study of tt-photon events with the cms-detector bad honnef 27. august 2007

Study of tt-Photon Events with the CMS-detector

Bad Honnef27. August 2007

Thomas Hermanns

III. Physikalisches Institut B

Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

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tt-Photon Events at the LHC

Cross section for tt-pair production at s= 14 TeV: 830 pb (NLO)

About 1 tt-pair per second (L=1033cm-2s-1)

Consider top-decays only via Vtb1

Consecutive decays of the W-bosons W+/-: electron or muon channel W-/+: two (light) quarks “Semileptonic or lepton+jets channel”

Branching fraction: 29.6%

High statistics appropriate for rare events in the realm of top-physics


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Motivation of the Analysis

Determination of photon spectra at the CMS-experiment Separation of photons radiated off top-quarks

Distinction of various QED-coupling scenarios Lorentz-invariant vertex parameterisation

SM prediction at Born level

U. Baur, A. Juste, L.H. Orr, D. Rainwater„Probing electroweak top quark couplings at hadron colliders“Physical Review D 71, 054013 (2005)

€

Γμttγ q,q, k2

( ) = −ie γμ F1Vγ k2

( ) +γ5F1Aγ k2

( )[ ]{ +σ μν2mt

q+q( )νiF2Vγ k2

( ) +γ5F2Aγ k2

( )[ ] ⎫ ⎬ ⎭

€

F1Vγ = 2

3

€

F2Vγ =Qt

gt − 22

= 0

€

F1Aγ = F2A

γ = 0


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The Feynman-Diagrams

Three classes for the hard process g + g t + t + Photon (8 diagrams) q + q t + t + Photon (8 diagrams for q=u,d) q + q + Photon t + t (8 diagrams for q=u,d)


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The Dataset

Cross section for 2 3 process (TopRex generator) E,min> 5 GeV: 16.8pb E,min> 100 GeV: 0.2pb Compare to: 830pb for top-quark-pair production (NLO)

Indistinguishable processes for photons radiated off incoming quarks and top-quarks in the case of annihilation

Study of a Pythia tt-dataset, to get photons radiated off top-quarks (final state radiation)


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Photons in Signal EventsSignal event

Semileptonically (electron and muon) decaying tt-pair Photon radiated off a top-quark (top photon)

CMS tt-inclusive dataset Total number of events: 3,900,000 events About 3,800 potential signal events (generator filter) Less than 1,000 events remaining after preselection cuts

Need for a private dataset

Filtering tt-semileptonic signal events on generator level E >10 GeV ||< 2.5 in events Cross-section: 0.1 pb

19,950 signal events


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The Event SelectionSelection criteria applied on reconstructed objects

1 electron or muon candidate pT>20 GeV/c Isolated in tracker and

calorimeter

2 jets candidates from b-quarks Iterative cone algorithm (R=0.5) pT>20 GeV/c

2 jets candidates from light quarks

Iterative cone algorithm (R=0.5) pT>20 GeV/c

1 photon candidate E >20 GeV/c Isolated in tracker and

calorimeter


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Photon Classes

Try to match of every preselected photon candidates to a MC-photon

Distance in (,)-plane: (Rgen-reco) < 0.05 Deviation of energy |Egen-reco| < 0.1 Egen

Signal Photons Match of reconstructed photon candidate to generator top-photon

Background Photons Match of reconstructed photon candidate to any generator photon

but the top-photon

Fake Photons No match of reconstructed photon candidate to any generator

photon


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Calorimeter Energy Deposits (Signal Events)

Hadronic over electromagnetic energy ratio Energy deposited in HCAL behind ECAL-supercluster

(R=0.3 around line through supercluster midpoint)

Implement cut at R=0.2 Remove large tails of mainly fake photon candidates

ECAL

HCAL

Super-cluster

E(had)= E(i)with i R-cone

Signal PhotonsFake PhotonsBkg. Photons


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Shower Shape Variables(Signal Events)

Spread of electromagnetic energy in calorimeter cells Energy of clusters (squares and rectangles)

Compare ratio of different shape variables Cut on E(3x3) divided by E(5x5) Lowest overlap between signal and background/fake

distributions (normalized)

ECAL cells



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Isolation in Silicon Tracker

(Signal Events)Veto against tracks in the vicinity of the photon direction

Number of tracks in a cone around photon candidate

Varying cone size as well as ratio of track momentum and photon energy

No tracks within R=0.2 with pTrack > 0.1 EPhoton

Fake PhotonsSignal Photons


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Numerical Results of the Photon Identification

10,022 out of 19,950 events fulfil minimum tt-photon requirements

Results of the photon identification S/B improved by a factor of about 26 less than 25% of all signal photons lost

Preselection

Ehad/EEM, Shower Shape

Tracker Isolation Efficiency

Signal 6,027 5,107 4,539 75,3%

Background 971 344 195 20,1%

Fake 40,934 6,741 1,046 2,6%

S/B 1/7.0 1/1.4 3.7/1


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Photon Spectrum (Signal Events)

Energy spectrum of signal, background and fake photons Spectrum of signal photons slightly harder Increase photon energy cut from 20 GeV to a higher value




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Background to tt-Events

Events with a similar decay structure W/Z+Jets Vector-Boson pair production W+Photon tt-dileptonic ...

Signal-Background Separation Strategy Reject background events via an appropriate tt-event solution

(using Top Quark Analysis Framework) Remove photons as efficient as background and fake photons in

signal event

Incorporation of background events recently started First Overview for WZ- and Z+4Jet Events


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Number of Photons

Signal Events WZ-Events Z+4 Jets-Events

After Preselection


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Numerical Results

Demand for a high energetic photon essential already at preselection step

WZ: 1,446 out of 190,000 events Z+4Jets: 3,735 out of 21,402 event

Rejection of background and fake photons comparable to signal events Preselect

ion

Calorimeter

Isolation

Tracker Isolation

Efficiency

Background Photons

WZ 140 56 35 25.0%Z + 4

Jets450 210 119 26.4%

Fake Photons

WZ 4,754 1,073 124 2.7%Z + 4

Jets15,171 4,528 354 2.3%


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Conclusions

Signal Photon Identification using Calorimeter Isolation and Tracker Isolation

Robust criteria to reject background and fake photons Comparable efficiencies for various input dataset

tt-background demanding a proper tt-reconstruction Preselection already gives a reasonable separation Using TQAF should reduce that component further

tt-Photon analysis should ... consider angular relations between photon and objects of the tt-

decay(probably correlated to tracker isolation)

respect kinematic constraints if the tt-photon decay chain

Compare current results with TopRex-Dataset More realistic description of physics


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Efficiency Purity


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TQAF

Top Quark Analysis Framework (TQAF) effort of the CMS-top-group to establish a common framework for

top-quark analysis integration of CMS-standard tools (electron-ID, kinematic

fit, ...) analysis code used and debugged by many people benefit from work one has to do but it was already done in the

past

Three-Layered-Structure production of top-objects independent of final state and analysis

goal• lepton identification, calibration of jets, ...

Building of event solutions assuming a certain event hypothesis• combing jets to build a W-Boson, top-quark, ...

Actual analysis• direct access to objects according to a certain event solution• criterion to select best solution: MC-matching solution


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R-Cuts(Generator Particles)

Photon radiated off elektron/muon

x-axis: R(photon - elektron/muon)

y-axis: R(photon - top-Quark)

Photon radiated off top-quark x-axis: R(photon - top-quark) y-axis: R(photon -

elektron/muon)


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Identification of top-Photons

Energy (reco./gen.) Pseudorapidity(reco./gen.)

-Angel (reco./gen.)


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Efficiency of Signal Photon Identification

High efficiency in identifying the signal photonsTight cuts on energy and distance in (,)-plane


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Identifikation der top-Photonen

Kriterien zur Identifikation E(reco-gen) (reco-gen) (reco-gen)

Energie der generierten top-Photonen

Energie der rekonstruierte Photonen

study of tt-photon events with the cms-detector bad honnef 27. august 2007

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