New Directions for New Dimensions: From Strings to Neutrinos to Axions to...

In this talk, I discuss recent developments concerning the possibility of large extra spacetime dimensions. After briefly reviewing how such dimensions can lower the fundamental GUT, Planck, and string scales, I then outline how these scenarios lead to a new higher-dimensional seesaw mechanism for generating neutrino oscillations --- perhaps even without neutrino masses. I also discuss how extra dimensions lead to new mechanisms contributing to the ``invisibility'' of the QCD axion. This talk reports on work done in collaboration with Emilian Dudas and Tony Gherghetta.Comment: 12 pages, LaTeX, 4 figures. Invited plenary talk given at PASCOS '99 (held at Lake Tahoe, California, 10-16 December 1999). To appear in the Proceeding

SPACETIME PROPERTIES OF (1,0) STRING VACUA

We discuss one of the generic spacetime consequences of having (1,0) worldsheet supersymmetry in tachyon-free string theory, namely the appearance of a ``misaligned supersymmetry'' in the corresponding spacetime spectrum. Misaligned supersymmetry is a universal property of (1,0) string vacua which describes how the arrangement of bosonic and fermionic states at all string energy levels conspires to preserve finite string amplitudes, even in the absence of full spacetime supersymmetry. Misaligned supersymmetry also constrains the degree to which spacetime supersymmetry can be broken without breaking modular invariance, and is responsible for the vanishing of various mass supertraces evaluated over the infinite string spectrum. [Talk delivered at Strings '95, based on material drawn from hep-th/9402006 and hep-th/9409114. To appear in Proceedings.]Comment: 6 pages, LaTeX, one encapsulated figur

Kaluza-Klein Masses and Couplings: Radiative Corrections to Tree-Level Relations

The most direct experimental signature of a compactified extra dimension is the appearance of towers of Kaluza-Klein particles obeying specific mass and coupling relations. However, such masses and couplings are subject to radiative corrections. In this paper, using techniques developed in previous work, we investigate the extent to which such radiative corrections deform the expected tree-level relations between Kaluza-Klein masses and couplings. As toy models for our analysis, we investigate a flat five-dimensional scalar \lambda\phi^4 model and a flat five-dimensional Yukawa model involving both scalars and fermions. In each case, we identify the conditions under which the tree-level relations are stable to one-loop order, and the situations in which radiative corrections modify the algebraic forms of these relations. Such corrections to Kaluza-Klein spectra therefore have the potential to distort the apparent geometry of a large extra dimension.Comment: 34 pages, LaTeX, 12 figure

Shape versus Volume: Making Large Flat Extra Dimensions Invisible

Much recent attention has focused on theories with large extra compactified dimensions. However, while the phenomenological implications of the volume moduli associated with such compactifications are well understood, relatively little attention has been devoted to the shape moduli. In this paper, we show that the shape moduli have a dramatic effect on the corresponding Kaluza-Klein spectra: they change the mass gap, induce level crossings, and can even be used to interpolate between theories with different numbers of compactified dimensions. Furthermore, we show that in certain cases it is possible to maintain the ratio between the higher-dimensional and four-dimensional Planck scales while simultaneously increasing the Kaluza-Klein graviton mass gap by an arbitrarily large factor. This mechanism can therefore be used to alleviate (or perhaps even eliminate) many of the experimental bounds on theories with large extra spacetime dimensions.Comment: 9 pages, LaTeX, 5 figure

How Strings Make Do without Supersymmetry: An Introduction to Misaligned Supersymmetry

We provide a non-technical introduction to "misaligned supersymmetry", a generic phenomenon in string theory which describes how the arrangement of bosonic and fermionic states at all string energy levels conspires to preserve finite string amplitudes even in the absence of spacetime supersymmetry. Misaligned supersymmetry thus naturally constrains the degree to which spacetime supersymmetry can be broken in string theory while preserving the finiteness of string amplitudes, and explains how the requirements of modular invariance and absence of physical tachyons affect the distribution of states throughout the string spectrum.Comment: 13 pages, uuencoded PostScript (with figures already embedded) [Talk presented at MRST '94: "What Next? Exploring the Future of High-Energy Physics" (held at McGill University, Montreal, Canada, 11--13 May 1994), and at PASCOS '94: "Particles, Strings, and Cosmology" (held at Syracuse University, Syracuse, NY, 19--24 May 1994). To appear in Proceedings published by World Scientific.