Searching for Supersymmetry with Final- State Photons at ATLAS
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Transcript of Searching for Supersymmetry with Final- State Photons at ATLAS
Searching for Supersymmetry with Final-
State Photons at ATLAS
UC Davis High-Energy
Physics Seminar
November 6, 2012
Bruce A. Schumm
Santa Cruz Institute for Particle Physics
University of California, Santa Cruz
Searches for SUSY with photons at ATLAS 2Bruce Schumm
SUSY States
SUSY posits a complete set of mirror states with SSUSY = |SSM – ½|
• Stabilize Higgs mass for GUTs
• Can provide reasonable dark-matter candidate
• Minimum of two Higgs doublets
Searches for SUSY with photons at ATLAS 3Bruce Schumm
To avoid lepton/baryon number violation can require that “SUSYness” is conserved, i.e., preserves a multiplicative “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 “1
0 ”, whose identity is not that relevant to phenomenology
R Parity
00 ~,
~,
~HW 0B
Searches for SUSY with photons at ATLAS 4Bruce Schumm
But we know that SUSY is broken…
SUGRA: Local supersymmetry broken by supergravity interactions
Phenomenology: LSP (usually 10) carries missing energy.
GMSB: Explicit intermediate-scale “messenger” gauge couplings to some number of “secluded” superfields mediate SUSY breaking.
Phenomenology: Gravitino ( ) LSP; NLSP is 10 or . Content of 1
0 germane.
AMSB: Higher-dimensional SUSY breaking communicated to 3+1 dimensions via “Weyl anomaly”.
Phenomenology: LSP tends to be , with 1+, 1
0 nearly degenerate.
SUSY Breaking
G~ ~
W~
Searches for SUSY with photons at ATLAS 5Bruce Schumm
Minimal GMSB has five parameters:
: Scale at which SUSY “kicks in” to cure quadratic divergences
Mmess: Scale of new gauge interactions
Nmess: Number of fields in secluded sector
tan: Ratio of <vev>s of two Higgs doublets
sgn(): Sign of SUSY Higgs mass parameter
“Minimal” (Standard) GMSB Phenomenology
Over most of GMSB parameter space, NLSP is a bino-like
neuatralino, i.e., the partner of the U(1) gauge boson
Searches for SUSY with photons at ATLAS 6Bruce Schumm
If the 10 NLSP is bino-like,
then it decays to -gravitino with BF cos2W.
Since SUSY states come in pairs (R-parity) this produces a striking + + ET
Miss signature
Photon Signatures in GMSB
For minimal GMSB (SPS8)
mgluino msquark >> m
“EW production” Associated leptons, jets
“SPS8 Trajectory” of GMSBSingle free parameter • tan = 15 • Mmess = 2• Nmess = 1• > 0
Searches for SUSY with photons at ATLAS 7Bruce Schumm
Strong vs. Electroweak Production
SUSY Breaking Scale (TeV)
probe high mass scale steep mass dependence (~M-8) beam energy vs. luminosity lower backgrounds; “scale chasing”
probe intermediate mass scales higher backgrounds benefit from high L.dt
mGMSB
EW
Strong
STRONG COUPLING
ELECTROWEAK COUPLING
However: if colored states are decoupled, EW production will dominate• Dedicated EW prod. analyses• Pure-EW simplified models (new!)
5 fb-1 reach
s = 7 TeV
Searches for SUSY with photons at ATLAS 8Bruce Schumm
• Preserve basic phenomen-ology (gravitino LSP and bino-like NLSP)
• Decouple everything else except for one higher-mass state that governs production (total transverse energy scale)
• Bino NLSP governs final decay step (ET
Miss, ET scales)
• For existing analyses, high-mass state is colored (gluino, squark)
Strong production
High mass scale
Generalized Gauge Mediation Scenarios
Start w/ minimal GMSB and decouple
most states
Searches for SUSY with photons at ATLAS 9Bruce Schumm
The high-mass (strongly-coupled) state can be either gluino or squark
Set limits in gluino-bino (squark-bino) plane
GGM Parameter Space
7 TeV Strong-Production Cross Sections
Searches for SUSY with photons at ATLAS 10Bruce Schumm
Further, one can relax assumption that 10 NLSP is bino-like.
What about a wino-Like NLSP?
Degenerate triplet 1, 1
0 Final state + lepton + ET
Miss
Both EW and strong production
Even-More Generalized Scenarios: Wino NLSP
EW Production at Wino (1 1
0) scaleStrong Production at gluino scale
Searches for SUSY with photons at ATLAS 11Bruce Schumm
If 10 is pure higgsino, no photons in final state
For certain range of admixture with bino, + bjet + MET is best channel
And Also: Higgsino-Like NLSP
Bino Mass
Br(χ->H+G)
Br(χ-> γ+G)
Br(χ-> Z+G)
B+MET Signif.
B+γ+MET Signif.
γγ+MET Signif.
390 0.129 0.703 0.167 0.53σ 3.38σ 6.61σ
395 0.208 0.634 0.158 0.763σ 3.95σ 6.87σ
400 0.32 0.537 0.144 0.754σ 3.27σ 5.1σ
405 0.449 0.425 0.126 0.971σ 3.22σ 4.31σ
410 0.57 0.322 0.109 1.07σ 3.14σ 2.78σ
415 0.666 0.24 0.094 1.12σ 2.29σ 1.9σ
420 0.738 0.179 0.083 1.13σ 1.83σ 1.35σ
425 0.789 0.136 0.075 1.25σ 1.74σ 0.923σ
430 0.826 0.105 0.069 1.31σ 1.23σ 0.543σ
435 0.853 0.082 0.065 1.23σ 0.941σ 0.531σ
Searches for SUSY with photons at ATLAS 12Bruce Schumm
For bino/higgsino state, choice > 1 suppresses 10 h + gravitino
Generic Single-Photon Signature
Signature:
• Single photon • ET
miss • Njets
Value of N under study (3?)
Searches for SUSY with photons at ATLAS 13Bruce Schumm
For all cases, c < 1 mm (photons point back towards origin)
A separate group is look at standard bino-like case for which c is a free parameter (not disfavored by cosmological considerations)
Requires a new photon reconstruction, more background-prone This is an analysis for which ATLAS is particularly well-suited ( segmentation of first layer of CAL)This analysis is not included in this talk! (but…)
Prompt vs. Non-Prompt Photons
Searches for SUSY with photons at ATLAS 14Bruce Schumm
Overall status of SUSY searches with photons:
No 2012 (8 TeV) results yet; 7 TeV results as follows:• Diphoton + MET: 35 pb-1,
35 pb-1, 1 fb-1 published, 5 fb-1 in pressAnnecy, Argonne, DESY, La Plata, Tokyo Tech, UCSC
• Photon + lepton + MET: 5 fb-1 conference resultUCSC
• Photon + bjet + MET: 5 fb-1 conference result in preparationUCSC, Technion
• Photon + Njet + MET: Just getting underwayLa Plata, UCSC
Summary of four signatures, and status
Searches for SUSY with photons at ATLAS 15Bruce Schumm
The ATLAS Detector
Non-prompt tracksPhoton conversions
Searches for SUSY with photons at ATLAS 16Bruce Schumm
The 2011 ATLAS Data Set
A total of 4.7 fb-1 deemed to be adequate for SUSY analyses
Searches for SUSY with photons at ATLAS 17Bruce Schumm
Expect 25-30 fb-1 at 8 TeV for 2012
2012 (8 TeV) Data is Accumulating Fast
Searches for SUSY with photons at ATLAS 18Bruce Schumm
The basic search: Strong (gluino) production, bino decay
36 pb-1 Analysis:
• Require two stiff, isolated photons (ET1 > 30 GeV, ET2 > 20 GeV)
Diphoton + MET: Strong Production
• Then ETmiss > 125 GeV separates signal from the backgrounds
Searches for SUSY with photons at ATLAS 19Bruce Schumm
Mi
Strong-Production “Scale Chasing”
With GGM gluino (squark) mass constrained to be high (> 500 GeV) a two-scale system emerges
• Bino mass sets ETmiss scale
m = 275 GeVm = 450 GeV
Searches for SUSY with photons at ATLAS 20Bruce Schumm
• Gluino mass sets total transverse energy scale ~ 1 TeV
Strong-Production “Scale Chasing”
HT does not include missing energy (wanted to maintain observational independence)
mgluino = 1000 GeVElectroweakproduction
Searches for SUSY with photons at ATLAS 21Bruce Schumm
Jet mireconstruction can
• Introduce fake photons
• Lead to large ETmiss
(,ETmiss)
Resulting ETmiss often
in direction of
(Small) sensitivity improvement from cutting on (,ET
miss)(,ET
miss) > 0.5
Searches for SUSY with photons at ATLAS 22Bruce Schumm
Strong Production• HT large• High-mass bino More ET
miss, less HT, larger (,ETmiss)
• Low-mass bino Less ETmiss, more HT, small (,ET
miss)
EW Production• HT small• Moderate ET
miss
• Larger (,ETmiss)
Diphoton + MET Singal Regions (A,B, and C)
A
C
B
Searches for SUSY with photons at ATLAS 23Bruce Schumm
“QCD” Backgrounds• (probably not dominant)• + jet; jet misreconstruction• Estimate from loose-photon sample scaled to data at low ET
miss
“EW” Backgrounds• W; W e; e misreconstruction• tt; t be; e misreconstruction• Estimate from e data rate, with e fake rate also from data
“Irreducible” Backgrounds• W; W e• Z; Z • Estimate directly from MC simulation
Diphoton + MET Backgrounds
Searches for SUSY with photons at ATLAS 24Bruce Schumm
Expected Background and Observed Signal
C
• Analysis nearly background-free in SRs A,B
• 2 events in SR C, consistent with background expectation (no HT cut for EW production search)
Searches for SUSY with photons at ATLAS 25Bruce Schumm
Define two control samples:• QCD: One loose-but-not-tight photon (dominated by EM-like jets)• QCD: One loose, one loose-but-not-tight photonNormalize to diphoton sample for 0 < ET
miss < 20 GeV
Estimating QCD Backgrounds
Normalization region
Background estimate
5 fb-1 control samples (no HT cut)
Searches for SUSY with photons at ATLAS 26Bruce Schumm
• Dominated by e misidentification• ET
miss distribution from data e sample• Scale by measured fake rate from (Ze)/(Zee)
Estimating Electroweak Backgrounds
5 fb-1 e sample (no HT
cut)
Just a reminder!
Searches for SUSY with photons at ATLAS 27Bruce Schumm
For example: SRB, low-mass Bino:• Acceptance ~20%• For background-free analysis, 95% CL is 3 events• 3/0.2 less than 15 events produced, or for 5 fb-1, < 3 fb
Assembling GGM Cross-section limits
Signal Acceptance (%)
Searches for SUSY with photons at ATLAS 28Bruce Schumm
And sure enough…
Derived GGM Cross-Section Limits
Searches for SUSY with photons at ATLAS 29Bruce Schumm
3 fb corresponds to production of ~1100 GeV gluino scale of limit
GGM Mass Limits
Choice of signal region (A or B) based on best expected limit (depends only on expectation from data-driven backgrounds and signal MC)
Searches for SUSY with photons at ATLAS 30Bruce Schumm
300 GeV (35%) limit increase from 1 fb-1 5 fb-1 (New signal regions, plus greater rejection of e fakes)
Gluino-bino and Squark-bino Mass Limits
Squark limits somewhat lower (lower cross-section)
Searches for SUSY with photons at ATLAS 31Bruce Schumm
• SPS8: Similar efficiency but higher background: < 5 fb-1
• NNLSP/NLSP scale 550/300 GeV• Hard to glean general sense from constrained model• 2012 analysis will include “EW grid” with Wino NNLSP and bino NLSP• Grid strategy developed with David Shih (Rutgers theory)• Optimization underway; will likely include to SRs (C,D) for high/low mass bino
SPS8 Limits and Electroweak Production
Searches for SUSY with photons at ATLAS 32Bruce Schumm
Single selection geared towards both EW and strong production
Make use of “transverse mass” (similar role to HT)
• High ET photon: ET > (100,85) GeV for (e,) channel
• Lepton with pT > 25 GeV
Photon + Lepton + MET Analysis
Searches for SUSY with photons at ATLAS 33Bruce Schumm
• Most backgrounds estimable from MC constrained by independent measurements (W, tt, tt, …).•Misreconstruction (jet,e ) plays smaller role• 15e, 11 channel events observed• 13e, 15 channel events expected
• BFs less favorable• Acceptance smaller (~5%)• Backgrounds higher Significantly weaker limits!
Photon + Lepton + MET Event Rates
Electron Channel
Muon Channel
Searches for SUSY with photons at ATLAS 34Bruce Schumm
Lepton + + MET Signal Region Distributions
Electron Channel Muon Channel
ElectroweakProduction
StrongProduction
Searches for SUSY with photons at ATLAS 35Bruce Schumm
Mi
Lepton + + MET Mass Limits (Wino NLSP)
ElectroweakProduction
StrongProduction
Searches for SUSY with photons at ATLAS 36Bruce Schumm
Wrap-up (Page 1)
Other two analyses still under development…• Photon + bjet + MET: Nearing completion for conference result• Photon + Njet + MET: Still exploring
Plan to have complete slate of analyses for 2012 analysis• Diphoton + MET: Moriond 2013, including EW grid (underway)• Photon + lepton + MET: Moriond 2013; multiple SRs• Photon + bjet + MET: Summer 2013• Photon + Njet + MET: Summer 2013
Searches for SUSY with photons at ATLAS 37Bruce Schumm
Naïve Projection for Diphoton + MET (25 fb-1 @ 8 TeV)
• 8 TeV / 7 TeV = 1.14 10% improvement in limit?• ~ M-8 x5 statistical gain 22% gain in limit?• Overall 30% gain in limit (???) would push gluino to ~1500 GeV and squarks to ~1250 GeV (seems a little optimistic?)
But clearly an interesting step forward.
For 14 (13?) TeV running
• Include c as third experimental parameter (along with production scale mass and NLSP mass)• ???
Wrap-Up (Page 2)
Searches for SUSY with photons at ATLAS 38Bruce Schumm
Back-Up
Searches for SUSY with photons at ATLAS 39Bruce Schumm
Mi
“
Searches for SUSY with photons at ATLAS 40Bruce Schumm
GGM (General GMSB): Diphoton Search
GGM and the Diphoton+ETmiss Analysis
Everything decoupled except• Gluino octet• Bino-like NLSP; B(0 G) ~ 1
Require two photons, ETmiss > 125 GeV
Observe: 5 eventsExpect: 4.1 0.6 (syst.) events
Mgluino > 805 GeV
for 50 < M0 < Mgluino
1 fb-1
Phys Lett. B 710 (2012), 519
Searches for SUSY with photons at ATLAS 41Bruce Schumm
Wrap-Up
Vibrant and expanding program of SUSY searches No discoveries claimed At 5 fb-1, colored sparticle limits above 1 TeV for some contexts Non-colored partner limits in 300-400 GeV range Increased consideration of “simplified” models (especially EW) Many analyses have yet to update to 5 fb-1
Speculation about 2012 reach (assume 20 fb-1 at s = 8 TeV) ~10% gain from increased s colored M-8 ~10% gain from statistics (background-limited)
• Somewhat better for EW production (not on PDF tails)• Ingenuity, “scale-chasing”
Guesstimate: ~25% increase in sensitive range (e.g. 1000 GeV limits increase to 1250 GeV if SUSY doesn’t exist at that scale)