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We investigate the production of photons from coherently oscillating, spatially localized clumps of axionic fields (oscillons and axion stars) in the presence of external electromagnetic ...fields. We delineate different qualitative behaviour of the photon luminosity in terms of an effective dimensionless coupling parameter constructed out of the axion-photon coupling, and field amplitude, oscillation frequency and radius of the axion star. For small values of this dimensionless coupling, we provide a general analytic formula for the dipole radiation field and the photon luminosity per solid angle, including a strong dependence on the radius of the configuration. For moderate to large coupling, we report on a non-monotonic behavior of the luminosity with the coupling strength in the presence of external magnetic fields. After an initial rise in luminosity with the coupling strength, we see a suppression (by an order of magnitude or more compared to the dipole radiation approximation) at moderately large coupling. At sufficiently large coupling, we find a transition to a regime of exponential growth of the luminosity due to parametric resonance. We carry out 3+1 dimensional lattice simulations of axion electrodynamics, at small and large coupling, including non-perturbative effects of parametric resonance as well as backreaction effects when necessary. We also discuss medium (plasma) effects that lead to resonant axion to photon conversion, relevance of the coherence of the soliton, and implications of our results in astrophysical and cosmological settings.
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We follow up the work, where in light of the Picard-Lefschetz thimble approach, we split up the real-time path integral into two parts: the initial density matrix part which can be ...represented via an ensemble of initial conditions, and the dynamic part of the path integral which corresponds to the integration over field variables at all later times. This turns the path integral into a two-stage problem where, for each initial condition, there exits one and only one critical point and hence a single thimble in the complex space, whose existence and uniqueness are guaranteed by the characteristics of the initial value problem. In this paper, we test the method for a fully quantum mechanical phenomenon, quantum tunnelling in quantum mechanics. We compare the method to solving the Schrödinger equation numerically, and to the classical-statistical approximation, which emerges naturally in a well-defined limit. We find that the Picard-Lefschetz result matches the expectation from quantum mechanics and that, for this application, the classical-statistical approximation does not.
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Direct numerical evaluation of the real-time path integral has a well-known sign problem that makes convergence exponentially slow. One promising remedy is to use Picard-Lefschetz theory to ...flow the domain of the field variables into the complex plane, where the integral is better behaved. By Cauchy’s theorem, the final value of the path integral is unchanged. Previous analyses have considered the case of real scalar fields in thermal equilibrium, employing a closed Schwinger-Keldysh time contour, allowing the evaluation of the full quantum correlation functions. Here we extend the analysis by not requiring a closed time path, instead allowing for an initial density matrix for out-of-equilibrium initial value problems. We are able to explicitly implement Gaussian initial conditions, and by separating the initial time and the later times into a two-step Monte-Carlo sampling, we are able to avoid the phenomenon of multiple thimbles. In fact, there exists one and only one thimble for each sample member of the initial density matrix. We demonstrate the approach through explicitly computing the real-time propagator for an interacting scalar in 0+1 dimensions, and find very good convergence allowing for comparison with perturbation theory and the classical-statistical approximation to real-time dynamics.
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We perform large-scale real-time simulations of a bubble wall sweeping through an out-of-equilibrium plasma. The scenario we have in mind is the electroweak phase transition, which may be ...first order in extensions of the Standard Model, and produce such bubbles. The process may be responsible for baryogenesis and can generate a background of primordial cosmological gravitational waves. We study thermodynamic features of the plasma near the advancing wall, the generation of Chern-Simons number/Higgs winding number and consider the potential for CP-violation at the wall generating a baryon asymmetry. A number of technical details necessary for a proper numerical implementation are developed.
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Inspired by Lefschetz thimble theory, we treat Quantum Field Theory as a statistical theory with a complex Probability Distribution Function (PDF). Such complex-valued PDFs permit the ...violation of Bell-type inequalities, which cannot be violated by a real-valued, non-negative PDF. In this paper, we consider the Classical-Statistical approximation in the context of Bell-type inequalities, viz. the familiar (spatial) Bell inequalities and the temporal Leggett-Garg inequalities. We show that the Classical-Statistical approximation does not violate temporal Bell-type inequalities, even though it is in some sense exact for a free theory, whereas the full quantum theory does. We explain the origin of this discrepancy, and point out the key difference between the spatial and temporal Bell-type inequalities. We comment on the import of this work for applications of the Classical-Statistical approximation.
To suppress the amount of brittle TiFe intermetallics in Ti-6Al-4V/304L joints, this study adopts the variable polarity cold metal transfer arc-brazing method using ERCuSi-A wires. The effects of the ...thermal distribution strategy on the welding process, penetration ratio, microstructure, and tensile strength are investigated by varying the electrode positive/electrode negative (EP/EN) ratio. With the ratio decreasing, more energy is conducted to the wire, thereby decreasing the high-temperature holding time. When the ratio decreases from 8:1 to 1:8, the thickness of the Ti-6Al-4V/seam transition zone containing a TiCu interlayer and Ti5Si3 intermetallic layer decreases from 118 to 81 μm. Along with the decrease in the interlayer thickness, the joint tensile strength initially increases from 216.28 to 261.69 MPa and then decreases to 211.29 MPa. A high EP/EN ratio results in a thick Ti-6Al-4V/seam transition zone that includes a Ti5Si3 layer. A low EP/EN ratio results in a thin Ti-6Al-4V/seam transition zone that includes Ti5Si3 distributed in the grain boundary. Both conditions reduce the tensile strength of the joints. The highest tensile strength is obtained when the ratio is 8:8, under which a thin transition zone and concentrated Ti5Si3 intermetallic layer can be generated.
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•More electrode negative cycle leads to less thermal applied on the base metal.•TiFe intermetallics are suppressed due to extremely low base metal dilution ratio.•The formation of a purified TiCu layer is owing to the consumption of Ti5Si3.
Wire feed speeds of 3.5, 4.5, and 5.5 m/min and offset positions of 1 and 2 were employed for this study with an ERCuSi-A weld wire. The microstructures of the joints, which include a Cu/Ti interface ...layer consisting of Ti2Cu, TiCu, and AlCu2Ti, a Cu-matrix seam consisting of Cu and petal-shaped Fe-Si-Ti intermetallics, and a Cu/Fe interface layer consisting of α-Fe and Cu, were studied. The formation enthalpy calculated from the Miedema model can explained the microstructure evolution mechanism. The interface thickness and ultimate tensile strength were found to increase with wire feed speed. The highest tensile strength of the joint was 294 MPa, fracturing at the Cu/Ti interface. Offsetting the welding torch to the TC4 side increased the amount and size of the Fe-Si-Ti intermetallics, degrading the tensile strength. Four fracture modes were proposed to differentiate the crack propagations in the joints, which were determined by the interfacial bonding strength and the Fe-Si-Ti intermetallics in the weld seam.
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We perform numerical simulations of Cold Electroweak Baryogenesis, including for the first time in the Bosonic sector the full electroweak gauge group SU(2) × U(1) and CP-violation. We find ...that the maximum generated baryon asymmetry is reduced by a factor of three relative to the SU(2)-only model of 1, but that the quench time dependence is very similar. In addition, we compute the magnitude of the helical magnetic fields, and find that it is proportional to the strength of CP-violation and dependent on quench time, but is not proportional to the magnitude of the baryon asymmetry as proposed in 2, 3. Astrophysical signatures of primordial magnetic helicity can therefore not in general be used as evidence that electroweak baryogenesis has taken place.
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