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Register a new userCompact extra-dimensions as solution to the strong CP problem
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We show that the strong CP problem can, in principle, be solved dynamically by adding extra-dimensions with compact topology. To this aim we consider a toy model for QCD, which contains a vacuum angle and a strong CP like problem. We further consider a higher dimensional theory, which has a trivial vacuum structure and which reproduces the perturbative properties of the toy model in the low-energy limit. In the weak coupling regime, where our computations are valid, we show that the vacuum structure of the low-energy action is still trivial and the strong CP problem is solved. No axion-like particle occur in this setup and therefore it is not ruled out by astrophysical bounds.
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We construct a theory in which the solution to the strong CP problem is an emergent property of the background of the dark matter in the Universe. The role of the axion degree of freedom is played by multi-body collective excitations similar to spin-waves in the medium of the dark matter of the Galactic halo. The dark matter is a vector particle whose low energy interactions with the Standard Model take the form of its spin density coupled to $G widetilde{G}$, which induces a potential on the average spin density inducing it to compensate $overline{theta}$, effectively removing CP violation in the strong sector in regions of the Universe with sufficient dark matter density. We discuss the viable parameter space, finding that light dark matter masses within a few orders of magnitude of the fuzzy limit are preferred, and discuss the associated signals with this type of solution to the strong CP problem.
We present a new solution to the strong CP problem in which the imaginary component of the up quark mass, $mathcal{I}[m_u]$, acquires a tiny, but non-vanishing value. This is achieved via a Dirac seesaw mechanism, which is also responsible for the generation of the small neutrino masses. Consistency with the observed value of the up quark mass is achieved via instanton contributions arising from QCD-like interactions. In this framework, the value of the neutron electric dipole moment is directly related to $mathcal{I}[m_u]$, which, due to its common origin with the neutrino masses, implies that the neutron electric dipole moment is likely to be measured in the next round of experiments. We also present a supersymmetric extension of this Dirac seesaw model to stabilize the hierarchy among the scalar mass scales involved in this new mechanism.
We consider a real scalar field with an arbitrary negative bulk mass term in a general 5D setup, where the extra spatial coordinate is a warped interval of size $pi R$. When the 5D field verifies Neumann conditions at the boundaries of the interval, the setup will always contain at least one tachyonic KK mode. On the other hand, when the 5D scalar verifies Dirichlet conditions, there is always a critical (negative) mass $M_{c}^2$ such that the Dirichlet scalar is stable as long as its (negative) bulk mass $mu^2$ verifies $M^2_{c}<mu^2$. Also, if we fix the bulk mass $mu^2$ to a sufficiently negative value, there will always be a critical interval distance $pi R_c$ such that the setup is unstable for $R>R_c$. We point out that the best mass (or distance) bound is obtained for the Dirichlet BC case, which can be interpreted as the generalization of the Breitenlohner-Freedman (BF) bound applied to a general compact 5D warped spacetime. In particular, in a slice of $AdS_5$ the critical mass is $M^2_{c}=-4k^2 -1/R^2$ and the critical interval distance is given by $1/R_c^2=|mu^2|-4k^2$, where $k$ is the $AdS_5$ curvature (the 5D flat case can be obtained in the limit $kto 0$, whereas the infinite $AdS_5$ result is recovered in the limit $Rto infty$).
We discuss the notion of mass, mostly for fermions, and its relation to the breaking of CP invariance, the natural symmetry of gauge interactions. In a first model, we show how compactification on a Vortex in 2 extra dimensions leads to a replication of generations in 3+1, with challenging mass patterns, and testable consequences in flavour-changing neutral currents (family-number conserving), both at low energies and at future colliders. In different model, we show how CP violation can result from compactification from 4+1 to 3+1 dimensions.
A different reason for the apparent weakness of the gravitational interaction is advanced, and its consequences for Hawking evaporation of a Schwarzschild black hole are investigated. A simple analytical formulation predicts that evaporating black holes will undergo a type of phase transition resulting in variously long-lived objects of reasonable sizes, with normal thermodynamic properties and inherent duality characteristics. Speculations on the implications for particle physics and for some recently-advanced new paradigms are explored.
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