Vector-like quarks t and partners

Transcription

1 Vector-like quarks t and partners Luca Panizzi University of Southampton, UK

2 Outline Motivations and Current Status 2 Couplings and constraints 3 Signatures at LHC

3 Outline Motivations and Current Status 2 Couplings and constraints 3 Signatures at LHC

4 hat are vector-like fermions? and where do they appear? The left-handed and right-handed chiralities of a vector-like fermion ψ transform in the same way under the SM gauge groups SU(3) c SU(2) L U() Y

5 hat are vector-like fermions? and where do they appear? The left-handed and right-handed chiralities of a vector-like fermion ψ transform in the same way under the SM gauge groups SU(3) c SU(2) L U() Y hy are they called vector-like?

6 hat are vector-like fermions? and where do they appear? The left-handed and right-handed chiralities of a vector-like fermion ψ transform in the same way under the SM gauge groups SU(3) c SU(2) L U() Y hy are they called vector-like? L = g ) (J µ+ µ + + J µ µ 2 Charged current Lagrangian

7 hat are vector-like fermions? and where do they appear? The left-handed and right-handed chiralities of a vector-like fermion ψ transform in the same way under the SM gauge groups SU(3) c SU(2) L U() Y hy are they called vector-like? L = g ) (J µ+ µ + + J µ µ 2 Charged current Lagrangian SM chiral quarks: ONLY left-handed charged currents { J µ+ = J µ+ L + J µ+ J µ+ R with L = ū L γ µ d L = ūγ µ ( γ 5 )d = V A J µ+ R =

8 hat are vector-like fermions? and where do they appear? The left-handed and right-handed chiralities of a vector-like fermion ψ transform in the same way under the SM gauge groups SU(3) c SU(2) L U() Y hy are they called vector-like? L = g ) (J µ+ µ + + J µ µ 2 Charged current Lagrangian SM chiral quarks: ONLY left-handed charged currents { J µ+ = J µ+ L + J µ+ J µ+ R with L = ū L γ µ d L = ūγ µ ( γ 5 )d = V A J µ+ R = vector-like quarks: BOTH left-handed and right-handed charged currents J µ+ = J µ+ L + J µ+ R = ū Lγ µ d L + ū R γ µ d R = ūγ µ d = V

9 hat are vector-like fermions? and where do they appear? The left-handed and right-handed chiralities of a vector-like fermion ψ transform in the same way under the SM gauge groups SU(3) c SU(2) L U() Y Vector-like quarks in many models of New Physics arped or universal extra-dimensions KK excitations of bulk fields Composite Higgs models VLQ appear as excited resonances of the bounded states which form SM particles Little Higgs models partners of SM fermions in larger group representations which ensure the cancellation of divergent loops Gauged flavour group with low scale gauge flavour bosons required to cancel anomalies in the gauged flavour symmetry Non-minimal SUSY extensions VLQs increase corrections to Higgs mass without affecting EPT

10 SM and a vector-like quark L M = M ψψ Gauge invariant mass term without the Higgs

11 SM and a vector-like quark L M = M ψψ Gauge invariant mass term without the Higgs Charged currents both in the left and right sector ψ L ψ R ψ L ψ R

12 SM and a vector-like quark L M = M ψψ Gauge invariant mass term without the Higgs Charged currents both in the left and right sector ψ L ψ R ψ L ψ R They can mix with SM quarks t u i b d i Dangerous FCNCs strong bounds on mixing parameters BUT Many open channels for production and decay of heavy fermions Rich phenomenology to explore at LHC

13 - L dt = 4.3 fb 95% CL exp. excl. 95% CL obs. excl. Ht+X [ATLAS-CONF-23-8] Same-Sign [ATLAS-CONF-23-5] Zb/t+X [ATLAS-CONF-23-56] b+x [ATLAS-CONF-23-6] Searches at the LHC CMS (t ) ATLAS (t ) - CMS preliminary s = 8 TeV 9.6 fb BR(b) BR(tZ) BR(tH) Observed T Quark Mass Limit [GeV] BR(T Ht) m T = 35 GeV m T = 5 GeV m T = 7 GeV m T = 4 GeV m T = 55 GeV m T = 75 GeV m T = 45 GeV m T = 6 GeV m T = 8 GeV ATLAS Preliminary Status: Lepton-Photon 23 s = 8 TeV, SU(2) (T,B) doub. SU(2) singlet m T = 65 GeV BR(T b) m T = 85 GeV Bounds from pair production between 6 GeV and 8 GeV depending on the decay channel Common assumption: mixing with third generation only

14 P P b b Example: b pair production t t + b + b Common assumption CC: b t Searches in the same-sign dilepton channel (possibly with b-tagging)

15 P P b b Example: b pair production t t + b + b Common assumption CC: b t Searches in the same-sign dilepton channel (possibly with b-tagging) If the b decays both into t and q P P b b t + jet b + P P b b jet + jet There can be less events in the same-sign dilepton channel!

16 Representations and lagrangian terms Assumption: vector-like quarks couple with SM quarks through Yukawa interactions

17 Representations and lagrangian terms Assumption: vector-like quarks couple with SM quarks through Yukawa interactions SM Singlets Doublets Triplets ( X ) X ( u ) ( c )( t ) (U) U ( U ) U U d s b (D) D ( D ) D D Y Y SU(2) L 2 and 2 3 U() Y q L = /6 u R = 2/3 d R = /3 2/3 -/3 7/6 /6-5/6 2/3 -/3 L Y y i u qi L Hc u i R y i d qi L Vi,j CKM Hdj R λ i u qi L Hc U R λ i d qi L HD R λ i u ψ LH (c) u i R λ i d ψ LH (c) d i R λ i q i L τa H (c) ψ a R

18 Representations and lagrangian terms Assumption: vector-like quarks couple with SM quarks through Yukawa interactions SM Singlets Doublets Triplets ( X ) X ( u ) ( c )( t ) (t ) t ( t ) t t d s b (b ) b ( b ) b b Y Y SU(2) L 2 and 2 3 U() Y q L = /6 u R = 2/3 d R = /3 L Y yi uv ū i 2 L ui R yi d v di 2 L V i,j CKM dj R 2/3 -/3 7/6 /6-5/6 2/3 -/3 λi uv ū i 2 L U R λi d v di 2 L D R λ i uv U 2 L u i R λi d v D 2 L d i R λ iv 2 ū i L U R λ i v d i L D R

19 Representations and lagrangian terms Assumption: vector-like quarks couple with SM quarks through Yukawa interactions SM Singlets Doublets Triplets ( X ) X ( u ) ( c )( t ) (t ) t ( t ) t t d s b (b ) b ( b ) b b Y Y SU(2) L 2 and 2 3 U() Y q L = /6 u R = 2/3 d R = /3 L Y yi uv ū i 2 L ui R yi d v di 2 L V i,j CKM dj R 2/3 -/3 7/6 /6-5/6 2/3 -/3 λi uv ū i 2 L U R λi d v di 2 L D R λ i uv U 2 L u i R λi d v D 2 L d i R λ iv 2 ū i L U R λ i v d i L D R L m M ψψ (gauge invariant since vector-like) Free parameters 4 4 or 7 M+3 λ i M+3λ i u + 3λ i d 4 M+3 λ i

20 Outline Motivations and Current Status 2 Couplings and constraints 3 Signatures at LHC

21 Flavour changing neutral currents in the SM Couplings Major consequences Z Z H H u t c = u c u R = u R t c L c L and flavour conserving neutral currents receive a contribution

22 Flavour changing neutral currents in the SM Couplings Major consequences Z Z H H u t c = u c u R = u R t c L c L and flavour conserving neutral currents receive a contribution Charged currents between right-handed SM quarks u R t R b R = u R d R d R and charged currents between left-handed SM quarks receive a contribution

23 Flavour changing neutral currents in the SM Couplings Major consequences Z Z H H u t c = u c u R = u R t c L c L and flavour conserving neutral currents receive a contribution Charged currents between right-handed SM quarks u R t R b R = u R d R d R and charged currents between left-handed SM quarks receive a contribution All proportional to combinations of mixing parameters

24 FCNC constraints Rare top decays t b b u, c Z t b u, c Z t u, c Z BR(t Zq) = O( 4 ) SM prediction BR(t Zq) < 4% measured at 5 fb Meson mixing and decay D { c u D { c u d i d i d i u} D c D { c u l + { c ν i D l u Z u} D c Z l + l

25 Zc c and Zb b couplings Flavour conserving NC constraints Z c, b c, b Direct coupling measurements: g q ZL,ZR = (gq ZL,ZR )SM (+δg q ZL,ZR ) Asymmetry parameters: A q = (gq ZL )2 (g q ZR )2 (g q ZL )2 +(g q = ASM ZR )2 q (+δa q ) Decay ratios: R q = Γ(Z q q) Γ(Z hadrons) = RSM q (+δr q ) Atomic parity violation Z u, d u, d eak charge of the nucleus Q = 2c ] [(2Z+N)(g u ZL g + gu ZR )+(Z+2N)(gd ZL + gd ZR ) = Q SM + δqvl Most precise test in Cesium 33 Cs: Q ( 33 Cs) exp = 73.2±.35 Q ( 33 Cs) SM = 73.5±.2

26 Constraints from EPT and CKM E precision tests t b Contributions of new fermions to S,T,U parameters CKM measurements Modifications to CKM relevant for singlets and triplets because mixing in the left sector is NOT suppressed The CKM matrix is not unitary anymore If BOTH t and b are present, a CKM for the right sector emerges

27 Higgs coupling with gluons/photons Production and decay of Higgs at the LHC g g q q q H H f f f γ γ New physics contributions mostly affect loops of heavy quarks t and q : κ gg = κ γγ = v g m ht t + v g t m hq q q The couplings of t and q to the higgs boson are: g ht t = m t v + δg ht t g hq q = m q v + δg hq q In the SM: κ gg = κ γγ = The contribution of just one VL quark to the loops turns out to be negligibly small Result confirmed by studies at NNLO

28 Outline Motivations and Current Status 2 Couplings and constraints 3 Signatures at LHC

29 Production channels Vector-like quarks can be produced in the same way as SM quarks plus FCNCs channels Pair production, dominated by QCD and sentitive to the q mass independently of the representation the q belongs to Single production, only E contributions and sensitive to both the q mass and its mixing parameters

30 Production channels Pair vs single production, example with non-sm doublet (X 5/3 t ) pair production Σ pb. single t single X m q, pair production depends only on the mass of the new particle and decreases faster than single production due to different PDF scaling current bounds from LHC are around the region where (model dependent) single production dominates

31 Decays SM partners Z H Z H t t b b Neutral currents u i + u i t t X 5/3 d i d i + b b Charged currents Y 4/3 d i u i Exotics X 5/3 ui, t + Y 4/3 d i, b Only Charged currents Not all decays may be kinematically allowed it depends on representations and mass differences

32 . Mixing ONLY with top Decays of t BR t t H t Z b Equivalence theorem at large masses: BR(qH) BR(qZ) Decays are in different channels (BR=% hypothesis now relaxed in exp searches) m t

33 Status: Lepton-Photon 23 s = 8 TeV, - L dt = 4.3 fb 95% CL exp. excl. 95% CL obs. excl. Ht+X [ATLAS-CONF-23-8] Same-Sign [ATLAS-CONF-23-5] Zb/t+X [ATLAS-CONF-23-56] b+x [ATLAS-CONF-23-6] SU(2) (T,B) doub. SU(2) singlet Status: Lepton-Photon 23 s = 8 TeV, - L dt = 4.3 fb 95% CL exp. excl. 95% CL obs. excl. Same-Sign [ATLAS-CONF-23-5] Zb/t+X [ATLAS-CONF-23-56] SU(2) (B,Y) doub. SU(2) singlet. Mixing ONLY with top Decays of t BR t t H t Z. b Equivalence theorem at large masses: BR(qH) BR(qZ) Decays are in different channels (BR=% hypothesis now relaxed in exp searches) Still, current bounds assume mixing with third generation only! m t BR(T Ht) m T = 35 GeV m T = 4 GeV m T = 45 GeV ATLAS Preliminary BR(B Hb) m B = 35 GeV m B = 4 GeV m B = 45 GeV ATLAS Preliminary m T = 5 GeV m T = 55 GeV m T = 6 GeV m T = 65 GeV m B = 5 GeV m B = 55 GeV m B = 6 GeV m B = 65 GeV m T = 7 GeV m T = 75 GeV m T = 8 GeV m T = 85 GeV m B = 7 GeV m B = 75 GeV m B = 8 GeV m B = 85 GeV BR(T b) BR(B t)

34 . Mixing ONLY with top Decays of t BR t t H t Z b m t Equivalence theorem at large masses: BR(qH) BR(qZ) Decays are in different channels (BR=% hypothesis now relaxed in exp searches) Still, current bounds assume mixing with third generation only!. Mixing with top and charm. Mixing ONLY with charm b BR t t H BR t c Z c H t Z c Z c H m t Decay to lighter generations can be sizable even if Yukawas are small! m t

35 Single Production based on arxiv:35.472, accepted by Nucl.Phys.B

36 From couplings to BRs Charged current of T (t ) L κ VL/R 4i g [ T L/R µ + γ µ d i L/R ] 2

37 From couplings to BRs Charged current of T (t ) L κ VL/R 4i g [ T L/R µ + γ µ d i L/R ] 2 Partial idth Γ(T d i ) = κ 2 V4i L/R 2 M3 g 2 64πm 2 Γ (M, m, m di = ) Assumption: massless SM quarks, corrections for decays into top (see )

38 From couplings to BRs Charged current of T (t ) L κ VL/R 4i g [ T L/R µ + γ µ d i L/R ] 2 Partial idth Γ(T d i ) = κ 2 V4i L/R 2 M3 g 2 64πm 2 Γ (M, m, m di = ) Assumption: massless SM quarks, corrections for decays into top (see ) Branching Ratio BR(T d i ) = V4i L/R 2 3 j= V4j L/R 2 κ 2 Γ V =,Z,H κ 2 V Γ V ζ i ξ

39 From couplings to BRs Charged current of T (t ) L κ VL/R 4i g [ T L/R µ + γ µ d i L/R ] 2 Partial idth Γ(T d i ) = κ 2 V4i L/R 2 M3 g 2 64πm 2 Γ (M, m, m di = ) Assumption: massless SM quarks, corrections for decays into top (see ) Branching Ratio BR(T d i ) = V4i L/R 2 3 j= V4j L/R 2 κ 2 Γ V =,Z,H κ 2 V Γ V ζ i ξ Re-expressing the Lagrangian g [ T L/R µ + γ µ d i L/R ] with κ T = 2 L κ T ζ i ξ Γ 3 i= VL/R 4i 2 κv 2 Γ V V

40 The complete Lagrangian L = κ T { ζ i ξ T Γ { ζ i ξ B + κ B Γ { ζ + i κ X Γ { ζ + i κ Y Γ + h.c. g [ T L µ + γµ d i ζ i ξz T L]+ 2 Γ Z g [ B L µ γµ u i ζ i ξz B L]+ 2 Γ Z g 2 [ X L + µ γ µ u i L] } } g [Ȳ L µ γ µ d i L] 2 g [ T L Z µ γ µ u i 2c L] g [ B L Z µ γ µ d i 2c L] ζ i ξh T Γ H M v [ T R Hu i ζ 3 ξh T L] Γ H } ζ i ξh B M Γ v [ B R Hd i L] H Model implemented and validated in Feynrules: m t v [ T L Ht R ] } 3 ζ i = i= ξ V = V=,Z,H T and B: NC+CC, 4 parameters each (ζ,2 and ξ,z ) X and Y: only CC, 2 parameters each (ζ,2 )

41 In association with top Cross sections (example with T) ( ) d i σ(t t) = κt 2 ξ Z ζ 3 σ Z3 T t 3 + ξ ζ i σ i T t i= d j In association with light quark T t q i q j Z T t σ(tj) = κ 2 T ( 3 ξ ζ i σ Tjet 3 i + ξ Z ζ i σ Tjet Zi i= i= ) q i q j T, Z q j q i q j Z T q k In association with gauge or Higgs boson ( σ(t{, Z, H}) = κ 2 T 3 ξ ζ i σ T 3 i + ξ Z ζ i σ TZ 3 i + ξ H ζ i σ i TH i= i= i= ) g q q T, Z g q T T, Z g u i u i T H g u i T T H The σ are model-independent coefficients: the model-dependency is factorised!

42 Cross sections Coefficients (in fb) for T and T with mass 6 GeV σ T t+ Tt Zi with top with light quark with gauge or Higgs σ T t+ Tt i σ Tj+ Tj Zi σ Tj+ Tj i σ TZ+ TZ i σ TH+ TH i σ T+ T i ζ = ζ 2 = ζ 3 = The cross section for pair production is 7 fb

43 Cross sections Embed the model-dependency into a consistent framework Benchmark Benchmark 2 Benchmark 3 Benchmark 4 Benchmark 5 Benchmark 6 κ =.2 κ =.7 κ = κ =.3 κ =. κ =.3 ζ = ζ 2 = /3 ζ = ζ 2 = ζ 3 = ζ = ζ 3 = /2 ζ 2 = ζ 3 = /2 (, 2/3) T (, /3) B (2, /6) T λ d = B (2, /6) T λu = B (2, /6) T λ d = λu B (2, 7/6) X T (2, 5/6) B Y (3, 2/3) X T B (3, /3) T B Y Flavour bounds are necessary to get the inclusive cross sections

44 Flavour vs direct search ATLAS search in the CC and NC channels Qq) BR(Q q) [pb] MadGraph, D-quark MadGraph, X -quark 95% C.L. upper limits: Observed Expected ±σ ±2σ Qq) BR(Q Zq) [pb] MadGraph, U-quark 95% C.L. upper limits: Observed Expected ±σ ±2σ σ(pp - σ(pp - -2 s = 7 TeV - Ldt = 4.64 fb ATLAS Preliminary -2 s = 7 TeV - Ldt = 4.64 fb ATLAS Preliminary m [GeV] Q m [GeV] Q Assumptions: mixing only with st generation and coupling strength κ = v M VL

45 Flavour vs direct search Comparison with flavour bounds.3 Doublet X T.3 Triplet X T B 5 ATLAS NC 5 Κ.5 Κ.5 ATLAS NC. ATLAS CC. Flavour bound.5 Flavour bound.5 ATLAS CC M GeV M GeV Assumptions: mixing only with st generation and coupling strength saturating flavour bounds Flavour bounds are competitive with current direct searches

46 Conclusions and Outlook After Higgs discovery, Vector-like quarks are a very promising playground for searches of new physics Fairly rich phenomenology at the LHC and many possibile channels to explore Signatures of single and pair production of VL quarks are accessible at current CM energy and luminosity and have been explored to some extent Current bounds on masses around 6-8 GeV, but searches are not fully optimized for general scenarios. Model-independent studies can be performed for pair and single production, and also to analyse scenarios with multiple vector-like quarks (work in progress, results very soon!)