Quartic Gauge Boson Couplings at Linear Colliders

We study new physics effects to the quartic gauge boson couplings formulated by the electroweak chiral Lagrangian. Five next-to-leading order operators characterize the anomalous quartic gauge interactions which involve pure Goldstone boson dynamics for the electroweak symmetry breaking. We estimate the typical size of these couplings in different strongly-interacting models and examine the sensitivity to directly probing them via the WWZ/ZZZ triple gauge boson production at the high energy linear colliders. The important roles of polarized e and e beams are stressed. We then compare the results with those from the W -pair production of the WW -fusion processes, and analyze the interplay of these two production mechanisms for an improved probe of the quartic gauge boson interactions. PACS number(s): 11.30.Qc, 11.15.Ex, 12.15.Ji, 14.70.–e Email: [email protected] † Email: [email protected] [email protected] Email: [email protected] 1. Electroweak Symmetry Breaking and Quartic Gauge Boson Couplings The validity of the Standard Model (SM) has been tested to a great precision up to the scale of O(100) GeV [1], while the electroweak symmetry breaking (EWSB) mechanism remains undetermined. If the EWSB sector is weakly coupled, such as in the supersymmetric theories, the effects of new physics at higher mass scales generally decouple from the low energy phenomena [2]. If the EWSB sector is strongly interacting instead, in lack of phenomenologically viable models for the strong dynamics [3], the effective field theory approach [4] provides the most general and economic description. In this case, the nondecoupling property of the electroweak sector enforces the new physics scale (Λ) to lie below or at 4πv ≃ 3.1 TeV, where v is the vacuum expectation value characterizing the EWSB. Below the possible heavy resonances [5] at the scale Λ, the new physics effects in the nondecoupling scenario can be systematically parametrized as the next-to-leading order (NLO) operators of the electroweak chiral Lagrangian (EWCL) [6]. There are in total fifteen NLO bosonic operators, which can modify the electroweak gauge boson interactions and induce the so-called “anomalous couplings” [7]. The contributions of all these NLO operators to various high energy processes have been recently classified by means of a global power counting analysis [8]. Five operators contain quartic gauge couplings (QGCs) which can involve the pure Goldstone boson interactions (according to the equivalence theorem [9]) and are thus of special importance for probing the EWSB. They are summarized as follows [6]:    L4 = l4 ( v Λ )2 [Tr(VμVν)] , L5 = l5 ( v Λ )2 [Tr(VμV)] ;    (SU(2)c−conserving)    L6 = l6 ( v Λ )2 [Tr(VμVν)]Tr(T Vμ)Tr(T Vν) , L7 = l7 ( v Λ )2 [Tr(VμV)]Tr(T Vν)Tr(T Vν) , L10 = l10 ( v Λ )2 1 2 [Tr(T Vμ)Tr(T Vν)]2 ;    (SU(2)c−violating) (1) where Vμ ≡ (DμU)U , DμU = ∂μU + WμU − UBμ, Wμ ≡ igW a μ τ/2, Bμ ≡ igBμτ/2, U = exp[iτπ/v] , and T ≡ Uτ3U † ; π is the would-be Goldstone boson field and W a μ , Bμ the gauge fields of the SU(2) ⊗ U(1) group. Note that the operators L6,7,10 violate the custodial SU(2)c symmetry even in the limit g ′ → 0. No current experiment ever reaches the sufficient energy threshold to directly probe these five operators which involve only