Final Presentation - DESY · Final Presentation Working with the... Philip Diessner, Julia Iturbe DESY Zeuthen September 6th, 2011

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


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).


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


ATLAS Detector

> Four major components

> Inner detector: momentum of each charged particle

> Calorimeter: energies

> Muon spectrometer

> Magnet system


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


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


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.


...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


Z→e+e-

>Clearer signal

>High cross section

→good statistics

>Comparable to leptonic top events (at least partly)


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


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


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


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


Isolation :

> Isolation of electron quantified with:

> Shape is different for electrons and jets

> Separation possible

ETconeETcone

ETcone

R0=∑RR0ET−ET electroncandidate


Getting the efficiencies

>Use Tag and Probe

> Fit signal and background

→ Number of signal events


Comparison of the two Methods

Only small differences →no bias introduced


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


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.)


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


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


The End

Thank you for your attention

> Questions can be sent to:

[email protected] or [email protected]

> 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:

[email protected] or [email protected]

mailto:[email protected]


http://www.atlas.ch/

http://press.web.cern.ch/public/en/LHC/LHC-en.html



Final Presentation - DESY · Final Presentation Working with the... Philip Diessner, Julia Iturbe DESY Zeuthen September 6th, 2011

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