KK Parity in Warped Extra Dimension

TL;DR

The paper introduces a warped $5$-D framework with a discrete KK parity by gluing two AdS$_5$ slices, aiming to address the little hierarchy problem while yielding a stable dark-matter candidate in the form of the lightest KK-odd particle. It analyzes two symmetric constructions, IR-UV-IR and UV-IR-UV, detailing how IR brane kinetic terms or fermion localizations can generate a meaningful splitting between the lightest KK-even and KK-odd modes, and it explores the resulting dark matter and collider phenomenology with a TeV-scale KK $Z$ as a central LKP possibility. While the IR-UV-IR setup can accommodate a viable LKP and dark matter, it requires careful handling of flavor constraints and perturbativity; the UV-IR-UV construction suffers gravitational instabilities (ghost radion and light graviton) and thus is less favorable. Overall, the work delineates how warped geometries can realize KK parity to yield new, testable collider signatures and a dark matter candidate, while highlighting key theoretical challenges and directions for further model-building and phenomenological studies.

Abstract

We construct models with a Kaluza-Klein (KK) parity in a five- dimensional warped geometry, in an attempt to address the little hierarchy problem present in setups with bulk Standard Model fields. The lightest KK particle (LKP) is stable and can play the role of dark matter. We consider the possibilities of gluing two identical slices of 5D AdS in either the UV (IR-UV-IR model) or the IR region (UV-IR-UV model) and discuss the model-building issues as well as phenomenological properties in both cases. In particular, we find that the UV-IR-UV model is not gravitationally stable and that additional mechanisms might be required in the IR-UV-IR model in order to address flavor issues. Collider signals of the warped KK parity are different from either the conventional warped extra dimension without KK parity, in which the new particles are not necessarily pair-produced, or the KK parity in flat universal extra dimensions, where each KK level is nearly degenerate in mass. Dark matter and collider properties of a TeV mass KK Z gauge boson as the LKP are discussed.

Figures (8)

• Figure 1: Three-site deconstruction of the warped extra-dimension.
• Figure 2: Left: Gauge boson wave functions along extra dimension for first even $(1_+)$ mode (red), first odd $(1_-)$ mode (blue) and zero $(0)$ mode (black). Middle: Profiles are zoomed near the IR brane and we added in dashed lines the level-2 KK modes for comparison. Right: Same as middle plot but switching off the IR BKT. The first odd mode is more strongly coupled to the IR brane while the first even mode is less suppressed than in the case with IR BKT.
• Figure 3: The position dependent propagator smeared with $a^{-1}$ (solid red). It hits the strong coupling scale at the second heavy KK mass. For comparison, IR brane-to-brane propagators in the absence of IR BKT (dashed blue).
• Figure 4: Profiles of first odd (black) and even (red) KK fermions with RH chirality for two values of $c$.
• Figure 5: KK mass spectrum. The first tower is for gauge bosons ($r_{IR}=4L$). The last three towers are for fermions with different $c$ parameters. The $n=1$ modes are black and the $n=2$ are pink. Each tower contains two sub-towers, the left one is for KK parity-odd modes, the right one for KK parity-even modes.