Sapporo, June 2007 Dan Tovey 1 Supersymmetry at ATLAS Dan Tovey University of Sheffield Dan Tovey...

11 Sapporo, June 2007Sapporo, June 2007Dan ToveyDan Tovey

Supersymmetryat ATLAS

Supersymmetryat ATLAS

Dan Tovey

University of Sheffield

Dan Tovey

University of Sheffield


ATLASATLAS

Length: ~45 m Radius: ~12 m Weight: ~7000 tons


SUSY @ ATLASSUSY @ 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)

MET

01

01

~

~


ATLAS SUSY StrategyATLAS SUSY Strategy

1) SUSY Discovery phase

2) Inclusive Studies– comparison of significance in inclusive channels,– measurement of SUSY Mass Scale.

3) Exclusive studies– model-dependent interpretation (e.g. mSUGRA DM),– less model-dependent DM,– universalities / flavour physics,– spin measurement (is it SUSY?),– ….

SUSY studies at ATLAS will proceed in three stages:


Stage 1:

SUSY Discovery


SUSY SignaturesSUSY Signatures• Q: What do we expect SUSY events @ LHC to look like?• A: Look at typical decay chain:

• Strongly interacting sparticles (squarks, gluinos) dominate production.

• Heavier than sleptons, gauginos etc. 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 ET

miss signature (c.f. Wl).

• Closest equivalent SM signature tWb.

lqq

l

g~ q~ l~

~

~

p p


ATLAS

5

• 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() and sign().

• Syst.+ stat. reach harder to assess: focus of current & future work.

Inclusive SearchesInclusive Searches

ATLAS

5


Monte Carlo StudiesMonte 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

ATLAS

10 fb-1

Meff=|pTi| + ET

miss

Meff=|pTi| + ET

miss


Background EstimationBackground 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 ET

miss signal region.

• Used by CDF / D0• Extrapolation is non-trivial.

– Must find variables uncorrelated with ETmiss

• Several approaches being developed.

ATLAS

One lepton mode BG @ 1fb-1

Effective mass (GeV)

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

Preliminary

Jets + ETmiss + 1 lepton

10 fb-1

Meff=|pTi| + ET

miss

Meff=|pTi| + ET

miss


W/Z+Jets BackgroundW/Z+Jets Background• Z + n jets, W l + n jets, W +

(n-1) jets ( fakes jet)• Estimate from Z l+l- + n jets (e or )• Tag leptonic Z and use to validate

MC / estimate ETmiss from pT(Z) & pT(l)

• Alternatively tag W l + n jets and replace lepton with 0l): – higher stats

– biased by presence of SUSY

ATLAS

Preliminary

ATLAS

Preliminary

(Zll)

(Wl)


Top BackgroundTop Background• Jets + ET

miss + 1 lepton: cut hard on mT(ET

miss, l)

• Rejects bulk of ttbar (semi-leptonic decays), W l + n jets

• Remaining background mostly fully leptonic top decays (ttbbllttbblttbb) with one top missed (e.g. hadronic tau)

Edge at W Mass

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

• tt(lnln)• tt(lnqq)• w+jet• sum of all BG• Pink: 　 SU3


Top BackgroundTop BackgroundSignal regionControl region

Transverse Mass (GeV)

Signal region

Control region

Missing ET (GeV)

Missing ET (GeV)

•tt(lnln)•tt(lnqq)• w•sum BG•SU3

normalization region

• One possibility use mT(W) – Divide mT into signal region +

control region (mT<100GeV)

– Use shape of control distribution for ETmiss or Effective Mass

– normalization factor determined in the low ETmiss region (100 - 200GeV)

ATLAS

PreliminaryATLAS

Preliminary

ATLAS

Preliminary


mT MethodmT Method

• With cut mT > 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

Effective Mass (GeV)

With SU3 signal

real BGestimated BG

real BGestimated BG

@1fb-1 @1fb-1

>800GeV24.8±1.622.0±0.9

>300GeV9.5±1.18.6±0.6

>800GeV24.8±1.660.8±2.5

Meff=|pTi| + ET

missMissing ET (GeV)

Effective Mass (GeV)

ATLAS

PreliminaryATLAS

Preliminary

ATLAS

Preliminary


QCD BackgroundQCD Background• Two main sources:

– fake ETmiss (gaps in acceptance, dead/hot

cells, non-gaussian tails etc.)– real ET

miss (neutrinos from b/c quark decays)

• Much harder with MC : simulations require detailed understanding of detector performance

• Strategy:1) Initially choose channels which minimise

contribution until well understood (e.g. jets + ET

miss + 1l)

2) Reject events where fake ETmiss likely:

beam-gas and machine background, bad primary vertex, hot cells, CR muons, ET

miss phi correlations (jet fluctuations), jets pointing at regions of poor response …

3) Cut hard to minimise contribution to background (at expense of stats).

4) Estimate background using data

Pythia dijets

SUSY SU3


All

QCD

Z SU3

Estimate (QCD)

22pb-1

QCD BackgroundQCD Background• Work in Progress: several ideas under study• Step 1: Measure jet smearing function from data

– Select events: ETmiss > 100 GeV, (ET

miss, jet) < 0.1

– Estimate pT of jet closest to EtMiss as

pTtrue-est = pT

jet + ETMiss

• Step 2: Smear low ETmiss multijet events with

measured smearing function

MET

jets

fluctuatingjet

Njets >= 4, p

T(j1,j2) >100GeV,

pT(j3,j4) > 50GeV

ETmiss

ATLAS

Preliminary

ATLAS

Preliminary


Stage 2:

Inclusive Studies


Inclusive StudiesInclusive Studies• First indication of mSUGRA

parameters from inclusive channels– Compare significance in jets + ET

miss + n leptons channels

• Detailed measurements from exclusive channels when accessible.

• Consider here two specific example points studied previously:

ATLAS

LHC Point 5 (A0 =300 GeV, tan()=2, >0)

Point SPS1a (A0 =-100 GeV, tan()=10, >0) Sparticle Mass (LHC Point 5) Mass (SPS1a)

qL ~690 GeV ~530 GeV

02 233 GeV 177 GeV

lR 157 GeV 143 GeV

01 122 GeV 96 GeV

~

~

~

~

Point m0 m1/2 A0 tan() sign() LHC Point 5 100 300 300 2 +1 SPS1a 100 250 -100 10 +1


Inclusive StudiesInclusive 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 ET

miss:

Meff=pTi| + ET

miss.

• Distribution peaked at ~ twice SUSY mass scale for signal events.

• Pseudo 'model-independent' measurement.

• Typical measurement error (syst+stat) ~10% for mSUGRA models for 10 fb-1 (PS).

Jets + ETmiss + 0 leptons

ATLAS

10 fb-1

Jets + ETmiss + 1 lepton


Stage 3:

Exclusive studies


Exclusive StudiesExclusive 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) 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 ( mass from decay spectrum, W (transverse) mass in Wl).

• Difference here is we don't know mass of neutral final state particles.

lqql

g~ q~ lR

~

~

~p p


Dilepton Edge MeasurementsDilepton Edge Measurements

~~02

~01

l ll

e+e- + +-

~

~

30 fb-1

atlfast

Physics TDR

Point 5

e+e- + +- - e+- - +e-

5 fb-1

SU3

ATLAS ATLAS

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

• When kinematically accessible

canundergo sequential two-body decay to

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

e+e- + +- - e+- - +e-

• Position of edge measured with precision ~ 0.5%

(30 fb-1).


Measurements With SquarksMeasurements 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.

~~

~

l ll

qL

q

~

~

~

bh

qL

q

~

b

llq edge1% error(100 fb-1)

lq edge1% error(100 fb-1)

llq threshold2% error(100 fb-1)

bbq edge

TDR,Point 5

TDR,Point 5

TDR,Point 5

TDR,Point 5

ATLAS ATLAS ATLAS ATLAS

1% error(100 fb-1)


‘Model-Independent’ Masses‘Model-Independent’ Masses

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

01 lR

02 qL

Mass (GeV)Mass (GeV)

Mass (GeV)Mass (GeV)

~

~

~

~

ATLAS ATLAS

ATLAS ATLAS

Sparticle Expected precision (100 fb-1)

qL 3%

02 6%

lR 9%

01 12%

~

~

~

~

LHCCPoint 5


Measuring Model ParametersMeasuring 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.

Point m0 m1/2 A0 tan() sign() LHC Point 5 100 300 300 2 +1 SPS1a 100 250 -100 10 +1

Parameter Expected precision (300 fb-1) m0 2% m1/2 0.6% tan() 9% A0 16%


Dark Matter ParametersDark Matter Parameters

Baer et al. hep-ph/0305191

LEP 2 No REWSB

LHC Point 5: >5 error (300 fb-1)

p=10-11 pb

p=10-10 pb

p=10-9 pb

• Can use parameter measurements for many purposes, e.g. estimate LSP Dark Matter properties (e.g. for 300 fb-1, SPS1a)– h2 = 0.1921 0.0053– log10(p/pb) = -8.17 0.04 SPS1a: >5

error (300 fb-1)Micromegas 1.1 (Belanger et al.)+ ISASUGRA 7.69

ATLAS

300 fb-1

p

ATLAS

300 fb-1

DarkSUSY 3.14.02 (Gondolo et al.)+ ISASUGRA 7.69

h2


Target ModelsTarget Models• SUSY (e.g. mSUGRA) parameter space strongly constrained by

cosmology (e.g. WMAP satellite) data. mSUGRA A0=0, tan() = 10, >0

'Bulk' region: t-channel slepton exchange - LSP mostly Bino. 'Bread and Butter' region for LHC Expts.

'Focus point' region: significant h component to LSP enhances annihilation to gauge bosons

~

Ellis et al. hep-ph/0303043

01

01

l

llR

~

~~

01

1

/Z/h1

~

~~

Disfavoured by BR (b s) = (3.2 0.5) 10-4 (CLEO, BELLE)

0.094 h2 0.129 (WMAP)

Slepton Co-annihilation region: LSP ~ pure Bino. Small slepton-LSP mass difference makes measurements difficult.

Also 'rapid annihilation funnel' at Higgs pole at high tan(), stop co-annihilation region at large A0


Coannihilation SignaturesCoannihilation Signatures

• ETmiss>300 GeV

• 2 OSSF leptons PT>10 GeV• >1 jet with PT>150 GeV• OSSF-OSOF subtraction applied

• ETmiss>300 GeV

• 1 tau PT>40 GeV;1 tau PT<25 GeV• >1 jet with PT>100 GeV• SS tau subtraction

• Small slepton-neutralino mass difference gives soft leptons – Low electron/muon/tau energy

thresholds crucial.

• Study point chosen within region:– m0=70 GeV; m1/2=350 GeV; A0=0;

tanß=10 ; μ>0;

• Decays of 02 to both lL and lR

kinematically allowed.– Double dilepton invariant mass edge

structure;– Edges expected at 57 / 101 GeV

• Stau channels enhanced (tan)– Soft tau signatures;– Edge expected at 79 GeV;– Less clear due to poor tau visible

energy resolution.

ATLAS

ATLAS

Preliminary

Preliminary

100 fb-1

100 fb-1

~~~


Focus Point SignaturesFocus Point Signatures• Large m0 sfermions are heavy

• Most useful signatures from heavy neutralino decay• Study point chosen within focus point region :

– m0=3550 GeV; m1/2=300 GeV; A0=0; tanß=10 ; μ>0

• Direct three-body decays 0n → 0

1 ll

• Edges give m(0n)-m(0

1) : flavour subtraction applied

~ ~~ ~

Z0 → ll

300 fb-1

→

ll~ ~

→ ll

~ ~

Preliminary

ATLAS

Parameter Without cuts Exp. value

M 68±92 103.35

M2-M1 57.7±1.0 57.03

M3-M1 77.6±1.0 76.41

M = mA+mB mA-mB


Dark Matter in the MSSMDark 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

h2

h2

σ(m)=5 GeV

σ(m)=0.5 GeV

SPA point

300 fb-1300 fb-1

σ(h2) vs σ(m)


Heavy Gaugino MeasurementsHeavy 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).

ATLAS100 fb-1 ATLAS

100 fb-1

ATLAS100 fb-1

ATLAS

SPS1a

SPS1a


SUSY Spin MeasurementSUSY 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 02

– relies on valence quark contribution to pdf of proton (C asymmetry)

– shape of dilepton invariant mass spectrum measures slepton spin

~

150 fb -1

spin-0=flat

Point 5

ATLAS

ll

llA

mlq

Straightline distn

(phase-space)

Point 5

Spin-½, mostly winoSpin-0

Spin-½

Spin-0

Spin-½, mostly bino

Polarise

MeasureAngle

ATLAS

150 fb -1


SupersummarySupersummary• 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


BACK-UP SLIDES


SupersymmetrySupersymmetry• Supersymmetry (SUSY) fundamental

continuous symmetry connecting fermions and bosons

Q|F> = |B>, Q|B> = |F>

• {Q,Q}=-2pgenerators 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.

LEPEWWGWinter 2006

mH<207 GeV (95%CL)


e

H-d

~H+

u

~H0

d

~H0

u

~0

4~0

3~0

2~0

1~

SUSY SpectrumSUSY Spectrum

• 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

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

H±H0A

G

e

bt

sc

du

g

W±Z

h

W±Z~ ~ ~

g~

G~

± 2~± 1

~

e

e

bt

sc

du~ ~

~

~

~ ~

~ ~ ~

~

~

~


SUSY & Dark MatterSUSY & Dark Matter• R-Parity Rp = (-1)3B+2S+L

• 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.

Universe Over-Closed

mSUGRA A0=0, tan() = 10, >0

Ellis et al. hep-ph/0303043

01

01

l

llR

~

~~

01

1

/Z/h1

~

~~

Disfavoured by BR (b s) = (3.2 0.5) 10-4 (CLEO, BELLE)

0.094 h2 0.129 (WMAP-1year)


SUSY @ ATLASSUSY @ ATLAS

• 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.

• 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.


Model FrameworkModel Framework• Minimal Supersymmetric Extension of the Standard Model (MSSM)

contains > 105 free parameters, NMSSM etc. has more 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 mSUGRA/CMSSM framework : unified masses and couplings at the GUT scale 5 free parameters(m0, m1/2, A0, tan(), sgn()).

• 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…

LHCC mSUGRA Points

1 2

3

45


RH Squark MassRH Squark Mass• Right handed squarks difficult as rarely decay via ‘standard’ 0

2 chain

– Typically BR (qR 01q) > 99%.

• Instead search for events with 2 hard jets and lots of ETmiss.

• Reconstruct mass using ‘stransverse mass’ (Allanach et al.):mT2

2 = min [max{mT2(pT

j(1),qT(1);m),mT

2(pTj(2),qT

(2);m)}]

• Needs 01 mass measurement as input.

• Also works for sleptons.

qT(1)+qT

(2)=ETmiss

~ ~

~qR

q~

ATLAS

ATLAS

ATLAS

30 fb-130 fb-1 100 fb-1

Left sleptonRight

squark

Right squark

~

~

SPS1a

SPS1aSPS1a

Precision ~ 3%


• 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)

• 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 < 1 ms (<< tU ~ 13.7 Gyr)

Inclusive StudiesInclusive Studies

Non-pointingphotons from 0

1G~ ~

GMSB Point 1b(Physics TDR)

LHC Point 5(Physics TDR)

R-Parity Conserved

R-Parity Violated

~

ATLAS

ATLAS


Decay ResimulationDecay 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. ET

miss)– 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?


Decay ResimulationDecay Resimulation• Select seed events:

– 2 leptons with pT>25GeV

– 2 jets with pT>50GeV

– ETmiss < <pT(l1),pT(l2)> (SUSY)

– Both m(lb) < 155GeV

• Solve 6 constraints for p()• Resimulate + merge with seed• Apply 1l SUSY cuts

m(lb)

cut

Pull (top pT)

Sample 5201,pT(top) > 200 GeVreco 11.0.4205,450 pb-1

GeV

Z

Estimate5201

ETmiss

>=2 jets, Normalisation to data under study

ATLAS

Preliminary

ATLAS

Preliminary

ATLAS

Preliminary

Sapporo, June 2007 Dan Tovey 1 Supersymmetry at ATLAS Dan Tovey University of Sheffield Dan Tovey...

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Transcript of Sapporo, June 2007 Dan Tovey 1 Supersymmetry at ATLAS Dan Tovey University of Sheffield Dan Tovey...