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Atlas TTbar to Tau Analysis
William P. Edson, Teeba Rashid Adviser: Mohammad Sajjad Alam State University of New York at Albany Anirvan Sircar Adviser: Dick Greenwood Loisianna Tech April 14, 2013
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Contents The Large Hadron Collider (LHC)
ATLAS (A Toroidal LHC Apparatus) Detector ttbar Production and Decay
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The LHC Proton–proton particle accelerator located at CERN (European Organization for Nuclear Research) Composed of two accelerating rings with cross-over points where collisions occur (collision points). CERN aerial view from Geneva airport (© CERN) Photograph: Jean-Luc Caron Date: Jun 1986
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ATLAS Detector A detailed computer-generated image of the ATLAS detector and it's systems [15]
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Top Pair Branching Fractions for ttbar decays[5]
LHC’s high energy will cause gluons to be the major source for ttbar production (~87%) Immediately decays due to large mass Decay is via weak force to Wb (BR = ).[4] W and b decay producing the particles found in the detectors Top Pair Branching Fractions for ttbar decays[5]
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Interaction Overview What we see in the detector for a ttbar → τ event: 2 b-jets Missing transverse energy (ETmiss) the presence of a lepton (muon or electron) or hadronic jets tau jet t W+ b tbar W- bbar ντ τ - l+, q’ νl, q
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Tau Hadronic Decays Hadronic decays compose approximately 64.79%
Two types of jets based on the number of charged hadron tracks (π’s or K’s) [7] 1-prong (77%) 3-prong (23%) Highly collimated resulting in a narrow particle shower recorded in the calorimeter π0 π+ π- Example 3-prong jet
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Boosted Decision Trees
Multivariate machine learning algorithm Makes cuts on a given set of variables does not discard samples which fail a cut Trained using MC and data samples identified as signal or background samples split into background and signal based on purity (P) [27] Events are assigned a score between 0 (background) and 1 (signal) Analysis uses two BDTs BDTe – remove electrons faking taus BDTj – remove jets faking taus
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Backgrounds QCD multi-jet W + jets Z + jets
reduced via ETmiss selection estimated contribution determined via data driven techniques W + jets reduced via HT selection and b-tag selection Z + jets ttbar decay (muons and electrons) reduced via Lepton veto cut and TauID QCD multi-jet not modeled well in MC following τ selection criteria
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Conclusions
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Thanks Dr. Sajjad Alam Dr. Pat Skupic Dr. Dick Greenwood
Catrin Bernius ttbar to tau group
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References https://twiki.cern.ch/twiki/bin/viewauth/Atlas/WorkBook
[1] Marion Lambacher, Study of fully hadronic ttbar decays and their separation from QCD multijet background events in the first year of the ATLAS experiment. July [2]CTEQ. [3] V. Chekelian, Standard model physics at HERA hep-ex/ [4] [5]Neil Collins, TopCross Section (Current Status and Early LHC Prospects). 10 March [6]Zofia Czyczula, Search for New Physics in Tau-pair Events in ATLAS at the LHC. June [7]Noel Dawe, Tau Identification with Boosted Decision Trees. 18 March 2010. [8]Bjorn Gosdzik, Tau Reconstruction at the ATLAS experiment. 22 January [9] [10] [11] [12] [13] [14] The ATLAS Liquid Argon Calorimeter at the LHC: Overview and Performance – Mathieu Aurousseau, [15] [16] ATLAS Tile calorimeter commissioning status and performance – Ana Henriques, [17] Muon Spectrometer Technical Design Report, [18] [19] The ATLAS Experiment at the CERN Large Hadron Collider [20] CERN faq LHC the guide. [21] Jane T. Bromley.,Investigation of the Operation of Resistive Plate Chambers. September [22] Joel Feltesse, Introduction to Parton Distribution Functions. Scholarpedia. 6 November [24] CMS collaboration, First Measurement of the top quark pair production in the dilepton channel with tau leptons in the final state in pp collisions at √s = 7 TeV. CMS-PAS-TOP-11-06 [25] Interactions.org, Particle Physics Glossary [26] David Griffiths, Introduction to Elementary Particles Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim [27] Noel Dawe, Dugan O’Neil, Tau Identification with Boosted Decision Trees. 15 July [28] Michael Wright, Exclusive Jet Study with Kt, AntiKt and Cambridge Aachen Jet Finders. 10 June [29] Mark R. Leach, The INTERNET Database of Periodic Tables [30] Arghir, S; Behar, S; Cavaliere, V; Coadou,Y ; Czodrowski, P; Flechl, M; Kopp, A; McCarn, A; zur Nedden, M ;Neubauer, M; Randle-Conde, A; Rozen, Y; Sekula, S; Takahashi, Y; Tomoto, M; Vickey, T; Xella, S, Data-driven estimation of the background to H+ searches with hadronic-tau final states. 31 March
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Back-ups
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LHC continued Internal pressure is kept at 10-13 atm [10]
Helps prevent collisions within beam-pipe between protons and gas molecules 1232 dipole magnets [10] Bends particle beams around circular path 392 quadrupole magnets [10] to Squeezes the proton bunches together to improve the probability of collision
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Semi-Conductor Tracker
Also called Silicon Microstrip Trackers Spatial resolution ~16μm in R-φ [19] Covers Region |η| < 2.5 [19] Operating Temperature ~-5 to -10˚C [19] Cooling Gas C3F8 Eight strip layers crossed by each track Barrel SCT Module [19]
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Silicon Pixel Detector
Pixel Size [19] (90%) 50 x 400μm (10%) 50 x 600μm Cover region |η| < 2.5 [19] Cooling Temperature ~-5 to -10˚C [19] Operating Gas C3F8 Typically 3 layers crossed by each track in Barrel region Barrel Pixel Module Schematic [19]
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How Silicon Detectors Work
Particles pass through the silicon (depletion) layer Electrons within the silicon are knocked away (leaving a free electron and a “hole”) Electrons and holes are pulled into contacts via an electric field produced by doping areas of the silicon Charge builds up on the contacts producing a current E - + n-type p-type E + - n-type p-type
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Transition Radiation Tracker
Gaseous straw tubes interleaved with transition radiation material Diameter 4mm [19] Intrinsic accuracy of ~130μm per straw [19] Covers Region |η| < 2.0 [19] Typically 36 hits per track Operates at room temperature Operating Gas [19] 70% Xe 27% CO2 3% O2 End-cap TRT module inner and outer ends [19]
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Electromagnetic Calorimeter
Electrodes – three copper layers separated by polyimide sheets Lead absorber plates Accordion-shaped geometry provides φ symmetry with no cracks Covers Region |η| < 3.2 (|η| < 2.5 precision physics region) [19] Liquid Argon: stability of response over time intrinsic radiation hardness intrinsic linear behavior Barrel Electromagnetic LAr Calorimeter [19]
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Hadronic End-Cap Calorimeter
32 identical wedge-shaped modules per wheel Gap between copper plates 8.5 mm [19] three electrodes per gap Covers Region 1.5 <|η| < 3.2 [19] Outer wheel radius 2.03 m [19] Inner wheel radius [19] 0.475 m in overlay region with FCal m Shares End-Cap cryostat with EMEC and FCal Liquid Argon chosen instead of tile due to high radiation levels in End-Cap Hadronic End-Cap Module [19]
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Hadronic module absorber matrix [19]
Forward Calorimeter Metal absorber matrix interposed with copper tubes centered around metallic rods Three modules per End-Cap: copper rods – EM measurements tungsten rods – hadronic interaction energy tungsten also Covers Region 3.1 < |η| < 4.9 [19] copper – optimizes resolution and heat removal tungsten – provides containment and minimizes spread of hadronic showers Hadronic module absorber matrix [19]
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How the Tile Calorimeter Works
Particles induce the production of ultraviolet light in the scintillation material Doping material (fluors) shifts the wavelength to that of visible blue light Wavelength-shifting fibers collect the light for the photomultipliers Tile calorimeter module [19]
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Cross-section and longitudinal view of MDT [19]
Monitored Drift Tubes Chambers of three to eight layers of drift tubes Average Resolution of 80 μm per tube (30 μm per chamber) [19] Covers Region [19] |η| < 2.0 innermost end-cap layer |η| < 2.7 Operating Gas Ar/CO2 (93/7) at 3 bar [19] Maximum drift time 700 ns [19] Cross-section and longitudinal view of MDT [19]
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Cathode Strip Chambers
“X” - Strips Wire Fiducial Mark Gas Seal Bar Seal Rubber 5 Composit Panels Lower Density Foam “X” – Readout Connector Wire Fixation Bar Assembly Holes Gas Inlet (Outlet) “Y” - Strips Multi-wire proportional chambers with 2 strip segmented cathodes perpendicular strips parallel strips Combine high spatial, time and double track resolution with high rate capability Cover Region 2 < |η| < 2.7 [19] Operating Gas Ar/CO2 [19] Four η and φ measurements per track CSC Design [19]
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Resistive Plate Chambers
Gaseous parallel electrode plate detector Electric field between the plates 4.9 kV/mm [19] Covers Region |η| ≤ 1.05 [19] Operating Gas Mixture [19] C2H2F4 → 94.7% Iso-C4H10 → 5% SF6 → 0.3% Low-pT (6 - 9 GeV) trigger signal hits inner two RPC layers High-pT (9 – 35 GeV) trigger signal hits in all three RPC layers Cross Section of an RPC [19]
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TGC triplet cross-section [19]
Thin Gap Chambers Multi-wire proportional chambers in either doublet or triplet form Four layers each end-cap Covers Region 1.05 ≤ |η| ≤ 2.4 [19] Distances [19] wire to cathode 1.4 mm wire to wire 1.8 mm Operating Gas mixture CO2, n-C5H12 TGC triplet cross-section [19]
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Proton Interactions The primary interaction for ttbar production is the strong interaction. As the energy of the reaction increases the amount of gluons within the proton (the sea) increases. Gluon distribution at different energies where x is the momentum fraction carried by the gluon within the proton.[3]
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Anti-kt Jet Algorithm Clustering algorithm
Distances are calculated between particles or pseudo-jets (dij) and between particles and the beam (diB) if dij minimum and less than some dcut, then i and j are combined if diB minimum, then i is labeled jet and removed from list Highest pT objects grouped first Depiction of the clustering conducted by the anti-kt algorithm dij = min (1/p2Ti, 1/p2Tj)∆R2ij/R diB = 1/p2iB
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1-prong Cut Flow Cut Flow for 1-prong ttbar to tau + muon analysis with statistical uncertainties [23]
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3-prong Cut Flow Cut Flow for 3-prong ttbar to tau + muon tau analysis with statistical uncertainties [23]
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Systematic Uncertainty Determination
Scale Factors selection event variation Muon momentum scale and resolution reconstructed μ pair invariant mass distributions of Z/γ* Jet Energy Scale (JES) combined information from test-beam, LHC collision and simulation Jet Reconstruction/Identification comparison between simulation and data Initial and Final-State Radiation (ISR/FSR) AcerMC generator interfaced with PYTHIA shower model varying parameters in range consistent with data MC Generator compare predictions with those from POWHEG PDF range of current PDF sets b-tag scale factor calibration studies using inclusive lepton and multi-jet final states TauID compare MC and data Z → ττ events with our selection requirements except Njets < 2 and MT < 20GeV Luminosity TauID difference in selection to remove W + jets events
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The Inner Detector Inner Detector is composed of three subdetectors:
Silicon Pixel Detector: precise measurement of vertices Semi-Conductor Tracker: measures particle momentum Transition Radiation Tracker: electron identification and path tracking Detailed computer generated view of the ATLAS Inner Detector [13]
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Liquid Argon Calorimeters
Liquid Argon Calorimeters (LAr Calorimeter) Electromagnetic Calorimeter (EMCal): precise measurements for photons and electrons (also used in their ID) Hadronic End-Cap Calorimeter (HEC): precise measurements for hadronic jets, as well as Missing Transverse Energy (ETiss). Forward Calorimeter (FCal): electromagnetic and hadronic energy measurements Tile barrel Tile extended barrel LAr hadronic end-cap (HEC) LAr electromagnetic end-cap (EMEC) Lar electromagnetic barrel LAr forward (FCal) Detailed computer generated view of the ATLAS Liquid Argon Calorimeter [14]
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Tile Calorimeter Doped polystyrene used as scintillating tiles
Steel absorber plates Over 460,000 scintillating tiles [19] Covers Region |η| < 1.7 [19] Measurements for both ETmiss and hadronic jets Jet information (from HEC, FCal and Tile Calorimeter) is used for jet reconstruction and identification doped – 1.5% PTP primary fluor, 0.044% POPOP secondary fluor Detailed computer generated view of the ATLAS Tile Calorimeter [16]
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Muon Spectrometer The Muon spectrometer is constructed of four different chamber types: Monitored Drift Tubes (MDTs) precise measurements of track position Cathode Strip Chambers (CSCs) Resistive Plate Chambers (RPCs) triggering and measurement of track coordinates Thin Gap Chambers (TGCs) same as RPCs but in end-cap region 3-D view of the muon system [17]
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ATLAS Magnet System Three superconducting magnet sub-systems:
Barrel Toroid 3-8 Tm over the central region End-cap Toroids: 3-8 Tm over the forward regions Solenoid: 2 Tesla within the central tracking volume Computer generated view of the ATLAS Magnet System [18]
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Standard Model Depiction (28)
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Quarks Elementary particles which come together to form hadrons
Fermions (have spin ±1/2) Color charged (red, green and blue) Fractional elementary charge of +2/3 or -1/3 Have associated antiparticles with the same mass and spin but opposite charge Antiparticles noted with the use of a bar above their symbol or following their name (tbar). Top Quark (t) Most massive of the known quarks (172.0±0.9±1.3GeV/c2 [4]) Third generation of quarks (with bottom) Charge of +2/3 [4]
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Leptons Elementary particles that are similar to quarks but vary in some very drastic ways. Fermions Do not carry color charge Integral elementary charge (-1 or 0) Have corresponding antiparticles Antiparticles noted by adding a + signifying their positive charge (e+) or using a bar above (υe) Taus (τ) Most massive of the known leptons ( ±0.16MeV/c2 [4]) Third generation of leptons Charge of -1 Each Lepton generation includes a corresponding neutrino with neutral charge and extremely small mass (~0).
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Particle Interactions and Gauge Bosons
Three major interactions and their mediating particles (the gauge bosons): Strong force Only occurs between color charged particles 8 gluons massless and elementary chargeless doubly color charged Electromagnetic force Only occurs between elementary charged particles photon Weak force Occurs between all quarks and leptons intermediate vector bosons W±, and Z Have mass and the W carries charge
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Protons Most stable baryons which exist
Positive elementary charge of +1 Mass of ± MeV/c2 [4] First generation quarks uud Parton distributions from CTEQ6M plotted at Q = 100 GeV. x is the fraction of proton momentum carried by the particle, and f(x) is the probability density. [22]
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Cross Section Measure of the likelihood that some specific interaction will occur Units barns (b) ~ m2 [25] Theoretically: σ = ∫dσ = ∫|b/sinθ(db/dθ)|dΩ [26] where b is the impact parameter (distance traveling particle moves in relation to the scattering particle’s center), θ is the polar angle and dΩ is the solid angle Equation used for measuring: σ = NEvents/L where NEvents are the number of events while L is the measured luminosity (number of particles per unit time per area) b dΩ
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Tau Importance Tau is an expected final state of previously undiscovered particles both within and beyond the Standard Model (SM): Higgs Boson: H → τ+τ- extra gauge bosons: Z’ → τ+τ- Minimal Supersymmetric Standard Model (MSSM) Higgs (charged Higgs): H+ → τ+ντ H τ+ τ Z’ τ+ τ
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ttbar Decays Continued
b quark decays form jets which can be readily identified W decays quarks (seen as jets) leptons and neutrinos Top Pair Branching Fractions for ttbar decays[5]
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ttbar Production LHC’s high energy will cause gluons to be the major source for ttbar production 87% by gluon fusion [1] ~13% mainly by qqbar annihilation [1] g t g tbar g g t g t tbar g tbar g q t g q tbar
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Tau Decays μ-νμντ 17.36±0.05% e-νeντ 17.85±0.05% π-ντ 10.91±0.07% π-π0ντ 25.51±0.09% π-2π0ντ 9.29±0.11% π-3π0ντ 1.04±0.07% h-h-h+ντ 9.80±0.08% h-h-h+π0ντ 4.75±0.06% Extremely short lifetime of (290±1)*10-15s [4] due to large mass Decays via the weak interaction Reconstructed from the daughter particles of its decay Main branching ratios of tau decays[4]. h± can stand for either K or π and
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Tau Decay Hadronic decays compose approximately 64.79%
Leptonic decays are almost impossible to distinguish as being produced via the tau decay versus any other process τ- W- ντ l νl τ- W- ντ d ubar
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Tau Reconstruction Calo-seeded: Begins with the detection of a particle shower within the calorimeter and then searches for associated tracks within a narrow cone. Track-seeded: Selects a good quality track and then looks for other tracks around the seed track within an isolated cone. The energy deposited in the calorimeter within this cone is also collected. Calo shower Calo seed Seed track Track cone Isolation cone
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ttbar to tau + mu t W+ b tbar W- bbar ντ τ μ+ νμ t W+ b tbar W- bbar
τ+ ντ
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BDTj Template Fit Results corresponding to a, b and c [23]
BDTj Contd χ2 fit of OS – SS BDTj distribution determined: (a * OS – b * SS) + c * Signal [23] OS, SS and Signal → templates a, b and c → free parameters to set the normalization of each template, also equivalent to the resultant number of events OS and SS templates weighted using scale factors determined from MC BDTj Template Fit Results corresponding to a, b and c [23] Weighting of OS and SS necessary due to the pT spectrum of tau candidates being different between ttbar and W + jets events
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OS – SS Plots BDTj (OS – SS) before ETmiss selection (a) τ1 (b) τ3 [23] BDTj (OS – SS) prior to b-tag selection (a) τ1 (b) τ2 BDTj (OS – SS) following b-tag selection (a) τ1 (b) τ3
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Signal Template Composition derived from MC [23]
Remaining Signal Signal Template Composition derived from MC [23] Contamination remains from: Z → ττ ttbar → μ + e Small predicted contributions are subtracted from signal prior to cross section calculation
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ttbar to tau + jets t W+ b tbar W- bbar ντ τ qbar q’ t W+ b tbar W-
τ+ ντ
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To Do Determine the Sources of possible background
Decide on the necessary MC samples Determine the Event Selection Make plots of the Data Determine how you will estimate the Background Plot the Background using MC Estimate the Uncertainty caused by Background Estimation Estimate the Uncertainty caused by Event Selection Criteria
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Background QCD multi-jet Single-top W → μ + jets W → e + jets
Z/γ* → μ + jets Z/γ* → e + jets Light jets (u and d)
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