SUSY SEARCHES WITH ATLAS ELIZABETH SHIERS

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 )

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

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

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

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

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

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

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

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

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

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

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

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!

Extra Slides

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

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

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

Electroweak comparison Disappearing track analysis Chargino travels before it decays into a neutralino and charged pion As mass of 𝜒 2 0 becomes closer to 𝜒 1 0 chargino lifetime is reduced, disappearing track cannot be used Low cross sections mean many points cannot pass preselection