Same-Sign Diletpon Signatures of Vector-like Quarks Chuan-Ren Chen (NTNU) 12/10/2015, NCTS Annual Theory Meeting In collaboration with H.-C. Cheng and I. Low,

Main Goal of LHC: SM -- Higgs Boson New Physics Beyond the SM

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5 What we have: same-sign dilepton “excess” in various searches observed same-sign dilepton has best fit SUSY SM tth same-sign dilepton (SS2L) Huang, Ismail, Low, Wagner, W + b b

CRC (NTNU)6 Fermion CRC, Cheng, Low, Realistic model: T-odd heavy b’ in Littlest Higgs model with T-parity DM: Dark Matter Other possibilities? exotic X 5/3 a vector-like b’ with odd quantum number wrt SM (existence of new TeV) less constrained, pair production

CRC (NTNU)7 production cross section of b’ at the LHC Simplified Model:

CRC (NTNU)8 Constraints?? collider signatures of b’ are similar to stop/sbottom in SUSY current limit on stop/sbottom from RUN I ATLAS, CERN-PH-EP

CRC (NTNU)9 Constraints?? collider signatures of b’ are similar to stop/sbottom in SUSY current limit on sbottom CMS, PAS SUS

CRC (NTNU)10 translating current limit of sbottom to limit of b’ (simplified model)

Littlest Higgs Model CRC (NTNU)11

particle spectrum CRC (NTNU)12 SM T-parity EvenT-parity Odd * parameters: f, k q, k l, λ 1, m h * The lightest T-odd particle is stable dark matter candidate

CRC (NTNU)13 production cross section of b’ at the LHC largest channel: Simplified Model: Realistic model: LHT

CRC (NTNU)14 Constraints?? collider signatures of b’ are similar to stop/sbottom in SUSY current limit on stop/sbottom from RUN I ATLAS, CERN-PH-EP

CRC (NTNU)15 comparing simplified model with Littlest Higgs with T-parity

CRC (NTNU)16 current limit on stop ATLAS, CERN-PH-EP

CRC (NTNU)17 translating current limit of stop to limit of t’ (simplified model)

CRC (NTNU)18 comparing simplified model with Littlest Higgs with T-parity

CRC (NTNU)19 Fitting CMS tth data production cross section is about 34 fb at 8 TeV LHC taking LHT

CRC (NTNU)20 Fitting CMS tth data exactly two same-sign leptons, Njet ≥ 4, Nbjet = 2 NOTE: for simplified model, i,.e.

CRC (NTNU)21 Projection on LHC RUN II with 13 TeV with the same cuts imposed in 8 TeV (compared with stop: ) enlarging b’ contribution large enhancement in production cross section most of same-sign events in the search of SM tth come from b’ !!

CRC (NTNU)22 b’ contributes about 0.4 (0.3) events in low (high) p T region, stop contributes about 3 (2.3) events in low (high) p T region.

CRC (NTNU)23 b’ contributes about 3.7 events stop contributes about 3.1 events.

CRC (NTNU)24 b’ contributes about 2.3 events stop contributes about 1.8 events.

CRC (NTNU)25 * A stop with ~550 GeV could fit the excess of SS2L in the search of tth. * “ Excesses “ in same-sign dilepton with b-jets signatures are seen in several searches both by CMS and ATLAS in LHC RUN I data. * A fermionic b’ with ~ 750 GeV in LHT (or 850 GeV in simplified model) explains the SS2L excess in tth search, too! * Large enhancement in b’ (stop) pair production at LHC RUN II, compared to SM tth. * Imposing harder kinematic could extract b’ signal in the search of tth at LHC RUN II.

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Cancellation CRC (NTNU)30

Little Higgs Model CRC (NTNU)31

Little Higgs Model w/ T-parity CRC (NTNU)32

Yukawa Sector CRC (NTNU)33 Dirac mass term with