2004 Xmas Meeting Sarah Allwood
WW Scattering at ATLAS
2004 Xmas Meeting Sarah Allwood
Introduction• An important goal of LHC is to investigate electroweak symmetry breaking.
• Without some new physics WLWL → WLWL violates perturbative unitarity at CoM E~1.2 TeV.
• Possibilities are:additional particle(s) with m ≤ 1TeV,and/or W and Z interactions becomestrong at E ~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 TeVin 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.
2004 Xmas Meeting Sarah 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→lqq ?
All investigated using kT and cone algorithms.
2004 Xmas Meeting Sarah 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 pT lepton
• high ETmiss and high pT of the leptonic W
reconstructed from these.
• 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), from the quarks that produced the W’s.
tt
2004 Xmas Meeting Sarah 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 kT algorithm.
• Underlying event included.
2004 Xmas Meeting Sarah Allwood
Jet Finding
Cone algorithm:Constructs cones of a fixed radius ΔR=√(Δη2 + Δφ2) around seed cells. Defines
these as jets.
Kt algorithm:
For each object, calculate dkl (~pT
2 of k with respect to l)
dkB (~pT2 of k with respect to the beam)
• 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.
2004 Xmas Meeting Sarah Allwood
Reconstructing the hadronic W
Mass of the highest pT 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.
kTcone • Best resolution
for kT: R=0.5
• Best resolution for cone: ΔR=0.7
2004 Xmas Meeting Sarah Allwood
Reconstructing the hadronic W, kT
• For the kT, use an R-parameter of 0.5 and get an extra cut from “subjet analysis”:
• Rerun kT algorithm in subjet mode on the cells in the highest pT jet.
• Clustering is stopped at a scale ycutpT
2 → clusters remaining are subjets.
• Scale at which jet is resolved into two subjets is ~mW
2 for a true W.
• Make a cut at 1.55<log(pT√y)<2.0.
• R=0.5 used for all other jet finding in the event.
2004 Xmas Meeting Sarah Allwood
Summary of analysis
• Select highest pT isolated lepton in event.
• Reconstruct leptonic W from lepton
and missing energy.
• Reject events with pTW < 320 GeV.
• Reconstruct hadronic W From two jets for the cone, From one jet and a subjet cut for the kT.
• Reject events with pTW < 320 GeV.
• Reject events outside the range mW±2σkT cone
s/b, cone
Efficiency, cone, %
s/b, kt Efficiency, kt, %
0.0008 6.92 0.0008 6.92
0.002 4.5 0.0009 5.55
0.006 3.8 0.007 3.46
2004 Xmas Meeting Sarah Allwood
Further cutsTop mass cut – reject events where m(W+jet)~mtop
Tag jet veto – require forward and backward jets with E > 300 GeV and |η| > 2.
pT cut – reject events with pT(WW+tag jets) > 50 GeV
Minijet veto – reject events that have more than one jet (pT > 15 GeV) in the central region
kT s/b KT efficiency
Cone s/b
Cone efficiency
0.015 3.09 0.013 3.28
1.04 1.47 0.93 1.46
1.31 1.08 1.38 1.06
1.45 1.06 1.55 1.01
2004 Xmas Meeting Sarah Allwood
Low luminosity results
conekT
Kt s/b Cone s/b
Kt effic Cone 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:
2004 Xmas Meeting Sarah 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 pT subsample will be generated. 1.5 million , including a subset with pT(t) > 500 GeV
• Generate 10000 events for each of the signals.• The emphasis is on the first year of running – i.e. low luminosity.
tt
2004 Xmas Meeting Sarah Allwood
Conclusions and further work
• The results depend on the cone radius and kT R-parameter used.
• For kT, can reconstruct hadronic W using one jet (due to the useful subjet analysis cut) and the optimum R-parameter to use is 0.5.
• kT 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 pT > 25GeV jets (was 15 GeV for low luminosity).
• But this is just fast simulation – next step is to look at full simulation.
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