“atlas susy searches” gianluca comune michigan state university on behalf of the atlas...

“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