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Searches for Vector Boson Scattering at the LHC

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

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Searches for Vector Boson Scattering at the LHC

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

  2. 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 • 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 ʃ L*dt= number of events produced • 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

  3. Vector Boson Scattering (VBS) Introduction • 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

  4. Motivations • 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) EWK EM P >> 1 Unitarize WW scattering

  5. W± and ZBosons • 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

  6. ZgChannel • Z boson decays into a fermion and its antiparticle • In this study the muondecay 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 neutrinochannel, as well as W lepton channels

  7. Et=37 GeV e  e e M(e,e) =91.2 GeV  Et=30 GeV e  e Et=51 GeV e Example of ISR event http://www.atlas.ch/photos/lhc.html 7

  8. Backgrounds • 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

  9. MVA – Multi-variant Analysis • 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. Image generated by Kushal Byatnal in TMVA

  11. VBS/QCD Comparison • Comparisons can tell us which variables to consider in the MVA analysis • Differences can be used to differentiate between signal and background Photon Pt (GeV) Pt(m+m- )/Pt(g ) GeV M(m+m- ) GeV

  12. GeV GeV

  13. Event Classification • 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)

  14. Final state Quark comparison • 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

  15. Going Forward • 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

  16. Summary • 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

  17. References • 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

  18. Backup Slides

  19. Motivations • 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

  20. ATLAS Detector • Muon detection: • Tracking detector • Charged particles bend in the magnetic field • Muon chambers • Photon detection: • Electromagnetic calorimeter

  21. Z Boson Information • 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

  22. Lepton Selection • 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 Pseudorapidity as a function of θ • η = -ln[tan(θ/2)]

  23. Photon Selection • 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  from jets and 0 • 0 -> g+g

  24. Jet Selection • 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

  25. Event Selection • 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

  26. Object Selection • Major differences: • Pt cut (26% vs. 34%) • Z0 (0.23% vs. 0.71%) • Eta (0.72% vs. 1.11%)

  27. 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%)

  28. Final state Quark comparison • 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 VBS QCD

  29. Top 10 Processes • 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

  30. 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

  31. 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

  32. 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

  33. MC Samples • 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

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