Same-Sign Diletpon Signatures of Vector-like Quarks Chuan-Ren Chen (NTNU) 11/03/2015, CYCU HEP seminar In collaboration with H.-C. Cheng and I. Low, 1511.*****

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

CRC (NTNU)3 preprint 31 Jul 2012 preprint 31 Aug 2012 July , both CMS and ATLAS announced discovery of new particle > 5 σ standard deviations Mass: GeV (CMS) (ATLAS) Main channel: γγ, ZZ (4 leptons)

CRC (NTNU)4 large cross section + low SM background current status at LHC: possible evidence of NP? ~ 2 TeV resonance in WW (2.6 σ ), WZ (3.6 σ ), ZZ (2.9 σ ) by ATLAS Others? Same-Sign Dilepton!

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7 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,

CRC (NTNU)8 Fermion CRC, Cheng, Low, 1511.***** Realistic model: T-odd heavy b’ in Littlest Higgs model with T-parity Dark Matter Other possibilities? exotic X 5/3 which decays into t and W a vector-like b’ with odd quantum number wrt SM pair production, less constrained

Littlest Higgs Model CRC (NTNU)9

Cancellation CRC (NTNU)10

Little Higgs Model CRC (NTNU)11

Little Higgs Model w/ T-parity CRC (NTNU)12

Yukawa Sector CRC (NTNU)13 Dirac mass term with

particle spectrum CRC (NTNU)14 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)15 production cross section of b’ at the LHC largest channel: Simplified Model: Realistic model: LHT

CRC (NTNU)16 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)17 Constraints?? collider signatures of b’ are similar to stop/sbottom in SUSY current limit on sbottom CMS, PAS SUS

CRC (NTNU)18 current limit on stop ATLAS, CERN-PH-EP

CRC (NTNU)19 translating current limit of sbottom to limit of b’ (simplified model)

CRC (NTNU)20 comparing simplified model with Littlest Higgs with T-parity

CRC (NTNU)21 translating current limit of stop to limit of t’ (simplified model)

CRC (NTNU)22 comparing simplified model with Littlest Higgs with T-parity

CRC (NTNU)23 Fitting CMS tth data production cross section is about 34 fb at 8 TeV LHC

CRC (NTNU)24 Fitting CMS tth data exactly two same-sign leptons, Njet ≥ 4, Nbjet = 2 NOTE: for simplified model, i,.e.

CRC (NTNU)25 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)26 * 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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CRC (NTNU)28 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)29 b’ contributes about 3.7 events stop contributes about 3.1 events.

CRC (NTNU)30 b’ contributes about 2.3 events stop contributes about 1.8 events.

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