Early SUSY Searches in Events with Leptons with the ATLAS-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 Fundamentals Supersymmetry and msugra SUSY Signatures and Backgrounds at the LHC Experimental Setup 2 Analysis Results Data-Monte Carlo Comparisons Background Studies: W+Jets K-Factors Expected 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 SUSY-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 uantum numbers giving observable mass eigenstates (4 neutralinos χ 0 i, 2 charginos χ± i ) Theoretical Motivation Higgs mass stabilization against uadratic 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 viable theory The msugra Model* SUSY breaking mediated by gravitational interactions Universal masses and couplings at GUT scale 5 free parameters: m 0, m 1/2, tanβ, A 0, σ(µ) Certain parameter choice fixes mass spectrum of the theory *other models (AMSB, GMSB, split SUSY) also considered in ATLAS Mass [GeV] 600 500 400 300 200 100 0 2 4 6 8 10 12 14 16 18 Log 10 (Q/1 GeV) H d H u M 2 M 3 M 1 sleptons suarks (µ 2 +m 0 2 ) 1/2 m 1/2 m 0 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, suark gluino production via strong interaction dominant SUSY Signatures χ 0 1 l Pairs of suarks and/or gluinos decay via long cascades resulting in g χ ± 1 W ν Hard jets from decays of gluinos and suarks Missing transverse energy due to the 2 LSPs escaping detector possibly Leptons from chargino, neutralino and slepton decays n leptons+k jets+ E T g χ 0 1 χ 0 2 χ 0 1 Z l l Leptonic final states Decays of charginos and neutralinos via (real or virtual) W and Z bosons Also decays via sleptons ( τ 1 ) possible g χ 0 1 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, suark gluino production via strong interaction dominant SUSY Signatures Pairs of suarks and/or gluinos decay via long cascades resulting in Hard jets from decays of gluinos and suarks Missing transverse energy due to the 2 LSPs escaping detector possibly Leptons from chargino, neutralino and slepton decays SUSY event in ATLAS n leptons+k jets+ E T Leptonic final states Decays of charginos and neutralinos via (real or virtual) W and Z bosons Also decays via sleptons ( τ 1 ) possible 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 m 0 -m 1/2 plane can be divided into two regions depending on m of suarks and gluinos: 2-body region: m 0 m 1/2 m g > m 3-body region: m 0 m 1/2 m g < m χ 0 1 l χ 0 1 l g χ ± 1 W ν g χ 0 2 Z l χ ± 1 W l g χ 0 1 χ 0 1 ν 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 t pair Most important SM background (same signature) t b W + W/Z+jets Fake leptons from jets in 2-lepton channel l W g ν t b W l ν Fake missing transverse enery l Z g l +... 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 10 31 cm 2 s 1 (design: 10 33 10 34 cm 2 s 1 ) Max. number of bunches achieved: 56 with 10 11 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 M eff Definition: M eff = N jets i=1 pjet i T + N lep i=1 plep i T + E T Measure of the primarily produced mass / SUSY mass scale Heavy SUSY particles lead to large M eff Transverse Mass M T Definition: MT 2 = 2plep T E T 2p lep T E T Important variable against W+Jets in 1-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 Reconstruction Missing Transverse Energy Top Reconstruction extraction of mass peak by solving kinematics of semileptonic t t decay reconstruction of leptonic W and χ 2 -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. calorimeter energy scale, lepton fake rate, reconstruction efficiencies, luminosity,... Large theoretical uncertainties from scale variations in Alpgen + Herwig Monte Carlo production 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 2 Result: 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 uncertainies Can 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: K p0 = 0.94±0.01, K p1 = 1.25±0.06, K p2 = 1.65±0.16, K p3 5 = 1.55± 0.29 Theory K pi (down,up): K p0 (0.92, 1.05), K p1 (1.20, 0.86), K p2 (1.43, 0.73), K p3 (1.67, 0.64), K p4 (1.93, 0.55), K p5 (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 p T 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 m 0 -m 1/2 -plane by designing distinct analyses for 2- and 3-body points Divide m 0 -m 1/2 -plane into different kinematic regions depending on mass difference of suarks and gluino 2-body region: all suarks (except for stops) lighter than gluino 3-body region: all suarks (except for stops) heavier than gluino Mixed region: all others Optimize cuts on E T, H T, M T, jet-multiplicity, jet p T seperately for both regions Goal: Maximize exclusion reach for a given luminosity Results for 10 pb 1 in 1 lepton channel 2-body region: H T > 250 GeV, E T > 160 GeV, #Jets 2, p jet2 T > 55 GeV, M T > 100 GeV 3-body region: H T > 340 GeV, E T > 110 GeV, #Jets 4, p jet4 T > 30 GeV, M T > 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 H T 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 limits 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 t, Z+Jets) Timo Müller (Universität Mainz) Early SUSY Searches in Events with Leptons with the ATLAS-Detector 17