Moderately suppressed dimension-five proton decay in a flipped SU(5) model

We study colored Higgsino-mediated proton decay (dimension-five proton decay) in a model based on the flipped SU(5) GUT. In the model, the GUT-breaking 10, $$ \overline{\mathbf{10}} $$ 10 ¯ fields have a GUT-scale mass term and gain VEVs through higher-dimensional operators, which induces an effective mass term between the color triplets in the 5, $$ \overline{\mathbf{5}} $$ 5 ¯ Higgs fields that is not much smaller than the GUT scale. This model structure gives rise to observable dimension-five proton decay, and at the same time achieves moderate suppression on dimension-five proton decay that softens the tension with the current bound on Γ(p → K+$$ \overline{\nu} $$ ν ¯ ). We investigate the flavor dependence of the Wilson coefficients of the operators relevant to dimension-five proton decay, by relating them with diagonalized Yukawa couplings and CKM matrix components in MSSM, utilizing the fact that the GUT Yukawa couplings are in one-to-one correspondence with the MSSM Yukawa couplings in flipped models. Then we numerically evaluate the Wilson coefficients, and predict the distributions of the ratios of the partial widths of various proton decay modes.

μ-distortion around stupendously large primordial black holes

In a variety of mechanisms generating primordial black holes, each black hole is expected to form along with a surrounding underdense region that roughly compensates the black hole mass. This region will propagate outwards and expand as a shell at the speed of sound in the homogeneous background. Dissipation of the shell due to Silk damping could lead to detectable μ-distortion in the CMB spectrum: if black holes are rare on the last scattering surface, the signal(s) would be pointlike; whereas if there are a sufficient number of them, we could have a uniform distortion in the CMB sky. While the current bound on the average μ-distortion is |μ̅| ≲ 10-4, the standard ΛCDM model predicts |μ̅| ∼ 10-8, which could possibly be detected in future missions. It is shown in this work that the non-observation of μ̅ beyond ΛCDM can place a new upper bound on the density of supermassive primordial black holes within the mass range 106 M ☉≲ M ≲ 1015 M ☉. Furthermore, black holes with initial mass M ≳ 1012 M ☉ could leave a pointlike distortion with μ ≳10-8 at an angular scale ∼ 1° in CMB, and its non-observation would impose an even more stringent bound on the population of these stupendously large primordial black holes.

Quasi-conformal models and the early universe

Extensions of the Standard Model and general relativity featuring a UV fixed point can leave observable implications at accessible energies. Although mass parameters such as the Planck scale can appear through dimensional transmutation, all fundamental dimension-4 operators can (at least approximately) respect Weyl invariance at finite energy. An example is the Weyl-squared term, whose consistency and observational consequences are studied. This quasi-conformal scenario emerges from the UV complete quadratic gravity and is a possible framework for inflation. We find two realizations. In the first one the inflaton is a fundamental scalar with a quasi-conformal non-minimal coupling to the Ricci scalar. In this case the field excursion must not exceed the Planck mass by far. An example discussed in detail is hilltop inflation. In the second realization the inflaton is a pseudo-Goldstone boson (natural inflation). In this case we show how to obtain an elegant UV completion within an asymptotically free QCD-like theory, in which the inflaton is a composite scalar due to new strong dynamics. We also show how efficient reheating can occur. Unlike the natural inflation based on Einstein gravity, the tensor-to-scalar ratio is well below the current bound set by Planck. In both realizations mentioned above, the basic inflationary formulæ are computed analytically and, therefore, these possibilities can be used as simple benchmark models.

Exploring the string axiverse and parity violation in gravity with gravitational waves

We show that the parametric resonance of gravitational waves (GWs) occurs due to the axion coherent oscillation and the circular polarization of GWs is induced by the Chern–Simons coupling. However, we have never observed these signatures in the data of GWs. Using this fact, we give stringent constraints on the Chern–Simons coupling constant [Formula: see text] and the abundance of the light string axion. In particular, we improved the current bound [Formula: see text] by many orders of magnitude.

Black holes in Einstein-Gauß-Bonnet-dilaton theory

Generalizations of the Schwarzschild and Kerr black holes are discussed in an astrophysically viable generalized theory of gravity, which includes higher curvature corrections in the form of the Gauss-Bonnet term, coupled to a dilaton. The angular momentum of these black holes can slightly exceed the Kerr bound. The location and the orbital frequency of particles in their innermost stable circular orbits can deviate significantly from the respective Kerr values. Study of the quasinormal modes of the static black holes gives strong evidence that they are mode stable against polar and axial perturbations. Future gravitational wave observations should improve the current bound on the Gauss-Bonnet coupling constant, based on observations of the low-mass x-ray binary A 0620-00.

TENSION BETWEEN SCALAR/PSEUDOSCALAR NEW PHYSICS CONTRIBUTION TO Bs → μ+μ- AND B → Kμ+μ-

New physics in the form of scalar/pseudoscalar operators cannot lower the semileptonic branching ratio Br (B → K μ+μ-) below its standard model value. In addition, we show that the upper bound on the leptonic branching ratio Br (Bs → μ+μ-) sets a strong constraint on the maximum value of Br (B → K μ+μ-) in models with multiple Higgs doublets: with the current bound, Br (B → K μ+μ-) cannot exceed the standard model prediction by more than 2.5%. The conclusions hold true even if the new physics couplings are complex. However, these constraints can be used to restrict new physics couplings only if the theoretical and experimental errors in Br (B → K μ+μ-) are reduced to a few per cent. The constraints become relaxed in a general class of models with scalar/pesudoscalar operators.