Measurement ofσ(pp ZZ/ZW) with the CMS Experiment at the LHC

D. Austin Belknap

University of Wisconsin – Madison

Preliminary Examination

May 1, 2012

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Outline

• Introduction

• Standard Model

• Electroweak Vector Bosons

• Experimental Setup

• Large Hadron Collider

• Compact Muon Solenoid

• Analysis

• Monte Carlo

• Event Selection

• Final Selection Summary

• Previous Results

• Conclusion

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The Standard Model

• Fermions

• Quarks – u, d, c, d, t, b

• Leptons – e, μ, τ, ν

• Force Carriers

• EM – photon (γ)

• Massless

• Weak – W±, Z

• 80.4 GeV/c and 91.2 GeV/c respectively

• Strong – g

• Massless

• Higgs Boson

• Spin-0 scalar particle

• Responsible for electroweak symmetry breaking

• Not yet observed

• Missing Pieces

• Dark Matter/Energy

• Gravity

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Particle Interactions

• Strong Force

• Carried by the gluons

• Binds quarks into mesons and baryons (e.g. protons/neutrons)

• Keeps atomic nuclei bound together

• Electromagnetic Force

• Carried by photons

• Binds electrons into atomic orbitals

• Binds atoms into molecules

• Weak Force

• Carried by Z and W bosons

• Responsible for radioactive decay

May 1, 2012

Proton Neutron

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Motivation for ZZ/ZW Study

• Standard Model

• Test of standard model weak gauge couplings

• Higgs Searches

• Higgs couples to weak bosons

• Standard Model ZZ production is a Higgs background

May 1, 2012

Higgs Branching Ratios

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Beyond the Standard Model

• Effective EWK Lagrangian

• Allows experimental results to be interpreted in a model-independent fashion

• Standard Model Values

• λ = 0, κ = 1, g1V = 1

• Deviations expressed as Δλ, Δκ, Δg1V

• No ZZV vertex – presence of a ZZZ vertex would be example of an anomalous trilinear gauge coupling (TGC)

• New Particles

• New particles could decay to electroweak vector bosons

May 1, 2012

V = Z, γ

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Luminosity and Cross-Section

• Cross-Section (σ)

• Describes the probability of a process occurring

• Has units of area (1 barn = 10-24 cm2)

• Luminosity (L)

• Describes the intensity of a particle beam

• Number of events (interactions) per unit area per unit time

• Integrated Luminosity (Lint) is the luminosity integrated over time

• Number of events per unit area

• Has units of inverse area (barn-1)

• Number of Events (N)

• N = σ x Lint

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Signal: ZZ/ZW Boson Production

• Leading Order Processes

• WZ – s/t-channel

• ZZ – t/u-channel

• At 7 TeV (LHC 2010-2011)

• σ(ppZZ)

• 6.4 ± 0.6 pb

• σ(ppWZ)

• 19.790 ± 0.088 pb

• At 8 TeV (LHC 2012)

• σ(ppZZ)

• 7.53 ± 0.01 pb

• σ(ppWZ)

• 22.02 ± 0.04 pb

May 1, 2012

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W and Z Boson Decays

May 1, 2012

• Z Decays

• fermion – anti-fermion pair

• Same Flavor

• e.g. positron-electron

• Decays to a pair of charged leptons 3 x (3.4%) of the time

• W Decays

• fermion – anti-fermion pair

• Lepton/matching neutrino

• Quarks – up-type/down-type

• Decays to a charged lepton-neutrino pair 3 x (10.8%) of the time

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Backgrounds

• ZZ Signal

• 6.4 pb

• 120k events

• WZ (background to ZZ)

• 19.8 pb

• 400k events

• ttbar

• 157 pb

• 3.1M events

• Z+Jets

• 3048 pb

• 61M events

May 1, 2012

*Event estimates are given at 20 fb-1

Cross section are given at COM 7 TeV

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The Large Hadron Collider• Overview

• Proton-proton collider, 27 km in circumference

• Located at CERN near Geneva, Switzerland

• Four Detectors: CMS, ATLAS, ALICE, LHCb

• Center of Mass Energy (COM)

• 14 TeV (Design)

• 7 TeV 2010-2011

• 8 TeV 2012

May 1, 2012

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Proton-Proton Collisions

May 1, 2012

Design 2010 2011 2012

Beam Energy (TeV) 7 3.5 3.5 4

Bunches/Beam 2835 368 1380 1380

Proton/Bunch(1011) 1.15 1.3 1.5 1.5

Peak Lumi.(1032 cm-2 s-1) 100 2 30 60

Integrated Lumi. (fb-1) 100/yr 0.036 6 15*

Pile-Up 25 ~1 20 30

Pile-Up – the number of proton interactions occurring during each bunch crossing

*expected values

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Compact Muon Solenoid

• Overview

• Located at Point-5 along the LHC ring in Cessy, France

• One of the two general-purpose detectors (CMS and ATLAS)

• Physics Goals

• Search for the Higgs

• Beyond the Standard Model Phenomena

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CMS Sub-Detectors

May 1, 2012

B-Field = 3.8 Tesla

Calorimeters contained within the solenoid

proton beam

Endcap

Barrel

Mass: 12,500TDiameter: 15.0mLength: 21.5m

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CMS Geometry

May 1, 2012

η

+z-zϕ

• Pseudorapidity (η) – Lorentz invariant quantity used to describe the angle of a particle's trajectory relative to the beam axis.

• Phi (ϕ) – Polar angle of a particle’s trajectory

• Transverse Momentum (pT) – The component of a particle’s momentum perpendicular to the beam

pT

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Tracker

• Overview

• Magnetic field (4T) bends the paths of charged particles

• Records paths of charged particles

• Measures pT, and charge

• Resolution

• pT Res. – δpT/pT ≈ (15 pT (TeV) 0.5) %⊕

• Pixel Detector

• Three barrel layers with radii of 4.4, 7.3, and 10.2 cm

• Two endcap disks

• Pixel size: 100 x 150 μm2

• Strip Detector

• Tracker Inner Barrel: 20 < r < 55 cm

• Min. cell size: 10 cm x 80 μm

• Tracker Outer Barrel: r > 55 cm

• Max. cell size: 25 cm x 180 μm

May 1, 2012

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Electromagnetic Calorimeter• Specifications

• PbWO4 scintillating crystals (B: 61200, EC: 7324)

• Resolution σ/E

• (Stochastic) + (Intrinsic) + (Noise)

• Radiation Length = 0.89 cm

• Moliere radius = 2.2 cm

• Light-yield: 30 photons/MeV

• Granularity (22 x 22 mm2)

• Δη x Δφ = 0.0175 x 0.0175 in Barrel

May 1, 2012

• Purpose

• Measures the energy of EM-interacting particles (e.g. e/γ)

Energy Resolution

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Hadronic Calorimeter

• Overview

• Measures the energy of, primarily, neutral hadrons (e.g. π0)

• Used for triggering

• Layered brass/scintillator tiles

• Quartz and steel in the endcap

• Three components within the solenoid

• Hadron Barrel (HB) – |η| < 1.4

• Hadron Endcap (HC) – 1.3 < |η| < 3.0

• Hadron Forward (HF) – 3.0 < |η| < 5.0

May 1, 2012

Energy Resolution• Resolution (σ/E)2

• (Stochastic) + (Intrinsic)

• |η| < 3

• 3 < |η| < 5

222

%5.5%115

E

E

222

%11%280

E

E

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Muon Spectrometer• Measures the pT and tracks of muons

• Improves resolution of high pT muons

• Located outside the magnet

• Interleaved with iron return yoke

• ~2 T magnetic field

• Drift Tubes (DT) – Barrel Section

• Cathode Strip Chambers (CSC) – Endcaps

• Resistive Plate Chambers (RPC)

May 1, 2012

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Level-1 Trigger

May 1, 2012

• Requirements

• 109 interactions/sec. 0.5 MB per bunch-crossing

• Must reduce the data rate while keeping interesting events

• Must make decisions every 25 ns within 4 μs

• L1 Trigger output – 100 kHz

• Achieved using custom electronics with hardware-based algorithms

• Triggers

• Calorimeter

• Electrons/Photons

• Jets

• Missing Energy

• Muons

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High-Level Trigger

• Purpose

• Further reduces the data rate

• 100kHz 300 Hz

• Uses commercial processors, and runs software-based algorithms

• HLT can run more robust, detailed algorithms

• Triggers Used for This Study

• Require two muons

• Each with pT > 7 GeV/c

• Require two electrons

• One electron with pT > 17 GeV/c

• Plus, one electron with pT > 8 GeV/c

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Analysis Outline

• Particle Reconstruction

• Muons

• Electrons

• Taus

• Simulation of pp ZZ

• Monte Carlo Samples at 7 TeV

• Normalize to 20 fb-1

• Initial Selection

• Reconstruct ZZ candidates

• Signal Selection

• Isolate ZZ signal from backgrounds

• Final Selection/Comparison to Prior Results

May 1, 2012

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Monte Carlo Generators

• MadGraph

• Matrix-element generator

• Standard Model, Higgs, MSSM, etc.

• Multiparton processes (necessary for proton collisions)

• Used to calculate hard interactions

• Pythia

• Generates events given MadGraph matrix-elements

• Initial/final state radiation

• Parton showers

• Beam Remnants

• Hadronization

May 1, 2012

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Monte Carlo Simulation

May 1, 2012

• Generate simulated proton-proton collision events

Madgraph + Pythia

• Simulate the detector response• Passage of particles through matter

GEANT

• Event selection and reconstruction• Produce ZZ/ZW events from MC

CMSSW

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Particle Flow Algorithm

• Combines information from all CMS sub-detectors to produce particle candidates

• charged/neutral hadrons, photons, muons, and electrons

• These particles, in turn, are used to produced higher-level observables

• e.g. Tau ID, lepton Isolation, Missing Energy, Jets, etc.

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Muon Reconstruction

• Standalone Muon

• Reconstructed from tracks in the muon spectrometer only (CSC/DT)

• Tracker Muon

• Tracks with pT > 0.5 GeV/c and p > 2.5 GeV/c are considered as possible muon candidates

• Must match at least one muon segment (CSC/DT)

May 1, 2012

• Global Muon

• Begins with standalone muon track, and fits with a tracker muon

• Accounts for energy loses as the muon travels through the detector

• Improves momentum resolution over “track-only reconstruction” at pT > 200 GeV/c

Muon pT Resolution

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Electron Reconstruction

• ECAL Driven Algorithm

• Finds ECAL superclusters with ET > 4 GeV

• A supercluster is a group of one or more clusters of energy deposits in the ECAL

• Takes into account the spread of energy in φ due to electrons radiating in the tracker from the B-field

• Matches superclusters to track seeds (pairs or triplets of hits)

• Electron tracks produced from these seeds

May 1, 2012

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Tau Reconstruction

• Tau Decays

• Hadronic Decay – Charged mesons (usually π±), up to two π0, and a neutrino

• Occurs ~ 2/3 of the time

• Tau reconstruction

• Relies on Particle-Flow (PF) objects

• Hadron Plus Strips (HPS)

• Starts with a PF Jet, and looks for possible tau decays within the jet

• Uses strips of electromagnetic particles to account for material effects

May 1, 2012

Single-Prong Three-Prong Single-Prong plus Strip

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ZZ Signal Characteristics

• ZZ

• Z1 (leading pT Z) μμ, ee

• Z2 (second Z) ττ, μμ, ee

• Taus

• Hadronic (τ) or leptonic (e,μ) decays

• Z2 ττ, τe, τμ, eμ

May 1, 2012

• Electrons

• ECAL hit matched with track

• Muons

• Global Muons – reconstructed from muon system + tracker

• Taus

• Leptonic + hadronic decays

Final-State Particles Diboson Channel

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Signal and Backgrounds (ZZ)

• Signal

• ZZ

• σ = 6.4 pb

• 1.2M generated events

• Background

• Z+Jets

• σ = 3048 pb

• 22M generated events

• ttbar

• σ = 157 pb

• 3.6M generated events

• ZW

• σ = 19.8 pb

• 1.2M generated events

May 1, 2012

All MC samples are normalized to 20 fb-1

All MC samples are generated at 7 TeV COM

All MC samples are generated using MadGraph+Pythia

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Initial Selection (ZZ)

May 1, 2012

• Require four lepton candidates

• Require two leptons (ee, μμ) with pT > 20 GeV and pT > 10 GeV

• Reconstructs the leading Z (Z1)

• Require two more leptons

• Reconstructs the second Z (Z2)

Lepton Pair

L1 pT (GeV) Min.

L1 |η| Max.

L2 pT (GeV) Min.

L2 |η| Max.

ee > 7 < 2.5 > 7 < 2.5

μμ > 5 < 2.4 > 5 < 2.4

ττ > 20 < 2.3 > 20 < 2.3

μτ > 10 < 2.4 > 20 < 2.3

eτ > 10 < 2.5 > 20 < 2.3

eμ > 10 < 2.5 > 10 < 2.4

• Initial Selection efficiency

• 1.2M generated ZZ events

• 890k events pass initial selection

• 74% efficiency

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Initial Selection for Taus/Electrons

May 1, 2012

• Muons and Electrons can fake Taus

• Apply muon and electron vetoes to all taus

• Remove tau candidate if it passes electron or muon identification

• A lepton pair from a Z decay that fakes a Tau will have an invariant mass peak at 91.2 GeV.

• Apply a mass cut to the second Z to avoid the peak

• 30 < Z2 Mass < 80 GeV

• All electrons are required to have 0 missed tracker hit

Signal Background

ZZ Top Z+Jets

ZW

Initial Pre-Selection 975 3163.3 12838.7 248.8

Z2 Mass/Vetoes 330.1 2188.2 9614.8 102.9

Electron Missed Hits 323.9 1939.2 7736.7 92.9

Reject Reject

Z2 ττ Invariant Mass

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Lepton Isolation

May 1, 2012

• Particle-Flow Relative Isolation

• Particles reconstructed with PF Algorithm

• Sums particle energies within a ΔR < 0.4 cone around the lepton axis

• Includes all charged particles

• Photons/neutral hadrons with ET > 0.5 GeV

• Inner veto cone with ΔR < 0.01 for photons and neutral hadrons

• Pile-up Correction

• Algorithm calculates energy density (ρ) due to Pile-Up

• Applied to electrons and muons

• IPFrel < 0.2

Reject

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Effect of Lepton Isolation

May 1, 2012

Signal Background

ZZ Top Z+Jets ZW

323.9 1939.2 7736.7 92.9

Signal Background

ZZ Top Z+Jets ZW

257.0 34.2 945.2 23.8

Before Lepton Iso.

Z1 μμ Invariant MassBefore Iso. After Iso.

After Lepton Iso.

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Tau Isolation

May 1, 2012

• Uses particles reconstructed with Particle Flow Algorithm

• Sums particle energies within a ΔR < 0.5 cone around the tau axis

• Sums hadron candidate energies

• pT > 0.5 GeV

• Must be within 0.2 cm of the primary vertex along z-axis.

• Sums photon candidate energies

• ET > 0.5 GeV

• Iτabs < 2.0

Reject

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Effect of Tau Isolation

May 1, 2012

Signal

Background

ZZ Top Z+Jets ZW

257.0 34.2 945.2 23.8

Signal Background

ZZ Top Z+Jets ZW

225.4 0.7 276.3 6.1

Z1 μμ Invariant MassBefore Iso. After Iso.

Before Tau Iso. After Tau Iso.

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Z1 Mass Requirement

May 1, 2012

Z1 μμ Invariant Mass • Applied to channels where Z1ee, μμ

• 60 < Z1 Mass < 120 GeV

Signal Background

ZZ Top Z+Jets ZW

225.4 0.7 276.3 6.1

Signal Background

ZZ Top Z+Jets ZW

220.3 0.0 138.3 5.5

Before Mass Requirement

After Mass Requirement

Reject

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Z2 Mass Requirement

May 1, 2012

• Applied to channels where Z2ee, μμ

• 60 < Z2 Mass < 120 GeV

Signal

Background

ZZ Top Z+Jets ZW

220.3 0.0 138.3 5.5

Z2 ee Invariant Mass

Signal Background

ZZ Top Z+Jets ZW

202.0 0.0 10.7 3.3

Z1 μμ Invariant Mass

Before Mass Req. After Mass Req.

Reject

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ZZ Signal Selection Summary

May 1, 2012

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Applied to: ZZ Top Z+Jets ZW

Initial Selection – 323.9 1939.2 7736.7 92.9

Lepton Isolation All leptons (not Taus) 257.0 34.2 945.2 23.8

Tau Isolation Z2 ττ, eτ, μτ 225.4 0.7 276.3 6.1

Z1 Mass Cut All Channels 220.3 0.0 138.3 5.5

Z2 Mass Cut Z2ee, μμ 202.0 0.0 10.7 3.3

• Normalized to 20 fb-1

• 7 TeV COM

• Significance• S/sqrt(S+B) = 13.7

• S/sqrt(B) = 54.0

• Signal Selection Efficiency• 62.4%

Signal Background

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Final Selection Summary

• ZZ Monte Carlo

• 202 signal events

• 14 background events

• Anomalous TGC

• A ZZZ vertex would proceed as an s-channel process

• Would lead to a broad increase in the 4l Invariant mass distribution

May 1, 2012

4μ Invariant Mass 4l Invariant Mass with ZZZ Vertex

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ZZ μμμμ Cross-Section

May 1, 2012

• Example Calculation from ZZ μμμμ Monte Carlo

• Agrees with the theoretical prediction of σ = 6.4 ± 0.6 pb at 7 TeV

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Previous Results: ATLAS/CDF• ATLAS

• 1.02 fb-1 at 7 TeV

• σ(ppZZ)* (2011)

• 8.4 +2.7/-2.3 +0.4/-0.7 ± 0.3 pb

• σ(ppZW)* (2012)

• 20.5 +3.1/-2.8 +1.4/-1.3 +0.9/-0.8 pb

• Tevatron (CDF)

• COM of 1.96 TeV

• σ(pp-barZZ)* (2011)

• 6 fb-1

• 2.3 +0.9/-0.8 ± 0.2 pb

• σ(pp-barZW)* (2012)

• 7.1 fb-1

• 3.93 +0.6/-0.53 +0.59/-0.46 pb

May 1, 2012

*(cross-section) ± (stat.) ± (syst.) ± (lumi.)

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CMS Previous Results

• Measurement of the WW, WZ and ZZ cross sections at CMS

• 2011 Result

• 1.1 ± 0.1 fb-1 at 7 TeV

• 4μ, 4e, 2e2μ, 2l2τ ZZ channels

• Recorded 9 events (ZZ)

• σ(ppWZ)*

• 17.0 ± 2.4 ± 1.1 ± 1.0 pb

• σ(ppZZ)*

• 3.8 +1.5/-1.2 ± 0.2 ± 0.2 pb

• Scaled to 20 fb-1

• 9 events 163.64 ± 54.5 events

• Consistent with 216.0 (S+B) events

• Significant improvement in statistics expected with 20 fb-1

May 1, 2012

*(cross-section) ± (stat.) ± (syst.) ± (lumi.)

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Conclusion

• Summary

• At 7 TeV with 20 fb-1, 202 ± 14 ZZ signal events are expected

• Scales to 238 ± 15 ZZ events at 8 TeV

• 6.3% statistical error

• 2011 CMS result at ~35% statistical error

• 2011 ALTAS result at ~30% statistical error

• Signal selection eliminates a considerable amount of the backgrounds: an estimated 14 background events remain

• Next Steps

• Implement WZ selection

• Run over Monte-Carlo for WZ signal and backgrounds

• Run over Monte-Carlo at 8 TeV

• Refine ZZ event selection

• Get data. Lots of data.

May 1, 2012

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Backup Slides

May 1, 2012