Using direct stop searches at ATLAS to constrain the parameter space of supersymmetric models Walter Hopkins University of Oregon September 2 2015 SLAC HEP Seminar JHEP 09 (2014) 015 arXiv:1506.08616 (submitted to EPJC) arXiv:1508.06608 (submitted to JHEP) Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 1 / 48
Outline • Quick intro to SUSY • The LHC and ATLAS • ATLAS SUSY analysis primer: stop searches • Stop searches results: simplified model interpretation • pMSSM interpretation Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 2 / 48
SUSY parameter space N=1 MSSM • SUSY is very broad and pMSSM NMSSM describes many models • Masses and scales of SUSY are not specified! SUSY T. Rizzo, SLAC Summer Institute 2012 NMSSM = Next-to-Minimal Supersymmetric Standard Model MSSM = Minimal Supersymmetric Standard Model pMSSM = phenomenological Minimal Supersymmetric Standard Model Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 3 / 48
Simplified models • SUSY has many tunable parameters ⇒ need to reduce • Even with experimental constraints (as g ,˜ ˜ g ,˜ ˜ q q in pMSSM with 19 free parameters) Solution: simplified models • Assume only select SUSY particle can be produced at LHC • Remainder is too massive (4-7 TeV) mass • Production cross section of light SUSY ˜ ˜ t t particle depends mainly on mass • Make simplified assumption of decay χ ± ˜ 1 chain • For example, only 1 or 2 decay modes χ 0 χ 0 ˜ ˜ possible for stops (˜ t ) Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 4 / 48
Stop quark production • Top/stop quark are important for hierarchy p problem solution ˜ t • Preference for stop masses below ∼ 1 TeV • Searching for direct stop pair production • QCD production ⇒ calculable • Highest production cross section after gluinos and p ˜ t light squarks σ LPCC SUSY WG 10 SUSY) [pb] 8TeV LHC data 5 10 ~ ~ * q q ∼ ∼ ± q= u,d,s,c 1 χ χ 0 L,R 4 10 ~ ~ g g -1 10 → 3 -1 10 (pp #events in 20 fb ~ ~ * t t σ -2 10 NLO(-NLL) 2 10 ∼ ∼ ~ ~ - + - χ + χ l l -3 10 10 -4 10 100 200 300 400 500 600 700 800 900 1000 SUSY sparticle mass [GeV] https://twiki.cern.ch/twiki/bin/view/LHCPhysics/SUSYCrossSections arXiv:1206.2892 Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 5 / 48
Stop decay mode m ˜ χ 0 where ˜ χ 0 χ 0 = Lightest • ˜ t 1 → t + ˜ Supersymmetric Particle (LSP) • LSP doesn’t interact with detector: missing energy • Final states contains 2 tops and missing energy • Top pair decay with 0, 1, or 2 leptons m ˜ t • Highlight 0-lepton (all hadronic) Simplified model parameters: search as example t , m ˜ m ˜ χ 0 t p ˜ t χ 0 ˜ 1 χ 0 ˜ ˜ t 1 p t Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 6 / 48
LHC • 27 km circumference in Geneva-land • 7 and 8 TeV proton-proton collisions ended in 2012 • 13 TeV coming happening now! • ATLAS: general purpose • CMS: general purpose • LHCb: b-quark physics • ALICE: Heavy-ion (lead-lead) Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 7 / 48
ATLAS Detector • Subdetectors highlight (for 0-lepton analysis): • Calorimeters: jets • Inner tracker: b quark identification Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 8 / 48
General outline of a SUSY search at ATLAS Signal region aims • Divide the signal “grid” into regions m ˜ χ 0 with similar final state kinematics • Identify backgrounds which can fake 200 your signal signature • Find handles to reject background 100 • Estimate background from Monte Carlo 600 m ˜ 200 400 (MC) simulations t • Control regions (CR): Normalize the observable 2 MC for specific background • Validation regions (VR): closer to SR SR3 and check normalization and shape SR2 CR2 VR2 • Unblind and look for excesses CR1 VR1 SR1 • If nothing is found, set limits on models observable 1 Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 9 / 48
Signal signature χ 0 t → t + ˜ • Direct stop production with each ˜ • Tops decay to W -boson + b -quark • 2 LSP’s results in large missing energy ( E miss ) T • 2 b-jets from top decay • More jets from W decay • Ideally: 6 jets (2 of which are b-jets) and missing energy • 2 Top masses can be reconstructed jet E miss T b t t jet jet b jet Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 10 / 48
Possible backgrounds Semi-leptonic t ¯ t ν E miss b • Dominated by τ decays T • Only 1 reconstructed top mass t τ jet t jet • τ ’s mimic jets but have less tracks associated with jet b • E miss near τ jet jet T Other backgrounds • Z/W + bb , cc from gluon • Z → νν • W → ℓν • Irreducible hadronic t ¯ t + Z → νν • Has 2 b-jets, 2 top masses, and E miss T Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 11 / 48
Discriminating signal from background Events / 20 GeV ATLAS Data 8 10 ∫ -1 t t s = 8 TeV, L dt = 20 fb 7 10 W+jets Preselection • Strongest discriminator is E miss ~ Single top χ ± χ σ × 6 0 T 10 m( t , , )= (550,300,150) GeV ( 100) 1 1 1 Diboson • Amount of E miss ~ in signal χ σ × 0 5 m( t , )= (500,200) GeV ( 100) 10 1 Z+jets T 1 depends on m ˜ t and m ˜ t t V 4 10 χ 0 Total SM 3 10 • Compressed region: low E miss T 2 10 t : high E miss • High m ˜ T 10 • Baseline E miss > 150 GeV T 1 • Highest E miss > 400 Data / SM 1.5 T 1 0.5 100 150 200 250 300 350 400 450 500 miss E [GeV] T Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 12 / 48
Semi-leptonic t ¯ t rejection: τ -veto • Semi-leptonic t ¯ t background mainly from t → b + W ( → τ + ν ) • Hadronic τ decay is dominant source of background • Most common decay into 1 and 3 pions • Identified by: • Jet with < = 4 tracks • ∆ φ between jet and E miss small (∆ φ < π/ 5) T • Events with such a τ jet are vetoed Jets Hadronic τ -decay Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 13 / 48
t rejection: m b , min More t ¯ T � p 1 T p 2 • m T = T cos (∆ φ ) • m b , min = m T between b-jet closest to E miss and E miss T T T t background has cut off at m b , min • t ¯ ∼ 175 GeV T Events / 25 GeV Data 2012 ATLAS =jets ∫ SM Total -1 =b-jets L dt=20.1 fb , s =8 TeV t t Single Top Common selection y 1000 t t +V W Z Diboson ~ ∼ 0 χ 50 x ( t , )=(600,1) GeV 1 ~ ∼ 1 ∼ + 0 χ χ 50 x ( t , , )=(400,200,100) GeV 1 1 1 500 x 0 2.0 Data / SM 1.5 E miss 1.0 T 0.5 0.0 100 200 300 400 500 b ,min m [GeV] T , τ -veto, and m b , min E miss , effectively reduce t ¯ t contribution T T Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 14 / 48
Signal region example: high m ˜ t Signal region aims m ˜ χ 0 Low Top p T High Top p T 200 W boost W 100 b b 600 m ˜ 200 400 t • At higher m ˜ t , tops can have high p T (boosted) • Jets from top become collimated • Can reconstruct top mass from jets with a certain cone • Same algorithm (anti- k t ) used in jet recon is used to form “top jets” but with different parameters ( R = 1 . 2) • Top masses reject background: 2 top masses should be present in signal • Semi-leptonic t ¯ t should only have 1 top • Z/W+jets should not have any tops Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 15 / 48
Control region example: t ¯ t Events / 15 GeV ATLAS Data 2012 200 ∫ -1 SM Total L dt=20.3 fb , s =8 TeV t t t CR [SRB] t • Require 1 lepton ⇒ Single Top 150 t t +V orthogonal to signal region W • Similar definition to signal Z 100 Diboson region • 2 b-jets, 50 • E miss > 150 GeV T 0 • Some modification to 2.0 Data / SM enhance t ¯ t purity 1.5 1.0 0.5 0.0 0 100 200 300 0 m [GeV] jet, R =1.2 Good agreement between MC and data Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 16 / 48
Final background composition (highlight) Low E miss High E miss , loose top reco , tight top reco T T tt = 0.49 ± 0.34 ttV = 0.50 ± 0.17 tt = 10.64 ± 1.90 Z = 0.68 ± 0.27 W = 0.06 ± 0.08 ttV = 1.80 ± 0.59 Z = 1.42 ± 0.53 Other = 0.63 ± 0.34 W = 0.95 ± 0.45 Other = 1.00 ± 0.35 Total: 2.4 ± 0.7 Total: 15.8 ± 1.9 Other contains: single top, dibosons, and multijet (negligible) Background contribution changes drastically for different kinematic regions Walter Hopkins (University of Oregon) Constraining SUSY with stop searches September 2 2015 17 / 48
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