1. ATLAS: Missing Transverse Energy in the Search for Supersymmetry
Christine McLean
The College of William and Mary
University of Michigan REU
Advisor: Dr. Richard Teuscher August 11, 2011

2. ATLAS – A Toroidal LHC ApparatuS
Multipurpose LHC Detector

3. The Search for Supersymmetry - SUSY
Solve Standard Model Problems
Hierarchy Problem
What keeps the Higgs Mass from Blowing Up?
100 GeV [Electroweak] vs. 10^19 GeV [Gravity]
Sparticles – Supersymmetric Partner Particles
Favored Model: mSUGRA
1. The Hierarchy problem
2. SM has no candidate for existing cold dark matter
3. It does not include gravity
Electroweak –W and Z; Gravity –Planck  large difference, higgs couples to everything, why doesn’t it blow up?
-mSUGRA: Super gravity mediated SUSY breaking -> breaking unequal masses

4. Goal: Supersymmetry Detection
Found from missing transverse energy in the detector
MET left by SM background and neutralinos
-built up plots of MC simulation data to aid in this detection

5. Missing ET – SM Background
W+Jets
Z+Jets
QCD jet (mismeasurement)
T Tbar
-jets have large cross-section
-t tbar  W
-decays
-picture is of a Z->ee candidate event

6. Early Plots – Jet Sample
-MonteCarlo simulation

7. Monte Carlo Plots – Background and Signal
Jet Pileup Bug
-looked at a lot of variables: MET and jet characteristics, Meff
-especially importants ones for analysis: MET and Meff
-- Meff importance: “It has also the interesting properties that for SUSY events it, the Meff distribution, peaks at a value which is strongly correlated with the mass of the pair of SUSY particles produced in the proton-proton interaction. It can therefore be used to quantify the mass-scale of SUSY events.”

8. Cuts for Signal Region A
Different cuts on the data for different SUSY production processes
Focused on signal region A: light- ˜q˜q production
Source:
Search for squarks and gluinos using final states with jets and missing transverse momentum with the ATLAS detector in sqrt(s) = 7 TeV proton-proton collisions

9. Important Cuts – Delta Phi and Effective Mass (Meff)
Meff = Jet Pt + MET; Meff >500 GeV
Delta Phi Absolute Value> 0.4
- Calorimeter resolution gives missing et b/c doesn’t measure jet back to back properly

10. Plots after Meff Cut

11. Plots After Last Official Cut

12. One Additional MET Cut – MET > 225 GeV
-this cut makes SUSY more visible and increases the signal significance

13. Data and Future
Full Monte Carlo Analysis Can be used to Compare to LHC Data
Z->ee Data to Look for SUSY in Two Lepton Mode
MC needs to be scaled to Data Luminosity First
ATLAS Work in Progress
Two-lepton mode
-Opposite sign dipletons -> Four or more jets
-Same sign dipletons -> Four or more jets
-describe what I learned here

14. Travels and Food

15. Acknowledgements
REU Leaders: Homer Neal, Steve Goldfarb, Jean Krisch, Junjie Zhu, and Lauren Rugani
Advisor: Richard Teuscher
ATLAS Canada Graduate Students: Travis Bain, Nikolina Ilic, and Reyhaneh Rezvani
Fellow REU and CERN summer students
NSF and CERN

16. Hierarchy Problem - Backup
Source: Travis Bain

17. Hierarchy Problem - Backup
Source: Travis Bain

18. Supersymmetry Unification - Backup
Source: Travis Bain

19. Monte Carlo - Backup
Event Generator– generates random numbers and decides whether event happens based on probability
ATLAS – after generating events, showers them through the detector to get a readout simulation

20. SM Missing ET Background and Cuts- Backup
Source: Nikolina Ilic

21. Cut Flow Table - Backup