# Presentation: 381535

## “ATLAS SUSY SEARCHES”

***Gianluca Comune***
***Michigan State University***
***On Behalf of the ATLAS Collaboration***
***PANIC 2005, Santa Fe’ 27/10/2005***

## LHC and ATLAS

***LHC***
***14 TeV CoM p-p collisions***
***Start of operations 04/2007***
***Total integ. luminosity 300 fb******-1***
***ATLAS***
- (A Toroidal LHC ApparatuS)
- General purpose detector
- Vast physics program
- Higgs, SUSY, Exotics, top, B physics...
***Staged ATLAS components:***
**One Pixel layer**
**Transition Radiation Tracker outer end-caps**
**Cryostat gap scintillators**
**Part of Muon drift tubes and half cathode strip layers**
**Part of forward shielding**
**Part of LAr read-out**
**Large part of trigger/DAQ CPUs**

- ATLAS

## SUSY and mSUGRA

**Every particle has a super-partner**
***“******Heaven” for particle physicists***
***MSSM Lagrangian depends on 105 parameters (!!)***
***Need to make some assumption to reduce the degree of freedom***
***mSUGRA depends on 5 (+1) parameters***
**M****0****, M****1/2****, A****0****, tan(****β****)****, sgn(****μ****), m****top**
**Assuming R parity conservation****         *****=> escaping LSP => large E******T******M******I******SS******  ******and scalar particles produced in pairs***
**Event cannot be fully reconstructed**
**SUSY is a bgd to itself**
**Various regions in the par. space**
**Coannihilation, Focus Point, Funnel, Bulk region**

**(Ellis et al., Phys. B565 (2003) 176)**

|  | M0 (GeV) | M1/2(GeV) | A0 | tanβ | sgn(μ) | mtop (GeV) |
| --- | --- | --- | --- | --- | --- | --- |
| Coannihilation | 70 | 350 | 0 | 10 | + | 175 |
| Focus point | 3550 | 300 | 0 | 10 | + | 175 |

## SUSY Production at LHC

***Production cross sections vary widely***
***From few to several hundreds pb******-1****** ***
***Actual kinematics and CS depend heavily on the chosen model***
***Long and complex decay chains***
***If R parity is conserved large E******T******M******I******SS***
***Powerful handle for Standard Model background removal***
***SUSY events have generally large jet multiplicity and large jet p******T***
***Depending on mass hierarchy multi lepton signatures as well***

## Inclusive Searches

** ****0 lept.**** **

***ATLAS Physics TDR***

**SM (PYTHIA)**

**10**** ****fb****-1**

***Discovery***
***Assuming luminosity 10******33****** cm******2 ******s******-1***
***1300 GeV => “1 week”***
***1800 GeV => “1 month”***
***2200 Gev => “1 year”***
***Backgrounds:***
***Real m******issing energy from SM processes with hard neutrino****** (******tt, ******W+jets, Z+jets******)***
***Fake missing energy from detector***
***Jet energy resolut******i******on (expec******ially non-gaussian  tails) critical****** ***

**(Fast parametric detector response) **

** ****1 jet with p****T ****>100 GeV****,**** 4 jets (p****T****>50 GeV) **
** ****E****T****M****ISS ****> max(100 GeV ,0.2M****eff****)**
** ****Transverse sfer****icity S****T****>0.2**
** ****No isolated muon or electron (p****T****>20 GeV)**

**1 TeV SUSY**** **

## Realistic Bgd Estimation

***Previous analysis uses Parton Shower for SM processes: ***
***=> badly underestimates hard jet emission***

***SM (ALPGEN+PYTHIA)***

- ATLAS

- ATLAS

***Recent ATLAS background studies:***
**-****hard process with exact ME computation**
**    ****-Alpgen, Sherpa (collinear and soft region through PS)**
**-****hadronization**** **
**    ****-HERWIG,PYTHIA**
**-Solve double counting problems**
**   ****-****MLM matching**

**Parton shower is a good model in collinear**
**region, but fails to describe hard jet emission**

**GeV**

***(p******T ******of hardest jet)***

## Inclusive Searches (2)

***High p******T****** jets are produced also in background processes***
**=> bad separation!!**
***E******T******M******I******SS******  ******excess can be ***
***E******T******M******I******SS ****** > 800 GeV***
***Need to optimize the selection***
***Meff still a good discovery signal (requiring 1 lepton)***

**0 leptons (preliminary)**

- ATLAS

**1 lepton**** **

**0 lepton mode**
**No leptons,  xEt****>****100GeV, >= 1 jet with pT>100GeV, >=4 jets with pT>50GeV, Transv. Sphericity >0.2**
**1 lepton mode**
**e,μ Pt >10 GeV, xEt****>****100 GeV, >= 1 jet with pT>100GeV, >=4 jets with pT>50GeV, Transv. Sphericity >0.2, Transverse mass between lepton and xEt >100GeV (to suppress W+N jets Background)**

- ATLAS

**Focus Point**
**4.2 fb-1**

**1 lepton**** **

- ATLAS

***SUSY production dominated by *********

***Red: signal***
***Black: bgd***

## Top Background estimate 

***The Top mass reasonably uncorrelated with E******T******M******I******SS***
***Select events w******ith m(lj) in top window ***
**apply W mass constraint **
**no b-tag used**
** ****Estimate combinatorial background with sideband subtraction. **
***Normalize to low E******T******M******iss ******reg******ion***** **
**SUSY contribution is small**
***Procedure gives estimate consistent with Top distribution also when SUSY ******i******s present***
**Z****+jets: big contribution from Z → **************
**Can use Z →ee, apply same cuts as analysis, substitute E****T****(ee) with E****T****miss**** and rescale by BRs.**

## SUSY Spectroscopy

***After SUSY is discovered it needs to be characterized***
***particle masses, spin ...***

***In every sequential double two body decay of the form***

***The maximum of the invariant Mass distribution is related to the initial particle masses through:    ***

***Use it on a “typical” SUSY decay chain    ***

***Formulas ******in Allanach et al., hep-ph/0007009***

## Leptonic Signatures

**ql(max)**

- ATLAS

**Larger of M(llq)**

**Coannhilation point  5.6 fb****-1**

- ATLAS

- ATLAS

**ql(min)**

- p

- p

***SM background negligible (could be a discovery signal)***
***Opposite-Flavour/Opposite Sign is subtracted****** (removes SUSY bgd)***

- ATLAS

- ATLAS

**Coannhilation P..**
**5.6 fb****-1**

- Mll (GeV)

- Mll (GeV)

- ATLAS

**Point 5a **
**4.37 fb****-1**

**Mod. Point 5**
**5.0 fb****-1**

**Black:t-tbar bgd**

## Tau Signatures

***Tau Signatures***

- Tau signatures play a very important role
- Tau BR relevant over a large portion of SUSY parameter space
- In stau coannihilation (              ) region is critical to reconstruct the stau mass (one tau is very soft)
- The relic dark matter density of the universe depends from the mass difference M1-M10 (very small)

- ATLAS

** ****Point  5A**
**4.4 fb-1**

- ATLAS

**(1 tau pT > 40 GeV, 1 Track pT>6 Gev**
**No other track pT > 1 GeV in R < 0.4)**

- Currently investigating a track seeded tau reconstruction algorithm

## SUSY Particle Masses 

***Once the edge values (and the errors) are known one can determine the SUSY particle masses***
- It is critical to understand how to fit all edges
- Work in progress
- Difficult to develope a true model independent approach
- More than one decay scenario (i.e. SUSY model) can lead to the same signature
- Need an independent measure of one of the SUSY particle to set the absolute scale

## Conclusions

**Few fb****-1**** of data should allow ATLAS to measure a clear excess over the SM contribution and reconstruct several mass relations.**** **
- this can be achieve in the first year of data taking depending on how quickly the detector and the SM backgrounds will be understood
**Large scale productions of Geant4 realistic detector simulated data**
- To understand detector systematics and prepare for real data analysis.
- Scan of parameter space to understand different problems
**Recent ATLAS (and CMS) collaboration efforts are focused on u****nderstanding of Standard Model backgrounds with the use of the latest Montecarlo tools**
**Developing strategies to validate the Montecarlo predictions with data. **

## Backup

**Jet should be matched to the parton generated with ME (R=0.7) except for the soft and collinear regions.**
**Blue show perfect matching between ME parton and jet.**
**Soft jet was emitted collinearly => Matched (Accepted)**
**One parton divided into 2 jets. (outside ME cone 0.7) => Not Matched**
**Event should be covered with 5jet ME (double counting) => Reject event**

**Matrix Element and double counting (MLM)**

***M. Mangano http://mlm.home.cern.ch/mlm***

## Other Background Sources 

***At startup calibration data will be limited***
***Miscalibrated detector is a source of ******E******T******M******iss****** ***

***QCD jets can add non gaussian tails to ******E******T******M******iss****** ***
***Very important given the CS***

## Other Endpoints

***Other Endpoints***

- ATLAS

**(using a mixed event technique for the SUSY bgd reduction)**

**Without t-tbar bgd**

**With t-tbar bgd**