H.P. Nilles

Selftuning and its footprints

Abstract We reconsider the selftuning idea in brane world models of finite volume. We notice that in a large class of selftuning models, the four-dimensional physics is sensitive to the vacuum energy on the brane. In particular the compactification volume changes each time the tension of the brane is modified: consequently, observable constants, as the effective Planck mass or masses of matter fields, change as well. We notice that the selftuning mechanism and the stabilization mechanism of the size of the extra dimensions are generically in apparent conflict. We focus on a selftuning model in six spacetime dimensions to analyze how the above considerations are explicitly realized.

Gauge corrections and FI-term in 5D KK theories

Abstract In the context of a five-dimensional N =1 Kaluza–Klein model compactified on S 1 / Z 2 × Z 2 ′ we compute the one-loop gauge corrections to the self energy of the (zero-mode) scalar field. The result is quadratically divergent due to the appearance of a Fayet–Iliopoulos term.

Low energy supersymmetry from the heterotic string landscape

We study possible correlations between properties of the observable and hidden sectors in heterotic string theory. Specifically, we analyze the case of the Z6-II orbifold compactification which produces a significant number of models with the spectrum of the supersymmetric standard model. We find that requiring realistic features does affect the hidden sector such that hidden sector gauge group factors SU(4) and SO(8) are favored. In the context of gaugino condensation, this implies low energy supersymmetry breaking.

Instabilities of bulk fields and anomalies on orbifolds

Abstract Bulk matter modes of higher-dimensional models generically become unstable in the presence of additional matter multiplets at the branes. This quantum instability is driven by localized Fayet–Iliopoulos terms that attract the bulk zero modes towards the boundary branes. We study this mechanism in the framework of a 5-dimensional S 1 / Z 2 orbifold and give conditions for the various possibilities of localization of (chiral) zero modes. This mechanism is quite relevant for realistic model building, as the standard model contains U(1) hypercharge with potentially localized FI-terms. The analysis is closely related to localized anomalies in higher-dimensional gauge theories. Five-dimensional gauge invariance of the effective action determines the anomaly constraints and fixes the normalization of Chern–Simons terms. The localization of the bulk modes does not effect the anomaly cancellation globally, but the additional heavy Kaluza–Klein modes of the bulk fields may cancel the Chern–Simons terms. We discuss also the potential appearance of the parity anomaly that might render the construction of some orbifold models inconsistent.

Spontaneous localization of bulk matter fields

Abstract We study models compactified on S 1 / Z 2 with bulk and brane matter fields charged under U (1) gauge symmetry. We calculate the FI-terms and show by minimizing the resulting potential that supersymmetry or gauge symmetry is spontaneously broken if the sum of the charges does not vanish. Even if this sum vanishes, there could be an instability as a consequence of localized FI-terms. This leads to a spontaneous localization of charged bulk fields on respective branes.

Localized anomalies in heterotic orbifolds

Abstract Recently spatially localized anomalies have been considered in higher-dimensional field theories. The question of the quantum consistency and stability of these theories needs further discussion. Here we would like to investigate what string theory might teach us about theories with localized anomalies. We consider the Z 3 orbifold of the heterotic E 8 ×E′ 8 theory, and compute the anomaly of the gaugino in the presence of Wilson lines. We find an anomaly localized at the fixed points, which depends crucially on the local untwisted spectra at those points. We show that non-Abelian anomalies cancel locally at the fixed points for all Z 3 models with or without additional Wilson lines. At various fixed points different anomalous U(1)s may be present, but at most one at a given fixed point. It is in general not possible to construct one generator which is the sole source of the anomalous U(1)s at the various fixed points.

Multiple anomalous U(1)s in heterotic blow-ups

Abstract The existence of multiple anomalous U(1)s is demonstrated explicitly in a blow-up version of a heterotic Z 3 orbifold. Another blow-up of the same orbifold supports further evidence for the type I/heterotic duality in four dimensions. It has a single anomalous U(1) which does not factorize universally. As multiple anomalous U(1)s as well as non-universal factorization have never been established on heterotic orbifolds explicitly, these findings might appear contradictory at first sight. Possible inconsistencies are avoided by reinterpreting a charged twisted state as a second non-universal localized axion. The mismatch between the charges of the orbifold and blow-up spectra is resolved by suitable field redefinitions. The anomaly of the field redefinitions corresponds to the difference of blow-up and heterotic orbifold anomalies.

Localized tadpoles of anomalous heterotic U(1)'s

Abstract We investigate the properties of localized anomalous U(1)s in heterotic string theory on the orbifold T 6 / Z 3 . We argue that the local four-dimensional and original ten-dimensional Green–Schwarz mechanisms can be implemented simultaneously, making the theory manifestly gauge invariant everywhere, in the bulk and at the fixed points. We compute the shape of the Fayet–Iliopoulos tadpoles, and cross-check this derivation for the four-dimensional auxiliary fields by a direct calculation of the tadpoles of the internal gauge fields. Finally we study some resulting consequences for spontaneous symmetry breaking, and derive the profile of the internal gauge field background over the orbifold.

Associated production of heavy quark flavor in e+e− annihilation☆

Abstract We evaluate the QCD contribution to heavy flavor production in e + e − annihilation to lowest order in α s and discuss the range of validity of an α s expansion. Inclusive quarkonium production in e + e − is also estimated for PETRA, PEP and LEP energies.

Matching the BPS spectra of heterotic Type I and Type I-prime strings

Abstract We give a detailed discussion of the matching of the BPS states of heterotic, type I and type I′ theories in d =9 for general backgrounds. This allows us to explicitly identify these (composite) brane states in the type I′ theory that lead to gauge symmetry enhancement at critical points in moduli space. An example is the enhancement of SO (16)× SO (16) to E 8 × E 8 .