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

ATLAS – A Toroidal LHC ApparatuS Multipurpose LHC Detector

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

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

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

Early Plots – Jet Sample -MonteCarlo simulation

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.”

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

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

Plots after Meff Cut

Plots After Last Official Cut

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

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

Travels and Food

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

Hierarchy Problem - Backup Source: Travis Bain

Hierarchy Problem - Backup Source: Travis Bain

Supersymmetry Unification - Backup Source: Travis Bain

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

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

Cut Flow Table - Backup