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Open AccessS-matrix positivity without Lorentz invariance: a case study
We investigate the analytic structure of scattering amplitudes in theories in which Lorentz invariance is spontaneously broken. We do so by computing and studying the S-matrix for a simple example: a superflui...
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Article
Open AccessApparently superluminal superfluids
We consider the superfluid phase of a specific renormalizable relativistic quantum field theory. We prove that, within the regime of validity of perturbation theory and of the superfluid effective theory, ther...
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Open AccessThe connection between nonzero density and spontaneous symmetry breaking for interacting scalars
We consider U(1)-symmetric scalar quantum field theories at zero temperature. At nonzero charge densities, the ground state of these systems is usually assumed to be a superfluid phase, in which the global sym...
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Article
Open AccessCosmology as a weak gravitational field and the trans-Planckian problem
At momenta much higher than the Hubble scale, the cosmological expansion can be thought of as a weak gravitational field. We consider QFT in a particularly convenient set of coordinates that makes this manifes...
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Open AccessIntegrating out beyond tree level and relativistic superfluids
We revisit certain subtleties of renormalization that arise when one derives a low-energy effective action by integrating out the heavy fields of a more complete theory. Usually these subtleties are circumvent...
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Open AccessA technical analog of the cosmological constant problem and a solution thereof
The near vanishing of the cosmological constant is one of the most puzzling open problems in theoretical physics. We consider a system, the so-called framid, that features a technically similar problem. Its st...
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Open AccessSolidity without inhomogeneity: perfectly homogeneous, weakly coupled, UV-complete solids
Solid-like behavior at low energies and long distances is usually associated with the spontaneous breaking of spatial translations at microscopic scales, as in the case of a lattice of atoms. We exhibit three ...
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Open AccessSpontaneously broken boosts and the Goldstone continuum
The spontaneous breaking of boost invariance is ubiquitous in nature, yet the associated Goldstone bosons are nowhere to be seen. We discuss why some subtleties are to be expected in the Goldstone phenomenon f...
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Article
Open AccessA multipole-expanded effective field theory for vortex ring-sound interactions
The low-energy dynamics of a zero temperature superfluid or of the compressional modes of an ordinary fluid can be described by a simple effective theory for a scalar field — the superfluid ‘phase’. However, w...
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Open AccessEffective string theory for vortex lines in fluids and superfluids
We discuss the effective string theory of vortex lines in ordinary fluids and low-temperature superfluids, by describing the bulk fluid flow in terms of a two-form field to which vortex lines can couple. We de...
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Open AccessZoology of condensed matter: framids, ordinary stuff, extra-ordinary stuff
We classify condensed matter systems in terms of the spacetime symmetries they spontaneously break. In particular, we characterize condensed matter itself as any state in a Poincaré-invariant theory that spont...
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Open AccessSpontaneously broken mass
The Galilei group involves mass as a central charge. We show that the associated superselection rule is incompatible with the observed phenomenology of superfluid helium 4: this is recovered only under the ass...
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Open AccessMore on gapped Goldstones at finite density: more gapped Goldstones
It was recently argued that certain relativistic theories at finite density can exhibit an unconventional spectrum of Goldstone excitations, with gapped Goldstones whose gap is exactly calculable in terms of t...
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Subluminal galilean genesis
We put forward an improved version of the Galilean Genesis model that addresses the problem of superluminality. We demote the full conformal group to Poincaré symmetry and dilations, supplemented with approximate
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Spontaneous symmetry probing
For relativistic quantum field theories, we consider Lorentz breaking, spatially homogeneous field configurations or states that evolve in time along a symmetry direction. We dub this situation “spontaneous sy...
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Erratum: energy’s and amplitudes’ positivity
We correct typos in eq. (2.41) and (2.42) of ref. [1]. This has several repercussions for the equations of section 2.3, but none for the subsequent sections or for the conclusions of the paper.
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Derrick’s theorem beyond a potential
Scalar field theories with derivative interactions are known to possess solitonic excitations, but such solitons are generally unsatisfactory because the effective theory fails precisely where nonlinearities r...
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The quantum mechanics of perfect fluids
We consider the canonical quantization of an ordinary fluid. The resulting long-distance effective field theory is derivatively coupled, and therefore strongly coupled in the UV. The system however exhibits a ...
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Energy’s and amplitudes’ positivity
In QFT, the null energy condition (NEC) for a classical field configuration is usually associated with that configuration’s stability against small perturbations, and with the sub-luminality of these. Here, we...
