1. SUSY SEARCHES WITH ATLAS
ELIZABETH SHIERS

2. What is SUSY? Relationship between bosons and fermions
Every particle there is a corresponding ‘sparticle’
Spin of a sparticle is changed by ½: Fermion -> Boson
Guage bosons also have partners named gauginos
Superpartners of Higgs are allowed to mix with gauginos
Forming charginos ( 𝝌 𝟏,𝟐 ± ) and neutralinos ( 𝝌 𝟏,𝟐,𝟑,𝟒 𝟎 )

3. Why look for SUSY? R-parity provides a dark matter candidate
Solution to the Higgs hierarchy problem
Running of couplings becomes unified at high energy, GUT theory
SUSY is a broken symmetry
Sparticles will have high masses on TeV scales

4. How to look for SUSY Large Hadron Collider, CERN
Collides high energy protons
Events are analysed at collision points in detectors

5. The ATLAS detector Multi layer detector consisting of:
The Inner Detector
Electromagnetic Calorimeter
Hadronic Calorimeter
Muon Spectrometer

6. The Inner Detector Measures momentum and charge of a particle
Encased inside a 2T solenoidal magnet
B-field directed along horizontal beam axis
Therefore transverse momentum determines bending
--> Unique Signature
Transition Radiation Tracker (TRT) – measures charge from ionised gas

7. The Calorimeters & Muon Spectrometer
ECAL - measures energy of particles produced in EM showers (i.e electrons and photons)
HCAL – high energy hadrons interact with matter and produce showers
Muon’s do not cause a shower in HCAL and are too heavy for ECAL
Muon’s lose energy via ionisation so dedicated muon spectrometer can detect them

8. The Trigger System
Detector has over 10^8 electronic channels  rejection factor of 10^7 needed
Event must first pass a trigger system
Trigger system consists of;
Level 1 – decision based on calorimeter data and muon spec data (jets, high transverse momentum muons)
Level 2 – Takes accepted data, and inner detector data (low energy electrons and heavy quarks)
Event Filter – Event is fully reconstructed and placed in storage

9. Scenarios searched for at ATLAS
Natural SUSY to constrain
Stops and sbottoms should be below 1TeV
R-parity means sparticles produced in pairs
R-parity means LSP has nothing to decay to
Search for missing transverse momentum
Three main R-parity conserving scenarios:
Squarks and Gluinos (largest cross section)
Third generation squarks
Electroweak (smallest cross section)
Use simplified models

10. Scenarios searched for at ATLAS
Squark & Gluino
e.g Squark pair production followed by a direct decay to quark and neutralino
Third Generation Squark
e.g Stop decaying to top and neutralino
Electroweak
e.g Chargino-pair pro- duction

11. Analysis and signatures
Analysis is specific to particles in the final state
Signal Regions: based on monte carlo and associated SM background, optimised for each SUSY model
Control Regions: optimised for background and least sensitive to SUSY
Validation Regions: Cross check SRs and CRs
Number of events in each region counted and compared to determine if there is any excess beyond SM
In the case of no excess, data is used to set exclusion limits
Direct squark production

12. ATLAS results in simplified models
Stop masses up to ~700GeV excluded, chimneys where mass difference between neutralino and stop is too close to the top mass
For 𝝌 𝟏 𝟎 =𝟏𝟎𝟎𝐆𝐞𝐕, 𝐬𝐮𝐪𝐚𝐫𝐤 𝐦𝐚𝐬𝐬 𝐮𝐩 𝐭𝐨 𝟖𝟓𝟎𝐆𝐞𝐕

13. ATLAS results in simplified models
Chargino production invloving WW, chargino masses up to ~180 GeV excluded
Chargino-neutralino involving WZ, neutralino masses up tp ~420 GeV excluded
~270 GeV for Wh
Lepton mediated:
Chargino masses ~470 GeV excluded
Neutralino masses ~700 GeV excluded

14. Conclusions and future plans
No evidence of SUSY so far
Many regions left to explore, especially the electroweak searches
13 TeV run excluded higher masses
Targeted chimey regions
Run 3 will run at 14 TeV and the search for SUSY continues!

15. Extra Slides

16. Comparison with PMSSM models
Simple models only consider a small number of particles
Cannot consider the whole MSSM as there would be 105 free parameters
Consider pMSSM which applies experimental constraints
19 free parameters (individual particle masses etc)
R-parity is conserved
All sparticle masses must be below 4TeV – experimental limitation
Each set of 19 parameters is called a model point
Must pass initial experimental constraints such as the Higgs mass
310,327 model points passed preselection and have been analysed
Three steps:
Must satisfy minimum cross section
Monte carlo generates event sample
Model points are then excluded if there is no excess beyond the standard model

17. Comparison with pMSSM models
Genrally consistent
pMSSM predicts existence of particles whose masses lie between neutralino and gluino
Lead to signatures not covered in simplified analysis
Gluino decays produce low energy quarks which fail to meet jet requirements
pMSSM considers monojet analysis which can detect these
Higher sensitivity in regions where gluino mass is close to neutralino mass

18. Direct stau production
Masses up to 109 GeV excluded for one neutralino mass scenario

19. Electroweak comparison
Disappearing track analysis
Chargino travels before it decays into a neutralino and charged pion
As mass of 𝜒 becomes closer to 𝜒 chargino lifetime is reduced, disappearing track cannot be used
Low cross sections mean many points cannot pass preselection