1. Discovering (and understanding) SUSY at the LHC…
Alan Barr University of Oxford
… an introduction (with apologies to the many people who’s work I have included
unreferenced and to those whom I have left out)

2. LHC physics is about to get very interesting!

3. ATLAS control room

4. Have lots of cosmics events (these from much earlier)

5. Last chance to visit LHC
Relatively well-known German physicist takes her chance

6. Motivational arguments
+SUSY
Log10 (μ / GeV)
1/α
Invisible mass
Visible mass
The value of prejudice rapidly diminishing
stop
higgs

7. Explorer/experimentalists rule:
How to make a discovery?
cMSSM
Which way to search?
Who knows what?
Other SUSY?
Extra Dimensions?
Explorer/experimentalists rule:
Try to COVER ALL BASES

8. Signature-based hunts
Experiments see:
Jets, leptons, missing energy, b-jets
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

9. Example SUSY search Assume R-parity Look for: “Typical” SUSY spectrum
Mass (GeV)
“Typical” SUSY spectrum
Assume R-parity
Look for:
Jets from squark & gluino decays
Leptons from gaugino & slepton decays
Missing energy from (stable) LSPs

10. SUSY event
Missing transverse momentum
Heavy quarks
Jets
Leptons

11. Cross-sections etc “Rediscover” Lower backgrounds “Discover”WW ZZ
“Discover”
Higher backgrounds

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

13. Typical search: inclusive distributions
Trigger on jets + missing energy
Plot “effective mass”
Look for non-SM physics at high mass
Signal BG

14. Standard Model backgrounds: measure from LHC DATA
Measure in Z -> μμ
Use in Z -> νν
R: Z -> nn B: Estimated R: 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

15. Estimating the backgrounds
Good match to “true” background
Search region
Control Region
More from Davide Costanzo later in this session

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

17. Reach in cMSSM? Rule out with 1fb-1 WMAP constraints
’Focus point' region: annihilation to gauge bosons
Reach in cMSSM?
mSUGRA A0=0,
tan(b) = 10, m>0
Slepton Co-annihilation region
Rule out with 1fb-1
'Bulk' region: t-channel slepton exchange
WMAP constraints

18. Multiple channels for discovery
Below the lines = discovered
Different final states

19. Slide based on Polesello
What might we then know?
Can say some things:
Undetected particles produced
missing energy
Some particles have mass ~ 600 GeV, with 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 …
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”
Slide based on Polesello

20. Mapping out the new world
LHC Measurement
SUSY
Extra Dimensions
Masses
Breaking mechanism
Geometry & scale
Spins
Distinguish from ED
Distinguish from SUSY
Mixings,
Lifetimes
Gauge unification?
Dark matter candidate?
Some measurements make high demands on:
Statistics ( time)
Understanding of detector
Clever experimental techniques

21. 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
Example method shown here
Alternatives also on the market
Comparable precision
Look for sensitive variables (many of them)
Extract masses

22. Stransverse mass (MT2) method

23. Measuring the shapes Better precision possible than for endpoints
Systematic uncertinties need to be controlled
Much work here recently…

24. SUSY spin measurements
The defining property of supersymmetry
Distinguish from e.g. similar-looking Universal Extra Dimensions
Difficult to LHC
No polarised beams
Missing energy
Inderminate initial state from pp collision
Nevertheless, we have some very good chances…
Slepton spin from angles in Drell-Yan production
Neutralino spin from angles in decay chains l+ ~ θ q _ l-
+ lots of other recent work in this area

25. Other ways of measuring spin
Vector
Gluino
Scalar
Fermion
Squark
Cross-section depends on spin
If mass scale can be measured then spin can be inferred

26. Dark matter relic density?
mSUGRA assumed
Use LHC measurements to “predict” relic density of observed LSPs
Caveats:
Cant tell about lifetimes beyond detector (need direct search)
Studies done so far in optimistic case (light sparticles)
To remove mSUGRA assumption need extra constraints:
All neutralino masses
Use as inputs to gaugino & higgsino content of LSP
Lightest stau mass
Is stau-coannihilation important?
Heavy Higgs boson mass
Is Higgs co-annihilation important?
More work is in progress
Probably not all achievable at LHC
ILC would help lots (if in reach)

27. Covering all the bases…
Host of other searches:
Light stop squarks
R-parity violating models
Dileptons/trileptons with missing energy
Taus with jets & missing energy, …
Single photons
Diphoton resonances
Heavy l resonances
Heavy flavour excesses
Monojets
Same sign Stops …
See e.g. CMS Physics TDR II 2006
ATLAS SUSY discovery chapter 2008

28. 10 TeV … LHC run 2008 10 TeV run need not be “just “commissioning”
Lots of physics and discovery potential

29. Conclusions

30. Extra rations

31. Gauge Mediated SUSY Breaking
Signature depends on Next to Lightest SUSY Particle (NLSP) lifetime
Interesting cases:
Non-pointing photons
Long lived staus
Extraction of masses possible from full event reconstruction
More detailed studies in progress by both detectors

32. R-hadrons Motivated by e.g. “split SUSY”
Heavy scalars
Gluino decay through heavy virtual squark very suppressed
R-parity conserved
Gluinos long-lived
Lots of interesting nuclear physics in interactions
Charge flipping, mass degeneracy, …
Importance here is that signal is very different from standard SUSY

33. Exotic WW scattering Reconstruct hadronic + leptonic W pair
The ultimate test of electroweak symmetry breaking
Not unitary above ~1 TeV if no new physics BG BG
signal
Reconstruct hadronic + leptonic W pair
Require forward jets
Veto jets in central region
Most difficult case: continuum signal
5- significance with 30 fb-1 in most difficult case

34. Dijet masses: Contact Interactions
Reduce systematics by using ratio à la DZero
New physics in the central region
“Calibration” sample at higher rapidity
Uncertainties from proton structure not negligible
Improve with LHC data?
Detector cross-calibration uncertainties to be determined from data
Estimates here

35. RS Gravitons & heavy bosons
1.5 TeV Randall-Sundrum graviton -» e+e-
Randall -Sudrum graviton spin p θ
graviton e
Graviton is spin-2
Angular distributions
Discovery
Find mass peak
Characterisation
Measure spin

36. Spectacular states : micro Black Holes
Large EDs
Micro black hole decaying via Hawking radiation
Photons + Jets + …
We will certainly know something funny is happening
Large multiplicities
Large ET
Large missing ET
Highly spherical compared to BGs
Theory uncertainty limits interpretation
Geometrical information difficult to disentangle
sphericity

37. Black hole interpretation?
Slide from Lester

38. Some of the 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

39. 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/

40. 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)

41. 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)

42. 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

43. 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

44. Light stops 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
M(bjj)
1.8 fb-1
M(bl)
1.8 fb-1
GeV
GeV
Points: simulated data
Histograms: signal events (MC truth)

45. New vector boson: W’
Transverse mass plot for W’ => μ