ATLAS TTBAR TO TAU ANALYSIS William P Edson
ATLAS TTBAR TO TAU ANALYSIS William P. Edson Adviser Muhammad Sajjad Alam Albany High Energy Physics Laboratory (AHEPL) November 22, 2012
Contents � � � � The Large Hadron Collider (LHC) ATLAS coordinate system ATLAS (A Toroidal LHC Apparatus) Detector Protons and quarks Particle interactions ttbar production and decays Tau decays ttbar to tau + mu
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). 26. 659 km [10] in circumference 50 to 175 m [9] underground Spans the border between France and Switzerland.
CERN aerial view from Geneva airport (© CERN) Photograph: Jean-Luc Caron Date: Jun 1986
Representation of the entire particle accelerator complex [20]
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
ATLAS coordinate System � The point of interaction (collision) is taken as the origin. The z-axis is defined by the direction of the incoming beam (toward point 8). The x-axis points to the center of the LHC and the y-axis points skyward. y x z
ATLAS coordinate System Cont. � � � Azimuthal angle (φ) the angle off the positive xaxis ranging from [-π, +π] in the x, y plane Polar angle θ measured from the positive z-axis. The pseudorapidity η is defined by the equation: y x z φ θ
The ATLAS Detector � � ATLAS detector was created to look for new physics in the highest currently possible energy range (14 Te. V center of mass). Requirements: 1. 2. 3. It must be sensitive to many possible channels of decay It must be able to measure particle momentum and position with high accuracy It must be able to accurately identify particles
A detailed computer-generated image of the ATLAS detector and it's systems [15]
The Inner Detector � Inner Detector is composed of three subdetectors: Silicon Pixel Detector: 1. a. precise measurement of vertices Semi-Conductor Tracker: 2. a. measures particle momentum Transition Radiation Tracker: 3. a. electron identification and path tracking Detailed computer generated view of the ATLAS Inner Detector [13]
Silicon Pixel Detector � Pixel Size [19] (90%) 50 x 400μm � (10%) 50 x 600μm � � � Cover region |η| < 2. 5 [19] Operating Temperature ~-5 to -10˚C [19] Operating Gas C 3 F 8 Typically 3 layers crossed by each track in Barrel region Barrel Pixel Module Schematic [19]
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] Operating Gas C 3 F 8 Eight strip layers crossed by each track Barrel SCT Module [19]
Transition Radiation Tracker � � � � Gaseous straw tubes interleaved with transition radiation material Diameter 4 mm [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% CO 2 3% O 2 End-cap TRT module inner and outer ends [19]
Liquid Argon Calorimeters � Liquid Argon Calorimeters (LAr Calorimeter) 1. Electromagnetic Calorimeter (EMCal): a. precise measurements for photons and electrons (also used in their ID) 2. Hadronic End-Cap Calorimeter (HEC): a. precise measurements for hadronic jets, as well as Missing Transverse Energy (Etmiss). 3. Tile barrel Forward Calorimeter (FCal): a. electromagnetic and hadronic energy measurements 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]
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]
Hadronic End-Cap Calorimeter � � 32 identical wedge-shaped modules per wheel Gap between copper plates 8. 5 mm [19] � � Covers Region 1. 5 <|η| < 3. 2 [19] Outer wheel radius 2. 03 m [19] Inner wheel radius [19] � � � three electrodes per gap 0. 475 m in overlay region with FCal 0. 372 m Shares End-Cap cryostat with EMEC and FCal Hadronic End-Cap Module [19]
Forward Calorimeter � � Metal absorber matrix interposed with copper tubes centered around metallic rods Three modules per End. Cap: 1. 2. 3. � copper rods – EM measurements tungsten rods – hadronic interaction energy tungsten also Covers Region 3. 1 < |η| < 4. 9 [19] Hadronic module absorber matrix [19]
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 Detailed computer generated view of the ATLAS Tile Calorimeter [16]
Muon Spectrometer � The Muon spectrometer is constructed of four different chamber types: Monitored Drift Tubes (MDTs) 1. a. precise measurements of track position Cathode Strip Chambers (CSCs) 2. a. precise measurements of track position Resistive Plate Chambers (RPCs) 3. a. triggering and measurement of track coordinates Thin Gap Chambers (TGCs) 4. a. same as RPCs but in end-cap region 3 -D view of the muon system [17]
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/CO 2 (93/7) at 3 bar [19] Maximum drift time 700 ns [19] Cross-section and longitudinal view of MDT [19]
Cathode Strip Chambers � Multi-wire proportional chambers with 2 strip segmented cathodes Wire “Y” - Strips “X” - Strips Lower Density Foam 5 Composit Panels Seal Rubber 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/CO 2 [19] Four η and φ measurements per track Gas Seal Bar “X” – Readout Connector Fiducial Mark Wire Fixation Bar Gas Inlet (Outlet) Assembly Holes CSC Design [19]
Resistive Plate Chambers � � Gaseous parallel electrode plate detector Electric field between the plates 4. 9 k. V/mm [19] Covers Region |η| ≤ 1. 05 [19] Operating Gas Mixture [19] � � Low-p. T (6 - 9 Ge. V) trigger signal � � C 2 H 2 F 4 → 94. 7% Iso-C 4 H 10 → 5% SF 6 → 0. 3% hits inner two RPC layers High-p. T (9 – 35 Ge. V) trigger signal � hits in all three RPC layers Cross Section of an RPC [19]
Thin Gap Chambers � � Multi-wire proportional chambers in either doublet or triplet form Four layers each endcap 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 CO 2, n-C 5 H 12 TGC triplet cross-section [19]
ATLAS Magnet System � Three superconducting magnet sub-systems: Barrel Toroid 1. a. 3 -8 Tm over the central region End-cap Toroids: 2. a. 3 -8 Tm over the forward regions Solenoid: 3. a. 2 Tesla within the central tracking volume Computer generated view of the ATLAS Magnet System [18]
Protons � � � Most stable baryons which exist Positive elementary charge of +1 Mass of 938. 272013± 0. 000023 Me V/c 2 [4] First generation quarks uud Flowing between and around the quarks is a “sea” of gluons and quark anti-quark pairs Parton distributions from CTEQ 6 M plotted at Q = 100 Ge. V. x is the fraction of proton momentum carried by the particle, and f(x) is the probability density. [22]
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. 3 Ge. V/c 2 [4]) Third generation of quarks (with bottom) Charge of +2/3 [4]
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 (1776. 82± 0. 16 Me. V/c 2 [4]) Third generation of leptons Charge of -1 Each Lepton generation includes a corresponding neutrino with neutral charge and extremely small mass (~0).
Particle Interactions and Gauge Bosons � Three major interactions and their mediating particles (the gauge bosons): Strong force 1. Only occurs between color charged particles 8 gluons a. b. � � massless and elementary chageless doubly color charged Electromagnetic force 2. Only occurs between elementary charged particles photon a. b. � massless and elementary chargeless Weak force 3. Occurs between all quarks and leptons intermediate vector bosons W±, and Z a. b. � Have mass and the W carries charge
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]
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 g g t t t g q q g t t
ttbar Decays � � � Immediately decays due to large mass Interaction occurs via weak force to Wb (BR = 0. 99+0. 09 -0. 08). [4] W and b decay producing the particles found in the detectors W+ t+ b Wtb
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]
Tau Decays � � � Extremely short lifetime of (290± 1)*10 -15 s [4] due to large mass Decays via the weak interaction Reconstructed from the daughter particles of its decay μ-νμντbar 17. 36± 0. 05% e-νeντbar 17. 85± 0. 05% π-ντbar 10. 91± 0. 07% π-π0ντbar 25. 51± 0. 09% π-2π0ντbar 9. 29± 0. 11% π-3π0ντbar 1. 04± 0. 07% h-h-h+ντbar 9. 80± 0. 08% h-h-h+π0ντbar 4. 75± 0. 06% Main branching ratios of tau decays[4]. h ± can stand for either K or π and
Tau Decays Continued � � Hadronically decays approximately 64. 79% Leptonic decays are almost impossible to distinguish as being produced via the tau decay versus any other process W- l νl τντ W- d u τντ
Tau Hadronic Jets � Two types of jets based on the number of charged hadron tracks (π’s or K’s) [7] � 1 -prong � 3 -prong � � (77%) (23%) Highly collimated resulting in a narrow particle shower recorded in the calorimeter Can also contain any number of neutral pions π+ π 0 ππ+ Example 3 prong jet
Interaction Overview � What we see in the detector for a ttbar → τ event: 1. 2. 3. 4. 2 b-jets Missing transverse energy (ETmiss) the presence of a lepton (muon or electron) or hadronic jets tau jet l, q’ + W νl, q b t+ t- b - W τ ντ -
ttbar to tau + mu Event Selection � � Single-muon trigger p. T > 18 Ge. V 5 track primary vertex Jets p. T > 20 Ge. V must pass jet-quality selections Only one isolated muon � � � No electrons 1 ‘loose’ tau candidate � � � � � p. T > 20 Ge. V Ecalo < 4 Ge. V in cone of ∆R = 0. 3 ∑p. T < 4 Ge. V in cone of ∆R = 0. 3 ∆R > 0. 4 from jet with p. T > 20 Ge. V ET > 20 Ge. V |η| < 2. 3 leading track p. T > 4 Ge. V isolated within ∆R(l, τ) < 0. 4 from muon and electron tracks BDTe > 0. 51 Njets ≥ 2 with p. T > 25 Ge. V and not within ∆R(jet, τ) < 0. 4 ETmiss > 30 Ge. V HT = ∑|ET| > 200 Ge. V At least 1 jet identified as a b-jet
1 -prong Cut Flow for 1 -prong ttbar to tau + muon analysis with statistical uncertainties [23]
3 -prong Cut Flow for 3 -prong ttbar to tau + muon tau analysis with statistical uncertainties [23]
Background � QCD multi-jet � expected to contribute equally to Same Sign (SS) and Opposite Sign (OS) � ttbar → μ + jets � background templates created from data (W+Jet SS and OS samples) ETmiss > 30 Ge. V MT > 30 Ge. V Njets > 3 SS and OS samples have equal contamination of b-jets and gluon jets
Signal Template � Created using MC simulation data � ttbar �Z → ττ � ttbar → μe + jets (BDTj cannot discern electrons)
BDTj
Why Taus? � Tau is an expected final state of previously undiscovered particles both within and beyond the Standard Model (SM): 1. 2. 3. � Higgs Boson: H → τ+ τextra gauge bosons: Z’ → τ+τMinimal Supersymmetric Standard Model (MSSM) Higgs (charged Higgs): H+ → τ + ντ Possibility that non-SM top decays may exist. τ+ H τ τ+ Z’ τ τ+ H+ ντ
Conclusions � � � The LHC and ATLAS will provide data necessary for the study of new physics at the currently highest energies. The strong interaction is expected to produce ttbar pairs. From the decay of the ttbar particles we hope to expand or knowledge on the decay to tau in hopes it will provide insight into new physics.
ttbar to tau group paper � [23] The ATLAS Collaboration. Measurement of the top quark pair production cross section in pp collisions at √s = 7 Te. V in μ + τ final states with ATLAS � Internal note: 19 July 2011. cdsweb. cern. ch/record/1368477 � Conference note: 22 August 2011. cdsweb. cern. ch/record/1376411
References � � � � � � https: //twiki. cern. ch/twiki/bin/viewauth/Atlas/Work. Book http: //atlas. web. cern. ch/Atlas/SUB_DETECTORS/TILE/ [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 2007. http: //edoc. ub. uni-muenchen. de/7603/1/Lambacher_Marion. pdf. [2]CTEQ. http: //www. phys. psu. edu/~cteq/. [3] V. Chekelian. Standard model physics at HERA. 2001. hep-ex/0107053. [4]http: //pdg. lbl. gov/ [5]Neil Collins. Top. Cross Section (Current Status and Early LHC Prospects). 10 March 2010. http: //www. ep. ph. bham. ac. uk/general/seminars/slides/neil-collins-100310. pdf [6]Zofia Czyczula. Search for New Physics in Tau-pair Events in ATLAS at the LHC. June 2009. http: //cdsweb. cern. ch/record/1204789/files/CERN-THESIS-2009 -078. pdf [7]Noel Dawe. Tau Identification with Boosted Decision Trees. 18 March 2010. [8]Bjorn Gosdzik. Tau Reconstruction at the ATLAS experiment. 22 January 2009. http: //wwwiexp. desy. de/studium/seminare/studsem/ws 2008/talks/vortrag 09 -gosdzik. pdf [9] http: //lhc-machine-outreach. web. cern. ch/lhc-machine-outreach/ [10] http: //public. web. cern. ch/public/en/lhc-en. html [11] http: //www. uslhc. us/ [12] http: //atlas. web. cern. ch/Atlas/Collaboration/ [13] https: //twiki. cern. ch/twiki/bin/viewauth/Atlas/Inner. Detector [14] The ATLAS Liquid Argon Calorimeter at the LHC: Overview and Performance – Mathieu Aurousseau, http: //cdsweb. cern. ch/record/1270159/files/ATL-LARG-SLIDE-2010 -119. pdf [15] http: //atlas. ch/ [16] ATLAS Tile calorimeter commissioning status and performance – Ana Henriques, http: //cdsweb. cern. ch/record/1213918 [17] Muon Spectrometer Technical Design Report, http: //atlas. web. cern. ch/Atlas/GROUPS/MUON/TDR/Web/TDR_chapters. html [18] http: //atlas-magnet. web. cern. ch/atlas-magnet/ [19] The ATLAS Experiment at the CERN Large Hadron Collider. 2008. http: //iopscience. iop. org/1748 -0221/3/08/508003 [20] CERN faq LHC the guide. http: //multimedia-gallery. web. cern. ch/multimedia-gallery/Brochures. aspx [21] Jane T. Bromley. Investigation of the Operation of Resistive Plate Chambers. September 1994. www. hep. man. ac. uk/theses/jane. pdf [22] Joel Feltesse. Introduction to Parton Distribution Functions. Scholarpedia. 6 November 2010. http: //www. scholarpedia. org/article/Introduction_to_Parton_Distribution_Functions
Questions
Back-ups
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 p-type E - + n-type p-type E + + - n-type
How the TRT Works � � Photons are emitted as the particles traverse the boundaries between the transition radiation material Charged particles passing through the straw ionize the gas it contains Photons produced as transition radiation also react with the gas The anode wire attracts the freed electrons which gives the measurement - + + + - - -
How Lar Calorimeters Work � � Particles interact with the absorber material Particles of lower energy are produced (showers) The charged particles cause ionization in the liquid argon Charge deposited at electrode and shower depth is proportional to total energy of initial particle Absorber Electrode + + - + - - - +
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]
How Resistive Plate Chambers Work � � � Charged particles passing through the gas gap ionize the gas trapped within The electric field pulls the electrons and ions towards the electrodes Charge deposited at electrodes is read out by the longitudinal and transverse strips Electrode Resistive Plates E PET Film + + - - + Readout Strips
Tau Reconstruction Current reconstruction method for the tau uses either calorimeterseeded, track-seeded algorithms or a combination of the two: 1. 2. Calo shower Calo seed Seed track ion lat Iso one ck c Tra ne co 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 (p. T > 6 Ge. V/c 2)[ 8] 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.
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