final presentation - desy · final presentation working with the... philip diessner, julia iturbe...

Final Presentation

Working with the...

Philip Diessner, Julia IturbeDESY ZeuthenSeptember 6th, 2011

Philip Diessner and Julia Iturbe | September 6th 2011 |

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



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