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We consider a decay of a false vacuum in flux compactifications of type IIB string theory and study a catalytic effect for a phase transition induced by a new type of impurities. We concentrate on the large N dual of a D5-brane/anti-D5-brane system which has a rich vacuum structure. We show that D3-branes wrapping the 3-cycles can form a dibaryon and make a bound state with a monopole. We find that these baryon-like objects can make the lifetime of the metastable vacuum shorter.
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Besides the string scale, string theory has no parameter except some quantized flux values; and the string theory Landscape is generated by scanning over discrete values of all the flux parameters present. We propose that a typical (normalized) probability distribution $P({cal Q})$ of a physical quantity $cal Q$ (with nonnegative dimension) tends to peak (diverge) at ${cal Q}=0$ as a signature of string theory. In the Racetrack Kahler uplift model, where $P(Lambda)$ of the cosmological constant $Lambda$ peaks sharply at $Lambda=0$, the electroweak scale (not the electroweak model) naturally emerges when the median $Lambda$ is matched to the observed value. We check the robustness of this scenario. In a bottom-up approach, we find that the observed quark and charged lepton masses are consistent with the same probabilistic philosophy, with distribution $P(m)$ that diverges at $m=0$, with the same (or almost the same) degree of divergence. This suggests that the Standard Model has an underlying string theory description, and yields relations among the fermion masses, albeit in a probabilistic approach (very different from the usual sense). Along this line of reasoning, the normal hierarchy of neutrino masses is clearly preferred over the inverted hierarchy, and the sum of the neutrino masses is predicted to be $sum m_{ u} simeq 0.0592$ eV, with an upper bound $sum m_{ u} <0.066$ eV. This illustrates a novel way string theory can be applied to particle physics phenomenology.
Leptoquarks extending the Standard Model (SM) are attracting an increasing attention in the recent literature. Hence, the identification of 4D SM-like models and the classification of allowed leptoquarks from strings is an important step in the study of string phenomenology. We perform the most extensive search for SM-like models from the non-supersymmetric heterotic string $mathrm{SO}(16)timesmathrm{SO}(16)$, resulting in more than 170,000 inequivalent promising string models from 138 Abelian toroidal orbifolds. We explore the 4D massless particle spectra of these models in order to identify all exotics beside the three generations of quarks and leptons. Hereby, we learn which leptoquark can be realized in this string setup. Moreover, we analyze the number of SM Higgs doublets which is generically larger than one. Then, we identify SM-like models with a minimal particle content. These so-called almost SM models appear most frequently in the orbifold geometries $mathbb Z_2timesmathbb Z_4$ (2,4) and (1,6). Finally, we apply machine learning to our dataset in order to predict the orbifold geometry where a given particle spectrum can be found most likely.
We obtain constraints from black hole superradiance in an ensemble of compactifications of type IIB string theory. The constraints require knowing only the axion masses and self-interactions, and are insensitive to the cosmological model. We study more than $2 cdot 10^5$ Calabi-Yau manifolds with Hodge numbers $1leq h^{1,1}leq 491$ and compute the axion spectrum at two reference points in moduli space for each geometry. Our computation of the classical theory is explicit, while for the instanton-generated axion potential we use a conservative model. The measured properties of astrophysical black holes exclude parts of our dataset. At the point in moduli space corresponding to the tip of the stretched K{a}hler cone, we exclude $approx 50%$ of manifolds in our sample at 95% C.L., while further inside the K{a}hler cone, at an extremal point for realising the Standard Model, we exclude a maximum of $approx 7%$ of manifolds at $h^{1,1}=11$, falling to nearly zero by $h^{1,1}=100$.
Problems of stabilizing moduli of the type--IIB string theory on toroidal orientifolds $T^6/Z_2$, in presence of worldvolume fluxes on various D-branes, are considered. For $Z_2$ actions, introducing either O9 or O3 planes, we rule out the possibility of moduli stabilization in a wide class of models with $mathcal{N}=1$ supersymmetry, characterized by the type of fluxes turned on along D-brane worldvolume. Our results, in particular, imply that Abelian worldvolume fluxes can not by themselves stabilize closed string moduli, in a consistent supersymmtric model, for above orientifold compactifications. We also discuss other $Z_2$ orientifolds of $T^6$ and show that certain other brane wrappings are also ruled out by similar consistency requirements. In specific setups we consider examples with D9-branes wrapping on a complex three-torus with its world-volume fluxes taken to be semi-homogeneous bundles and D7-branes wrapping holomorphic four-cycles of the complex three-torus carrying world-volume fluxes.
We conjecture a general upper bound on the strength of gravity relative to gauge forces in quantum gravity. This implies, in particular, that in a four-dimensional theory with gravity and a U(1) gauge field with gauge coupling g, there is a new ultraviolet scale Lambda=g M_{Pl}, invisible to the low-energy effective field theorist, which sets a cutoff on the validity of the effective theory. Moreover, there is some light charged particle with mass smaller than or equal to Lambda. The bound is motivated by arguments involving holography and absence of remnants, the (in) stability of black holes as well as the non-existence of global symmetries in string theory. A sharp form of the conjecture is that there are always light elementary electric and magnetic objects with a mass/charge ratio smaller than the corresponding ratio for macroscopic extremal black holes, allowing extremal black holes to decay. This conjecture is supported by a number of non-trivial examples in string theory. It implies the necessary presence of new physics beneath the Planck scale, not far from the GUT scale, and explains why some apparently natural models of inflation resist an embedding in string theory.
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