luminosity Results For 100 fb-1 Sarah Allwood

  15. Summary • With 100 fb-1 a wide range of resonances can be observed, and their spins measured. • kT and cone results give similar efficiencies and signal/background. • Final signal/background > 1 in all cases. • Using R-parameter of 0.5 instead of 1 has improved signal/background and made some cuts less sensitive to underlying event. • Reconstructing the hadronically decaying W as 1 jet followed by a subjet cut is similar to the cone 2 jet reconstruction. Sarah Allwood

  16. Extra Slides Sarah Allwood

  17. Underlying Event Switch off multi-parton interactions in Pythia and compare to ATLAS default… ATLAS default: mstp(82) = 4 parp(82) = 1.9 parp(90) = 0.16 Sarah Allwood

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