Improved BSM sensitivity in diboson processes at linear colliders

Abstract

We study W + W − and Zh final states at future linear e + e − colliders; designing analyses specific to the various final state polarizations allows us to target specific beyond the Standard Model (BSM) effects, parametrized in the form of dimension-6 operators. We find that CLIC can access effects roughly an order of magnitude smaller than HL-LHC or ILC, and two orders of magnitude smaller than LEP. These results are interpreted in the context of well-motivated BSM scenarios-at weak and strong coupling-where we expect correlated effects in Drell-Yann processes. The latter turn out to have better discovery potential, although the diboson processes provide additional discriminating power, potentially furnishing a way to measure the spin and coupling of BSM states. arXiv:1909.01937v1 [hep-ph] 4 Sep 2019 1 MotivationStandard Model (SM) precision tests are at the core of present and future collider programs. While interesting as confirmation of our unprecedented control of SM computations, they serve an exciting additional purpose: a way to search for new structure lurking beyond the SM (BSM). Indeed, heavy dynamics-beyond the direct reach of collidersleave an imprint on lower energy processes, in the form of deformations of SM interactions. These can be described by an Effective Field Theory (EFT), which parametrises the most general deviations from the SM and, at the same time, captures the effects of general, heavy BSM dynamics.The leading such effects, associated with dimension-6 operators in the effective Lagrangian, in general behave schematically as σ ∼ σ SM (1+E 2 /Λ 2 ), with E a characteristic energy scale of the process and Λ the physical scale associated with the EFT operator. This suggests two modes of exploration: i) on a SM resonance-such as the Z-pole studied at LEP1-where σ SM is maximal, statistical uncertainty the smallest, and the experiment becomes sensitive to tiny departures from the SM, or ii) at high-energy, where the BSM effect is larger and less precision is needed to access effects of a given size. In this article we focus on experiments of the latter type; in particular, we study non-resonant 2 → 2 processes, which are the simplest processes, with the largest cross sections, with access to the high-energy regime. Processes with more particles can be interesting to test operators with more legs, such as those that modify Higgs couplings [1].Particularly interesting for the BSM discovery potential they offer are diboson V V and V h (V ( ) = W, Z) final states. Indeed, new dynamics in the Higgs sector-such as Higgs compositeness [2-6]-alter the behaviour of V h processes [7-9] and, according to the equivalence theorem, also enter in processes with longitudinal V V [10][11][12]. BSM in the gauge sector [13,14] instead affects V V processes with transverse polarisations. Finally, light fermion substructure, as implied in models of fermion compositeness [15,16], can modify the initial light quark or lepton current, although this option seems to be disfavoured by tests of flavour and CP violatio...

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