1. Zγ Generator and Background Studies
Lindsey Gray
University of Wisconsin at Madison
EWK: Multiboson Meeting
9 April, 2009

2. Lindsey Gray, UW Madison
Zγ Production
Direct Zγ coupling = zero
In Standard Model
Two Channels
Photon radiated by quark
“Outer Zγ”
Photon radiated by lepton
“Inner Zγ”
Useful for calibration
Mz = Mllγ
Rate affected by Trilinear Gauge Coupling
Rate accurately predicted in SM
Look for excess outer Zγ
“Inner Zγ”
Say FSR is useful for calibration
ISR is signal
l = e, μ
“Outer Zγ”
Signal 
Lindsey Gray, UW Madison
9 April, 2009

3. New Physics Accessible With Zγ
Possible causes, if excess is observed:
Composite vector boson models
Could give electric dipole moment to Z
Higgs  Zγ
Any model which adds particles that decay into Zγ.
4th Generation of Quarks
4th Generation of Quarks can be seen through production of ultraheavy vector quarkonium with constituent quarks ‘Q’ (mass of roughly 1 TeV. The pseudoscalar “eta” quarkonium gives no contribution to the cross section. Initial legs shown as gluons since at the LHC, and for quarkonium in general, the production mode is primarily gluon fusion.  
Leads to increase in cross section.
Lindsey Gray, UW Madison
9 April, 2009

4. Anomalous Electric Dipole Moment
is the center of mass energy.
‘O’ is the combination of interacting fields
‘j’ denotes vector or axial vector couplings
‘n’ is the order of the correction
‘f’ is the Anomalous Coupling constant (AC)
‘Λ’ is the scale of new physics interactions
Standard Model Zγ
3.3 pb per lepton channel
Shown: anomalous electric dipole moment. (f63 = .08)
Enhanced rate of Zγ
3.4 pb per lepton channel
Λ = 1 TeV
4.3 pb per lepton channel
Λ = 5 TeV
The A-EDM term in the vertex function is proportional to the emitted photon energy. Comes from terms like Z_{mu,sigma}F^{mu,rho}Z_{sigma,rho}, add in levi-civita symbol to get all terms.
The magnetic quadrupole term goes as (Z momentum) * (emitted photon energy) but gets scaled by another factor of 1/mZ^2 comes from terms ZFZF, permuting lorentz indices as needed.
Λ = 5 TeV
√s =10 TeV
Λ = 1 TeV
Plots generated with program described in:
U. Baur, T. Han, J. Ohnemus
Phys. Rev. D 57, 2923 (1998)
Standard Model Outer Zγ
Lindsey Gray, UW Madison
9 April, 2009

5. Search for Excess Energetic Photons from Outer Zγ
Anomalous couplings and new resonances enhance Zγ production cross-section
Enhancement occurs for events with energetic photons
Rare in SM Outer Zγ
Excess of high ET photons compared to standard model indicates new physics
√s =10 TeV
Λ = 5 TeV
Λ = 1 TeV
Standard Model Outer Zγ
Lindsey Gray, UW Madison
9 April, 2009

6. Current Anomalous Coupling Limits
Place limits by modeling Zγ photon ET of various anomalous coupling strengths
Determine what range of coupling strengths is consistent with ET distribution seen in data
Tevatron measured limits on anomalous couplings:
New Physics Scale Λ > 1 TeV
Determined by Tevatron mass reach.
Anomalous Coupling f63 < .083
At Λ = 1 TeV
Anomalous Coupling
2.0 fb-1
Limits on CP Conserving Vector (h3) and Axial Vector (h4) Couplings
Jianrong Deng, Al Goshaw, Thomas Phillips
Jan 31, 2008
Lindsey Gray, UW Madison
9 April, 2009

7. High Level Electron Trigger
Electron HLT
Find groups of energy in ECAL
Reconstruct tracks near deposit
Match energy deposit to tracks
Recover energy losses to bremsstrahlung by extending included calorimeter area in phi direction.
For Zγ study at LHC startup:
Use isolated electron trigger to tag possible Zγ events.
Isolate electrons by summing nearby calorimeter deposits to check for activity.
Electron pT >15 GeV
Electron E_T threshold may drop to 10 GeV by LHC start up (only say if someone asks). •
Lindsey Gray, UW Madison
9 April, 2009

8. Muon High Level Trigger
Muon HLT
Find track in muon system
Reconstruct tracks in tracker pointing towards muon system track
Match muon system track to tracker track
For Zγ study at LHC startup:
Zγ events use non-isolated muon trigger to tag possible events.
Muon pT > 5 GeV μ-
Side Note: The matching criteria can be set to use chi-square minimization matching instead of delta-R matching. •
Lindsey Gray, UW Madison
9 April, 2009

9. Photon Reconstruction
Photons reconstructed from collections of associated crystals with energy in ECAL called SuperClusters.
Starts from a “seed” crystal of > 1 GeV
Make 5x5 crystal “seed cluster” if seed crystal is local maximum
Add up to 17 1x5 rows in each direction in phi, keeping rows with energy sum > .1 GeV
All SuperClusters are Photon candidates
ET > 10 GeV
H/E < .2
Requires no matched pixel detector hit.
The presence in CMS of material in front of the calorimeter results in bremmstrahlung and photon conversions. Because of the strong magnetic field the energy reaching the calorimeter is spread in φ. The spread energy is clustered by building a cluster of clusters, a "supercluster", which is extended in φ. R9
The shower shape variable R9 is effective in distinguishing photon conversions in the material of the tracker. Photon candidates with large values of R9 either did not convert or converted late in the tracker and have good energy resolution. Photons converting early have lower values of R9 and worse energy resolution.
The variable R9 has been shown to be very useful also in discriminating between photon sand jets. This occurs both because of the conversion discrimination – either of the photonsfrom a pi0 can convert – and because, looking in a small 3×3 crystal area inside the supercluster, the R9 variable can provide very local isolation information about narrow jets. R9
Crystal f
Crystal h
■ Crystals in Seed Cluster
■ Other crystals within Supercluster
--- Supercluster boundary
Lindsey Gray, UW Madison
9 April, 2009

10. Photon Identification
Photon reconstruction begins with SuperCluster > 10 GeV.
Other particles can create a 10 GeV SuperCluster.
Jets fragmenting primarily to π0
.001 of jets fake photons (# jets to # photons ~ 1000:1)
Electrons
Photon ID selects reconstructed photons passing various quality cuts.
HCAL < 10 GeV, near reconstructed photon.
ECAL < 10 GeV near reconstructed photon.
Require < 2 tracks near reconstructed photon.
Require that 80% of ECAL energy is within 3x3 crystals.
Electrons appear more spread out in phi than direct photons due to bending in magnetic field.
■ Crystals in Seed Cluster
■ Other crystals within Supercluster
--- 3x3 region
--- Supercluster boundary
Note that the cuts mentioned here are very loose cuts which were chosen to provide the greatest photon acceptance without regard to purity. We will see that we need much improvement to pick out the outer Zgamma signal.
Chris Seez , photon workshop talk.
Crystal f
Crystal h
Lindsey Gray, UW Madison
9 April, 2009

11. Electron Reconstruction
Calorimeter Reconstruction
Create superclusters of ECAL energy to include bremmstrahlung photons.
ET > 4 GeV
H/E < .1
Tracker Reconstruction
Require calorimeter deposit matched to reconstructed track, ΔR < .15
pT > 3 GeV
Pixels
Tracker
Strips ET pT • e- γ
Lindsey Gray, UW Madison
9 April, 2009

12. Lindsey Gray, UW Madison
Muon Reconstruction
Standalone Reconstruction
Muon system only
Tracker Reconstruction
Match tracks to regions in the calorimeter consistent with a minimum ionizing particle.
Match within
Global Reconstruction
Match tracker tracks to muon system tracks by minimizing a ‘quality’ variable.
‘Δd’ is distance between end of tracker track and beginning of muon track
Standalone Muon
Track
Inner Detector
You need to explain the difference here between the HLT muon reconstruction and the offline muon reconstruction.
Track Quality
Lindsey Gray, UW Madison
9 April, 2009

13. Zγ & Z+Jets Event Simulation
Zγ generated with Pythia 6.409
LO matrix element cross section calculation
Higher order initial (final) state radiation is approximated
Z+jets background generated with MadGraph
Matrix element cross section calculation for Z + N ≤ 4 Jets
Detector simulated using Full Simulation (GEANT) for signal and FastSim for background.
GEANT simulates passage of particles through matter.
FastSim is a parameterization of GEANT CMS simulation with faster execution time.
Detector simulation
GEANT 4 FastSim
Hard scattering
Pythia MadGraph
Hadronization, showers, IFSR
PYTHIA
Reconstruction of event
CMSSW
Lindsey Gray, UW Madison
9 April, 2009

14. Zγ Generator Comparison
Pythia
Baur Generator
LO or NLO QCD Matrix Element LO
NLO
Anomalous Couplings No
Yes
Tunable
Pythia Based Hadronization
Interface to Pythia in Progress
Number of Zγ Events vs. Photon ET
Baur Zγ Generator
Developed by Dr. Ulrich Baur (U. Buffalo) et al.
Calculates NLO Zγ cross section using Monte Carlo
Tunable anomalous couplings & new physics scale Λ
Accurately models photon ET for outer Zγ
Baur SM Outer Zγ
Pythia SM Outer Zγ
Events
Lindsey Gray, UW Madison
9 April, 2009

15. Comparing Baur to Tevatron Data
CDF measures inner & outer Zγ
= 4.6 ± 0.2 (stat) ± 0.3 (sys) pb
= 1.2 ± 0.1 (stat) ± .17 (sys) pb
D0 measures inner & outer Zγ as well
= 4.4 ± .27 (stat) ± .27 (sys) pb
All measurements agree with Baur MC predictions:
= 4.5 ± 0.4 pb (Inner + Outer Zγ)
= 1.21 ± 0.1 pb (Outer Zγ Only)
2.0 fb-1
Note that these numbers are for tevatron energies, LHC Outer Zγ cross section is ~5 times higher. But Z+jets is a factor of 14 times larger at LHC
Baur
Anomalous
Coupling
1.1 fb-1
Lindsey Gray, UW Madison
9 April, 2009

16. Z+Jets Background to Outer Zγ
1 in 1000 jets fragment primarily to π0
σZ+Jets = 251 Tevatron (to leptons)
σZ+Jets = 3700 LHC (to leptons)
Similar kinematics to Outer Zγ
CDF Z+Jets pT measurement matches NLO MCFM well.
MCFM: Monte Carlo for Femtobarn Measurement (dev. by CDF Collab.)
Give accurate background prediction for CDF Zγ measurement
CMS Z+Jets will be measured in 200 pb-1
Expect more Z + multiple jets
Aside: Due to more gluons being present in protons at LHC, the compton-like ( q + gluon -> q + Z) channel diagram contributes more than at Tevatron. Also, the quark bremsstrahlung diagram (with Z in s-channel) will contribute in a different way than at Tevatron since the anti-quark in this case is coming from the sea instead of being a valence quark of an anti-proton (vastly different PDFs!). These differences force the Z+jets cross section to be remeasured at LHC since we cannot simply extrapolate from the Tevatron results due to these different diagrams contributing.
Lindsey Gray, UW Madison
9 April, 2009

17. Zγ Signal and Z+Jets Background
Require electrons, muons and photons to be within the tracker and to pass trigger. (-2.5 < η < 2.5)
Require e±: ET > 15 GeV & μ±: pT > 5 GeV
Removes poorly reconstructed e± and μ±.
Starting With:
105 Signal
28k Bkg [200pb-1]
Zγ -> eeγ MC
Z+jets MC
Add % and number left. Annouce that I’m doing this.
Purity?
Low tail coming from faked leptons being associated with leptons from Z.
Change title to explain Z+jets and Zgamma background!
Zγ->μμγ MC
Z+jets MC
200pb-1
200pb-1 e μ
Lindsey Gray, UW Madison
9 April, 2009

18. Cut on Dilepton Invariant Mass
Require dilepton mass near Z peak (70 < Mll < 100)
Majority of signal Zs are on shell
Suppresses Inner Zγ
What’s Left:
85 Signal, 81%
21k Background, 75%
Reject
Reject
Add % and number left. Annouce that I’m doing this pb ^-1!!!!!!
Purity?
Low tail coming from faked leptons being associated with leptons from Z. e μ
Zγ -> eeγ MC
Z+jets MC
Zγ->μμγ MC
Z+jets MC
Lindsey Gray, UW Madison
9 April, 2009

19. Selecting Signal Photons: H/E
Hcal-to-Ecal energy ratio of a reconstructed photon.
Jets have a larger hadronic energy fraction.
Hence, so do many jets that fake photons.
Cut at H/E = .025
What’s Left:
75 Signal, 74%
9k Background, 33%
% rejection and total left! # of background # of signal every plot. Combine converted and unconverted plots.
Z+jets Zγ
EM Supercluster g
ECAL
jet
ECAL
HCAL
reject
Supercluster
Lindsey Gray, UW Madison
9 April, 2009

20. Selecting Signal Photons: R9
Cut on ratio of E3x3 to Esupercluster ( “R9”)
EM deposits from Jets will be more spread out.
Except energetic π0’s
Cut at r9 = .90
What’s Left:
45 Signal, 43%
2k Background, 7.9%
Z+jets Zγ
200pb-1 f h
reject
Lindsey Gray, UW Madison
9 April, 2009

21. Selecting Signal Photons: Track Isolation
Count number of reconstructed tracks in a cone near the photon with pT > .5 GeV
Faked photons have more tracks in .4 ΔR cone.
Cut at Number of Tracks = 2
What’s Left:
41 Signal, 39%
1.5k Background, 5.3%
Cut at 2 retains conversions.
Z+jets Zγ
Isolation
Cone
ΔR=0.4 γ
reject
Lindsey Gray, UW Madison
9 April, 2009

22. Selecting Signal Photons: ET Isolation
Σ (Hcal ET + Ecal ET + Track pT)/ET, Supercluster in annulus around reconstructed photon.
Faked photons have more energy and tracks in the .06 < ΔR < .4 annulus.
Cut at (Isolation Sum)/ET = .4
What’s Left:
25 Signal, 24%
480 Background, 1.5%
Z+jets Zγ
Signal Cone
ΔR=0.06
Isolation
Cone
ΔR=0.4 γ
reject
Lindsey Gray, UW Madison
9 April, 2009

23. Selecting Signal Photons: Phi Width
Since π0 -> γγ, faked photons will appear wider in phi due to the opening angle between the photons.
Cut at Phi Width < .015
What’s Left:
17 Signal, 16%
300 Background, 1.0%
The rejection factor may look bad here but this is an artifact of running out of statistics. In raw numbers there are actually more signal events than background, and more signal events are lost than background events so it appears that this cut is not doing much. Essentially the Z+Jets sample space is not well-filled enough.
Z+jets Zγ f h
reject
Lindsey Gray, UW Madison
9 April, 2009

24. Selecting Signal Photons: Minimum ΔRlγ
ΔRlγ > 1.3 cut applied after previous photon cuts.
Further rejects Z+Jets background and Inner Zγ
Added advantage of avoiding singularity in the Zγ cross section from photon collinearity
Improves cross section prediction
What’s Left:
9 Signal, 8.5%
38 Background .13%
E, mu bigger, try making DeltaR cut larger? Final answer on photons,,,, indicate e
reject
reject
Z+jets Zγ
Z+jets Zγ μ
Lindsey Gray, UW Madison
9 April, 2009

25. Cut on llγ Invariant Mass
Inner Zγ events with large photon ET can pass ΔRlγ cut.
Dilepton+photon invariant mass will be near Z mass.
Majority of outer Zγ will be outside of Z peak.
Cut at Mllγ > 105 GeV
What’s Left:
8 Signal, 7.6%
10 Background .035%
Title of slide -> Invariant Mass cut … all titles go to cut being applied. Combine current title with third bullet.
Z+jets Zγ
Z+jets Zγ e μ
reject
reject
Lindsey Gray, UW Madison
9 April, 2009

26. Summary of Signal & Background
Zγ (Outer, Anom. Coup.)
Z+jets
Initial Sample
105
28k
70 < Mll < 100 GeV 85
21k
Photon ID Cuts 18
310
ΔR(l±, γ) > 1.3 9 36
Mllγ > 105 GeV
8 (11) 10
200pb-1
Lindsey Gray, UW Madison
9 April, 2009

27. Conclusion and Next Steps
Signal to background is roughly 1:1 on Z peak.
8 (11, with Anom. Coup.) Events & 10 Background
Next Steps:
Improve Signal-to-Background to 2:1
Measure Z+Jets background in data in tandem with Zγ measurement and apply fake rate
200 pb-1 analysis allows SM Zγ measurement
Sensitive to new physics
Assuming maximum allowed anomalous coupling, events & 1 background (NLO prediction)
Photon ET > 100 GeV
3 events make it into the detector and pass cuts for f = .1, 5 TeV scale. With .7 background 3 sigma!!!
Note that this is a back of the envelope calculation, since I cannot simulation AC’s in my current setup.
0.011 events make it into the detector and pass cuts for no AC. .7 background.
Lindsey Gray, UW Madison
9 April, 2009

28. Backup Slides

29. Anomalous Coupling: Helicity Angle
Sensitive to new physics
Introduction of new physics can change favored polarization.
Angle between Z momentum in lab frame and daughter lepton in rest frame.
Spins of daughter particles related to Z polarization.
Different distributions for longitudinal and transverse Z
Different daughter spin combinations required.
@ 10 TeV
Standard Model
Tevatron Limit
Λ = 5 TeV scale
Quartic coupling WWZgamma l^+, l^- to “positive lepton”
Lindsey Gray, UW Madison
9 April, 2009

30. Resonance Search: Dalitz Plot
New physics manifests as bands in the Dalitz Plot.
Shown: SM Zγ
Higgs -> Zγ
Would appear as an enhanced segment along the Z band.
Lindsey Gray, UW Madison
9 April, 2009

31. Higgs Branching Ratios
Higgs branching ratio to Zγ is phase space heavily suppressed.
Dominated by vector boson pair production
Probability for all three particles to be in fiducial region small.
Lindsey Gray, UW Madison
9 April, 2009

32. Selecting Signal Photons: ECAL Isolation
Σ (Ecal ET) in annulus around reconstructed photon.
As jets are more spread out than photons, the ET sum will be larger for jets with a large EM fraction.
Cut at 80% signal acceptance -> Ecal Isolation < 7.5 GeV
Define annulus size.
Isolation = 7.5 GeV
Z+jets Zγ
Scaled to 100pb-1
reject
Lindsey Gray, UW Madison
9 April, 2009

33. Selecting Signal Photons: Track ET Isolation
Σ (Track ET) in annulus around reconstructed photon.
Prompt and converted photons will have fewer energetic tracks near than a jet.
Cut at ~75% signal acceptance -> Track Isolation < 3 GeV
Define annulus size.
Isolation = 3 GeV
Z+jets Zγ
Scaled to 100pb-1
reject
Lindsey Gray, UW Madison
9 April, 2009

34. Selecting Signal Photons: Number of Nearby Tracks
Reconstructed photons embedded within jets will have more nearby tracks than isolated photons.
Cut at 80% signal acceptance -> # Tracks < 3
#Tracks = 3
Z+jets Zγ
Scaled to 100pb-1
reject
Lindsey Gray, UW Madison
9 April, 2009

35. Integrated Luminosity
LHC Expected Yield
Month
LHC Status
Protons/Bunch
Peak Luminosity
Integrated Luminosity 1
Beam Commissioning
First Collisions 2
Pilot Physics
3x1010
1.2x1030
nb-1 3
5x1010
3.4x1030
~ 2pb-1 4
2.5x1031
~ 13pb-1 5
7x1010
4.9x1031
~ 25pb-1 6
50 ns Bunches
4.0x1031
~ 21pb-1 7
1.1x1032
~ 60pb-1 8 9
Total:
200 – 300 pb-1
Lindsey Gray, UW Madison
9 April, 2009