Mono Vector-Quark Production at the LHC Haiying Cai Department of Physics, Peking University arxiv: 1210.5200 Particle Physics and Cosmology KIAS, November 5-9, 2012
Introduction Vector-like quark exists in many models of new physics beyond the Standard Model: Little Higgs model, Composite Higgs model and strong dynamic models, etc. The chiral top quarks need one pair of vector like partners to stablize the EW scale or for EW symmetry breaking. collider phenomenology of heavy quarks has been studied: pair production of heavy quarks via gluon fusions gg X 5/3 X5/3, B B analysis is conducted in same-sign dilepton final states R.Contino and G.Servant, 2008 associate production with one light quark mediated by W, Z qq jt, jb, jx 5/3, jy 4/3 Atre, Azuelos, Carena et al. 2011
mono production of heavy quark as an s-channel resonance. qg T, B, X 5/3, Y 4/3 I only consider the mono production of heavy top quark T. This process can be induced by FCNC anomalous g-q-t couplings. Outline of this talk: Review varieties of vector-quark models Electroweak constraints on model parameters Collider simulation I: pp T tg Collider simulation II: pp T bw + Conclusion
Vector Like Quark Models Q (m) U 1 D 1 D 2 D X D Y T X T Y ( ) ( ) ( ) X U U X D U D U D D U Y D Y SU(2) L 0 0 2 2 2 3 3 Y 2/3-1/3 1/6 7/6-5/6 2/3-1/3 D 2 : standard doublet vector-quarks with one up type quark and one down type quark. D X : non-standard doublet vector-quarks with one up type quark and one exotic 5/3 charged heavy quark X. D Y : non-standard doublet vector-quarks with one up type quark and one exotic 4/3 charged heavy quark Y. Aguila, Victoria and Santiago, 2000
Quark Mixing via Yukawa Terms L Y,SM = y u q L H c u R y d q L Hd R L Y,new = λ u q L H c U R λ u D2L H c u R λ d D2L Hd R λ u DXL Hu R λ u q L τ a H c TXR a λ u q L τ a HTY a R L mass = MŪLU R M D 2L D 2R M D XL D XR M T XL T XR M T Y L T Y R vector-quarks mix with chiral quarks via Yukawa interactions. mixing patterns are determined by their SU(2) L U(1) Y assignments. gauge singlets and triplets mix in the same pattern and doublets mix in another pattern. Cacciapaglia, Deandrea, Harada and Okada, 2010
Mixing with Third Generation For simplicity, we consider the scenario that the vector-quarks mix only with the third-generation quark in the SM. t L,R = cos θ u L,Rt L,R + sin θ u L,RT L,R, T L,R = sin θ u L,Rt L,R + cos θ u L,RT L,R, b L,R = cos θ d L,Rb L,R + sin θ d L,RB L,R, B L,R = sin θ d L,Rb L,R + cos θ d L,RB L,R, Defining x t = λ u v/ 2 and x b = λ d v/ 2, those mixing angles depend on x t, x b and gauge invariant mass M.
Mixing Pattern: singlet/triplet x t M m t x t t L T R T L t R t L T R < H > < H > < H > sin θ u L x t M sin θ u R m t M x t M
Mixing Pattern: doublets m t x t x t M t L t R T L t R T L T R < H > < H > < H > sin θ u L m t M sin θ u R x t M Compared with singlet/triplet senario, the LH and RH mixing angles are simply exchanged in doublet senarioes. x t M
Electroweak Constraints Quark mixing would inevitably modify the W -t-b and Z-b-b couplings in the SM, which in turn severely constrain the model parameters. Mass splitting among the fermions will contribute to T parameter through the vacuum polarization of the gauge bosons: T = 4π (Π s 2 c 2 MZ 2 11 (0) Π 33 (0)) Charged and neutral currents are modified according to weak isospin of vector-quarks. (Note heavy top s isospin is not always 1/2) T W+ W W T W 3 3 b t SM contribution is substructed and divergence is cancelled.
Electroweak precision measurements: T = 0.05 ± 0.11, mainly put constraints on singlet and doublet models. M.Peskin and T.Takeuchi, 1990 L. Lavoura and J. Silva, 1993 Mixing between the vector bottom quarks and the chiral bottom quarks induces δgl NP and δgr NP, related to the deviation of R b: ( gl δr b = 2R b (1 R b ) gl 2 + δg g2 L NP + g ) R R gl 2 + δg NP g2 R R Electroweak precision measurements: δr b = 0.00051 ± 0.00066, mainly put constraints on triplet vector-quark models.
600 U 1 model 800 D X model 700 500 600 T = 0.17 400 500 T = -0.07 xt GeV 300 T = 0.17 xt GeV 400 300 200 200 T = -0.07 100 100 600 600 800 1000 1200 1400 1600 1800 2000 M GeV D 2 model 200 600 800 1000 1200 1400 1600 1800 2000 M GeV T X model 500 x b = 20 GeV 150 T = 0.17 400 xt GeV 300 xt GeV 100 R b =0.00117 200 T = 0.17 50 100 600 800 1000 1200 1400 1600 1800 2000 M GeV 0 600 800 1000 1200 1400 1600 1800 2000 M GeV
Chromo-magnetic-dipole Couplings In the approach of Effective Field Theory, all the possible dimension-6 effective operators describing the strong magnetic g-q-q interaction are: L gqq = κ u,r Λ 2 + κ u,l Λ 2 + κ X,L Λ 2 + κ X,R Λ 2 q L σ µν λ A S u,r φg A µν + κ d,r Λ 2 q L σ µν λ A S d,r φg A µν D 2L σ µν λ A u R φg A µν + κ d,l Λ 2 D 2L σ µν λ A d R φg A µν D XL σ µν λ A u R φg A µν + κ Y,L Λ 2 D Y L σ µν λ A d R φg A µν T XR σ µν λ A ( φτ I q L ) G A µν + κ Y,R Λ 2 T Y R σ µν λ A ( φτ I q L )G A µν The cross section of mono production of heavy quark depends on the effective g-q-t couplings. Possible decaying channels: T tg, T bw +, T tz and T th. In the collider simulation sections we are going to analyze the first two.
Collider Simulation I Mono heavy top production and decaying into t plus one jet: ug T tg, t bl + ν, Signature suffers from a few SM backgrounds as follows: Single top productions q q W t b bq tq, b q t q qq, gq tjj (gb tw (jj)) W + two jets productions W bj, W b b, W jj, W Z(b b) t t production Signature is analyzed in the simplified scenario x t = 0.
Event Selection I Basic event-selection cuts (b-tagging is required) : p b T > 20 GeV, η b < 2.5, p l T > 20 GeV, η l < 2.5, R jj > 0.4, R jl > 0.4. Veto cuts are demanded: p T (j, l ± ) < 10 GeV or η(j, l ± ) > 3.5. Either falling into a large rapidity region or carrying a too small transverse momentum to be detected
0.16 0.14 0.12 signal + W j j + W b b + W b j + W Z t j t j j t b t b j t t 0.16 signal + W j j + W b b + W b j + 0.14 W Z t j t j j t b 0.12 t b j t t T 1/ σ d σ / d p 0.1 0.08 0.06 1/ σ d σ / d H T 0.1 0.08 0.06 0.04 0.04 0.02 0.02 0 0 100 200 300 400 500 600 p (GeV) of the leading jet T 0 0 100 200 300 400 500 600 700 800 900 1000 (GeV) H T Figure 1: P T distribution in the left panel and H T distribution in the right panel after basic and veto selection cuts. Each plot is normalized.
Hard P T and H T cuts: p T (j 1st ) > 200 GeV, H T > 400 GeV. Longitudinal momentum of neutrino is reconstructed according to: p ν (x) = E T (x), p ν (y) = E T (y), m 2 W = (p l + p ν ) 2. Two mass window cuts are imposed to optimize the signal events: M W = m lν m W < 10 GeV M t = m lνb m t < 10 GeV
Significance 0.50 0.45 Σ= 5 0.40 fl,r 0.35 0.30 Σ = 3 0.25 0.20 500 600 700 800 900 1000 M T GeV Figure 2: Contour of discovery potential at the 14 TeV LHC with an integrated luminosity of 1 fb 1. Red line represents the 3 σ exclusion limit; Blue line represents the 5 σ discovery limit.
Top Quark Polarization The behavior of the angle between the lepton in the rest frame of the top quark and the top quark moving direction in the center of mass frame can be factorized out. 1 σ dσ d cos θ tl = 1 2 (1 + a cos θ tl), with a = 1 for doublet T -quark and a = 1 for singlet/triplet T -quark. singlet/triplet T -quark left-handed polarized top quark doublet T -quark right-handed polarized top quark Jezabek and Kuhn, 1989 Mahlon and Parke, 1996
0.06 0.06 0.05 0.05 1/ σ d σ / d cosθ 0.04 0.03 0.02 1/ σ d σ / d cosθ 0.04 0.03 0.02 0.01 0.01 0 1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 cosθ t l 0 1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 cosθ t l Figure 3: cos θ tl distribution with no cut and after mass-window cuts: (a) singlet/triplet T -quark, (b) doublet T -quark.
Collider Simulation II The following process has the collider signature of bl + E T : gq T, T bw + bl + ν Main SM backgrounds (very less) are: (1) W + j (2) W + bj, qb q t(bw + ) (3) q q t(bw + ) b Basic cuts are: Hard cuts are: p b T > 50 GeV, η b < 2.0, p l T > 20 GeV, η l < 2.4, R bl > 0.7. p T (b) > 200 GeV, R(l +, ν) < 1.5.
0.16 signal W + jet 0.3 signal + W + jet 0.14 W + b + jet t + j 0.25 + W + b + jet t + j T 1/ σ d σ / d p 0.12 0.1 0.08 0.06 t + b 1/ σ d σ / d R 0.2 0.15 0.1 t + b 0.04 0.02 0.05 0 0 50 100 150 200 250 300 350 400 450 500 p (GeV) of the leading jet T 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 + R ( l ν ) Figure 4: P T distribution in the left panel and R(l +, ν) distribution in the right panel after the basic and veto selection cuts. Each plot is normalized.
Significance: S B Significance: S B 120 40 100 D 2 model f L =0.3 x b =20GeV 35 T X model f R = 0.3 30 xt GeV 80 60 Σ = 5 Σ = 3 xt GeV 25 20 Σ = 8 Σ = 5 40 Σ = 8 15 Σ = 3 20 10 500 600 700 800 900 1000 M T GeV 500 600 700 800 900 1000 M T GeV Figure 5: Significance contour at the 14 TeV LHC with an integrated luminosity of 1 fb 1. In the doublet scenario x t decreases with M; In the triplet scenario x t increases with M.
Leptonic angular distribution The differential cross section with respect to cos θ l for the ug T bl + ν in the c.m. frame in the limit of m W s are found to be: 1 ˆσ 1 ˆσ dˆσ(u R g bl + ν) d cos θ l = dˆσ(u L g bl + ν) d cos θ l = + gw L 2 gw L 2 + gr W 2 (1 cos θ l ) + gw R 2 gw L 2 + gr W 2 O(m 2 W /s) cos θ l gw L 2 gw L 2 + gr W 2 (1 + cos θ l ) + gw R 2 gw L 2 + gr W 2 O(m 2 W /s) cos θ l gw R 2 gw L 2 + gr W 2 (1 + cos θ l ) gw R 2 gw L 2 + gr W 2 (1 cos θ l ) Note that the O(m 2 W /s) correction is only for the term proportional to the right-handed T -b-w coupling.
0.04 0.04 0.035 0.035 0.03 0.03 1/ σ d σ / d cosθ 0.025 0.02 0.015 1/ σ d σ / d cosθ 0.025 0.02 0.015 0.01 0.01 0.005 0.005 0 1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 cosθ l 0 1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 cosθ l Figure 6: The cos θ l distribution between the lepton and gluon moving directions with no cut and after the mass-window cut in the models with dominate gw L coupling: (a) the singlet T -quark, (b) the doublet T -quark (when there is no heavy bottom quark or its effect can be ignored).
Conclusion Electroweak precision measurements impose strong bounds on the model parameters of vector-quark models. The leptonic angular distribution is a favored analyzing power for identifying the chiral property of couplings, therefore distinguish varieties of vector-quark models. It is promising to explore the existence of vector-quarks in mono production channel, depending on specific assumptions. model independent approach pair production
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