# Presentation: 504661

## Searching for Supersymmetryat the LHC

***Searching for ******Supersymmetry******at the LHC***

- Sven Vahsen (Lawrence Berkeley Lab)

***UC Berkeley Physics 290E, Oct 28 2008***

*http://www.icepp.s.u-tokyo.ac.jp/~kenta/analysis.html*

- http://home.slac.stanford.edu/pressreleases/2006/20060821.htm

## Outline

- Motivation for SUSY
- Limits from Tevatron and LEP
- If SUSY exist, how to find it at the LHC?
- If SUSY discovered, what more can we learn at LHC?

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Motivation

## The SUSY/LHC Dream Scenario

**The SUSY/LHC Dream Scenario**

- Discover TeV scale Supersymmetry (SUSY) early!
- Spend LHC + SLHC years measuring SUSY !

- 2008

**2010**

**2012**

**2014**

**2016**

**2018**

**2020**

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## The Standard Model of Particle Physics

- Successful theory of fundamental interactions since early 1970s
- Survived numerous experimental tests
- Only Higgs missing
- LHC built to look for Higgs and Physics beyond the Standard Model...

*H*

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Supersymmetry (SUSY)

- Well motivated extension of the Standard Model
- Standard Model particles have supersymmetric partners
- Differ by 1/2 unit in spin
- Higgs sector: h,H,A,H+/- (skip today)
- EW-gaugino+higgsino mixing  2 charginos             4 neutralinos

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## SUSY Breaking

- No sparticles observed to date  if SUSY exists, then it is somehow broken
- MSSM: >100 free parameters
- Models of SUSY breaking mechanism
- mSUGRA
- GMSB
- AMSB
- Split SUSY
-  reduced number of model parameters
- These determine sparticle masses and phenomenology

**SU**

**SY**

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Keeps corrections to Higgs mass small. Requires wino and stop masses ~ few hundred GeV
Unifies gauge couplings at large Q2. Requires sparticle masses ~ few hundred GeV
Can provide plausible WIMP Dark matter candidates. Cosmological arguments prefer WIMP mass ~ hundred GeV 

- Unifies gauge couplings at large Q2. Requires sparticle masses ~ few hundred GeV
- Can provide plausible WIMP Dark matter candidates. Cosmological arguments prefer WIMP mass ~ hundred GeV

**Why We Like SUSY**

- With SUSY

- SM

**without SUSY**

**with SUSY**

-              Energy [GeV]

- W. de Boer, C. Sander
- Phys.Lett.B (2004)

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## R-Parity conservation  Stable LSP

- R = (-1)2j+3B+L  	SM particles have R=1, superpartners R=-1
- Even number of SUSY particles for every vertex
- SUSY particles always produced in pairs
- Get decay chains as below
*Lightest SUSY particle* (LSP) cannot decay, hence potential WIMP Dark Matter candidate

- q

- g

- ~

- qL

- ~

- l

- ~

- 

- ~

- p

- p

- q

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Is the Dark Matter Supersymmetric?

**Is the Dark Matter Supersymmetric?**

- http://home.slac.stanford.edu/pressreleases/2006/20060821.htm

- 

- ~

- ?

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## SUSY / Dark Matter Connection: LSP Relic Density

- WIMPs thermally produced
- Universe expands and cools, WIMP density reduced through annihilation
- Eventually density is too low for annihilation process to keep up with expansion rate.
- Freeze-out density depends on annihilation cross-section
- Need ~ weak cross-section and mWIMP~100 GeV to get DM density
- Amazingly, SUSY LSP good match!

- Freeze-out

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## mSUGRA and Dark Matter

- mSUGRA
- SUSY masses unify at GUT-scale m0, m1/2
- tanβ, A0, sign(μ)
- Neutralino LSP
- Four regions with ****NEUTRALINO  ****DM  due to enhanced annihilation in early universe

*Pseudo-projection* – no units!

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Exclusion Limits from Tevatron and LEP

## Squark and Gluino Mass Limits 

- Best limits from D0
- mgluino  > 308 GeV
- msquark > 379 GeV
- Evaluated within mSUGRA

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Older Experimental limits

**Older Experimental limits**

- Many other best limits still from LEP

- PDG 2006??

- PDG2006

- See update on next slide

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## If SUSY exist, how to discover it at the LHC?

## Tevatron  LHC

- Geneva

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Will the LHC be a SUSY Factory?

- If SUSY exists at the TeV scale, expect copious production of squarks and gluinos
- Just QCD, nearly independent of SUSY model

- σ (ppSUSY) calculated at NLO
- √s=14TeV, mSUSY ~ 0.5-1.0 TeV   σppSUSY ~ 1-100 pb

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Tevatron  LHC

**Tevatron ******** LHC**

- Win twice when moving to LHC
- σSUSY increases 20000(!) for mgluino=400 GeV
- S/B improves
- SUSY-discovery challenge
- reject SM by factor of ~1011
- understand SM events that survive SUSY selection

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## 

Two sparticles initially
Cascade decays down to LSP: jets, leptons
LSP escapes undetected: large ETmiss 

 Canonical SUSY signature: ETmiss, high-pT jets, often leptons

- Two sparticles initially
- Cascade decays down to LSP: *jets, leptons*
- LSP escapes undetected: *large E**T**miss *
-  Canonical SUSY signature: ETmiss, high-pT jets, often leptons

- q

- g

- ~

- qL

- ~

- l

- ~

- 

- ~

- p

- p

**Experimental Signature**

- q

- mSUGRA bulk region

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Most Generic SUSY signature: Missing ET + jets

**Most Generic SUSY signature: Missing E****T**** + jets**

- Event selection
- Jets 1 with pT>100 GeV
- Jets 2,3,4 with pT>  50 GeV
- ETMISS > 100 GeV
- ETMISS > 0.2 Meff
- Transverse Sphericity > 0.2
- Δφ(jet1,2,3, ETmiss) > 0.2
- veto events with isolated leptons
- Plot Effective Mass variable
- Meff = Σ|pTi| + ETmiss

- Excess at large Meffective potential discovery of SUSY

- Events with several hard,
- (often miss-measured) QCD jets: Monte Carlo predictions have large systematic uncertainties

- 1 fb-1

- mSUGRA bulk region

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Measuring Standard Model Backgrounds

***Measuring***** Standard Model Backgrounds**

- Have to estimate Standard Model background passing SUSY selection using data-driven techniques
- Example:  Select Z→ll and replace charged leptons by neutrinos
- Obtain shape of ETMISS, Meffective

- 1 fb-1

- mSUGRA bulk region

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Detector Performance Crucial

- QCD multijet is background with largest cross-section
- Rejecting this relies on good ETMISS resolution
- Can spoil ETMISS resolution with
- Miss-measured jets in cracks
- Calorimeter noise & inefficiencies
- Beam Halo & Cosmic Background
- Need to “clean up”
- use calorimeter timing, ETMISS from tracks vs calo, veto bad runs
- Won’t be easy to get there, took a lot of time at Tevatron

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## SUSY Search strategy

- Assuming background measurement have been carried out, and detector performance is under control...
- ... apply event selection, look for excess events at high Meffective
- Choose exact cuts to maximize number of discoverable models
- Works beyond mSUGRA, because other models typical produce additional final state particles

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## L ~ 1 fb-1: SUSY Discovery Potential

- Full simulation of backgrounds
- Includes expected systematic (JES, background estimation)
- Good chance of finding TeV scale SUSY with 1fb-1 of data
- Dream scenario!
- SUSY at higher mass scales could still show up later, but would make detailed studies difficult
- Ultimate LHC reach ~ 3 TeV

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Mass reach versus integrated luminosity 

- SUSY search most exciting during first LHC years!
- TDR result: Reach gluino / squark masses of
- 1.0 TeV with 0.1 fb-1
- 1.5 TeV with 1.0 fb-1
- 2.0 TeV with 10  fb-1
- 	(Outdated study, but shows nature of luminosity dependence)

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Missing ET + jets + 1 lepton

**Missing ET + jets + ****1 lepton**

- Require an additional lepton (=electron or muon)
- Drawbacks: less inclusive, slightly lower rate
- Advantages:
- higher S/B
- Better QCD multijet rejection, simpler background composition
- Transverse mass provides additional handle on remaining backgrounds
*If standard model backgrounds larger, or detector performance worse than expected, 1-lepton mode may be more powerful than 0-leptons*

- (1 lepton)

- (0 leptons)

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## 3rd Generation: τ- and b-jet modes

- Tau mode

**3****rd**** Generation: **τ- and b-jet modes

- Can require either b-tagged jet or (hadronic) τ in event selection
- Lower signal efficiency, but higher purity
- Dominant background is ttbar
- Improved discovery potential for high tanβ models (“SU6” on right), which have increased decays into 3rd generation

- b-jet mode

- 100 pb-1 SU3?

- 1 fb-1 SU3?

- 1fb-1

- 1fb-1

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## GMSB: photons

- Lightest SUSY particle (LSP) is Gravitino
- 2nd lightest (NLSP) in GMSB is Neutralino or Slepton
- Typical signatures for neutralino case
- Etmiss + jets + di-photons (prompt or non-pointing)

- Prompt photon case:
- High-pt photons after SUSY selection almost background free
- SUSY Discovery easy for this class of GMSB models

- 1fb-1 (after SUSY selection)
- very preliminary

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## What if squarks, sleptons are heavy?

- mSUGRA example
- Direct gaugino production dominates
- Signature:
- 3 isolated leptons + ET-miss
- few jets, at low PT (ISR only)
- Discovery: 10-100 fb-1 (under study)

- 10fb-1
- very preliminary

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## If SUSY found at LHC, what next?

## Home in on SUSY model

- Inclusive studies provide first constraints on model parameters
- Meffective   Mass scale
- Relative Significance in 0,1,2 lepton channels  m0,m1/2
- 3rd generation   tan

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Most promising: Opposite sign, same flavor di-leptons from single neutralino decay

Subtract background (from Standard Model and SUSY itself) using flavor information
 e+e- + μ+μ– - e+μ- - e-μ+(after efficiency correction)
Low background, relatively high statistics

- Subtract background (from Standard Model *and SUSY itself*) using flavor information
-  e+e- + μ+μ– - e+μ- - e-μ+(after efficiency correction)
- Low background, relatively high statistics

**Mass reconstruction**

- Position of mass-edge sensitive to combination of sparticle masses

- l+

- l-

- mSUGRA bulk region

- 1 fb-1

*In favorable models, can measure large part of mass spectrum at LHC – see future talk!*

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Do we have a dark matter candidate?

**Do we have a dark matter candidate?**

**ZEPLIN-2**

**Cross-section (cm****2****)**

- WIMP Mass [GeV/c2]

*Same WIMP in lab and space?*

*Calculate* neutralino mass m and cross section σ-nucleon
- Compare with direct detection

**                ****log****10**** (************p****si**** / 1pb)**

- 300 fb-1

*NOW*

*2009?*

*2012?*

- 300 fb-1

*BULK*

*FOCUS*

*FUNNEL*

*COAN.*

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Do we have a dark matter candidate?

*Calculate* neutralino relic density
- What fraction of the dark matter is neutralinos?** ** (****= ****DM ?)

** ****** h2 = 0.192 ± 0.005 (stat) 		                ± 0.006 (sys)
- 	(simulated point is *pre-WMAP)*

**No. MC Experiments**

**********h****2**

- 300 fb-1

*Caveat: this precision depends on assuming very constrained SUSY** **b**reaking scenario. In more general scenario: much looser constraints.*

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Spin Determination

- Measure spin of new particles to distinguish e.g. UED and SUSY
- Method that seems to work for many SUSY models: direct slepton* *production
- Occurs through s-channel spin-1 process only
- Characteristic angular  distribution in production

- q

- q

- _

- e+

- e-

- Z/γ

- e+

- e-

- ~

- ~

- ~

- ~

- Normalised cross-sections

- AJB hep-ph/0511115

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Nice idea, but...

**Nice idea, but...**

- Low x-section
- Background from Standard Model and SUSY itself
- Needs lots of data and favorable SUSY model
- Plot on left: 500 fb-1 !

- SUSY   PS   UED

- SUSY “data”

- mSUGRA bulk region

- AJB hep-ph/0511115

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## The Dream Scenario

**The Dream Scenario**

- Discover TeV scale Supersymmetry (SUSY) early!
- Spend LHC + SLHC years measuring SUSY !

- 2008

**2010**

**2012**

**2014**

**2016**

**2018**

**2020**

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Conclusion

- Convincing SUSY search requires detailed understanding of Detector and Standard Model events passing SUSY selection
- If SUSY exists at TeV-scale (where it seems most motivated), LHC should find it early (~1fb-1)
- In this case, rates at LHC would be high, allowing detailed studies of SUSY in large number of final states
- Degree of progress at LHC will then depend on Nature’s benevolence in breaking SUSY
- Favorable scenarios suggest SUSY mass reconstruction and precise calculations of m, σ**** possible with ~300fb-1 of data
- When combined with Astroparticle & Cosmology measurements, this would reveal the relation of the SUSY LSP to the dark matter
- If SUSY mass scale is higher, LHC may still find SUSY, but detailed studies may not be possible

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Backup slides

## A Toroidal LHC Apparatus (ATLAS): All of detector important for SUSY search!

_**Electromagnetic Calorimeter**_

**Muon spectrometer**

**Inner Detector**

_**Hadronic Tile Calorimeter**_

**Solenoid**

**Toroid**

- A Toroidal LHC Apparatus (ATLAS): All of detector important for SUSY search!

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Fitting for edges after flavor subtraction

- Method sensitive to any sleptons lighter than 2nd neutralino

- 1 fb-1

- 18 fb-1

- SUSY

- Standard Model

- l+

- l-

- 0.5 fb-1

- Two light sleptons,
- Coannihilation region

*Bulk region*

*Low mass point*

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## mSUGRA

- Behavior determined by only 4 parameters + a sign at GUT scale
- m0   	common scalar mass
- m1/2 	common higgsino/gaugino mass
- A0 	common higgs-sfermion-sfermion coupling
- sgn(μ)  sign of susy conserving Higgs mass
- tanβ    vu/vd ratio of Higgs VEV’s
- These determine masses and couplings of sparticles at weak scale via renormalization group equations [RGEs]

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E

## Predicted Background Uncertainties

- Current ATLAS SUSY group prediction for *accuracy* with which we will estimate Standard Model background after SUSY selection
- Used 50% (Z,W,top) and 100%(QCD) uncertainties for 100pb-1

|  | statistical | systematic | combined σN/N @1 fb-1 |
| --- | --- | --- | --- |
| Z+4 jets | 13% | 8% | 15.3% |
| W+4 jets | 4% | 16% | 16.5% |
| Top | 4% | 15% | 15.5% |
| QCD | 5% | 75% | 75.2% |

- Sven Vahsen, LBNL

- UC Berkeley Physics 290E