Z? Generator and Background Studies

1
Z? Generator and Background Studies

• Lindsey Gray
• University of Wisconsin at Madison
• EWK Multiboson Meeting
• 9 April, 2009

2
Z? Production

• Direct Z? coupling zero
• In Standard Model
• Two Channels
• Photon radiated by quark
• Outer Z?
• Photon radiated by lepton
• Inner Z?
• Useful for calibration
• Mz Mll?
• Rate affected by Trilinear Gauge Coupling
• Rate accurately predicted in SM
• Look for excess outer Z?

Inner Z?
l e, µ
Outer Z?
Signal ?
3
New Physics Accessible With Z?
Possible causes, if excess is observed

• Composite vector boson models
• Could give electric dipole moment to Z
• Higgs ? Z?
• Any model which adds particles that decay into
  Z?.
• 4th Generation of Quarks

?
?
Leads to increase in cross section.
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) ? is the scale of new
physics interactions

• Standard Model Z?
• 3.3 pb per lepton channel
• Shown anomalous electric dipole moment. (f63
  .08)
• Enhanced rate of Z?
• 3.4 pb per lepton channel
• ? 1 TeV
• 4.3 pb per lepton channel
• ? 5 TeV

? 5 TeV
vs 10 TeV
? 1 TeV
Plots generated with program described in U.
Baur, T. Han, J. Ohnemus Phys. Rev. D 57, 2923
(1998)
Standard Model Outer Z?
5
Search for Excess Energetic Photons from Outer Z?

• Anomalous couplings and new resonances enhance Z?
  production cross-section
• Enhancement occurs for events with energetic
  photons
• Rare in SM Outer Z?
• Excess of high ET photons compared to standard
  model indicates new physics

vs 10 TeV
? 5 TeV
? 1 TeV
Standard Model Outer Z?
6
Current Anomalous Coupling Limits

• Place limits by modeling Z? 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 ? gt 1 TeV
• Determined by Tevatron mass reach.
• Anomalous Coupling f63 lt .083
• At ? 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 http//www-cdf.fnal.gov/physics/ewk/2008/Zgam
ma/
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 Z? study at LHC startup
• Use isolated electron trigger to tag possible Z?
  events.
• Isolate electrons by summing nearby calorimeter
  deposits to check for activity.
• Electron pT gt15 GeV

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 Z? study at LHC startup
• Z? events use non-isolated muon trigger to tag
  possible events.
• Muon pT gt 5 GeV

µ-
9
Photon Reconstruction

• Photons reconstructed from collections of
  associated crystals with energy in ECAL called
  SuperClusters.
• Starts from a seed crystal of gt 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 gt .1 GeV
• All SuperClusters are Photon candidates
• ET gt 10 GeV
• H/E lt .2
• Requires no matched pixel detector hit.

R9
Crystals in Seed Cluster Other crystals
within Supercluster --- Supercluster boundary
10
Photon Identification

• Photon reconstruction begins with
  SuperCluster gt 10 GeV.
• Other particles can create a 10 GeV
  SuperCluster.
• Jets fragmenting primarily to p0
• .001 of jets fake photons ( jets to photons
  10001)
• Electrons
• Photon ID selects reconstructed photons passing
  various quality cuts.
• HCAL lt 10 GeV, near reconstructed photon.
• ECAL lt 10 GeV near reconstructed photon.
• Require lt 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
11
Electron Reconstruction

• Calorimeter Reconstruction
• Create superclusters of ECAL energy to include
  bremmstrahlung photons.
• ET gt 4 GeV
• H/E lt .1
• Tracker Reconstruction
• Require calorimeter deposit matched to
  reconstructed track, ?R lt .15
• pT gt 3 GeV

12
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.
• ?d is distance between end of tracker track and
  beginning of muon track

13
Z? ZJets Event Simulation

• Z? generated with Pythia 6.409
• LO matrix element cross section calculation
• Higher order initial (final) state radiation is
  approximated
• Zjets 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
14
Z? Generator Comparison
Pythia Baur Generator
LO or NLO QCD Matrix Element LO NLO
Anomalous Couplings No Yes
Tunable Yes Yes
Pythia Based Hadronization Yes Interface to Pythia in Progress
Number of Z? Events vs. Photon ET

• Baur Z? Generator
• Developed by Dr. Ulrich Baur (U. Buffalo) et al.
• Calculates NLO Z? cross section using Monte Carlo
• Tunable anomalous couplings new physics
  scale ?
• Accurately models photon ET for outer Z?

Baur SM Outer Z? Pythia SM Outer Z?
Events
15
Comparing Baur to Tevatron Data
Baur

• CDF measures inner outer Z?
• 4.6 0.2 (stat) 0.3 (sys) pb
• 1.2 0.1 (stat) .17 (sys) pb
• D0 measures inner outer Z? as well
• 4.4 .27 (stat) .27 (sys) pb
• All measurements agree with Baur MC predictions
• 4.5 0.4 pb (Inner Outer Z?)
• 1.21 0.1 pb (Outer Z? Only)

2.0 fb-1
Baur
Anomalous Coupling
1.1 fb-1
16
ZJets Background to Outer Z?

• Zjets
• 1 in 1000 jets fragment primarily to p0
• sZJets 251 pb _at_ Tevatron (to leptons)
• sZJets 3700 pb _at_ LHC (to leptons)
• Similar kinematics to Outer Z?
• CDF ZJets pT measurement matches NLO MCFM well.
• MCFM Monte Carlo for Femtobarn Measurement (dev.
  by CDF Collab.)
• Give accurate background prediction for CDF Z?
  measurement
• CMS ZJets will be measured in 200 pb-1
• Expect more Z multiple jets

17
Z? Signal and ZJets Background

• Require electrons, muons and photons to be within
  the tracker and to pass trigger. (-2.5 lt ? lt 2.5)
• Require e ET gt 15 GeV µ pT gt 5 GeV
• Removes poorly reconstructed e and µ.

Starting With 105 Signal 28k Bkg 200pb-1
Z? -gt ee? MC Zjets MC
Z?-gtµµ? MC Zjets MC
200pb-1
200pb-1
µ
e
18
Cut on Dilepton Invariant Mass

• Require dilepton mass near Z peak (70 lt Mll lt
  100)
• Majority of signal Zs are on shell
• Suppresses Inner Z?

Whats Left 85 Signal, 81 21k Background, 75
e
µ
Z? -gt ee? MC Zjets MC
Z?-gtµµ? MC Zjets MC
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

Whats Left 75 Signal, 74 9k Background, 33
Zjets Z?
EM Supercluster
ECAL
g
jet
ECAL HCAL
reject
Supercluster
20
Selecting Signal Photons R9

• Cut on ratio of E3x3 to Esupercluster ( R9)
• EM deposits from Jets will be more spread out.
• Except energetic p0s
• Cut at r9 .90

Whats Left 45 Signal, 43 2k Background, 7.9
Zjets Z?
200pb-1
f
h
reject
21
Selecting Signal Photons Track Isolation

• Count number of reconstructed tracks in a cone
  near the photon with pT gt .5 GeV
• Faked photons have more tracks in .4 ?R cone.
• Cut at Number of Tracks 2

Whats Left 41 Signal, 39 1.5k Background, 5.3
Zjets Z?
reject
22
Selecting Signal Photons ET Isolation

• S (Hcal ET Ecal ET Track pT)/ET, Supercluster
  in annulus around reconstructed photon.
• Faked photons have more energy and tracks in the
  .06 lt ?R lt .4 annulus.
• Cut at (Isolation Sum)/ET .4

Whats Left 25 Signal, 24 480 Background, 1.5
Zjets Z?
reject
23
Selecting Signal Photons Phi Width

• Since p0 -gt ??, faked photons will appear wider
  in phi due to the opening angle between the
  photons.
• Cut at Phi Width lt .015

Whats Left 17 Signal, 16 300 Background, 1.0
Zjets Z?
f
h
reject
24
Selecting Signal PhotonsMinimum ?Rl?

• ?Rl? gt 1.3 cut applied after previous photon
  cuts.
• Further rejects ZJets background and Inner Z?
• Added advantage of avoiding singularity in the Z?
  cross section from photon collinearity
• Improves cross section prediction

Whats Left 9 Signal, 8.5 38 Background .13
e
reject
reject
Zjets Z?
Zjets Z?
µ
25
Cut on ll? Invariant Mass

• Inner Z? events with large photon ET can pass
  ?Rl? cut.
• Dileptonphoton invariant mass will be near Z
  mass.
• Majority of outer Z? will be outside of Z peak.
• Cut at Mll? gt 105 GeV

Whats Left 8 Signal, 7.6 10 Background .035
Zjets Z?
Zjets Z?
e
µ
reject
reject
26
Summary of Signal Background
Signal
Background
Z? (Outer, Anom. Coup.) Zjets
Initial Sample 105 28k
70 lt Mll lt 100 GeV 85 21k
Photon ID Cuts 18 310
?R(l, ?) gt 1.3 9 36
Mll? gt 105 GeV 8 (11) 10
200pb-1
27
Conclusion and Next Steps

• Signal to background is roughly 11 on Z peak.
• 8 (11, with Anom. Coup.) Events 10 Background
• Next Steps
• Improve Signal-to-Background to 21
• Measure ZJets background in data in tandem with
  Z? measurement and apply fake rate
• 200 pb-1 analysis allows SM Z? measurement
• Sensitive to new physics
• Assuming maximum allowed anomalous coupling,
  3 events 1 background (NLO prediction)
• Photon ET gt 100 GeV

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.

_at_ 10 TeV Standard Model Tevatron Limit ? 5
TeV scale
30
Resonance Search Dalitz Plot

• New physics manifests as bands in the Dalitz
  Plot.
• Shown SM Z?
• Higgs -gt Z?
• Would appear as an enhanced segment along the Z
  band.

31
Higgs Branching Ratios

• Higgs branching ratio to Z? is phase space
  heavily suppressed.
• Dominated by vector boson pair production
• Probability for all three particles to be in
  fiducial region small.

32
Selecting Signal Photons ECAL Isolation

• S (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 -gt Ecal Isolation lt
  7.5 GeV

Isolation 7.5 GeV
Zjets Z?
Scaled to 100pb-1
reject
33
Selecting Signal Photons Track ET Isolation

• S (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 -gt Track Isolation
  lt 3 GeV

Isolation 3 GeV
Zjets Z?
Scaled to 100pb-1
reject
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 -gt Tracks lt 3

Tracks 3
Zjets Z?
Scaled to 100pb-1
reject
35
LHC 2009-2010 Expected Yield
Month LHC Status Protons/Bunch Peak Luminosity Integrated Luminosity
1 Beam Commissioning First Collisions
2 Pilot Physics 3x1010 1.2x1030 100-200 nb-1
3 5x1010 3.4x1030 2pb-1
4 5x1010 2.5x1031 13pb-1
5 7x1010 4.9x1031 25pb-1
6 50 ns Bunches 3x1010 4.0x1031 21pb-1
7 50 ns Bunches 5x1010 1.1x1032 60pb-1
8 50 ns Bunches 5x1010 1.1x1032 60pb-1
9 50 ns Bunches 5x1010 1.1x1032 60pb-1
Total 200 300 pb-1