“ATLAS SUSY SEARCHES”

Gianluca ComuneMichigan State University

On Behalf of the ATLAS Collaboration

PANIC 2005, Santa Fe’ 27/10/2005

LHC and ATLAS• LHC

– 14 TeV CoM p-p collisions– Start of operations 04/2007– Total integ. luminosity 300 fb-1

• ATLAS– (A Toroidal LHC ApparatuS)– General purpose detector– Vast physics program

• Higgs, SUSY, Exotics, top, B physics...

Staged ATLAS components:• One Pixel layer• Transition Radiation Tracker outer end-caps• Cryostat gap scintillators• Part of Muon drift tubes and half cathode strip layers• Part of forward shielding• Part of LAr read-out• Large part of trigger/DAQ CPUs

SUSY and mSUGRA• Every particle has a super-partner

– “Heaven” for particle physicists• MSSM Lagrangian depends on 105

parameters (!!)– Need to make some assumption to reduce the

degree of freedom• mSUGRA depends on 5 (+1) parameters

M0, M1/2, A0, tan(β), sgn(μ), mtop– Assuming R parity conservation

=> escaping LSP => large ETMISS and

scalar particles produced in pairs• Event cannot be fully reconstructed• SUSY is a bgd to itself

– Various regions in the par. space• Coannihilation, Focus Point, Funnel, Bulk region

(Ellis et al., Phys. B565 (2003) 176)

M0 (GeV) M1/2(GeV) A0 tanβ sgn(μ) mtop (GeV)

Coannihilation 70 350 0 10 + 175Focus point 3550 300 0 10 + 175

SUSY Production at LHC

• Production cross sections vary widely– From few to several hundreds pb-1

• Actual kinematics and CS depend heavily on the chosen model– Long and complex decay chains

• If R parity is conserved large ETMISS

– Powerful handle for Standard Model background removal• SUSY events have generally large jet multiplicity and large jet pT• Depending on mass hierarchy multi lepton signatures as well

pg~

Lq~qq

l~0

2χ~01χ~

l l

p

(stau Coannihilation point)

Inclusive Searches

0 lept.

ATLAS Physics TDR

SM (PYTHIA)

10 fb-1

• Discovery– Assuming luminosity 1033 cm2 s-1

• 1300 GeV => “1 week”• 1800 GeV => “1 month”• 2200 Gev => “1 year”

• Backgrounds:– Real missing energy from SM processes with

hard neutrino (tt, W+jets, Z+jets)– Fake missing energy from detector– Jet energy resolution (expecially non-gaussian

tails) critical (Fast parametric detector response)

• 1 jet with pT >100 GeV, 4 jets (pT>50 GeV) • ET

MISS > max(100 GeV ,0.2Meff)• Transverse sfericity ST>0.2• No isolated muon or electron (pT>20 GeV)

1 TeV SUSY

Realistic Bgd EstimationPrevious analysis uses Parton Shower for SM processes:

=> badly underestimates hard jet emission

SM (ALPGEN+PYTHIA)

Recent ATLAS background studies:-hard process with exact ME computation -Alpgen, Sherpa (collinear and soft region through PS)-hadronization -HERWIG,PYTHIA-Solve double counting problems

-MLM matching

Parton shower is a good model in collinearregion, but fails to describe hard jet emission

GeV

(pT of hardest jet)

Inclusive Searches (2)• High pT jets are produced also

in background processes=> bad separation!!

• ETMISS excess can be

– ETMISS > 800 GeV

– Need to optimize the selection• Meff still a good discovery

signal (requiring 1 lepton)

0 leptons (preliminary)

1 lepton

• 0 lepton mode– No leptons, xEt＞ 100GeV, >= 1 jet with pT>100GeV, >=4 jets with pT>50GeV, Transv. Sphericity >0.2

• 1 lepton mode– e,μ Pt >10 GeV, xEt＞ 100 GeV, >= 1 jet with pT>100GeV, >=4 jets with pT>50GeV, Transv. Sphericity >0.2,

Transverse mass between lepton and xEt >100GeV (to suppress W+N jets Background)

Focus Point4.2 fb-1

1 lepton

SUSY production dominated by

Red: signalBlack: bgd

Top Background estimate

• The Top mass reasonably uncorrelated with ET

MISS

• Select events with m(lj) in top window – apply W mass constraint – no b-tag used– Estimate combinatorial background with

sideband subtraction. • Normalize to low ET

Miss region – SUSY contribution is small

• Procedure gives estimate consistent with Top distribution also when SUSY is present

• Z+jets: big contribution from Z → – Can use Z →ee, apply same cuts as analysis,

substitute ET(ee) with ETmiss and rescale by

BRs.

Blue: tt (MC@NLO)Green: SUSYDots: top estimate

Preliminary

Full Simulation 0.5 fb-1

SUSY Spectroscopy• After SUSY is discovered it needs to be characterized

– particle masses, spin …

pg~

Lq~qq

l~02χ~

01χ~

l l

p

• In every sequential double two body decay of the form

• The maximum of the invariant Mass distribution is related to the initial particle masses through:

• Use it on a “typical” SUSY decay chain

Formulas in Allanach et al., hep-ph/0007009

Leptonic Signatures

ql(max) Larger of M(llq)Coannhilation point 5.6 fb-1

ql(min)

minllqllq

maxllq mmm

p g~

Lq~qq

l~02χ~

01χ~

l l

p

• SM background negligible (could be a discovery signal)• Opposite-Flavour/Opposite Sign is subtracted (removes SUSY bgd)

Coannhilation P..5.6 fb-1

Mll (GeV) Mll (GeV)

Point 5a 4.37 fb-1

Mod. Point 55.0 fb-1

Black:t-tbar bgd

Coannihilation point 20 fb-1

Tau Signatures• Tau signatures play a very important role

– Tau BR relevant over a large portion of SUSY parameter space

– In stau coannihilation ( ) region is critical to reconstruct the stau mass (one tau is very soft)

• The relic dark matter density of the universe depends from the mass difference M1-M1

0 (very small)

γττ~χ~ 101

Point 5A4.4 fb-1

m

(1 tau pT > 40 GeV, 1 Track pT>6 GevNo other track pT > 1 GeV in R < 0.4)

• Currently investigating a track seeded tau reconstruction algorithm

SUSY Particle Masses • Once the edge values

(and the errors) are known one can determine the SUSY particle masses– It is critical to understand how

to fit all edges• Work in progress

– Difficult to develope a true model independent approach

• More than one decay scenario (i.e. SUSY model) can lead to the same signature

• Need an independent measure of one of the SUSY particle to set the absolute scale

m10 (GeV)

m 2

0 (G

eV)

(GeV)

(GeV)

~Lq

~ 01

~R

~L ~ 0

2

Conclusions• Few fb-1 of data should allow ATLAS to

measure a clear excess over the SM contribution and reconstruct several mass relations. – this can be achieve in the first year of data taking depending on

how quickly the detector and the SM backgrounds will be understood

• Large scale productions of Geant4 realistic detector simulated data– To understand detector systematics and prepare for real data

analysis.– Scan of parameter space to understand different problems

• Recent ATLAS (and CMS) collaboration efforts are focused on understanding of Standard Model backgrounds with the use of the latest Montecarlo tools

• Developing strategies to validate the Montecarlo predictions with data.

Backup• Jet should be matched to the parton generated with

ME (R=0.7) except for the soft and collinear regions.– Blue show perfect matching between ME parton and jet.– Soft jet was emitted collinearly => Matched (Accepted)– One parton divided into 2 jets. (outside ME cone 0.7) => Not Matched

• Event should be covered with 5jet ME (double counting) => Reject event

Matrix Element and double counting (MLM)

M. Mangano http://mlm.home.cern.ch/mlm

Other Background Sources • At startup calibration

data will be limited• Miscalibrated

detector is a source of ET

Miss

• QCD jets can add non gaussian tails to ET

Miss

– Very important given the CS

Coannhilation Point 5.6 fb-1

Other Endpoints

qqqqL

~~ 01

~~1

~ (using a mixed event technique for the SUSY bgd reduction)

Without t-tbar bgdWith t-tbar bgd

20.6 fb-1No cuts

2.6 excess

01

~03,2

~

Focus Point 4.2 fb-1