Top squark search in the 1-lepton channel with CMS GDR Terascale - PowerPoint PPT Presentation

Top squark search in the 1-lepton channel with CMS GDR Terascale October 30th, 2013 Alexandre Aubin IPHC, Strasbourg

Context Super-symmetry (R-parity conserved) Why top squarks ? 2 • Standard model extension • Boson ↔ fermion symmetry • Provides dark matter candidate • Solves the hierarchy problem • "Natural" SUSY ( m ˜ t ∼ 1 TeV)

Context Assumptions focus here on 1-lepton channel (CMS-SUS-13-011, arXiv :1308.1586) higgsinos combination of gauginos and parameters reduces complexity to 2-3 Direct stop pair production 3 two decay modes considered, same final state • Simplified SUSY models • BR = 100% χ 0 and ˜ χ ± are linear • ˜ t ∗ → bbWW + ˜ χ 0 ˜ pp → ˜ t ˜ χ 0

Selection Pre-selection Drell-Yan) Backgrounds Cut & count and BDT analysis after Signal region 4 χ 0 ˜ pp → ˜ t ˜ t ∗ χ 0 → bbWW + ˜ χ 0 ˜ χ 0 → ℓ + qqbb + ν ℓ ˜ • 1 lepton ( e / µ ) • ≥ 4 jets, ≥ 1 b-tag • MET ≥ 100 GeV • second-lepton vetos (isolated track, hadronic τ ) M T = m T ( ℓ, MET ) > 120 GeV • t ¯ t → ℓℓ • t ¯ t → ℓ + jets, single t • W ( → ℓν ℓ ) + jets • rare (diboson, triboson, t ¯ t + V , Selected t ¯ t → ℓℓ events

Discriminating variables 5 M W T 2 1 = 0 , ( p 1 + p l ) 2 = p 2 { [ ]} p T p T 2 = � E mis 1 + � , p 2 2 = M 2 � W , M W T T 2 = min m y consistent with : ) 2 = ( p 2 + p b 2 ) 2 = m 2 ( p 1 + p l + p b 1 y

Discriminating variables 6 H ratio T ∑ MET side p T ( jet ) = H same side as MET T H ratio = ∑ p T ( jet ) T H total T

Discriminating variables 7 min ∆Φ min ∆Φ = min { ∆ φ ( MET, j 1 ) , ∆ φ ( MET, j 2 ) }

Discriminating variables X X X X X Variable X X X X MET 8 X χ 0 χ ± ˜ ˜ t → t ˜ t → b ˜ M W T 2 min ∆Φ HT ratio T hadronic top χ 2 on-shell t leading b-tagged jet p T off-shell t ∆ R ( ℓ , leading b-tagged jet) lepton p T off-shell W • Cut & count (cross-check) • Boosted decision trees (primary) • Different regions ↔ specific kinematics

Background estimation Key points 9 • t ¯ t → ℓℓ modelization • M T tail control

Background estimation 10 CR-2 ℓ • Strong dependence to ISR/FSR description • Agreement with the data is okay

Background estimation 11 CR-0 b • M T tail underestimated by MC for W +jets • Correction with a scale-factor = 1.2 ± 0.3

Results 12 ˜ χ 0 t → t ˜ ⇒ no excess is observed

Results 13 ˜ χ ± t → b ˜ ⇒ no excess is observed

Interpretation Hypothesis reminder Results also available for 14 ˜ χ 0 ˜ χ ± t → t ˜ t → b ˜ m ˜ χ ± = 0 . 25 m ˜ t + 0 . 75 m ˜ χ 0 • Signal is ˜ t pair production only m ˜ χ ± = 0 . 75 m ˜ t + 0 . 25 m ˜ χ 0 • Unpolarized tops (see next slide) • BR is 100% (see back-up)

15 Interpretation ˜ χ 0 ˜ χ ± t → t ˜ t → b ˜ • Polarization effects affects signal acceptance

Perspectives GeV 16 • Other analysis providing complementary results • Ongoing combination with 0 ℓ and 2 ℓ channels • 14 TeV projections predicts 5 σ discovery potential up m ˜ t ∼ 750-950

Conclusion squarks masses up to 650 GeV. forward to the 13-14 TeV. Thank you for your attention, stay tuned ! 17 • Hot topic for SUSY searches and naturalness constrains • Analyzed the full 8 TeV data recorded by CMS. BDT probes top • Natural SUSY far to be ruled out • Ongoing combination with 0-lep and 2-lep channels, and looking

Back-up 18

ATLAS results 19

Kinematic variables 20

Signal regions 21

BDT and cut & count 22

Results (event yields) 23 ˜ χ 0 t → t ˜ ˜ χ ± t → b ˜

Results (BR variation) 24

Systematics 25