Searches for Supersymmetry at ATLASmctp/SciPrgPgs/events/2012/higgs/talks/Bianco.pdf · Final...

Searches for Supersymmetry at ATLAS

M. BiancoM. Bianco

(INFN & University of Salento)

on behalf of the ATLAS Collaboration

17/04/2012 Michele Bianco (Spring-2012) 1

The ATLAS experiment at the LHCATLAS detector:

• cylindrical geometry with ~4π coverage

• inner detector with a silicon pixel detector,

silicon microstrip detector, and transition

radiation tracker, inside 2 T solenoid field

• electromagnetic and hadronic

calorimeters extending to |η| < 4.9

• muon spectrometer |η| < 2.7 within a

toroidal magnetic system

17/04/2012 2Michele Bianco (Spring-2012)

toroidal magnetic system

2010 2011 2012

pp √s 7 TeV 7 TeV 8 TeV

Data on tape 45 pb-1 5.25 fb-1 ~ 15 fb-1*

(* expected)

SupersymmetryNew symmetry between fermions and bosons�A superpartner for every SM particle differing

by half unit of spin

�At least 2 Higgs doublets

Motivation

• SUSY can solve the naturalness problem

• Unification of forces at a high energy scale

• If R-parity is conserved, the lightest supersymmetric

particle (LSP) is stable! Good Dark Matter candidate


particle (LSP) is stable! Good Dark Matter candidate

� SUSY particles are produced in pairs

SUSY Production @ LHC

Strong production

Direct stop/sbottom

Weak production

Searching for SUSY at the LHC

LHC is a proton-proton collider• Large cross-section of colored sparticle

production (depending on mass)

• Cascade decays from squarks/gluinos

Final states are characterized by: multi-jet + ETmiss + X

Here we will discuss:squark-squark production (heavy gluino):


squark-squark production (heavy gluino):

final states with 2 or more jets → 0-lepton analysis

gluino-gluino and squark-gluino production (heavy squark, chargino→ neutralino):

long decay chains, many jets and (possibly) leptons. → 0 and 1 -lepton+multijet

Direct sbottom production:

exactly 2b-jets and ETmiss from the neutralinos → 2 b-jets only (veto on third jet)

Direct stop production :

several t or b quarks and neutralinos → 2 same-flavor leptons + jets + ETmiss

Gauge mediated stop and sbottom production:

several t or b quarks and neutralinos → b-jets, one or several leptons, light jets

Long-lived particle / Disappearing track:

distinguish signals by a strange object or mass → ETmiss trigger/muon trigger

0-lepton and (2-6) jets + ETmiss

• This analysis search for squark/gluino production in final states with jets, ET

miss , and no leptons

• Sensitive to final states from squarks decaying directly (≥ 2 jets) to longer decay chains (≤ 6 jets)

• Discrimination signal/background is based on the effective mass meff sensitive to the SUSY mass scale, meff is defined as:

ATLAS-CONF-2012-033 (4.7 fb-1)


• 11 inclusive signal regions (SR), depending on jet multiplicities and meff

– From 2 to 6 jet multiplicities and various combination of meff cuts

– Optimized to achieve maximal reach over (msquark,mgluino) plane, and to enhance sensitivity to models with compressed spectra (small mass splitting)

• Backgrounds from multi-jet processes kept under control through cuts on the minimum azimuthal angle (∆φ) between the jets and ET

miss

0-lepton and (2-6) jets + ETmiss ATLAS-CONF-2012-033

(4.7 fb-1)

Different

mass scales

Gluinos/cascadesSquarks Compr. Spec.


For each signal region, 5

control regions Multijet BG determined from

data-driven smearing method,

non QCD BG, estimated from

background-enriched control

regions (CR) through transfer

factors (TF) taken from MC


(4.7 fb-1)

Observed meff(incl.) distribution for signal regions A, D and E


� Good agreement is observed between the data and the SM prediction, with no

significant excess with respect to SM predictions


(4.7 fb-1)

Non-disjoint SRs: exclusion based on best expected significance

MSUGRA/CMSSMGluino-squark-neutralino model


● In the simplified model, gluino masses below 940 GeV and squark masses below 1380 GeV are excluded at the 95% confidence level.

● In the MSUGRA/CMSSM models, values of m1/2 < 300 GeV are excluded for all values of m0, and m1/2 < 680 GeV for low m0.

● Equal mass squarks and gluinos are excluded below 1400 GeV in both scenarios.


(4.7 fb-1)

Search for gluino production in final states with several jets, ETmiss

and no leptons.( This analysis differs from the 0-lepton and (2-6)jets analysis for much lower MET

cut because of multijet triggers )


�Six non-exclusive Signal Regions

�The j55 and j80 signal regions are motivated by different triggers

�HT scalar sum of the transverse momenta of all jets with pT > 40 GeV and |η| < 2.8

�√HT correlated with ETmiss resolution


(4.7 fb-1)

Main background sources:

– multi-jet processes (including fully hadronic tt), estimated from data in control regions with lower jet multiplicities

– 'Leptonic‘ processes: tt (semi and full-leptonic) and W/Z+jets, estimated from data (when possible) in control regions, and extrapolated to signal region using MC with transfer function



(4.7 fb-1)


Most discrepant 7j80

p-value 0.07

Signal region results

1-lepton + jets + ETmiss ATLAS-CONF-2012-041

(4.7 fb-1)

• Search for strong production of squarks/gluinos pair in events containing:� jets

�ETmiss

� one isolated lepton (electron or muon)

• Two different searches:

– Soft lepton to probe models with compressed spectra:

• 7(6) < pT < 25 (20) GeV for electrons (muons)

– Hard lepton to probe higher SUSY mass scales:

• pT > 25 (20) GeV for electrons (muons)


• pT > 25 (20) GeV for electrons (muons)

• 2 exclusive signal regions: 3, 4 jets

• Signal/background discrimination provided by transverse masse mT and the inclusive effective mass meff;

• Backgrounds:tt, W+jets (dominant) Z+jets, single top, dibosons:

� tt, W+jets: estimated in CR and calculated in SR via TF

� Other backround: QCD estimated entirely from the data by matrix method, Z+jets, single-top and diboson production is estimated almost purely from simulation


(4.7 fb-1)

• Fit inputs (poisson distributed):

– Observed number of W+jets and tt incontrol regions (binned in jet multiplicities);

– Transfer factors (TF) for W+jets and ttfrom MC, cross-contamination and signal contamination in control regions

• Background in signal regions from (over-constrained) simultaneous fit based on profile likelihood method;


signal contamination in control regions taken into account;

– Number of multi-jet events in signal/control regions: entirely data-driven

– Minor backgrounds in signal/control regions from MC.

• Fit results are checked in validation regions, kinematically similar to signal regions;

• Fit free parameters: overall normalization for W+jets and tt;

• Uncertainties treated as nuisance parameters (gaussian distributed)


(4.7 fb-1)

�Significantly improved from previous analysis

�No significant excess observed with respect to SM predictions


Near-degenerate region, big sensitivity

improvement from soft lepton analysis

Summary of results in some benchmark model

ATLAS-CONF-2012-033 (4.7 fb-1)

Up to 6 jets + ETmiss

ATLAS-CONF-2012-037 (4.7 fb-1)


ATLAS-CONF-2012-037 (4.7 fb-1)

Up to 9 jets + ETmiss

ATLAS-CONF-2012-041 (4.7 fb-1)

1 lepton + jets + ETmiss

3rd Generation SUSY Searches

� An important motivation for SUSY: “naturalness” => stabilize the

Higgs mass without massive fine tuning

� 3rd generation squarks (t, b) could be light

� 2 ways to search for them: � direct pair production

� gluino cascade decays

~ ~


Direct production

Through gluino decays

Direct sbottom production arXiv:1112.3832

�Analysis to probe direct sbottom production at the LHC with

�Signature: exactly two b-jets and ETmiss from the neutralinos

�Discriminant variable: boost-corrected cotransverse mass (MCT),

�Proposed in JHEP 0804, 034 (2008) and JHEP 1003, 030 (2010)

�In two pair-produced heavy states δ which decay via δ→ v α:

0

11~~χbb →


�In two pair-produced heavy states δ which decay via δ→ viα:

�MCT(bb) distributions have endpoints defined by mδ, mα:

� For :

� For SUSY: proportional to in decays( )1

011

~2

~~bb

mmmχ

− 0

11~~χbb →

Three signal regions considered: MCT > 100, 150 and 200 GeV

GeVbbM CT 0.135)( 21

max =bb


Backgrounds•Diboson, and associated production ( ) (1-10% of the total background) :

estimated using MC

•Main backgrounds: semi data-driven estimation using transfer factors

o Top and W/Z in association with heavy flavor jets (W+hf, Z+hf) with

(irreducible) and

tt WWttZttWtt , ,

νν→Z

τν→W

Results�Systematic uncertainty on the background: 21%-44% (dominated by the finite statistics in


�Systematic uncertainty on the background: 21%-44% (dominated by the finite statistics in

control regions).

�Data in good agreement with the SM prediction in all signal regions.


�Expected and observed exclusion

limits, in the sbottom-neutralino

mass plane.

�The reference point indicated on

the plane corresponds to the

MSSM scenario with sbottom and

neutralino masses of 300 GeV and


neutralino masses of 300 GeV and

100 GeV, respectively.

�Results are compared to previous

exclusion limits from Tevatron

experiments.

�Results from LEP cover the

region with sbottom mass below

100 GeV.

Direct stop production ATLAS-CONF-2012-036 (2.05 fb-1)

GMSB scenario with gravitino LSP (mG < 1 keV), neutralino NLSP�Analysis signature:

2 same-flavor leptons + jets + ETmiss

�Trigger:

electron / muon + jet

�Selection:

86 < mll < 96 GeV

first jet pT> 60 GeV, one more pT> 50 GeV

1 b-tagged jet p > 50 GeV


1 b-tagged jet pT> 50 GeV

ETmiss > 50 (80) GeV

�Backgrounds:

�The main SM processes contributing to the background are, in order of importance, top quark

pair and single top quark production, followed by Z+hf and W+jets

�Top background is evaluated using CRs. Estimation in each SR is obtained by multiplying

the number of events observed in the corresponding CR – corrected using simulations for

non-top backgrounds – by a TF.

�Z+hf process is estimated from MC simulation and validated in a CR where events passing

all SR selection criteria except for a reversed ETmiss cut (ET

miss < 50 GeV) are considered.

�W+jets background estimate via data-driven method.

Direct stop production ATLAS-CONF-2012-036 (2.05 fb-1)

The results are in agreement with the

SM prediction.

Stop masses up to 310 GeV are

excluded for 115 GeV < < 230 GeV

at 95% C.L., reaching an exclusion of

< 330 GeV for = 190 GeV.

01χ̂

m

01χ̂

m1~t

m


Gluino Mediated Stop and SbottomATLAS-CONF-2012-003 (2.05 fb-1)

Stop with 0-1 lepton and b-jets + ETmiss

�Final state with several top or bottom quarks and neutralinos

�Signature: b-jets, ETmiss one or several leptons, light jets

SM background �Non-QCD background:�Semi data-driven estimation from control regions using transfer factors

�QCD background:�Less than 5% of the total background

�Estimated from data using a pass-fail matrix method

Discriminant variables:


Signal Region definition

Discriminant variables:

Transverse mass: , effective mass


Results

Distribution of the

effective mass for the

1-electron (left) and

1-muon (right)

channel in SR1-D.


channel in SR1-D.

Sig Reg SM Background Data

SR1-D (electron) 39±12 43

SR1-D (muon) 38±14 38

SR1-E (electron) 8.1±3.4 11

SR1-E (muon) 6.3±4.2 6

Number of events in

signal regions

Good agreement of

data with the SM

estimation


•MSSM scenario Gtt model Gtb model


•MSSM scenario

(considering both and

production )

•Gluino masses below 620

GeV are excluded for stop

masses up to 440 GeV

Gtt model

(simplified models considering

only production)

•Gluino masses below 700 GeV

are excluded for

•Neutralino masses below

160 GeV are excluded for

GeVm 10001ˆ

=χ

GeVmg 700~ =

Gtb model

(simplified models considering

only production)

•Gluino masses below 710 GeV

are excluded for

•Neutralino masses below

200 GeV are excluded for

GeVm 10001ˆ

=χ

GeVmg 600~ =

gg~~

tt~~ gg~~ gg~~

Long-lived particles

LLP are not an exotic theory: they are naturally expected in some models!

• Anomaly Mediated Symmetry Breaking (AMSB), very small mass splitting

• 3-body decay via very heavy particle (Split SUSY)

• Gauge Mediated Symmetry Breaking (GMSB, stau), very small coupling

• RPV coupling 10 -(5-7) -> long-lived LSP

Trigger based on ETmiss , multi jets and muon signatures in order to be able to


T

distinguish signals by a strange object or mass.

(1) Slow, large dE/dx

(2) Slow, stopped

(3) Disappearing track

(4) Kinked track

(5) displaced track Lon

ger

life

tim

e~ 1000 mm

~ 100 mm

~ 10 mm

Disappearing trackATLAS-CONF-2012-034 (4.7 fb-1)

In AMSB scenarios, the lightest gaugino is the wino, and the lightest chargino

and neutralino are the charged and neutral winos; becomes slightly heavier

due to radiative corrections involving electroweak gauge bosons in the loops.

Masses of charged and neutral winos are highly degenerate and this leads to a

significant lifetime for the lightest chargino.

±

1~χ

±

1~χ0

1~χ


Background:

�Interacting hadron track

�Badly reconstructed track

Disappearing trackATLAS-CONF-2012-034 (4.7 fb-1)

The pT spectrum of the observed candidates

track is found to be consistent with the

expectation from SM background processes.


Constraints on AMSB chargino mass and lifetime are set: a chargino having a mass

below 90 (118) GeV and 0.2 (1) < < 90 (2) ns is excluded at 95% CL.

The result also gives a new constraint for a chargino having a mass up to 118 GeV.

±1

~χτ

Summary and conclusions

Summary

�ATLAS has performed many SUSY searches.

�So far, no indication of BSM physics in any of them:

� Most stringent limits set on popular models, but there is still a

long way till light SUSY is excluded

Outlook

�Update all analyses to ~5 fb-1 collected in 2011 data taking


ATLAS SUSY public resultshttps://twiki.cern.ch/twiki/bin/view/AtlasPublic/SupersymmetryPublicResults

�Update all analyses to ~5 fb-1 collected in 2011 data taking

�Searches for other signatures are coming

�Present searches will keep going

�more and refined Signal Regions

�refined background estimates

�2012 Data coming now!!!!

Searches for Supersymmetry at ATLASmctp/SciPrgPgs/events/2012/higgs/talks/Bianco.pdf · Final...

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