Top Quark Production at the Large Hadron Collider Fourth Year Project Seminar Top Quark Production at the Large Hadron Collider Jack Goddard 20th March 2009

Fourth Year Project Seminar Outline Background The Top Quark ATLAS Aim Method Backgrounds Efficiency Results Conclusion 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar The Top Quark Mt = 171.2 ± 2.1 GeV (PDG) About the same mass as a gold atom Point-like to scales ≥10-18m Discovered at the Tevatron Expected to couple to electroweak gauge symmetry breaking forces Free parameter in the standard model Constrains the mass of the Higgs arXiv:0712.0929 [hep-ex] 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Top Pair Production Fully Hadronic Leptonic Semi-leptonic Fully Hadronic BR = 44.4% Semi-leptonic Leptonic BR = 4.9% BR = 29.6% 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar The ATLAS Detector Multipurpose detector IP1 J. Pequenao CERN-GE-08031012 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar The Inner Detector 3 Layers of Pixel Detectors 4 layers of Semiconductor Tracker (SCT) Transition Radiation Tracker (TRT) |η| < 2.5 20th March 2009 Fourth Year Project Seminar

The Electromagnetic Calorimeter Lead Liquid Argon Sampling Calorimeter. Accordion-shaped kapton electrodes and lead absorber plates. |η|<2.5 devoted to precision physics. Segmented into 3 sections. 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Aim of project Decay Branching Ratio t → bW+ 0.998 t → sW+ ≈1.9x10-3 t → dW+ ≈ 10-4 If: Dominant H+ decays: Number of electrons/muons produced in top decay would be reduced, by non SM processes. Rll/l would be less than SM prediction Ree/e ≈ 1/19 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Method ATHENA 14.2.22 Monte Carlo Samples Producing Histograms in ROOT Select top quark decays Pre-Selection Cuts Electron Identification Cuts Jet Cuts Background Studies Efficiency Studies 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Pre-Selection Cuts For a sample enriched with leptonic and semi-leptonic decays: ETmiss > 20 GeV Electron with pT > 20 GeV At least 2 b-tagged jets with pT > 20 GeV All in |η| < 2.5 Differentiated by: A second electron with pT > 20 GeV With opposite charge to the original electron 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Pre-Selection Cuts 20th March 2009 Fourth Year Project Seminar

Electron Identification Cuts Efficiency (%) Loose (had and middle) 88 Medium (Loose + strips + trk qual + trk match) 77.2 Tight (Medium + TRT) 64 20th March 2009 Fourth Year Project Seminar

Electron Identification Cuts: Loose Type Description Value Acceptance of the detector Pseudorapidity cut |η| < 2.5 Hadronic Leakage Ratio of ET in the first sampling of the hadronic to ET of ECal RET <0.045 Second Layer of the ECal Ratio of cell energies in 3x7 vs 7x7 cells Rη > 0.910 Rφ > 0.5 Lateral Width of the shower wlats < 0.0160 Simple Electron ID based on information from the calorimeters Gives good identification efficiency, but low background rejection 20th March 2009 Fourth Year Project Seminar

Electron Identification Cuts: Loose 20th March 2009 Fourth Year Project Seminar

Electron Identification Cuts: Medium Strip based cuts effective at rejecting π0 → γγ Showers studied in a ΔηxΔφ = 0.125 x 0.2 window Type Description Value First Layer of the ECal Difference between energy in the 2nd largest deposit and energy of the minimal value between 1st and 2nd maxima. ΔES > 0.3 Second largest energy deposit (normalised to the cluster energy) Rmax2< 0.53 Total Shower Width wstot < 4.10 Shower width for 3 strips around maximum strip ws3 < 0.8 Fraction of energy outside core of 3 central strips Fside < 0.8 Track Quality Number of hits in the pixel detector Npix> 0 Number of B-layer hits Nb-lay > 0 Number of hits in the pixels and SCT Npix+SCT> 8 Transverse Impact Parameter A0 < 1mm 20th March 2009 Fourth Year Project Seminar

Electron Identification Cuts: Medium 20th March 2009 Fourth Year Project Seminar

Electron Identification Cuts: Tight Makes use of all the electron ID cuts available Includes the loose and medium cuts Type Description Value Isolation Ratio of ET in a cone ΔR<0.2 to the total cluster energy ET /Etot < 0.8 Vertex-layer Number of hits in the vertexing layer Nvert > 0 Track Matching Δη between the cluster and the track Δη < 0.005 Δφ between the cluster and the track Δφ < 0.02 Ratio of cluster energy to the track momentum 0.7 < E/p < 5.0 TRT Total number of hits in the TRT NTRT > 15 Ratio of number of high threshold hits to the total number of hits in the TRT RTRT > 0.08 20th March 2009 Fourth Year Project Seminar

Electron Identification Cuts: Tight 20th March 2009 Fourth Year Project Seminar

Effect of all the Cuts Missing ET cut Jet cuts Electrons from semi-leptonic decays Electron medium cuts Electron η cut Electrons from leptonic decays Electron pT cut Selected Electrons Electron loose cuts 20th March 2009 Fourth Year Project Seminar

Backgrounds Background from other processes Background from other top decays t →bτυτ Hadronic channel Decay Cross-section (pb) Total top pair production 834 Backgrounds to Semi-leptonic Decays W + jets → lν + jets 7.8 x 103 W + W → lν + jets 17.1 W + Z → lν + jets 3.4 Backgrounds to both leptonic and Semi-leptonic Decays Z + jets → ll + jets 1.2 x 103 W + bb → ll + jets 66.7 Z + Z → ll + jets 9.2 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Tau Background Under investigation Either: Tighten pT cut to 30 GeV or Account for electrons from taus in systematic errors 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Efficiency Look at the MC truth information Compare to reconstructed data NID = number of reconstructed electrons Ntruth = number of true electrons at the generator level 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Ree/e From Reconstruction: Ree/e = 44/839 = 0.0524 ± 0.0081(stat) From Truth: Ree/e = 428/8636 = 0.0496 ± 0.0025(stat) R = 1/19 = 0.0526 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Conclusion Leptonic and Semi-leptonic decays selected Ree/e = 0.0524 ± 0.0081(stat) Some systematic errors being investigated Efficiency and Purity Studies ongoing. 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar References “Precision Electroweak Measurements and Constraints on the Standard Model” (arXiv:0712.0929) : The LEP Collaborations and LEP Electroweak Working Group. “ATLAS Detector and Physics Performance Technical Design Report” The ATLAS Collaboration. “Reconstruction and Identification of Electrons” ATLAS CSC Note: D. Zerwas & D. Froidevaux 20th March 2009 Fourth Year Project Seminar

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Backup Slides

Fourth Year Project Seminar Cross-section An accurate measurement of BR(W → lv) is of interest for the cross-section calculation: 20th March 2009 Fourth Year Project Seminar

Transition Radiation Tracker Based on use of straw detectors 70% Xe, 20% CO2, 10% CF4 Xenon gas used to detect transition-radiation photons created in a radiator between the straws Allows a large number of measurements (typically 36) to be made on every track 20th March 2009 Fourth Year Project Seminar

Sampling Calorimeters 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar B-tagging Hadrons with b-quarks have a long lifetime – allows distinct primary and secondary vertices to be seen Compared to other jets b-jets are: Higher multiplicities Higher invariant mass Contains low energy leptons with momentum perpendicular to the jet 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar The Monte Carlo Sample Run 5200 Contains only leptonic and semi-leptonic tt(bar) events Electron, Muon and Tau events Contains 1861 files of 250 events 465,250 events in total 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Ree/e = 1/19 ? Due to a leptonic decays that produce a electron and a muon or tau 20th March 2009 Fourth Year Project Seminar

Fourth Year Project Seminar Error Calculation 20th March 2009 Fourth Year Project Seminar