Peaky blinders: searches for t t resonances James Ferrando - PowerPoint PPT Presentation

Outline Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future Why Top quarks? Summary Signatures and Models Peaky blinders: searches for t ¯ t resonances James Ferrando University of Glasgow Elementary Particle Physics Group Seminar University of Birmingham 11 th December 2013 Searches for t ¯ James Ferrando t resonances 1/ 65

Outline Introduction Outline The Standard Model Recent Searches What’s wrong with the SM? The Future Why Top quarks? Summary Signatures and Models Why search for t ¯ t resonances? Review of t ¯ t resonance searches Looking towards the future Will focus on the details of ATLAS searches but also show the best results from the competition Searches for t ¯ James Ferrando t resonances 2/ 65

Outline The Standard Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future Model Why Top quarks? Summary Signatures and Models The Standard Model (SM) of particle physics: Fermionic matter: Three generations of quarks Three generations of leptons Gauge Bosons: Four Force carriers : γ (EM), W ± , Z (Weak), g (strong) The Higgs Boson to give mass ”Was she pretty?” asked the bigger of the small girls. ”Not as pretty as any of you,” said the bachelor, ”but she was horribly good.” The storyteller - H. H. Munro (Saki) Searches for t ¯ James Ferrando t resonances 3/ 65

Outline Introduction SM Problems The Standard Model Recent Searches What’s wrong with the SM? The Future Why Top quarks? Summary Signatures and Models So what’s wrong with the Standard Model? No Dark Matter candidates Not enough CP violation to explain the observed matter-antimatter imbalance The Higgs boson has still not been observed No gravity Particle masses are not understood Is there physics beyond the Standard Model? Searches for t ¯ James Ferrando t resonances 4/ 65

Outline Introduction The LHC The Standard Model Recent Searches What’s wrong with the SM? The Future Why Top quarks? Summary Signatures and Models Where to look for answers? The Large Hadron Collider at CERN 27 km circumference ring Currently collides protons at centre-of-mass energy 8 TeV Four detectors installed around the ring An excellent environment to test the Standard Model and search for new Physics Triviality/Unitarity constraints on some SM cross sections imply a Higgs Boson or something else at an energy scale < 800 GeV Searches for t ¯ James Ferrando t resonances 5/ 65

Outline LHC Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future Detectors Why Top quarks? Summary Signatures and Models What equipment to use? A Toroidal Large ApparatuS (ATLAS) 4 Detectors: 2 General Purpose ATLAS CMS Two Specialised ALICE - Heavy ion LHCb - CP violation ATLAS with full solid angle coverage, excellent charged particle tracking, particle ID and energy measurement is well-suited for TeV-Scale physics (and so is CMS of course) Searches for t ¯ James Ferrando t resonances 6/ 65

Outline Introducing: Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future The Top Quark Why Top quarks? Summary Signatures and Models The top quark was discovered at TeVatron in 1995 Extremely heavy for a fundamental particle: Similar mass to a gold atom ∼ 35 times heavier than the next heaviest quark (the bottom quark) Usually produced in a t ¯ t pair with its partner the anti-top Could it provide a gateway to new physics? Searches for t ¯ James Ferrando t resonances 7/ 65

Outline Top and Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future BSM Physics Why Top quarks? Summary Signatures and Models Many BSM scenarios on the market Large top mass ( m t ≈ 173 GeV ) → top often plays a special role in BSM theories BSM physics often has consequences for the third generation quarks Some examples: Add new heavy quarks: Often decay to tops or look like heavy tops Incorporate Gravity using Extra Dimensions: Many models predict new states with strong coupling to the top Exotic Higgs Bosons: large coupling to the top SUSY: naturalness prefers top-partners not too far from m t Searches for t ¯ James Ferrando t resonances 8/ 65

Outline Hints of Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future New Physics? Why Top quarks? Summary Signatures and Models Extra motivation: TeVatron p ¯ p data t = N ( y t ¯ > 0) − N ( y t ¯ t t < 0) t t A t ¯ N ( y t ¯ > 0) + N ( y t ¯ t t < 0) t t Tevatron collides p and ¯ p producing t ¯ t t a measure of how much the A t ¯ t prefers the p direction p -value of such a large slope 0.00646 (CDF) “Strengthens the case that new physics plays a role in t ¯ t production” Searches for t ¯ James Ferrando t resonances 9/ 65

Outline A Top Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future Factory Why Top quarks? Summary Signatures and Models Measurements of top properties at TeVatron statistically limited The LHC t ¯ t production cross-section is much larger In effect the LHC is a top quark factory TeVatron: < 8 × 10 4 (10 fb − 1 ) top pairs per experiment ∼ 10 years running LHC: > 6 × 10 6 top pairs per experiment in 2011-12 At the LHC many top quark studies are possible that were not feasible at TeVatron Searches for t ¯ James Ferrando t resonances 10/ 65

Outline Top Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future Signatures Why Top quarks? Summary Signatures and Models t g t g g t g g t t q t In the SM, top decays t q g approximately 100% t → Wb q t q t Classified according to the W decays Searches for t ¯ James Ferrando t resonances 11/ 65

Outline New Physics Introduction The Standard Model Recent Searches What’s wrong with the SM? The Future with Tops Why Top quarks? Summary Signatures and Models Broadly speaking can study new physics in t ¯ t in three different ways Look for anomalous production of tops Look for unexpected behaviour in top quark decays Directly search for new particles decaying to tops (and possibly something else) This talk focuses on the latter, searching for a peak in the m t ¯ t distribution from production of new particles that decay to t ¯ t pairs Searches for t ¯ James Ferrando t resonances 12/ 65

Outline Introduction t ¯ t resonances I The Standard Model Recent Searches What’s wrong with the SM? The Future Why Top quarks? Summary Signatures and Models A wealth of peaky new physics signals from different scenarios: Extra dimensions (Bulk RS) : Excitations of gluon ( g KK )/ graviton ( G KK ) preferentially decay to t ¯ t Topcolor-assisted Technicolor : Strong EWSB model via a top condensate - expect top- π ( H -like) and top- ρ ( Z ′ -like ) the latter heavy enough to decay to t ¯ t Composite Higgs scenarios : Usually require (naturalness) extra heavy-fermions, and commonly heavy “gluons” that decay to t R or new heavy fermions depending on the masses BSM Higgs : New heavy pseudoscalar Higgs-like particles in, e.g. the MSSM, would also have a large t ¯ t branching ratio Searches for t ¯ James Ferrando t resonances 13/ 65

Outline Introduction t ¯ t resonances II The Standard Model Recent Searches What’s wrong with the SM? The Future Why Top quarks? Summary Signatures and Models Searches so far have focused on two benchmark scenarios: Topcolor-assisted technicolor (TC2) q t TC2 → t ¯ Z ′ t Spin-1 Color singlet Narrow width (1.2%) modelled with SSM Z ′ (3%) width q t hep-ph/9911288, LO Z’ Branching Ratios TC2 0.5 Eur. Phys. J. C (2012) 72 2072 0.45 u u t t Branching Ratio d d b b 0.4 RS Kaluza-Klein Gluon g KK → t ¯ 0.35 t 0.3 0.25 Spin-1 0.2 0.15 color octet 0.1 hep-ph/9911288 - Harris et. al, Model IV, f =1.0, f =0.0 1 2 Including correction from Ferrando and Frandsen used in wide (10-15%) 0.05 Eur. Phys. J. C (2012) 72, 2072 - Harris and Jain 0 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 BR ( g KK → t ¯ t ) ∼ 92.5% m(Z’ ) [TeV] TC2 JHEP 0709 (2007) 074 Searches for t ¯ James Ferrando t resonances 14/ 65

Introduction Selecting t ¯ t Early ATLAS searches @ 7 TeV Recent Searches ATLAS searches with the Full 7 TeV data The Future ATLAS and CMS searches with 8 TeV data Summary First t ¯ t resonance search at the LHC selected tops in a familar way (ATLAS - Eur.Phys.J. C72 (2012) 2083) dilepton channel l +jets channel Two isolated leptons Isolated electron or muon ll = ee , e µ, µµ Missing Transverse ee or µµ : M ll outside M Z momentum ( E miss ) T window 4 or more jets (inclusive e µ : Require large H T Anti- K T , R = 0 . 4) or, 3 jets M ll > 10 GeV and one jet has mass > 60 GeV E miss T At least 1 b -tagged jet 2 or more jets H T is the scalar sum of P T of all hard objects in event. Searches for t ¯ James Ferrando t resonances 15/ 65