TASI Lectures on Remnants from the String Landscape

Abstract

Superstring theories are very promising theoretically, but the enormous landscape of string vacua and the (likely) very large underlying string scale imply that they may never be tested directly. Nevertheless, concrete constructions consistent with the observed world frequently lead to observable remnants, i.e., new particles or features that are apparently accidental consequences of the ultraviolet theory and that are typically not motivated by specific shortcomings of the standard models of particle physics or cosmology. For example, moduli, axions, large extended gauge sectors, additional Z gauge bosons, extended Higgs/Higgsino sectors, and quasi-chiral exotics are extremely common. They motivate alternative cosmological paradigms and could lead to observable signatures at the LHC. Similar features can emerge in other standard model extensions, but in the stringy case they are more likely to occur in isolation and not as part of a more complete TeV-scale structure. Conversely, some common aspects of the infinite "landscape" of field theories, such as large representations, are expected to be very rare in the string landscape, and observation of features definitively in the swampland could lead to falsification. In this article, common stringy remnants and their phenomenology are surveyed, and implications for indirectly supporting or casting doubt on string theory are discussed.1

Notes on anomalies, elliptic curves and the BS-D conjecture

“…String compactifications admitting gauge theories with matter multiplets in higher representations have been constructed in the past, for example using higher level Kac-Moody algebras for heterotic compactifications that are free on the world-sheet [26]. It is also possible to get adjoint matter multiplets in IIA string theory, and higher representations seem to be possible in F -theory but this has not been seriously explored [27]. It would be interesting to carry out such a search for these types of models, perhaps using the ideas in [28] .…”

Moduli stars

“…We find that the dimensionless compactness for the LVS and KKLT volume moduli is included in the range 8 × 10 −6 C 1.39 , (3.70) 21 The correct value of the parameter α is expected to be the outcome of numerical studies of the formation of the stars, and is related to the distribution of these compact objects. 22 The formation time of the binary is larger than the formation time of the star which is in turn larger than 1/m. However the estimate in eq.…”

“…String theory offers many dark matter candidates and quasi-stable particles, to name a few: matter fields from a hidden sector, moduli (including the dilaton and the many axions), the gravitino (see for instance [22]). But even if the particle decays relatively early in the history of the Universe it may still give rise to (relatively) long-lived compact objects that contribute to the energy density of the Universe for some time and may leave observational signatures such as GWs.…”

Cosmology in the presence of multiple light moduli

“…At this stage, the density of non-relativistic matter is given by the contribution from the oscillations of the modulus ϕ 1 only. This can be written as 6) where in going from the first line to the second, we have set the initial displacement of ϕ 1 to be of the order of m pl . Similarly, the energy density of radiation at that time is…”

“…We will focus on the simplest case -dark matter produced as a result of decay of the moduli. We find that in the absence of annihilation processes, dark matter is overproduced (the dilution mechanism for dark 1 Consistency requirements often force a large number of hidden sectors in string compactifications, see e.g., [5,6]. 2 Situations where the lightest modulus is much lighter than all others can naturally arise if a symmetry principle is operational such as in the Large Volume Scenario [11].…”