1. Discovering and Exploring the New World SUSY Searches with ATLAS
Alan Barr, UCL

2. Signature-based hunts
Astro/cosmo motivation for model-independent signatures
We’re pretty sure there are WIMPs out there
LHC produces Dark Matter + something visible
Invisible particle could be:
Lightest SUSY particle
Lightest KK particle
Lightest generic parity-odd particle
Signature:
Missing energy + Xvis + Xvis
Benefit: Same search finds multiple different models
Drawback: You ain’t so sure what you’ve got when you find it

3. (S)particles SM SUSY quarks (L&R) leptons (L&R) neutrinos (L&?)
squarks (L&R) sleptons (L&R) sneutrinos (L&?)
Spin-1/2
Spin-0
After Mixing
 Z0 W±
gluon
B W0
Bino Wino0 Wino±
gluino
Spin-1
4 x neutralino
Spin-1/2
gluino h0 H0 A0 H± ~
H0 H± ~
2 x chargino
Spin-0
Extended higgs sector
(2 doublets)

4. SUSY Dark Matter
mSUGRA A0=0,
tan(b) = 10, m>0
Slepton Co-annihilation region: LSP ~ pure Bino. Small slepton-LSP mass difference makes measurements difficult.
'Focus point' region: significant h component to LSP enhances annihilation to gauge bosons ~
Ellis et al. hep-ph/
Disfavoured by BR (b  s) = (3.2  0.5)  (CLEO, BELLE)
c01 t1 t
g/Z/h ~
'Bulk' region: t-channel slepton exchange - LSP mostly Bino. 'Bread and Butter' region for LHC Expts.
c01 l lR ~
Also 'rapid annihilation funnel' at Higgs pole at high tan(b), stop co-annihilation region at large A0
0.094    h2  0.129
(WMAP)

5. Example SUSY spectrum Assume R-parity “Standard search” Look for:
Mass (GeV)
Assume R-parity
“Standard search” Look for:
Jets from squark & gluino decays
Leptons from gaugino & slepton decays
Missing energy from (stable) LSPs
Enhanced taus at large tan beta
Enhanced b-jets at large tan beta
“Typical” SUSY spectrum

6. An Event

7. Lots of channels to look at
Below the lines = discovered
Different final states
CMS Physics TDR vol II makes good reading!

8. https://twiki.cern.ch/twiki/bin/view/Atlas/CscSusyNotes
Where are we now?
SUSY1: Data-driven Estimation of Z/W backgrounds to SUSY
SUSY2: Data-driven Estimation of top Backgrounds to SUSY
SUSY3: Data-driven Estimation of QCD Backgrounds to SUSY
SUSY4: Estimation of Heavy Flavor backgrounds and associated systematic
SUSY5: Searches and inclusive studies for SUSY events
SUSY6: Exclusive measurements for SUSY events
SUSY7: Gaugino direct productions
SUSY8: Studies for Gauge mediated SUSY
Reconstruction
Detector response
Model backgrounds
Measure backgrounds

9. Importance of detailed detector understanding
Et(miss)
Lesson from the Tevatron
GEANT simulation already shows events with large missing energy
Jets falling in “crack” region
Calorimeter punch-through
Vital to remove these in missing energy tails
Large effort in physics commissioning

10. First landfall? Inclusive distributions
Trigger on jets + missing energy
Plot “effective mass”
Look for non-SM physics at high mass
Signal BG

11. Precise measurement of SM backgrounds: the problem
SM backgrounds are not small
There are uncertainties in
Cross sections
Kinematical distributions
Detector response

12. Standard Model backgrounds: measure from LHC DATA
Measure in Z -> μμ
Use in Z -> νν
R: Z -> nn
B: Estimated
Example: SUSY BG
Missing energy + jets from Z0 to neutrinos
Measure in Z -> μμ
Use for Z -> 
Good match
Useful technique
Statistics limited
Go on to use W => μ to improve

13. Observation on measuring backgrounds “from the data”

14. Independent variables?

15. It’s not just ETMiss!
Potential large object reco systematics in busy environment
Jet reconstruction
Lepton isolation
B-jet efficiencies
Tau reconstruction
Jet veto for gauginos …
Trigger
We need to measure these separately
TTbar is “nearest thing” in many cases

16. Triggers?
Pb-1
Recall that we are interested in only one event in a billion

18. R-hadrons in detectors
Signatures:
High energy tracks (charged hadrons)
High ionisation in tracker (slow, charged)
Characteristic energy deposition in calorimeters
Large time-of-flight (muon chambers)
Charge may flip
Trigger:
Calorimeter: etsum or etmiss
Time-of-flight in muon system
Overall high selection efficiency
Reach up to mass of 1.8 TeV at 30 fb-1
GEANT simulation of pair of R-hadrons
(gluino pair production)

19. GMSB: non-pointing g event
h cluster
(1st layer – IP)
h rec poiting
(egamma obj) g G ~
c01 ~

20. ?

21. What might we have found out?
Assume we have MSSM-like SUSY with m(squark)~m(gluino)~600 GeV
See excesses in these distributions
Can’t say “we have discovered SUSY”
Can say some things:
Undetected particles produced
missing energy
Some particles mass ~ 600 GeV, couplings similar to QCD
Meff & cross-section
Some of the particles are coloured
jets
Some of the particles are Majorana
excess of like-sign lepton pairs
Lepton flavour ~ conserved in first two generations
e vs mu numbers
Possibly Yukawa-like couplings
excess of third generation
Some particles contain lepton quantum numbers
opposite sign, same family dileptons …
Fun for later!
Slide based on Giacomo’s

22. Imperatives Concentrate on: CSC notes! Understanding the systematics
Detector, trigger, reconstruction
Measuring the backgrounds
Many to consider
Optimising the search strategies
Power + credibility!
Exercising your sea-legs
CSC notes!

23. Told you we’d find India!

24. Some sources CMS Physics TDR, Volume II (recent)
CERN-LHCC
ATLAS Physics TDR (older)
CERN-LHCC
Physics at the LHC 2006
Programme
SLAC School 06
Polesello, Hinchliffe
SUSY06
Polesello, Spiropulu
Missing ET tails:
Paige
SM background
Okawa et al,
WMAP constraints
Ellis et al
SUSY mass extraction
Gjelsten et al
SUSY Spin:
Barr
Exotic SUSY
Parker
Dark Matter
Nojiri et al
R-hadrons
Kraan et al
Hellman et al
WW scattering
Stefanidis
GMSB
Zalewski, Prieur
RS Graviton: Allanach et al, Traczyk
Black Holes
Charybdis, Tanaka, Brett, Lester
Stephanidis

25. Extra rations

26. SUSY mass scale

27. SUSY mass measurements
Try various decay chains
Extracting parameters of interest
Difficult problem
Lots of competing channels
Can be difficult to disentangle
Ambiguities in interpretation
Experiments have been studying this for ~ a decade
Lots of effort has been made to find good techniques
Look for sensitive variables (many of them)
Extract masses

28. SUSY production cross-sections

29. Constraining masses with cross-section information
edges
inclusive cross-section ptmiss > 500
combined
Combine with Markov Chain MC
Edges best for mass differences
Formulae contain differences in m2
Overall mass- scale hard at LHC
Cross-section changes rapidly with mass scale
Use inclusive variables to constrain mass scale
E.g. >500 GeV ptmiss
Lester, Parker, White hep-ph/

30. Focus point
Lari et al
Heavy Scalars
Light gluino, gaugino
300 fb-1

31. More on GMSB
Negligible contribution from the SM backgrounds (consistent with TDR)
 Trigger efficiencies of the signal is crucial for the discovery potential
(background rejection, rate estimates would be the next step)
G1a (L=90TeV)
G1a (L=90TeV)
<After Requiring>
Meff > 400GeV
EtMiss>0.1Meff
two leptons
BG Total
BG Total g1 g2
Leading Photon Pt (GeV)
2nd Leading Photon Pt (GeV)

32. Baryonic R-Parity Violation
Decay via allowed where m( ) > m( )
Use extra information from leptons to decrease background.
Sequential decay of to through and producing Opposite Sign, Same Family (OSSF) leptons
Test point

33. Leptonic R-Parity Violation
RPV has less missing Et Neutralino -> stau tau stau -> tau mu qq Large rate of taus - smoking gun
Stau LSP
Phillips

34. Light stops Lari, Polesello
Stop pair production: 412 pb (PROSPINO, NLO)
Dominant (~100%) stop decay: t → c+ b → c01 W* b
Final state is very similar to top pair production events.
4 jets, 2 of which b-jets, one isolated lepton, missing energy
All of them softer (on average) than in top pair production
Invariant mass combinations will not check out with top, W masses
Lari,
Polesello
M(bjj)
1.8 fb-1
M(bl)
1.8 fb-1
GeV
GeV
Points: simulated data
Histograms: signal events (MC truth)