Prospects for SUSY at ATLAS and CMS

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Supersymmetryat ATLAS
Dan Tovey University of Sheffield
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ATLAS
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(No Transcript)
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SUSY _at_ ATLAS

• Many motivations for low-scale SUSY
• Gauge hierarchy problem
• Higgs mass
• Coupling constant unification
• Dark Matter candidate
• Focus here on R-Parity Conserving models
• Dark Matter Candidate ? ETmiss signature
• NB ETmiss signature valid for any DM candidate
  (not just SUSY)

c01
MET

c01
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ATLAS SUSY Strategy

• SUSY studies at ATLAS will proceed in three
  stages

• SUSY Discovery phase
• Inclusive Studies
• comparison of significance in inclusive channels,
• measurement of SUSY Mass Scale.
• Exclusive studies
• model-dependent interpretation (e.g. mSUGRA DM),
• less model-dependent DM,
• universalities / flavour physics,
• spin measurement (is it SUSY?),
• .

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• Stage 1
• SUSY Discovery

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SUSY Signatures

• Q What do we expect SUSY events _at_ LHC to look
  like?
• A Look at typical decay chain

• Strongly interacting sparticles (squarks,
  gluinos) dominate production.
• Heavier than sleptons, gauginos etc. g cascade
  decays to LSP.
• Long decay chains and large mass differences
  between SUSY states
• Many high pT objects observed (leptons, jets,
  b-jets).
• If R-Parity conserved LSP (lightest neutralino in
  mSUGRA) stable and sparticles pair produced.
• Large ETmiss signature (c.f. Wgln).
• Closest equivalent SM signature tgWb.

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Inclusive Searches

• Use 'golden' Jets n leptons ETmiss discovery
  channel.
• Map statistical discovery reach in mSUGRA m0-m1/2
  parameter space.
• Sensitivity only weakly dependent on A0, tan(b)
  and sign(m).
• Syst. stat. reach harder to assess focus of
  current future work.

5s
5s
ATLAS
ATLAS
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Monte Carlo Studies

• Monte Carlo background estimates subject to
  significant systematics
• Original studies used Parton Shower
• Recent studies use ALPGEN MPFS generator MLM
  matching to PS
• Re-assessment of significances

Jets ETmiss 0 leptons
MeffSpTi ETmiss
ATLAS
10 fb-1
MeffSpTi ETmiss
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Background Estimation

• Inclusive signature jets n leptons ETmiss
• Main backgrounds
• Z n jets
• W n jets
• QCD
• ttbar
• Greatest discrimination power from ETmiss
  (R-Parity conserving models)
• Generic approach to background estimation
• Select background calibration samples (control
  region)
• Extrapolate into high ETmiss signal region.
• Used by CDF / D0
• Extrapolation is non-trivial.
• Must find variables uncorrelated with ETmiss
• Several approaches being developed.

One lepton mode BG _at_ 1fb-1

• Blue tt(?lnln)
• Green tt(?lnqq)
• Red w
• Black sum BG
• Pink SU3

Preliminary
ATLAS
MeffSpTi ETmiss
Effective mass (GeV)
Jets ETmiss 1 lepton
10 fb-1
MeffSpTi ETmiss
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W/ZJets Background

• Z g nn n jets, W g ln n jets, W g tn (n-1)
  jets (t fakes jet)
• Estimate from Z g ll- n jets (e or m)
• Tag leptonic Z and use to validate MC / estimate
  ETmiss from pT(Z) pT(l)
• Alternatively tag W g ln n jets and replace
  lepton with n (0l)
• higher stats
• biased by presence of SUSY

(Z?ll)
ATLAS
Preliminary
(W?ln)
ATLAS
Preliminary
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Top Background

• Jets ETmiss 1 lepton cut hard on mT(ETmiss,
  l)
• Rejects bulk of ttbar (semi-leptonic decays), W g
  ln n jets
• Remaining background mostly fully leptonic top
  decays (tt?bblnln, tt?bbtnln, tt?bbtntn) with one
  top missed (e.g. hadronic tau)

Type of Event Number of Events
Electron-Electron 67
Muon-Muon 80
Tau-Tau 14
Electron-Muon 152
Electron-Tau 100
Muon-Tau 109
Electron-Hadron 77
Muon-Hadron 174
Tau-Hadron 7
ATLAS
Preliminary
Edge at W Mass

• tt(?lnln)
• tt(?lnqq)
• wjet
• sum of all BG
• Pink SU3

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Top Background
Control region
Signal region
Signal region
ATLAS
ATLAS
Preliminary
Preliminary
Missing ET (GeV)
Transverse Mass (GeV)
Control region
ATLAS

• One possibility ? use mT(W)
• Divide mT into signal region control region
  (mTlt100GeV)
• Use shape of control distribution for ETmiss or
  Effective Mass
• normalization factor determined in the low ETmiss
  region (100 - 200GeV)

Preliminary

• tt(?lnln)
• tt(?lnqq)
• w
• sum BG
• SU3

Missing ET (GeV)
normalization region
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mT Method
_at_1fb-1
_at_1fb-1
real BG estimated BG
real BG estimated BG
gt300GeV 9.51.1 8.60.6
gt800GeV 24.81.6 22.00.9
ATLAS
ATLAS
Preliminary
Preliminary
MeffSpTi ETmiss
Effective Mass (GeV)
Missing ET (GeV)
With SU3 signal

• With cut mT gt 100 GeV, BG well reproduced in
  absence of SUSY signal (statistical error 10)
• With SUSY signal estimated BG increased by factor
  2.5.
• Rejection of SUSY contamination next issue

gt800GeV 24.81.6 60.82.5
ATLAS
Preliminary
Effective Mass (GeV)
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QCD Background

• Two main sources
• fake ETmiss (gaps in acceptance, dead/hot cells,
  non-gaussian tails etc.)
• real ETmiss (neutrinos from b/c quark decays)
• Much harder with MC simulations require
  detailed understanding of detector performance
• Strategy
• Initially choose channels which minimise
  contribution until well understood (e.g. jets
  ETmiss 1l)
• Reject events where fake ETmiss likely beam-gas
  and machine background, bad primary vertex, hot
  cells, CR muons, ETmiss phi correlations (jet
  fluctuations), jets pointing at regions of poor
  response
• Cut hard to minimise contribution to background
  (at expense of stats).
• Estimate background using data

Pythia dijets
SUSY SU3
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QCD Background

• Work in Progress several ideas under study
• Step 1 Measure jet smearing function from data
• Select events ETmiss gt 100 GeV, Df(ETmiss, jet)
  lt 0.1
• Estimate pT of jet closest to EtMiss as
• pTtrue-est pTjet ETMiss
• Step 2 Smear low ETmiss multijet events with
  measured smearing function

fluctuating jet
Njets gt 4, pT(j1,j2) gt100GeV, pT(j3,j4) gt 50GeV
22pb-1
ATLAS
ATLAS
Preliminary
Preliminary
All QCD Z?nn SU3 Estimate (QCD)?
ETmiss
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• Stage 2
• Inclusive Studies

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Inclusive Studies

• First indication of mSUGRA parameters from
  inclusive channels
• Compare significance in jets ETmiss n leptons
  channels
• Detailed measurements from exclusive channels
  when accessible.
• Consider here two specific example points studied
  previously

ATLAS
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Inclusive Studies

• First SUSY parameter to be measured may be mass
  scale
• Defined as weighted mean of masses of initial
  sparticles.
• Calculate distribution of 'effective mass'
  variable defined as scalar sum of masses of all
  jets (or four hardest) and ETmiss
• MeffSpTi ETmiss.
• Distribution peaked at twice SUSY mass scale
  for signal events.
• Pseudo 'model-independent' measurement.
• Typical measurement error (syststat) 10 for
  mSUGRA models for 10 fb-1 (PS).

Jets ETmiss 0 leptons
ATLAS
10 fb-1
Jets ETmiss 1 lepton
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• Stage 3
• Exclusive studies

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Exclusive Studies

• With more data will attempt to measure weak scale
  SUSY parameters (masses etc.) using exclusive
  channels.
• Different philosophy to TeV Run II (better S/B,
  longer decay chains) g aim to use
  model-independent measures.
• Two neutral LSPs escape from each event
• Impossible to measure mass of each sparticle
  using one channel alone
• Use kinematic end-points to measure combinations
  of masses.
• Old technique used many times before (n mass from
  b decay spectrum, W (transverse) mass in Wgln).
• Difference here is we don't know mass of neutral
  final state particles.

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Dilepton Edge Measurements

• When kinematically accessible c02 can undergo
  sequential two-body decay to c01 via a
  right-slepton (e.g. LHC Point 5).
• Results in sharp OS SF dilepton invariant mass
  edge sensitive to combination of masses of
  sparticles.
• Can perform SM SUSY background subtraction
  using OF distribution
• ee- mm- - em- - me-
• Position of edge measured with precision 0.5
• (30 fb-1).

• m0 100 GeV
• m1/2 300 GeV
• A0 -300 GeV
• tan(b) 6
• sgn(m) 1

ee- mm- - em- - me-
ee- mm-
Point 5
ATLAS
ATLAS
30 fb-1 atlfast
5 fb-1 SU3
Physics TDR
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Measurements With Squarks

• Dilepton edge starting point for reconstruction
  of decay chain.
• Make invariant mass combinations of leptons and
  jets.
• Gives multiple constraints on combinations of
  four masses.
• Sensitivity to individual sparticle masses.

bbq edge
llq threshold
1 error (100 fb-1)
2 error (100 fb-1)
TDR, Point 5
TDR, Point 5
TDR, Point 5
TDR, Point 5
ATLAS
ATLAS
ATLAS
ATLAS
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Model-Independent Masses

• Combine measurements from edges from different
  jet/lepton combinations to obtain
  model-independent mass measurements.

c01
lR
ATLAS
ATLAS
Mass (GeV)
Mass (GeV)


c02
qL
ATLAS
ATLAS
LHCC Point 5
Mass (GeV)
Mass (GeV)
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Measuring Model Parameters

• Alternative use for SUSY observables (invariant
  mass end-points, thresholds etc.).
• Here assume mSUGRA/CMSSM model and perform global
  fit of model parameters to observables
• So far mostly private codes but e.g. SFITTER,
  FITTINO now on the market
• c.f. global EW fits at LEP, ZFITTER, TOPAZ0 etc.

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Dark Matter Parameters

• Can use parameter measurements for many purposes,
  e.g. estimate LSP Dark Matter properties (e.g.
  for 300 fb-1, SPS1a)
• Wch2 0.1921 ? 0.0053
• log10(scp/pb) -8.17 ? 0.04

Baer et al. hep-ph/0305191
LHC Point 5 gt5s error (300 fb-1)
SPS1a gt5s error (300 fb-1)
Micromegas 1.1 (Belanger et al.) ISASUGRA 7.69
DarkSUSY 3.14.02 (Gondolo et al.) ISASUGRA 7.69
scp10-11 pb
scp10-10 pb
Wch2
scp
scp10-9 pb
300 fb-1
300 fb-1
No REWSB
LEP 2
ATLAS
ATLAS
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Target Models

• SUSY (e.g. mSUGRA) parameter space strongly
  constrained by cosmology (e.g. WMAP satellite)
  data.

mSUGRA A00, tan(b) 10, mgt0
Slepton Co-annihilation region LSP pure Bino.
Small slepton-LSP mass difference makes
measurements difficult.
Ellis et al. hep-ph/0303043
Disfavoured by BR (b ? s?) (3.2 ? 0.5) ?
10-4 (CLEO, BELLE)
'Bulk' region t-channel slepton exchange - LSP
mostly Bino. 'Bread and Butter' region for LHC
Expts.
Also 'rapid annihilation funnel' at Higgs pole at
high tan(b), stop co-annihilation region at large
A0
0.094 ? ? ? h2 ? 0.129 (WMAP)
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Coannihilation Signatures

• Small slepton-neutralino mass difference gives
  soft leptons
• Low electron/muon/tau energy thresholds crucial.
• Study point chosen within region
• m070 GeV m1/2350 GeV A00 tanß10 µgt0
• Decays of c02 to both lL and lR kinematically
  allowed.
• Double dilepton invariant mass edge structure
• Edges expected at 57 / 101 GeV
• Stau channels enhanced (tanb)
• Soft tau signatures
• Edge expected at 79 GeV
• Less clear due to poor tau visible energy
  resolution.

• ETmissgt300 GeV
• 2 OSSF leptons PTgt10 GeV
• gt1 jet with PTgt150 GeV
• OSSF-OSOF subtraction applied

100 fb-1
ATLAS
Preliminary

• ETmissgt300 GeV
• 1 tau PTgt40 GeV1 tau PTlt25 GeV
• gt1 jet with PTgt100 GeV
• SS tau subtraction

100 fb-1
ATLAS
Preliminary
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Focus Point Signatures

• Large m0 ? sfermions are heavy
• Most useful signatures from heavy neutralino
  decay
• Study point chosen within focus point region
• m03550 GeV m1/2300 GeV A00 tanß10 µgt0
• Direct three-body decays c0n ? c01 ll
• Edges give m(c0n)-m(c01) flavour subtraction
  applied

M mAmB m mA-mB
Parameter Without cuts Exp. value
M1 6892 103.35
M2-M1 57.71.0 57.03
M3-M1 77.61.0 76.41
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Dark Matter in the MSSM

• Can relax mSUGRA constraints to obtain more
  model-independent relic density estimate.
• Much harder needs more measurements
• Not sufficient to measure relevant (co-)
  annihilation channels must exclude all
  irrelevant ones also
• Stau, higgs, stop masses/mixings important as
  well as gaugino/higgsino parameters

Nojiri et al., JHEP 0603 (2006) 063
s(Wch2) vs s(mtt)
Wch2
Wch2
s(mtt)5 GeV
s(mtt)0.5 GeV
300 fb-1
300 fb-1
SPA point
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Heavy Gaugino Measurements

• Potentially possible to identify dilepton edges
  from decays of heavy gauginos.
• Requires high stats.
• Crucial input to reconstruction of MSSM
  neutralino mass matrix (independent of SUSY
  breaking scenario).

ATLAS
SPS1a
ATLAS
ATLAS
ATLAS
100 fb-1
100 fb-1
100 fb-1
SPS1a
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SUSY Spin Measurement

• Q How do we know that a SUSY signal is really
  due to SUSY?
• Other models (e.g. UED) can mimic SUSY mass
  spectrum
• A Measure spin of new particles.
• One proposal use standard two-body slepton
  decay chain
• charge asymmetry of lq pairs measures spin of c02
• relies on valence quark contribution to pdf of
  proton (C asymmetry)
• shape of dilepton invariant mass spectrum
  measures slepton spin

Point 5
ATLAS
150 fb -1
mlq
spin-0flat
150 fb -1
ATLAS
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Supersummary

• The LHC will be THE PLACE to search for, and
  hopefully study, SUSY from next year onwards at
  colliders (at least until ILC).
• SUSY searches (preparations) will commence on Day
  1 of LHC operation.
• Many studies of exclusive channels already
  performed.
• Lots of input from both theorists (new ideas) and
  experimentalists (new techniques).
• Big challenge for discovery will be understanding
  systematics.
• Big effort now to understand how to exploit first
  data in timely fashion

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• BACK-UP SLIDES

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Supersymmetry

• Supersymmetry (SUSY) fundamental continuous
  symmetry connecting fermions and bosons
• QaFgt Bgt, QaBgt Fgt
• Qa,Qb-2gmabpm generators obey
  anti-commutation relations with 4-mom
• Connection to space-time symmetry
• SUSY stabilises Higgs mass against loop
  corrections (gauge hierarchy/fine-tuning problem)
• Leads to Higgs mass 135 GeV
• Good agreement with LEP constraints from EW
  global fits
• SUSY modifies running of SM gauge couplings just
  enough to give Grand Unification at single scale.

LEPEWWG Winter 2006
mHlt207 GeV (95CL)
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SUSY Spectrum

• SUSY gives rise to partners of SM states with
  opposite spin-statistics but otherwise same
  Quantum Numbers.

• Expect SUSY partners to have same masses as SM
  states
• Not observed (despite best efforts!)
• SUSY must be a broken symmetry at low energy
• Higgs sector also expanded

h
ne
e
d
u
nm
m
s
c
nt
t
b
t
G
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SUSY Dark Matter

• R-Parity Rp (-1)3B2SL
• Conservation of Rp (motivated e.g. by string
  models) attractive
• e.g. protects proton from rapid decay via SUSY
  states
• Causes Lightest SUSY Particle (LSP) to be
  absolutely stable
• LSP neutral/weakly interacting to escape
  astroparticle bounds on anomalous heavy elements.
• Naturally provides solution to dark matter
  problem
• R-Parity violating models still possible ? not
  covered here.

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SUSY _at_ ATLAS

• LHC will be a 14 TeV proton-proton collider
  located inside the LEP tunnel at CERN.
• Luminosity goals
• 10 fb-1 / year (first 3 years)
• 100 fb-1/year (subsequently).
• First data this year!
• Higgs SUSY main goals.

• Much preparatory work carried out historically by
  ATLAS
• Summarised in Detector and Physics Performance
  TDR (1998/9).
• Work continuing to ensure ready to test new ideas
  with first data.
• Concentrate here on more recent work.

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Model Framework

• Minimal Supersymmetric Extension of the Standard
  Model (MSSM) contains gt 105 free parameters,
  NMSSM etc. has more g difficult to map complete
  parameter space!
• Assume specific well-motivated model framework in
  which generic signatures can be studied.
• Often assume SUSY broken by gravitational
  interactions g mSUGRA/CMSSM framework unified
  masses and couplings at the GUT scale g 5 free
  parameters

• (m0, m1/2, A0, tan(b), sgn(m)).
• R-Parity assumed to be conserved.
• Exclusive studies use benchmark points in mSUGRA
  parameter space
• LHCC Points 1-6
• Post-LEP benchmarks (Battaglia et al.)
• Snowmass Points and Slopes (SPS)
• etc

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RH Squark Mass

• Right handed squarks difficult as rarely decay
  via standard c02 chain
• Typically BR (qR g c01q) gt 99.
• Instead search for events with 2 hard jets and
  lots of ETmiss.
• Reconstruct mass using stransverse mass
  (Allanach et al.)
• mT22 min maxmT2(pTj(1),qTc(1)mc),
  mT2(pTj(2),qTc(2)mc)
• Needs c01 mass measurement as input.
• Also works for sleptons.

qTc(1)qTc(2)ETmiss
ATLAS
ATLAS
30 fb-1
100 fb-1
30 fb-1
Right squark
SPS1a
ATLAS
SPS1a
Right squark
SPS1a
Left slepton
Precision 3
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Inclusive Studies

• Following any discovery of SUSY next task will be
  to test broad features of model.
• Question 1 Is R-Parity Conserved?
• If YES possible DM candidate
• LHC experiments sensitive only to LSP lifetimes lt
  1 ms (ltlt tU 13.7 Gyr)

LHC Point 5 (Physics TDR)
R-Parity Conserved
R-Parity Violated
ATLAS

Non-pointing photons from c01gGg

• Question 2 Is the LSP the lightest neutralino?
• Natural in many MSSM models
• If YES then test for consistency with
  astrophysics
• If NO then what is it?
• e.g. Light Gravitino DM from GMSB models (not
  considered here)

GMSB Point 1b (Physics TDR)
ATLAS
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Decay Resimulation

• Second approach Decay Resimulation
• Goal is to model complex background events using
  samples of tagged SM events.
• Initially we will know
• a lot about decays of SM particles (e.g. W, top)
• a reasonable amount about the (gaussian)
  performance of the detector.
• rather little about PDFs, the hard process and
  UE.
• Philosophy
• Tag seed events containing W/top
• Reconstruct 4-momentum of W/top (x2 if e.g.
  ttbar)
• Decay/hadronise with e.g. Pythia
• Simulate decay products with atlfast(here) or
  fullsim
• Remove original decay products from seed event
• Merge new decay products with seed event (inc.
  ETmiss)
• Perform standard SUSY analysis on merged event
• Technique applicable to wide range of channels
  (not just SUSY)
• Involves measurement of (top) pT useful for
  exotic-top searches?

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Decay Resimulation

• Select seed events
• 2 leptons with pTgt25GeV
• 2 jets with pTgt50GeV
• ETmiss lt ltpT(l1),pT(l2)gt (SUSY)
• Both m(lb) lt 155GeV
• Solve 6 constraints for p(n)
• Resimulate merge with seed
• Apply 1l SUSY cuts

m(lb)
Z?mm
cut
ATLAS
Preliminary
GeV
ETmiss
Sample 5201, pT(top) gt 200 GeV reco
11.0.4205, 450 pb-1
ATLAS
Pull (top pT)
Preliminary
gt2 jets, Normalisation to data under study
ATLAS
Preliminary