57 research outputs found
Non-Abelian discrete gauge symmetries in 4d string models
We study the realization of non-Abelian discrete gauge symmetries in 4d field
theory and string theory compactifications. The underlying structure
generalizes the Abelian case, and follows from the interplay between gaugings
of non-Abelian isometries of the scalar manifold and field identifications
making axion-like fields periodic. We present several classes of string
constructions realizing non-Abelian discrete gauge symmetries. In particular,
compactifications with torsion homology classes, where non-Abelianity arises
microscopically from the Hanany-Witten effect, or compactifications with
non-Abelian discrete isometry groups, like twisted tori. We finally focus on
the more interesting case of magnetized branes in toroidal compactifications
and quotients thereof (and their heterotic and intersecting duals), in which
the non-Abelian discrete gauge symmetries imply powerful selection rules for
Yukawa couplings of charged matter fields. In particular, in MSSM-like models
they correspond to discrete flavour symmetries constraining the quark and
lepton mass matrices, as we show in specific examples.Comment: 58 pages; minor typos corrected and references adde
Transplanckian axions !?
We discuss quantum gravitational effects in Einstein theory coupled to
periodic axion scalars to analyze the viability of several proposals to achieve
superplanckian axion periods (aka decay constants) and their possible
application to large field inflation models. The effects we study correspond to
the nucleation of euclidean gravitational instantons charged under the axion,
and our results are essentially compatible with (but independent of) the Weak
Gravity Conjecture, as follows: Single axion theories with superplanckian
periods contain gravitational instantons inducing sizable higher harmonics in
the axion potential, which spoil superplanckian inflaton field range. A similar
result holds for multi-axion models with lattice alignment (like the
Kim-Nilles-Peloso model). Finally, theories with axions can still achieve a
moderately superplanckian periodicity (by a factor) with no higher
harmonics in the axion potential. The Weak Gravity Conjecture fails to hold in
this case due to the absence of some instantons, which are forbidden by a
discrete gauge symmetry. Finally we discuss the realization of
these instantons as euclidean D-branes in string compactifications.Comment: 46 pages, 6 figures. Added references, clarifications, and missing
factor of 1/2 to instanton action. Conclusions unchange
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