2004 Xmas MeetingSarah Allwood WW Scattering at ATLAS.

2004 Xmas Meeting Sarah Allwood

WW Scattering at ATLAS


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.


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.


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.

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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.


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.


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


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.


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


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


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:


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.

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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.

2004 Xmas MeetingSarah Allwood WW Scattering at ATLAS.

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