searches for vector boson scattering at the lhc
DESCRIPTION
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 PresentationTRANSCRIPT
Searches for Vector Boson Scattering at the LHC
Aaron WebbMentors: Al Goshaw, Andrea Bocci
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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
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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
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
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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
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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
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
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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
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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
10 7/31/2014Image generated by Kushal Byatnal in TMVA
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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
12 7/31/2014GeV
GeV
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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
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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
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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
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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
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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
Backup Slides
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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
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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
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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
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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
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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
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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
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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%)
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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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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
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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
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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
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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
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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
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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