1. Supersymmetry at the LHC: Status and a Thought or Two about the Future
RPM Seminar
LBNL
October 27, 2016
Supersymmetry at the LHC: Status and a Thought or Two about the Future
Bruce A. Schumm
Santa Cruz Institute for Particle Physics
University of California, Santa Cruz

2. References…
I’ve been too lazy to note each result, reference-by-reference. They are all available from the following two public WEB pages, CMS and ATLAS, respectively (the CMS URL isn’t accidentally truncated, even though it seems it must be!):
Bruce Schumm

3. Stabilize the Higgs Mass
SUSY Motivations I
Stabilize the Higgs Mass
If we try to situate SU(3)SU(2)U(1) in a grand-unified theory, the Higgs self-energy term wants to be at the unification (e.g. Planck) scale.
Elegant solution: posit a mapping
Fermion  Boson
between SM particles and an as-of-yet undiscovered set of “superpartners” with identical couplings but opposite spin-statistics.
Provides a cancellation of the dangerous vacuum contributions to the Higgs self-energy term.
In fact, SUSY naturally prefers a light Higgs, which is what we observe!
Bruce Schumm

4. Natural Dark Matter Candidate
SUSY Motivations II
Natural Dark Matter Candidate
To avoid lepton/baryon number violation can require that “SUSYness” is conserved, i.e., preserves an additive “parity” quantum number R such that RSM = +1; RSUSY = -1
If you can’t get rid of SUSYness, then the
lightest super-symmetric particle (LSP) must
be stable  dark matter, missing energy
LSP is typically a “neutralino” (dark matter
must be neutral); admixture of ,
known as “10, whose identity is not that
relevant to phenomenology
Note: This review will be restricted to searches to RP-conserving process. Missing energy (ETmiss) is assumed to always be in use as a discriminating variable.
Bruce Schumm

5. Gauge Coupling Unification
SUSY Successes I: Unification
Gauge Coupling Unification
SM + desert
MSSM
MSSM = Minimal Supersymmetric Model = minimal set of states and couplings needed to preserve known phenomenology when SM is embedded in SUSY framework
Bruce Schumm

6. SUSY Successes II: Higgs Mass
Xt is a combination of fundamental GUT-scale SUSY parameters (masses, trilinear coupling strength); m~t2 an effective squared stop mass
Bruce Schumm
Baer, Barger, Savoy, arXiv: [hep-ph]

7. A Striking Aspect of Nature: mtop >>> mbottom
Bruce Schumm

8. Baer, Barger, Savoy, arXiv:1502.04127 [hep-ph]
SUSY Successes III: Top Mass
Higgs scalar potential contribution m2Hu must be negative to generate EWSB (“Mexican Hat” potential)
GUT-scale unification sets m2Hu > 0; but radiative corrections arise in its evolution to the EW scale: mH
t = ln(Q2) (RGE scale)
ft = top-mass Yukawa coupling
S expected to be small
SUSY+EWSB “predicts”
100 < mt < 200 GeV
Baer, Barger, Savoy, arXiv: [hep-ph]
Bruce Schumm

9. Formal Motivation for SUSY
The set of spacetime symmetries of 3+1 dimensional field theory form the Poincare Group.
Fermions fit within the fundamental representations of the Poincare group; bosonic representations are formed from tensor products of the spinor representations  fermions and bosons have different formal statures
Extensions of the Poincare group that attempt to establish a reciprocity between fermionic & bosonic representations (e.g. the “cover” group SL(2,C) ) generally introduce a symmetry transformation between bosons and fermions  SUSY
I think one can certainly say it's pretty, and unifies the representations in an intriguing way.  It's certainly in the category of things "it would be a shame if nature didn't take advantage of". - Michael Dine
In addition, SUSY may be a necessary condition for quantizing gravity*.
We do put stock in our formal structures
SU(3) flavor states (“8-fold way”)
Black holes
“A typical stationary point of some string effective action is unstable unless it is at least approximately supersymmetric” M. Dine again…

10. But where are these states?  SUSY must be broken!
SUSY States
SUSY posits a complete set of mirror states with SSUSY = |SSM – ½|
But where are these states?
 SUSY must be broken!
Bruce Schumm

11. SUGRA: Local supersymmetry broken by supergravity interactions
SUSY Breaking
SUGRA: Local supersymmetry broken by supergravity interactions
Phenomenology: LSP (usually 10) carries missing energy.
GMSB: Explicit couplings to intermediate-scale (MEW <  < MGUT) “messenger” gauge interactions mediate SUSY breaking.
Phenomenology: Gravitino ( ) LSP; NLSP is 10 or . Content of 10 germane.
AMSB: Higher-dimensional SUSY breaking communicated to 3+1 dimensions via “Weyl anomaly”.
Phenomenology: LSP tends to be , with 1+, 10 nearly degenerate.
But still, we would naturally expect these states to be within spitting distance of the EW scale…
Bruce Schumm

12. mGMSB Strong vs. Electroweak Production
STRONG COUPLING
However: if colored states are decoupled, EW production will dominate
Dedicated EW prod. analyses
Pure-EW simplified models
 probe high mass scale
steep mass dependence (~M-10)
beam energy vs. luminosity
lower backgrounds
mGMSB
ELECTROWEAK COUPLING
s = 7 TeV
5 fb-1 reach EW
Strong
 probe intermediate mass scales
higher backgrounds
benefit from high L.dt
SUSY Breaking Scale  (TeV)
Bruce Schumm

13. e.g. General Gauge Mediation (GGM):
Classes of Models
Minimal Models
Simplified Models
GUT unification  few parameters
mSUGRA/CMSSM, GMSB
e.g. GMSB:
Λ: SUSY breaking scale
Mmes: Messenger scale
N5: Number of messenger fields
tan: Ratio of vev for the two Higgs doublets
Cgrav: Gravitino mass parameter
sgn(): Sign of higgsino mass term
e.g. the “SPS 8” model is just a specific set of choices for the GMSB parameters.
e.g. General Gauge Mediation (GGM):
Start with GMSB and decouple all but a few states; properties of these state become “free”
Bruce Schumm
ICHEP 2016: SUSY with Photons and Taus

14. SUSY States: A Closer Look
Neutralinos (n0) formed from linear combina-tions of neutral Wino, Bino, Higgsino states
Charginos (n) formed from linear combina-tions of charged Wino Higgsino states
SUSY Higgs sector has two complex doublets  five physical degrees of freedom
Bruce Schumm

15. The LHC
Bruce Schumm

16. ATLAS (and ~CMS) Luminosity Integration
No Results yet
Some Results
Many Results
Bruce Schumm

17. The ATLAS Detector
Non-prompt tracks
Photon conversions
Bruce Schumm

18. CMS Detector
Bruce Schumm

19. Photons, leptons, (tagged) jets, composites (boosted objects)
SUSY Tools I
From the tracks and clusters in the detector, we assemble a collection of objects to be used as tools in our search for SUSY above pesky SM backgrounds.
Photons, leptons, (tagged) jets, composites (boosted objects)
The more specific you can be, the easier it is to reject backgrounds, but the more model dependent your analysis
ETmiss: Transverse momentum imbalance
LSP escapes detection (RP conserving SUSY)
Meff, HT, etc: Transverse energy scale
Strong production can reach high mass scales
“Scale chasing”
Bruce Schumm

20. Favorite Discriminating Variables
x : Minimum  separation between ETmiss vector and any object of type X.
LSP produced in intermediate-to-high mass decay
Separation between LSP and decay sibling
Jet backgrounds tend to have small separation (combinatoric)
Generic Kinematics:
Transverse mass (and its descendants), Razor variables, T…
Tend to exploit generic kinematic features of high-mass production with invisible decay products
Empirically-oriented approach
Bruce Schumm

21. Here We Go…
OK – Let’s Find SUSY!
Bruce Schumm

22. Obvious place to start: gluino production…
SUSY Cross Sections
Obvious place to start: gluino production…
Bruce Schumm

23. SUSY 101: Gluino Production Searches
High cross-section gluino production is natural place to start
Most simple of all models is gluino  10 + X through virtual gluon or squark channel.
If a squark channel, then can
be dominated by particular
flavor of jets (ttbar, bbar, qqbar)
Free parameters of the model
are gluino, LSP (10) masses
Bruce Schumm

24. Basic Gluino Searches
CMS: Very basic search, requiring 3 jets and various amounts of HT and Etmiss (targeting different regions in the (mg,m) space)
At each point in the space, set limit with HT,Etmiss requirement giving the best expected limit.
Bruce Schumm

25. Basic Gluino Searches (cont’d)
Limits on gluino mass (in this context) in TeV range
Limits generally less constraining than expected  consistent excess of observed events relative to expectations.
What might ATLAS have seen?
Bruce Schumm

26. No excess observed… ATLAS Gluino Searches via t,b
Include requirement of multiple b jets
More sensitive
More model-dependent
No excess observed…
Bruce Schumm

27. 0 leptons, 2-6 jets (and of course Etmiss)
ATLAS Gluino Searches, Flavor-Blind
0 leptons, 2-6 jets (and of course Etmiss)
Similar sensitivity to CMS (slight excess in one region)
Bruce Schumm

28. So, lightest stop can’t decouple; must be at EW scales
Light Stop: the Celebrity Squark
Why so?
Argument: The stabilization of the Higgs mass (centerpiece of SUSY motivation) must be dominated by stop, since couplings are largest
So, lightest stop can’t decouple; must be at EW scales
However… Howie Baer, ICHEP 2016: “The idea that a light stop is a necessary requirement of naturalness has been debunked” (ask him for details…).
 Stop searches demoted from “critical” to “highly motivated”…
Bruce Schumm

29. Stop Search Landscape (ATLAS Approach)
Depending on stop-10 mass splitting m, top may or may not appear in decay chain
0-lepton, 1-lepton analysis for large m
Multi-lepton analysis for lower m
Bruce Schumm

30. Putting it all together…
Stop masses towards 900 GeV can be excluded, but only for light 10.
Analysis more challenging when top-decay channel is closed
Small region for m0 addressed by monojet (“MJ”) analysis; more on this soon.
Observed limits below expected…
Bruce Schumm

31. CMS and Light Stop
Somewhat different approach from CMS… But results largely the same
Again, observed limits a bit below expectation…
These low-mass production scale studies benefit well from increased statistics
Bruce Schumm

32. Event Variable Example: T and Compressed Stop
Also require HT = jetsET > GeV
T = ETj2/MT
j2 = subleading jet
MT = dijet transverse mass
Force two jets for this calculation
(excess again…)
Particularly sensitive to
mstop  m in ~ 300 GeV range
Randall & Tucker, arXiv:
Bruce Schumm

33. EW SUSY is intrinsically more difficult to find Small cross section
Electroweak SUSY
Possible scenario: only accessible states are not strongly coupled (gauginos, higgsinos, sleptons)
EW SUSY is intrinsically more difficult to find
Small cross section
Worse signal-to-SM-background ratio
In fact, there are some suggestions that accessible states will be weakly interacting:
Dark matter is weakly interacting
If it is borne out, (g-2) points to weakly-coupled virtual state
Bruce Schumm

34. SUGRA-inspired EW Searches
Key to EW searches has been leptons (plus photons when we get to Gauge Mediation)
CMS and ATLAS:
Two opposite sign leptons, or three leptons not all of same charge; not necessarily of same flavor.
Kinematic cuts to remove W,Z backgrounds (mT etc…)
Bruce Schumm

35. Multi-lepton searches for EW SUSY
CMS update with 13 fb-1 of 13 TeV data
“Compressed” region
Slepton ½ way between production and LSP mass
Slepton just above LSP mass
Sensitive to masses approaching 1 TeV, but significant model dependence
“Compressed region” challenging
Bruce Schumm

36. Compressed SUSY Spectra
Compressed spectra in SUSY quite plausible (some would even say preferred):
Naturalness again:
Mass parameter associated with set of Higgsino states should be at EW scale. i.e., nearly-degenerate set of lightest gauginos
Cosmology: Having multiple degrees of freedom at WIMP scale more readily produces observed dark-matter abundance (“co-annihilation”)
Bruce Schumm

37. Nearly-Degenerate EW Scenarios
Common (production) scale for 1, 20
Explore reach in m(20,10)  0
New dedicated trigger (last 10 fb-1):
ETmiss > 50 GeV
2 muons > 3 GeV
Offline Etmiss > 125 GeV
Also require at least 100 GeV of jet activity to suppress high cross-section backgrounds
Relies somewhat on ISR to boost system & enhance observables
Over all SRs, expect 36 events, see 19
Bruce Schumm

38. Signature: Single lepton + Higgs
Other Gaugino Decay Channels: Higgsinos
Signature: Single lepton + Higgs
Limits are still very forgiving
Bruce Schumm

39. Highly Degenerate SUSY: the Monojet Analysis
Pure Etmiss trigger; offline requirements include
Etmiss > 250 GeV
Jet with pT > 250 GeV
Can search for stop in the region m  mc
Low-mass production  will benefit greatly from increasing luminosity
Bruce Schumm

40. LSP-Only Scenarios
What if the only accessible SUSY partner is a single, lightest neutralino (0)?
SM gauge structure (U(1) is abelian; explicit algebra of SU(2)) forbids leading-order pair production of any admixture of Bino or neutral Wino states.
One possibility is a multiplet of effectively-degenerate charged and neutral higgsino states (also motivated by AMSB SUSY breaking; sensitivity would be provided by the monojet analysis (has this ever been explored?).
Otherwise, LSP-only scenarios seem to be the unique purview of direct-detection experiments!
Bruce Schumm

41. Gauge Mediation Scenarios
Generally speaking, gauge mediation has several predominant characteristics:
LSP is the nearly-massless gravitino
NLSP is either
Stau
Neutralino, typically all or mostly bino-like
 Photonic and tau-onic final states particularly common for gauge-mediation-inspired searched
Bruce Schumm

42. Diphoton + Etmiss Analysis (3.2 fb-1)
(mg,m) focus points for optimization:
(1500, 100) for low-mass 10
(1500,1300) for high-mass 10
No significant difference found for optimal selection for 3 fb-1 at 13 TeV
 single diphoton+Etmiss SR 1 2 1 2
Run I Results
Significant requirements only on Etmiss and transverse mass scale Meff
 fully efficient even for mbino  mgluino and mbino  0
Bruce Schumm

43. No Events Observed in SR
Diphoton + Etmiss Results (3.2 fb-1)
Optimization calls for demanding requirements on Etmiss and Meff leaving little background
Gluino mass limits (for case of purely bino-like NLSP) in range of GeV
No Events Observed in SR
Bruce Schumm

44. Photon + Jets Analysis (13.3 fb-1)
Separate optimization for:
low-mass bino (“L”; many jets in cascade)
high-mass bino (“H”; large Etmiss)
Run I Results L H
Optimization reckoned by considering two points in each region (see “L” and “H” above)
Bruce Schumm

45. Photon + Jets Results (13.3 fb-1) PRELIMINARY
FIRST PUBLIC PRESENTATION
At these high mass scales (Meff > 2000 TeV) there is little SM background.
Observation of 3 events (SRL) when 0.78  0.18 are expected is 2% likely.
Combined-SR gluino mass limits (for case of higgsino/bino NLSP with 50/50 /Z branching) as high as 2 TeV
 ATLAS-CONF
Bruce Schumm

46. Multi-Tau Analysis (3.2 fb-1)
2 low-background multi- SRs geared towards mSUGRA model with high gluino mass and large mass gap
1 multi- SR geared towards GMSB model
Bruce Schumm

47. mGMSB Limits for Tau Analyses (3.2 fb-1)
Nominal gluino mass limits as high as 2.3 TeV, but note that *L=0.7 events for gluino pair production at this mass  Limit coming from lower-mass EW production, plus GMSB constraints!
Bruce Schumm

48. Some Pet Projects at SCIPP
Bruce Schumm

49. Some Pet Projects at SCIPP
Bruce Schumm

50. Some Pet Projects at SCIPP
Bruce Schumm

51. So Where Do We Stand?
Bruce Schumm

52. So Where Do We Stand? How “ruled out” is SUSY?
One approach: “Radiatively Driven Natural SUSY” in the SUGRA framework. Note that this framework
admits gauge-coupling unification
predicts a light Higgs
strongly prefers a heavy top
Within this framework
Measure of fine tuning provided by
(H. Baer& Associates, Phys. Rev. D87 (115028), 2013).
Bruce Schumm

53. “Natural” SUSY Spectrum
Stolen from H. Baer
Bruce Schumm

54. So Where Do We Stand?
Bruce Schumm

55. What are the luminaries saying?
Stepping (Way) Back…
What are the luminaries saying?
Rumors of my death have been greatly exaggerated…
I never… no… I never said that. NO.
SUSY is dead?
Bruce Schumm

56. And so the Search Goes On…
Bruce Schumm

57. BackUp
BackUp…
Bruce Schumm

58. Post-LEP SUGRA mH Prediction
SUSY partners below 20 TeV
SUSY partners below 5 TeV
Bruce Schumm

59. ISO: The Stop Squark
Bruce Schumm

60. Mono-X experiments for LSP-only scenarios EW production
Back-Up
Some things to cover
Gluinos
Squarks – light stop
Mono-X experiments for LSP-only scenarios
EW production
Razor/Edge analyses Maybe CMS CMS-SUS ?…
GMSB
Small-splitting case
R-parity violation
Non-pointing, tracklets, low-pt leptons
Bruce Schumm

61. e.g. General Gauge Mediation (GGM)
Classes of Models
Minimal Models
General Models
GUT unification: few parameters
mSUGRA/CMSSM
mGMSB
e.g. mSUGRA:
m0: GUT scale common scalar mass
m½: GUT scale common gaugino mass
tan: Ratio of Higgs doublet VEVs
A0: Common trilinear coupling
Sgn(): Higgs mass term
e.g. General Gauge Mediation (GGM)
Bruce Schumm