Z Production at CMS with vs 10 TeV

1
Z? Production at CMS with vs 10 TeV

• Lindsey Gray
• University of Wisconsin at Madison
• Preliminary Exam

2
Outline

• Standard Model
• Z? Production
• Large Hadron Collider
• Compact Muon Solenoid Experiment
• Z? Event Simulation Analysis
• Previous Results
• CMS MC Studies
• Summary and Future Steps

3
The Standard Model

• 24 fundamental matter particles anti-particles
• Spin ½
• Describes 3 forces
• Mediated by spin 1 particles
• Z photon do not couple
• Tree level ZZZ ZZ? cross sections are zero
• Higgs boson
• Not experimentally found
• Gives mass to other SM particles

4
Z? Production

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

Inner Z?
l e, µ
Outer Z?
Signal ?
5
New Physics Accessible With Z?
If excess is observed, possible causes are

• 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.
6
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?
• 4.3 pb per lepton channel
• ? 5 TeV
• 3.4 pb per lepton channel
• ? 1 TeV

vs 10 TeV
? 5 TeV
? 1 TeV
Standard Model Outer Z?
Plots generated with program described in U.
Baur, T. Han, J. Ohnemus Phys. Rev. D 57, 2923
(1998)
7
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?
8
Previous Z? Studies

• Experiments based at FNAL
• Tevatron Collider
• Proton-antiproton
• vs 1.96 TeV
• Both make Z? cross section measurement
• Place exclusions on Z? anomalous coupling
  strength

9
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/
10
The Large Hadron Collider
11
Proton Interactions at LHC
(5 TeV, 2009-10)
7
Design
2009-10 Luminosity lt 1032 cm-2s-1
2009-10 Expect pb-1
Luminosity L particle flux/time Interaction
rate Cross section ? effective area of
interacting particles sZ? 6.7 pb
Plan analysis for 200 pb-1
12
The Compact Muon Solenoid
13
Particle Detection in CMS

• Photons
• Isolated deposits in ECAL
• Quiet HCAL
• No nearby track
• Electrons
• Isolated deposits in ECAL
• Quiet HCAL
• Nearby Track
• Muons
• Track in Tracker and Muon System
• Minimum Ionizing in Calorimeters

14
Silicon Tracker

• Used to measure curvature of charged particles
  for ? lt 2.5
• Si pixel detectors closest to IP

• Si strip detectors in outer barrel and endcap
  discs
• 200m2 of Si strip detectors
• Immersed in 3.8T magnetic field

• L 1.2m radius
• Resolution
• dMZ .566, 45 GeV Muon Tracks
• Used to isolate photons, electrons and muons

T
15
Electromagnetic Calorimeter

• 76,000 PbWO4 crystals
• High density (8.6 g cm-3)
• Small radiation length (9 mm)
• 26 ?0 per crystal
• Small Moliere radius (22 mm)
• Coverage for ? lt 3
• Used to find ? and e
• Low noise
• Resolution
• dMZ 1.3 for 45 GeV Electrons
• Calorimeter only

Noise
Intrinsic
Stochastic
16
Hadronic Calorimeter

• Sampling calorimeter for measuring jets and
  missing energy
• Hermetic coverage for ? lt 5.0
• Brass scintillator sampling calorimeter for ?
  lt 3.0
• Steel quartz fiber forward calorimeter for 3.0
  lt ? lt 5.0
• Completes missing energy measurement
• Scintillating tile outside magnet
• Samples uncontained showers
• Used to isolate leptons and photons

Barrel Endcap resolution
HF
Forward resolution
17
Muon System

• 3 Technologies
• Resistive Plate Chambers
• 0 lt ? lt 2.5
• Cathode Strip Chambers
• 0.9 lt ? lt 2.5
• Drift Tubes
• 0 lt ? lt 1.2
• Operates in return field
• 1.8T
• Protected by calorimeters
• 10 ? before muon system
• .8 standalone resolution for 45 GeV muons
• Tracks in muon system matched to tracker
• High efficiency for Z -gt µµ

CSC
DT
RPC
18
Trigger

• Level 1 Trigger
• Custom hardware operating at crossing frequency
• 3µs decision latency
• 100 KHz output
• High Level Trigger
• Run on commodity computing farm
• Use robust fast versions of offline
  reconstruction.
• Better rejection
• Improved measurement of Level-1-tagged particles
• Output rate of 100 Hz

_at_ 1034 cm-2s-1
19
Level One Trigger

• Triggers
• e/?
• Jet
• Missing ET
• Muon

20
L1 e/? Trigger
21
L1 Muon Trigger

• RPC muon trigger
• Match RPC hits to trigger patterns.
• CSC DT Chamber Level
• Combine hits into segments and find segment
  directions
• CSC DT muon triggers
• Look for set of segments in muon system which
  have the same ?.
• Measure ?f between segments.
• Global muon trigger
• Combines results of subsystems.
• Best four L1 muons chosen.

CSCs 0.9 lt ? lt 2.4
DTs ? lt 1.2
22
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

23
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-
24
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
25
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 (jetsphotons 10001)
• Electrons
• Photon ID selects reconstructed photons passing
  various isolation cuts.
• HCAL lt 10 GeV, near reconstructed photon.
• ECAL lt 10 GeV near reconstructed photon.
• Require lt 3 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
26
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

27
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
  extrapolation and beginning of muon track

28
Z? Perturbation Theory

• M2 is related to the probability of a hard
  scatter interaction.
• Order is determined by power of coupling constant
  kept in S matrix expansion
• Leading Order (LO) Outer Z? Matrix Element
• Only contributing graph is photon emission
• Next-to-Leading Order (NLO) Outer Z? Matrix
  Element
• Radiative Corrections
• Vertex Correction

29
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
30
Z? Generator Comparison

• 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?
31
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
32
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

33
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
34
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
35
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
36
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
37
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
38
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
39
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
40
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
41
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
42
Summary of Signal Background
Signal
Background
Zjets SM Z? Anom. Coup.
200pb-1
43
Conclusion and Next Steps

• Signal to background is roughly 11 on Z peak.
• 8 (11, with Anom. Coup.) Events 10 Background
• Next Steps
• Optimize cut based analysis
• Implement multivariate analysis
• Will improve Signal-to-Background to 21
• 200 pb-1 analysis allows SM Z? measurement
• Sensitive to new physics
• Assuming maximum allowed anomalous coupling,
  3 events 1 background (NLO prediction)
• Using current analysis but require photon ET gt
  100 GeV

44
Backup Slides
45
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
46
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.

47
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.

48
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
49
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
50
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
51
LHC 2009-2010 Expected Yield