Effects of non-standard tri-linear couplings in photon-photon collisions (I) γγ → W+Wt-

Abstract

The effect of anomalous couplings in γγ → W + W − is studied for different energies of the γγ mode of the next linear collider. The analysis based on the maximum likelihood method exploits the variables in the four-fermion semi-leptonic final state. Polarised differential cross sections based on the complete set of diagrams for these channels with the inclusion of anomalous couplings are used and compared to an approximation based on γγ → W + W − with full spin correlations. To critically compare these results with those obtained in e + e − we perform an analysis based on the complete calculation of the four-fermion semi-leptonic final state. The anomalous couplings that we consider are derived from the next-to-leading order operators in the non-linear realisation of symmetry breaking. quite difficult to disentangle a W W γ and a W W Z coupling at the e + e − mode, whereas obviously only the former can be probed at the γγ mode. However, as we will argue, the electroweak precision measurements at the Z peak are a sign that there should be a hierarchy of couplings whereby the symmetries one has observed at the present energies indicate that the W W γ and the W W Z should be related. If this is so, though γγ → W + W − is unique in unambiguously measuring the W W γ couplings, it is somehow probing the same parameter space as e + e − → W + W − . In such an eventuality one should then enquire about how to exploit the γγ mode, and whether the reaction γγ → W + W − can give more stringent constraints than in the e + e − mode. In addition it is useful to investigate whether one may gain by combining the results of the γγ analysis with those obtained in 1 the e + e − mode. These two aspects will be addressed in the present paper.Although there have been numerous studies that dealt with the subject of the tri-linear anomalous couplings in γγ → W + W − [11], they have all been conducted at the level of the γγ → W + W − cross section. As we will show, even if one assumes reconstruction of the helicities of the W , restricting the analysis of the γγ → W + W − at the level of the cross section, where one only accesses the diagonal elements of the W W density matrix, it is not possible to maximally enhance the effect of the anomalous couplings. Indeed, as one expects in an investigation of the Goldtsone sector, the new physics parameterised in this context by an anomalous magnetic moment of the W , ∆κ γ , affects principally the production of two longitudinal W bosons. In the γγ mode this affects predominantly the J Z = 0 amplitude by providing an enhanced coupling of the order γ = s γγ /M 2 W ( √ s γγ is the γγ centre of mass energy). Unfortunately the same standard amplitude (the J Z = 0 with two longitudinal W's) has the factor 1/γ and therefore the interference is not effective in the sense that the genuine enhanced coupling γ brought about by the new physics is washed out. This is in contrast with what happens in the e + e − mode where the interference is fully effective. Nonetheless, the enhanced coupling could still be ex...

Effective-Lagrangian approach to γγ → WW I: Couplings and amplitudes

“…In order to give the amplitudes in a convenient way we distinguish between longitudinal and transverse W helicities. Thus, in the following These results agree with [22] apart from an overall factor of −i. Note their different definition of γ (they use γ = s/m…”

“…In the same reference also the advantages of the different approaches are surveyed and a discussion of the relation to other work on anomalous electroweak gauge-boson couplings in e + e − annihilation [1,[9][10][11][12][13][14][15][16][17] is given. In the form-factor approach implications from a future γγ collider for anomalous couplings were discussed in [17][18][19][20][21][22][23][24][25]. At tree level anomalous triple gauge-boson couplings from the γW W vertex but not from the ZW W vertex contribute.…”

The Photon Collider at Tesla

“…However, the higher event rate does not necessarily provide better bounds on anomalous couplings. In some models electroweak symmetry breaking leads to large deviations mainly in longitudinal W L W L pair production [119] …”

Gold-plated processes at photon colliders

“…While there have been extensive analyses [80] of anomalous triple gauge boson couplings at e + e − colliders, the corresponding processes γe → W ν and γγ → W W have generally received less attention in the literature [81,82,20] and the higher order processes were not considered in detail.…”

“…However, the higher event rate does not necessarily provide better bounds on anomalous couplings. In some models electroweak symmetry breaking leads to large deviations mainly in longitudinal W L W L pair production [82]. On the other hand the large cross section of the reaction γγ → W + W − is due to transverse W T W T pair production.…”