Study of tt-Photon Events
with the CMS-detector
Bad Honnef
27. August 2007
Thomas Hermanns
III. Physikalisches Institut B
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 Vtb1
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
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
2
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


  
 

tt q, q, k 2  ie   F1V k 2   5 F1A
k2 




2

2
q

q
iF
k


F
k



2V
5 2A
2mt

 
  
 
SM prediction at Born level

F1V 


Thomas Hermanns
2
3

F2V
 Qt
gt  2
0
2

F1A
 F2A  0

U. Baur, A. Juste, L.H. Orr, D. Rainwater
„Probing electroweak top quark
couplings at hadron colliders“
Physical Review D 71, 054013 (2005)
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
3
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)
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
4
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)
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
5
Photons in Signal Events
Signal 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
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
6
The Event Selection
Selection 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
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
7
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
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
8
Calorimeter Energy Deposits
(Signal Events)
Hadronic over electromagnetic energy ratio

E(had)= E(i)
Energy deposited in HCAL behind ECAL-supercluster
(R=0.3 around line through supercluster midpoint)
with i  R-cone
Implement cut at R=0.2

Remove large tails of mainly fake photon candidates
Signal Photons
Fake Photons
Bkg. Photons
HCAL
Supercluster
ECAL
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
9
Shower Shape Variables
(Signal Events)
Spread of electromagnetic energy in calorimeter cells

Energy of clusters (squares and rectangles)
ECAL cells
Compare ratio of different shape variables


Cut on E(3x3) divided by E(5x5)
Lowest overlap between signal and background/fake
distributions (normalized)
Signal Photons
Fake Photons
Bkg. Photons
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
10
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
Signal Photons
Thomas Hermanns
Fake Photons
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
11
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
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
12
Photon Spectrum
(Signal Events)
Energy spectrum of signal, background and fake photons


Spectrum of signal photons slig htly harder
Increase photon energy cut from 20 GeV to a higher value
Signal Photons
Fake Photons
Bkg. Photons
Thomas Hermanns
Bad Honnef, 27. September 2007
Signal Photons
Fake Photons
Bkg. Photons
III. Physikalisches Institut B
13
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
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
14
Number of Photons
Signal Events
WZ-Events
Z+4 Jets-Events
After Preselection
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
15
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
Background
Photons
Fake
Photons
Thomas Hermanns
Preselection
Calorimeter
Isolation
Tracker
Isolation
Efficiency
WZ
140
56
35
25.0%
Z+4
Jets
450
210
119
26.4%
WZ
4,754
1,073
124
2.7%
Z+4
Jets
15,171
4,528
354
2.3%
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
16
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
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
17
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
18
Efficiency  Purity
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
19
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
•

Building of event solutions assuming a certain event hypothesis
•

lepton identification, calibration of jets, ...
combing jets to build a W-Boson, top-quark, ...
Actual analysis
•
•
Thomas Hermanns
direct access to objects according to a certain event solution
criterion to select best solution: MC-matching solution
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
20
R-Cuts
(Generator Particles)
Photon radiated off top-quark


x-axis: R(photon - top-quark)
y-axis: R(photon - elektron/muon)
Photon radiated off elektron/muon


x-axis: R(photon - elektron/muon)
y-axis: R(photon - top-Quark)
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
21
Identification of top-Photons
Energy (reco./gen.)
Pseudorapidity
(reco./gen.)
-Angel (reco./gen.)
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
22
Efficiency of
Signal Photon Identification
High efficiency in identifying the signal photons
Tight cuts on energy and distance in (,)-plane
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
23
Identifikation der top-Photonen
Energie der rekonstruierte Photonen
Energie der generierten top-Photonen
Kriterien zur Identifikation



E(reco-gen)
(reco-gen)
(reco-gen)
Thomas Hermanns
Bad Honnef, 27. September 2007
III. Physikalisches Institut B
24