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Final Presentation
Working with the...
Philip Diessner, Julia IturbeDESY ZeuthenSeptember 6th, 2011
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 2
Introduction
> Brief introduction to LHC and ATLAS Experiment
> ATLAS detector
>Motivation: Top Physics
>Method
>Results: Calculated efficiencies and scaling factor of tight cut using the Z→e+e- events and introducing the isolation variable
ETcone
0.2
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 3
LHC
> Huge scientific instrument near Geneva
> World's largest and most powerful particle accelerator
> Purpose: find or exclude the Higgs boson and look for physics beyond the SM (push knowledge foward).
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 4
ATLAS Experiment
> One of the two general-purpose detectors at LHC
> Investigates a wide range of physics: search for the Higgs boson, extra dimensions, particles that could make up dark matter...
> ATLAS records sets of measurements on the particles created in collisions - their paths, energies, and their identities.
> 3000 scientists, 174 institutions, 38 countries
The ATLAS Collaboration
> One of the two general-purpose detectors at LHC
> Investigates a wide range of physics: search for the Higgs boson, extra dimensions, particles that could make up dark matter...
> ATLAS records sets of measurements on the particles created in collisions - their paths, energies, and their identities.
> 3000 scientists, 174 institutions, 38 countries
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 5
ATLAS Detector
> Four major components
> Inner detector: momentum of each charged particle
> Calorimeter: energies
> Muon spectrometer
> Magnet system
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 6
Tracking detector system
> Pixel detectors: high granularity, high precision set of measurements. 80 million pixels cover an area of 1.7 m2
> Semiconductor Tracker (SCT): 8 layers of silicon microstrip detectors
> Transition Radiation Tracker (TRT): straw detectors, electron identification using Xenon gas
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 7
Calorimeter
> Metal plates (absorbers) and sensing elements (se)
> Interactions in the abs transform the incident energy into a "shower" of particles that are detected by the se.
> Inner sections: se = liquid argon.
> Outer sections: se = tiles of scintillating plastic.
"shower" of particles
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 8
Motivation : Top Physics
> The study of top quarks produced at the LHC provides validation of the SM and could play an important role in the discovery of new physics.
> (SM) tt pair production is an important bkg for searches of the Higgs boson.
> tt study → New physics that modifies the production and/or decay of top quarks.
> The study of top quarks produced at the LHC provides validation of the SM and could play an important role in the discovery of new physics.
> (SM) tt pair production is an important bkg for searches of the Higgs boson.
> tt study → New physics that modifies the production and/or decay of top quarks.
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 9
...Top Physics : boosted tops
> High energy events → tt with much higher invariant mass than the combined mass of the top quarks → decay products are boosted in the direction of flight of the quark
b-jet electron
Excess of energy boosts the top
Light jets
b jet
b jetp
p
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 10
Z→e+e-
>Clearer signal
>High cross section
→good statistics
>Comparable to leptonic top events (at least partly)
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 11
Method: Tag and Probe
> Tag
one electron passing certain selections
Need strong identification
> Probe
Low reconstruction level
Z→e+e-
Convenience of the method:In wanted event are two electrons
Select a good oneNeed little evidence for second one
M ee= pe1 pe22;
pe=E ,p
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 12
Monte Carlo Simulation
>Comparison between experiment and theory
>Creating random events out of theory and look at results
>Can't simulate reality perfectly
> Introduce Scale Factor
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 13
Cuts and Efficiencies
>Many electron candidates at container level (track connected to shower region)
> Introducing identification cuts
Lowering jet background
Also lowering electron number
>Describing this with an efficiency
>Use it for the Scale Factor
eff =N cut
N container
SF=eff dataeff MC
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 14
Everything's good?
R=2
2 ; =−ln tan
2
>Distance between electron and jet in η-φ-space:
ΔR<0.4
> Invariant mass not good for small distances
> Still, well studied
>Needed for comparison with new methods
>Use Tag and Probe
> Fit signal and background
→ Number of signal events
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 15
Isolation :
> Isolation of electron quantified with:
> Shape is different for electrons and jets
> Separation possible
ETconeETcone
ETcone
R0=∑RR0ET−ET electroncandidate
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 16
Getting the efficiencies
>Use Tag and Probe
> Fit signal and background
→ Number of signal events
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 17
Comparison of the two Methods
Only small differences →no bias introduced
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 18
Result: Scale Factors as function of η and φ
>Relatively uniform distribution as to be expected
> Almost mirror-symmetric
> Fluctuations depending on active detector material crossed by particles
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 19
Result: Efficiencies and Scale Factor as function of ΔR
SF(ΔR<0.4)=0.745 ±0.045(stat.) ±0.290(sys.)
Data
Roodecay
Roodecay
SF(ΔR<0.4)=0.745
>
±0.290(sys.)
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 20
Uncertainties for Isolation ΔR<0.4
Systematic Problems:
> Finding right function
>Mass window
>Cuts on tag electron
Statistical Problems:
>Not enough data
>No stable fit
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 21
Conclusion
> It was possible to find a scale factor for small distances between electrons and jets
> things need to be done:
Find right functions for signal and background distribution
Data accumulation for better statistics
Apply to case of boosted tops
Philip Diessner and Julia Iturbe | September 6th 2011 | Page 22
The End
Thank you for your attention
> Questions can be sent to:
julia.mariana.iturbe.ponce@desy.de or philip.diessner@desy.de
> References:
ATLAS Experiment, URL: http://www.atlas.ch
CERN – The Large Hadron Collider URL: http://press.web.cern.ch/public/en/LHC/LHC-en.html
Feynman Diagrams for Top Physics Talks and Notes, URL: http://www-d0.fnal.gov/Run2Physics/top/top_public_web_pages/top_feynman_diagrams.html
Thank you for your attention
> Questions can be sent to:
julia.mariana.iturbe.ponce@desy.de or philip.diessner@desy.de
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