Early SUSY Searches in Events with Leptons with the ATLAS ... · distributions is ﬂat: ofﬁcial...

Early SUSY Searches in Events with Leptons with theATLAS-Detector

Timo Müller

Johannes Gutenberg-Universität Mainz

2010-29-09 EMG Annual Retreat 2010

Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 1

Outline

1 Introduction and FundamentalsSupersymmetry and mSUGRASUSY Signatures and Backgrounds at the LHCExperimental Setup

2 Analysis ResultsData-Monte Carlo ComparisonsBackground Studies: W+Jets K-FactorsExpected Exclusion Limits

3 Summary and Outlook


Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup

Basics of Supersymmetry

SUSY is a new symmetry which relates fermions to bosons via a S USY-transformation:

Q |boson〉 = |fermion〉 , Q |fermion〉 = |boson〉

◮ All SM particles have SUSY-partners with spin difference of ±1/2◮ SUSY-partners have same electric charges, weak isospin and color degrees of freedom

Minimal Supersymmetric Standard Model (MSSM):◮ Each SM boson gets one fermionic superpartner → gauginos◮ Each SM fermion gets two bosonic superpartners (handedness) → sfermions◮ Higgsinos and ew. gauginos mix due to same quantum numbers giving observable mass

eigenstates (4 neutralinos χ̃0i , 2 charginos χ̃±

i )

Theoretical Motivation

◮ Higgs mass stabilization against quadratic divergent loop corrections (hierachy problem)◮ Can provide unification of gauge couplings◮ LSP as cold dark matter candidate in SUSY theories with exact R-parity conservation◮ Invariance under local SUSY-transformations causes existence of a spin 2 particle (graviton)



Supersymmetry Breaking

Masses of SM and SUSY particles cannot be degenerate (so far no SUSY particles observed)⇒ SUSY must be broken at low energies

SUSY breaking is a critical point◮ Breaking mechanism unknown ⇒ parametrization of SUSY breaking effects◮ MSSM has more than 100 new free parameters ⇒ theory becomes highly non predictive

⇒ Need to impose constraints to obtain a phenomenoligically v iable theory

The mSUGRA Model*

◮ SUSY breaking mediated bygravitational interactions

◮ Universal masses and couplings atGUT scale

⇒ 5 free parameters: m0, m1/2, tan β, A0,σ(µ)

◮ Certain parameter choice fixes massspectrum of the theory

*other models (AMSB, GMSB, split SUSY) also considered in ATLAS2 4 6 8 10 12 14 16 18

Log10

(Q/1 GeV)

0

100

200

300

400

500

600

Mass

[GeV

]

m0

m1/2

(µ2+m

0

2)1/2

squarks

sleptons

M1

M2

M3

Hd

Hu



Production and Decay of SUSY Particles

If scalar masses not too heavy, squark gluino production via strong interaction dominant

SUSY Signatures

Pairs of squarks and/or gluinos decay vialong cascades resulting in

◮ Hard jets from decays of gluinos andsquarks

◮ Missing transverse energy due to the 2LSPs escaping detector

◮ possibly Leptons from chargino, neutralinoand slepton decays

⇒ n leptons + k jets +�ET

Leptonic final states

◮ Decays of charginos and neutralinos via(real or virtual) W and Z bosons

◮ Also decays via sleptons (τ̃1) possible

g̃

q̃

q

χ̃01

q q

q̃ χ̃±

1

χ̃01

Wν

l

g̃

q̃

q

g̃ q̃∗

q q

q̃ χ̃02

χ̃01

q

Zl

l

q

χ̃01



Production and Decay of SUSY Particles

If scalar masses not too heavy, squark gluino production via strong interaction dominant

SUSY Signatures

Pairs of squarks and/or gluinos decay vialong cascades resulting in

◮ Hard jets from decays of gluinos andsquarks

◮ Missing transverse energy due to the 2LSPs escaping detector

◮ possibly Leptons from chargino, neutralinoand slepton decays

⇒ n leptons + k jets +�ET

Leptonic final states

◮ Decays of charginos and neutralinos via(real or virtual) W and Z bosons

◮ Also decays via sleptons (τ̃1) possible

SUSY event in ATLAS



2- And 3-Body Region

◮ m0-m1/2 plane can be divided into two regions depending on ∆m of squarks and gluinos:

2-body region: m0 ≪ m1/2 ⇒ mg̃ > mq̃

g̃

q̃

q

q̃

q

χ̃±

1 W ∗

l

ν

q

χ̃±

1

χ̃0

1

χ̃0

1

W ∗l

ν

3-body region: m0 ≫ m1/2 ⇒ mg̃ < mq̃

g̃

q̃

q

q̃∗

q

χ̃0

2 Z∗

l

l

χ̃0

1

g̃ q̃∗

q q q

χ̃0

1



Standard Model Backgounds

leptonic decay of t t̄ pair

◮ Most important SM background (samesignature)

t

b

W+

q

q′

t̄

b̄

W−

ν

l−

W/Z+jets

◮ Fake leptons from jets in 2-lepton channel

q

q̄′

W

l

ν

g

◮ Fake missing transverse enery

q

q̄

Z

l−

l+g

... more backgrounds

◮ QCD

◮ Single top

◮ Diboson (WW, ZZ, WZ)



The LHC

◮ Proton-proton collider currently running at√

s = 7TeV (design:√

s = 14TeV)◮ Peak luminosity achieved: 3, 5 × 1031cm−2s−1 (design: 1033–1034cm−2s−1)◮ Max. number of bunches achieved: 56 with 1011 protons per bunch (design: 2808)◮ So far ≈ 7pb−1 of data recorded by ATLAS under stable conditions



The ATLAS-Detector


Analysis Results Data-MC Comparisons W+Jets K-Factors Expected Exclusion Limits

Important Variables

Effective Mass Meff

◮ Definition:Meff =

∑Njetsi=1 pjet i

T +∑Nlep

i=1 plep iT +�ET

◮ Measure of the primarily producedmass / SUSY mass scale

◮ Heavy SUSY particles lead to largeMeff

Transverse Mass MT

◮ Definition: M2T = 2plep

T �ET − 2p lepT · �ET

◮ Important variable against W+Jets in1-lepton channel



Top ReconstructionMissing Transverse Energy Top Reconstruction

◮ extraction of mass peak by solvingkinematics of semileptonic t t̄ decay

◮ reconstruction of leptonic W andχ2-minimization to select jets

◮ still limited by statistics



Systematic Uncertainties/Scale Variations

◮ Crucial to understand systematic uncertainties in searches for new physics e.g. calorimeterenergy scale, lepton fake rate, reconstruction efficiencies, luminosity, ...

◮ Large theoretical uncertainties from scale variations in Alpgen + Herwig Monte Carloproduction

◮ W+Jets is an important background for leptonic SUSY searches◮ Study uncertainties on W+Jets by varying renormalization scale and factorization scale up

and down by a factor of 2Result:

◮ Shapes in each parton bin do not change for differing scales◮ Only the cross section changes⇒ Ratio of distributions with nominal cross section applied to varied samples and nominal

distributions is flat: official, high scale, low scale



Measuring W+Jets K-Factors from Data

◮ Difference between the cross sections can be applied like a K-Factor◮ Differences between the sets of K-Factors for up and down variation can be considered as

systematic uncertainiesCan we determine W+Jets K-Factors from data more precisely?

◮ Jet multiplicity distribution is sensitive to Matrix Element parton◮ Fit Monte Carlo jet multiplicity distribution to data in W+Jets enriched region by varying

K-Factors for different parton bins◮ Keep overall normalization fixed

before fitting after fitting

Measurement : Kp0 = 0.94 ± 0.01, Kp1 = 1.25 ± 0.06, Kp2 = 1.65 ± 0.16, Kp3−5 = 1.55± 0.29

Theory K pi (down,up) : Kp0(0.92, 1.05), Kp1(1.20, 0.86), Kp2(1.43, 0.73), Kp3(1.67, 0.64),Kp4(1.93, 0.55), Kp5(2.16, 0.49)



Jet Spectra

Jet pT distributions with fitted K-Factors



Optimization Strategy

Idea: Exploit kinematic differences of regions in m0-m1/2-plane by designing distinct analyses for2- and 3-body points

◮ Divide m0-m1/2-plane into different kinematic regions depending on mass difference ofsquarks and gluino

◮ 2-body region : all squarks (except forstops) lighter than gluino

◮ 3-body region : all squarks (except forstops) heavier than gluino

◮ Mixed region : all others◮ Optimize cuts on�ET , HT , MT ,

jet-multiplicity, jet pT seperately for bothregions

◮ Goal: Maximize exclusion reach for agiven luminosity

Results for 10 pb −1 in 1 lepton channel◮ 2-body region: HT > 250 GeV,�ET > 160 GeV, #Jets ≥ 2, pjet2

T > 55 GeV, MT > 100 GeV◮ 3-body region: HT > 340 GeV,�ET > 110 GeV, #Jets ≥ 4, pjet4

T > 30 GeV, MT > 100 GeV



Expected Exclusion with 2.7 pb -1

◮ Calculate 95% confidence limit on the signal cross section using CLs method◮ Use shape of HT distribution as input to improve reach◮ Combine different leptonic final states: e, µ, ee, eµ, µµ

Soon will be sensitive beyond existing LEP and Tevatron limi ts


Summary and Outlook

Summary and Outlook

Summary◮ SUSY is a promising BSM theory◮ Scale variations are an important source of systematic uncertainties for SUSY searches◮ W+Jets K-Factors can be extracted from data◮ Generic kinematic properties can be exploited to optimize analysis◮ ATLAS will soon be sensitive to SUSY beyond existing limits

Outlook◮ Extend analysis to more general SUSY models◮ Extend systematic studies to other backgrounds (t t̄ , Z+Jets)


Early SUSY Searches in Events with Leptons with the ATLAS ... · distributions is ﬂat: ofﬁcial...

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