searches for vector boson scattering at the lhc

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Searches for Vector Boson Scattering at the LHC Aaron Webb Mentors: Al Goshaw, Andrea Bocci

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Searches for Vector Boson Scattering at the LHC. Aaron Webb Mentors: Al Goshaw, Andrea Bocci. LHC / ATLAS Introduction. The Large Hadron Collider accelerates protons to high energies and focuses them to head-on collisions. Several layers of detectors record the results - PowerPoint PPT Presentation

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Page 1: Searches for Vector Boson Scattering at the LHC

Searches for Vector Boson Scattering at the LHC

Aaron WebbMentors: Al Goshaw, Andrea Bocci

Page 2: Searches for Vector Boson Scattering at the LHC

2

LHC / ATLAS Introduction

7/31/2014

The Large Hadron Collider accelerates protons to high energies and focuses them to head-on collisions. Several layers of detectors record the results Particle type, energy, and location are all

recorded Using data collected at in 2012

Center of mass energy 8 TeV Integrated luminosity (cross section) of 20.3 fb-1

σ L*dt = number of events producedʃ A Monte Carlo event generator, Sherpa, is used

to simulate event collisions Allows us to compare theory and data Access to “truth” values – can compare to

reconstructed data http://www.atlas.ch/photos/lhc.html

Page 3: Searches for Vector Boson Scattering at the LHC

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Vector Boson Scattering (VBS) Introduction

7/31/2014

A vector boson is a particle with spin 1 photons, W+/- and Z bosons VBS is when two vector bosons scatter off

of one another Vector Boson scattering allows us to:

Test electroweak symmetry breaking Better understand the Higgs mechanism Look for physics beyond the standard

model

Page 4: Searches for Vector Boson Scattering at the LHC

Motivations

7/31/2014

VBS can be used to study spontaneous electroweak symmetry breaking Complete EWK symmetry is broken at

low energies, replaced by EM subgroup Believed to be a result of the Higgs mechanism

Process by which W and Z bosons acquire mass Unbroken part of the symmetry results in a massless

photon

Non-unitarity of WW scattering Without the Higgs, at high energies the probability of WW scattering

becomes nonphysical (>1)

P >> 1

Unitarize WW scattering

𝑆𝑈 (2 ) ×𝑈 (1 )→𝑈 (1)

EWK EM

Page 5: Searches for Vector Boson Scattering at the LHC

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W± and Z Bosons

7/31/2014

Force carriers of the electroweak force W can have a charge of +/-1, while the Z is neutral Both are spin 1

W and Z bosons bosons are massive (80.4 and 91.2 GeV), and short-lived (~10^-25 s) Have to look at the decay products to study them

Use kinematic selection criteria to determine which particles (leptons) came from W or Z decay

isolate the relevant events, and reconstruct them Use relativistic mechanics, conservation laws E.g. W decays to 1 lepton, 1 neutrino

Look for events with a high energy lepton, and missing transverse energy of the neutrino

Page 6: Searches for Vector Boson Scattering at the LHC

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Zg Channel

7/31/2014

Z boson decays into a fermion and its antiparticle In this study the muon decay channel is used

p + p -> Z(m+ m-) + g + 2 jets

Things we’re looking for: two high energy jets Two oppositely charged muons High Pt photon

Studying the leptonic decay channel Z->mumu

Others in the group studying electron and neutrino channel, as well as W lepton channels

Page 7: Searches for Vector Boson Scattering at the LHC

Et=37 GeV

7/31/201477

g

e

e

g ee

e

e

g

Et=51 GeV

Et=30 GeV

M(e,e) =91.2 GeV

Example of ISR event

http://www.atlas.ch/photos/lhc.html

Page 8: Searches for Vector Boson Scattering at the LHC

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Backgrounds

7/31/2014

QCD processes (i.e. strong force interactions) represent the main background of this channel Same final state as VBS Very large compared to signal

Effectively differentiating between signal and background is essential

Example QCD Process VBS Process with the same final state

Page 9: Searches for Vector Boson Scattering at the LHC

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MVA – Multi-variant Analysis

7/31/2014

Multi-variant analysis techniques are used to optimize signal efficiency with respect to a given background Multi-dimensional methods can often allow for better

background separation than looking at single variables individually

TMVA is an MVA program within a root environment Giving TMVA a signal sample and a background sample will

“train” it TMVA develops a weighting algorithm

Each event is given a probability of being signal vs. background

Page 10: Searches for Vector Boson Scattering at the LHC

10 7/31/2014Image generated by Kushal Byatnal in TMVA

Page 11: Searches for Vector Boson Scattering at the LHC

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VBS/QCD Comparison

7/31/2014Pt(m+ m- )/Pt(g ) GeV

Photon Pt (GeV)

M(m+ m- ) GeV

Comparisons can tell us which variables to consider in the MVA analysis Differences can be used to

differentiate between signal and background

Page 12: Searches for Vector Boson Scattering at the LHC

12 7/31/2014GeV

GeV

Page 13: Searches for Vector Boson Scattering at the LHC

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Event Classification

7/31/2014

Trying to see which processes are most common, and therefore most relevant Looking at final state quarks

Use Sherpa MC’s truth level data to classify final state quarks Pdg: particle classification codes

Negatives correspond to antiparticles (e.g. -2 is ū)

Down 1 Bottom 5

Up 2 Top 6

Strange 3 Gluon 21

Charm 4

Page 14: Searches for Vector Boson Scattering at the LHC

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Final state Quark comparison

7/31/2014

• Truth level information used to identify particles

• Gluons account for a major fraction of QCD events

• ~60% contain gluons• Looking into whether

we can distinguish quark jets from gluon jets

VBS

QCD

Page 15: Searches for Vector Boson Scattering at the LHC

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Going Forward

7/31/2014

Study still in early phases Analyze full MC sample will next, followed by the real data

set Use MVA to develop QCD/VBS discrimination

Look for more variables to differentiate between VBS and background processes

Pursue polarization studies Potential source of QCD/VBS discrimination Can be used to study Z boson structures and couplings

Page 16: Searches for Vector Boson Scattering at the LHC

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Summary

7/31/2014

VBS allows us to study central features of the standard model Test couplings that are sensitive to the predictions of

electroweak symmetry breaking Better understand the Higgs mechanism behind EWKSB Search for anomalous gauge couplings indicative of physics

beyond the standard model E.g. direct coupling of the photon to the Z would be indicative of

some internal structure within the Z

Despite large backgrounds, lepton VBS channels appear to be good candidates for studying these key features of the SM, and search for new physics

Page 17: Searches for Vector Boson Scattering at the LHC

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References

7/31/2014

Kuss, I., and E. Nuss. "Gauge Boson Pair Production at the LHC: Anomalous Couplings and Vector Boson Scattering." The European Physical Journal C 4.4 (1998): 641-60. Web.

Djouadi, Abdelhak. "The Anatomy of Electroweak Symmetry Breaking." Physics Reports 457.1-4 (2008): 1-216. Web.

Feynman Diagrams created using JaxoDraw

Page 18: Searches for Vector Boson Scattering at the LHC

Backup Slides

Page 19: Searches for Vector Boson Scattering at the LHC

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Motivations

7/31/2014

Higgs is necessary for massive vector particles (W and Z bosons) explain their mass (extra DOF in longitudinal direction)

Goldstone’s theorem: “A theory with spontaneous symmetry breaking must have a massless scalar particle in its spectrum.” This massless scalar particle is a Higgs (not the SM Higgs) spontaneous symmetry breaking in EWK

Non-unitarity of WW scattering Cross section calculated from Feynman Diagrams violates unitarity Unitarized by the Higgs

Page 20: Searches for Vector Boson Scattering at the LHC

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ATLAS Detector

7/31/2014

Muon detection:• Tracking detector

• Charged particles bend in the magnetic field

• Muon chambers

Photon detection:• Electromagnetic

calorimeter

Page 21: Searches for Vector Boson Scattering at the LHC

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Z Boson Information

7/31/2014

Branching Ratios W:

Electron/neutrino: 10.46% Muon/neutrino: 10.5% Tau/neutrino: 10.75% Hadrons: 68.32%

Z: 20.5% neutrinos 10.2% Leptons

3.4% for each, electrons, muons and taus 69.2% hadrons

Page 22: Searches for Vector Boson Scattering at the LHC

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Lepton Selection

7/31/2014

Require two oppositely charged muons

Et > 25 GeV |η| < 2.4

Lorentz invariant angle between the beam and the particle

Muon-Muon separation ΔR > 0.3 Measured as

PtCone30/Pt < 0.15 Isolation cut

Muon+muon invariant mass > 40 GeV

Misc. corrections

η = -ln[tan(θ/2)]

Pseudorapidity as a function of θ

Page 23: Searches for Vector Boson Scattering at the LHC

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Photon Selection

7/31/2014

Et > 15 GeV

Et cone < 4 GeV Isolation cut

|η| < 2.37 Photon-Muon separation ΔR > 0.7 Require the photon to be well-identified and isolated

from other particles• Narrow energy cluster, with no/small energy leakage into hadronic

calorimeter• Cut on shower shape variables to discriminate g from jets and 0

• 0 -> g + g

Page 24: Searches for Vector Boson Scattering at the LHC

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Jet Selection

7/31/2014

pt > 30 GeV |η| < 4.5 jet vertex fraction cut check overlap with photons check overlap with electrons Misc. Corrections

veto jets if is LOOSERBAD BCH cleaning

Page 25: Searches for Vector Boson Scattering at the LHC

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Event Selection

7/31/2014

• Difference in jets selection is unsurprising

• Different pileup weights come from different MC generations (MC12a vs. MC12 b)

• Different mu values

• Unexpected differences between the object selection

• Have to look at kinematics in more detail

VBS QCDInitial events 10000 10000PileUp weights 10366.7 13508.3Misc. Corrections 10261.4 13340.6Trigger 8162.17 8640.27Muon Selection 3455.73 2642.91M(ll)>40GeV 3452.17 2642.91Trigger matching (muons) 3475.7 2670.62One good photon with Et > 15 GeV 863.345 450.38Njet>=2 534.767 19.364Mjj of leading jets >150GeV 370.379 10.7657Mjj of two leading jets >500GeV 193.744 0Delta rapidity of two leading jet >2.4 179.587 0

veto events with IsBadTightBCH jets 0 0

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Object Selection

7/31/2014

Muon SelectionVBS QCD

# of events

Percent cut

# of events

Percent cut

no cut 20189 0 17175 0.00author (Staco CB) 18077 10.46 15224 11.36

quality (tight) 18077 0 15224 0.00

MCP 17845 1.15 15038 1.08

eta requirement 17700 0.72 14848 1.11

pt > 25 GeV 12412 26.19 9000 34.05

z0 requirement 12365 0.23 8878 0.71

d0 significance 12308 0.28 8843 0.20

Track isolation 12046 1.30 8675 0.98

Total 40.33% 49.49%

• Major differences:– Pt cut (26% vs. 34%)– Z0 (0.23% vs. 0.71%)– Eta (0.72% vs.

1.11%)

Page 27: Searches for Vector Boson Scattering at the LHC

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Major differences: Photon:

Ambiguity resolver (0.56% vs. 0)

Loose ID cut (1.39% vs. 0.39%)

Jets: Pt cut (72% vs. 89%) LOOSERBAD (0.99% vs.

0.45%) BCH cleaning (0.58% vs.

0.16%)

Photon Selection   VBS Percent  QCD Percent no cut 141302 0.00 97701 0.00Et > 15 GeV 7520 94.68 4325 95.57quality bit 7472 0.03 4295 0.03eta range 6754 0.51 3833 0.47photon cleaning 6752 0.00 3830 0.00Ambiguity resolver 6729 0.56 3827 0.00Loose ID cut 4764 1.39 3444 0.39Overlap removal 4037 0.51 3127 0.32Tight photon ID 3205 0.59 2472 0.67isolation 2763 0.31 2270 0.21Total 98.59 97.68

Jet Selection   VBS Percent  QCD Percent no cut 80607 0.00 58187 0.00pt > 30 GeV 22104 72.58 6202 89.34eta 22084 0.02 6197 0.01jet vertex fraction 20476 1.99 4978 2.09overlap photons 19173 1.62 4171 1.39overlap electrons 19126 0.06 4147 0.04LOOSERBAD 18330 0.99 3888 0.45BCH cleaning 17861 0.58 3797 0.16Total 77.84 93.47

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Page 29: Searches for Vector Boson Scattering at the LHC

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Final state Quark comparison

7/31/2014

VBS

QCD

• Truth level – before any cuts• Cuts used:

• ISR• Invariant mass >

182 GeV• dr_egv>0.2 &&

abs(eta_gv)<2.47 && abs(eta_ev)<2.7 && abs(eta_nv)<2.7

• Invariant mass jj > 500 GeV

• JJ invariant mass cut not applied for QCD

• Only 2 events within the QCD sample pass

Page 30: Searches for Vector Boson Scattering at the LHC

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Top 10 Processes

7/31/2014

VBS QCD1. ud 58.39% ug 27.31%2. dđ 10.63% gg 8.49%3. dc 7.55% uū 5.90%4. uū 6.36% gū 5.54%5. uu 4.67% cc* 4.80%6. uc* 4.07% dg 4.80%7. ds* 3.87% uu 3.69%8. us 2.18% đg 3.6959. ūđ 1.19% gs 2.95%10. c*đ 0.79% s*g 2.95%Total: 99.70% 72.32%

• Plan to look at the most common events

• Find out what processes they correspond to

• Gluons account for a major fraction of QCD events

• Looking into whether we can distinguish quark jets from gluon jets

Page 31: Searches for Vector Boson Scattering at the LHC

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Polarization Studies The Z boson has spin 1

It can be polarized in a particular direction

Preference for spin in a particular direction could be indicative of anomalous gauge couplings E.g. coupling to the photon May also be useful in

differentiating VBS from QCD

Anomalous gauge couplings

Page 32: Searches for Vector Boson Scattering at the LHC

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Process Angular distribution of decay products, (m+ m-),

is determined by the polarization By determining the angular distribution we can

reconstruct the polarization of the Z• Lorentz transform the 4-vectors of the 2 muons into

the rest frame of the Z• Plot the angular distribution of the 2 muons

Page 33: Searches for Vector Boson Scattering at the LHC

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Preliminary Results SM predicts isotropy in cos(θ)

fr is spin in the direction of travel, fl spin in the opposite direction, f0 spin perpendicular

Plot of cosθ is similar to predicted result (isotropy) Excess at the extremes

cosθ

Page 34: Searches for Vector Boson Scattering at the LHC

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MC Samples

7/31/2014

VBS Sample: /eos/atlas/atlascerngroupdisk/phys-sm/Vgamma_skim/

CutFlow/NTUP_SMWZ.01413658._000001_zmumuVBS.root.1

QCD Sample:

/eos/atlas/atlascerngroupdisk/phys-sm/Vgamma_skim/CutFlow/NTUP_SMWZ.01110562._000001_mumugamma.root.1

Generator Cuts: Leptons: pT > 15 GeV, M(lepton, lepton) > 20 GeV Jets: pT > 15 GeV, DeltaR(jet, jet) > 1.0