Finding Conversion Electrons Using Tracker

1
Finding Conversion Electrons Using Tracker

• Wing To, Victor Pavlunin, David Stuart
• UC, Santa Barbara

• Introduction Why photon conversions?
• Method How to find them?
• Single Photon Sample
• MinBias Sample
• QCD Jets Sample
• Electron ID variables
• Conclusion

2
Introduction

• This is a follow up on Victor Pavlunins EGamma
  Meeting Dec. 2007.
• In start-up data we need to validate Electron ID
  variables.
• EE/TrkP and HadE/EE.
• ?f(track, EBC) and ??(track, EBC).
• Need a sample of electrons from first data.
• Z?ee, pure but small production rate due to µb
  cross section.
• Photon are in every events from p-zero decays.
• Find Photon Conversions with only Tracker
  variables.
• These conversion electrons can make an unbias
  calibration and validation of Electron ID
  Variables.

EBC Ecal Basic Clusters
3
Method Finding Conversion with Tracks

• To find conversions in the pixel part of the
  tracker we use CTF Tracks seeded by first 2
  layers of TIB.
• Electrons from conversion have
• small ?f, ?cot(?), and ?z0
• displaced vertex so d0 will be non-zero.
• sum of the 3 Momenta of two tracks points away IP.

cm
cm

• At ?f 0. The tracks are right on top of each
  other.
• Displaced Vertex of 20 cm in Y. Conversion
  occurred in TIB1.

cm
cm
Fig. 4ab Conversion Candidate Tracks
4
Method cuts used to find conversions

• Find appropriate cuts variables using 10000,
  30GeV Single Photons and compare the cut
  variables to 1M MinBias Events.

? MinBias Events
Photons Events?
MinBias Events ?
Photons Events ?
Normalized d0charge distribution of Tracks (cm)
Normalized ?cot(?) Distribution of Tracks

• Fig 4a Single Photon Events (Blue) have small
  ?cot(?) compare to MinBias Evts (Red)

• Fig 4b Conversion occurs off IP. The d0 will be
  non-zero for conversion electrons. By convention
  d0charge is always positive for conversion
  electrons.

5
Method cuts used to find conversions
Photons Events?
MinBias Events ?

• Table 5b Cuts on two tracks used to find
  conversions.
• Track parameter z0 is not well measured due to
  pixel less tracking

Normalized Track Cross (TX) distribution of
Tracks (cm)

• Fig 5a Distance between two tracks at their
  minimal approach (TX).
• ?f is arbitrary for two tracks since track_f
  changes.
• Use a derived variable Track Cross distance (TX).

6
Simulated Single Photons

• 105 Single Photons Events with Pt 2-30 GeV
• Every Track in event is an electron
• ECal Filter 2 EBC of 5 GeV
• Conversions found.
• 606 tight conversions, 561 (92) matched to
  SimTracks.
• 902 loose conversions, 798 (88) matched to
  SimTracks.
• Unrealistic but it can be used to check our other
  results.

cm/cm

• Match conversion tracks to SimTracks with the
  same Pt, ? and f within their respective errors
  at vertex.
• Scroll between Slide 6 and 7 to see match-up in
  XY-plane

7
Simulated Single Photons

• 105 Single Photons Events with Pt 2-30 GeV
• Every Track in event is an electron
• ECal Filter 2 EBC of 5 GeV
• Conversions found.
• 606 tight conversions, 561 (92) matched to
  SimTracks.
• 902 loose conversions, 798 (88) matched to
  SimTracks.
• Unrealistic but it can be used to check our other
  results.

cm/cm

• Match conversion tracks to SimTracks with the
  same Pt, ? and f within their respective errors
  at vertex.
• Scroll between Slide 6 and 7 to see match-up in
  XY-plane

8
Simulated MinBias

• Lets try to find Conversions in MinBias Sample
• Similar to first data at the LHC.
• Fewer tracks per event.
• Tracks are generally soft, most tracks dont
  reach ECal.
• Low pt tracks scatters larger of their
  momentum.
• Needs to run large number of Events, so we use an
  EBC filter 2 x 2GeV
• Tight cuts for higher purity
• Found only 12 conversions out of 106 MinBias
  Evts.
• 7 (58) Match both SimTrks.
• Loose cuts for higher statistics
• Found only 24 conversions out of 106 MinBias
  Evts.
• 12 (50) Match both SimTrks.
• Need to Simulate 100M MinBias to get good
  statistics at this rate.
• Need sample with more photons.

9
Simulated QCD Jets

• QCD Jet Events 105 with Pt 50 GeV
• More photons per event.
• More p K can fake electron tracks.
• Photons made from p0 will be inside Jets.
• QCD Jet Conversions
• 1504 tight conversions,1265 (84) matched to
  SimTracks.
• 2515 loose conversions, 1752 (70) matched to
  SimTracks.
• 2515 is enough to start looking at Electron ID
  variables.

10
Results in QCD Jets Sample

• First Lets look at something simple. R in the
  XY-plane where the conversion occur.
• Structures seen 5cm, 8cm, 11cm are each layer
  of Pixel Tracker.
• The first Layer of TIB is found at 22cm

Fig 10a Distribution of R in XY plane (Rxy) for
conversion candidates.
Conversion Rxy in Jet Events (cm)
11
Results in QCD Jets Sample

• We need to connect the sample of electrons to
  Electron ID variables.
• First propagate the track to the radius of the
  ECal. Then search for the nearest EBC.

Fig 11a ?f(track, EBC) for conversion in Jets
Fig 11b EM Energy / Trk Outer P for conversion
in Jets

• The nearest EBC is sharply peaked around zero
  with small tail.
• E/P is peaked at 1 as expected for electrons.
  However left side has a large shoulder.
• These are not pure electrons. We need a method
  to remove the background from plots.

12
Background Prediction

• Predict the background by extrapolating from
  background dominate region into signal region.
• Use ?Cot(?) for example.
• Fig. 10a shows ?Cot(?) of all tracks. The blue is
  the signal region where the red is the background
  region.

• Zooming into the background region, we can see
  that the background is linear. Then we can simply
  extrapolate a straight line into the signal
  region.

?
?
Fig. 10a ?Cot(?) between 2 tracks
Fig. 10b ?Cot(?) between 2 tracks zoomed
13
QCD Jet Sample with Bkg Subtraction

• Fig 12 a b
• Background subtraction removed most of the
  conversion that occur at R lt 5 cm and between 15
  to 22 cm.

?

• Fig 12 c d
• The left shoulder was also removed by the
  background prediction

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QCD Jet Sample with Bkg Subtraction
Fig 13 a b HadE/EE tail was reduced but still
exist
?
?
Fig 13 c d Most of the side band were remove
by background subtraction in both ?f and ??
distributions.
?
?
Fig 13 e f ?? has a const cut at 0.05 which is
seen here on the left.
?
?
15
Compare to Single Photon Events
?
?ConversionR Photons
?
?
Fig. 15a ConvR Jets bkg_sub photon events.
Fig. 15b E/P Jets with bkg_sub photon events.
?

• Photon events have lower number of entries and
  histograms are rescaled accordingly.
• H/E still has a tail after Bkg subtraction.
• Conversion occurs inside Jets. Hadrons are
  sometimes right on top of electrons.
• Granularity of HCal 4x of ECal. Overlap to
  nearby hadrons is more likely.

?
Fig. 15c H/E Jet with bkg sub photon events.
16
Compare to Single Photon Events
?
?
?
?
Fig. 16a ?f(track, EBC) Jets with background
subtraction photon events.
Fig. 16b ??(track, EBC) Jets with background
subtraction photon events.
17
Conclusion

• Tracker Only Conversion Finder finds conversions
  in Single Photons, MinBias and Jet Events.
• The purity depends on how tight we cut.
• Background prediction gives good fake subtraction
  in large data sample.
• Startup Data 100 Hz ( 8M evts / day )
• At Ideal MinBias 100 conv / day.
• Real Data will have EGamma HLT Triggers
• 1x, 2x Ecal hits within 5x5 crystals with energy
  gt 5 GeV. Simulated by ECal Filtering.

18
Extra

• Pre-Search ECal Filtering
• First used, 2 Basic Clus of 2GeV in 0.03 ?.
• Second, two EBC of 5 GeV to simulate HLT.

• If we use the background part of the Jet Sample
  and reconstruct the invariant mass of two tracks
  using m 0.140 GeV (pion mass since most tracks
  are pions.)
• We find a mass peak at 0.500 GeV (K_short).

Evt
GeV
19
Extra Extra
cm

• Calibration between the tracker and ECal can be
  done by plotting ?f(track, EBC) as a function of
  ? or vice versa.
• Tracker Material can be mapped using conversion.
  Conversions occurs in high density area such as
  support structures and cables.

cm
20
Extra Extra Extra

• Separate the Jet Sample into one with Low H/E
  (lt0.5) and High H/E (gt1.0).
• Low H/E obviously are electron from E/P plot.
• High H/E have some electron but contain a lot of
  non-EM particles.

• H/E as a function of E/P in Jet Events.
• Most events are Low in H/E.
• High H/E event then to have E/P less than 1.