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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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 2
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)
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 3
Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 4
Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 5
Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 5
Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 6
Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup
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)
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 7
Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 8
Introduction and Fundamentals SUSY and mSUGRA Signatures and Backgrounds Experimental Setup
The ATLAS-Detector
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 9
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 10
Analysis Results Data-MC Comparisons W+Jets K-Factors Expected Exclusion Limits
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 11
Analysis Results Data-MC Comparisons W+Jets K-Factors Expected Exclusion Limits
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 12
Analysis Results Data-MC Comparisons W+Jets K-Factors Expected Exclusion Limits
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)
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 13
Analysis Results Data-MC Comparisons W+Jets K-Factors Expected Exclusion Limits
Jet Spectra
Jet pT distributions with fitted K-Factors
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 14
Analysis Results Data-MC Comparisons W+Jets K-Factors Expected Exclusion Limits
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 15
Analysis Results Data-MC Comparisons W+Jets K-Factors Expected Exclusion Limits
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
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 16
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)
Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 17