Flavour Condensates in Brane Models and Dark Energy

In the context of a microscopic model of string-inspired foam, in which foamy structures are provided by brany point-like defects (D-particles) in space-time, we discuss flavour mixing as a result of flavour non-preserving interactions of (low-energy) fermionic stringy matter excitations with the defects. Such interactions involve splitting and capture of the matter string state by the defect, and subsequent re-emission. Quantum fluctuations of the D-particles induce a non-trivial space-time background; in some circumstances this could be akin to a cosmological Friedman-Robertson Walker expanding-Universe, with weak (but non-zero) particle production. Furthermore the D-particle medium can induce an MSW type effect. We have argued previously, in the context of bosons, that the so-called flavour vacuum is the appropriate state to be used, at least for low-energy excitations, with energies/momenta up to a dynamically determined cutoff scale. In this work we evaluate the flavour-vacuum expectation value (condensate) of the stress-energy tensor of the (1/2)-spin fields with mixing in an effective low-energy Quantum Field Theory in this foam-induced curved space-time. We demonstrate, at late epochs of the Universe, that the fermionic vacuum condensate behaves as a fluid with negative pressure and positive energy, but alone it cannot lead to present-day accelerating Universes. One needs flavoured boson contributions for this purpose.Comment: 19 pages revtex, three eps figures incorporate

Modeling Resistive Switching in Nanogranular Metal Films

Films produced by assembling bare gold clusters well beyond the electrical percolation threshold show a resistive switching behavior whose investigation has started only recently. Here we address the challenge to charaterize the resistance of a nanogranular film starting from limited information on the structure at the microscopic scale by the means of Bruggeman's approach to multicomponent media, within the framework of Effective Medium Approximations. The approach is used to build a model that proves that the observed resistive switching can be explained by thermally regulated local structural rearrangements

Flavour-Condensate-induced Breaking of Supersymmetry in Free Wess-Zumino Fluids

Recently we argued that a particular model of string-inspired quantum space-time foam (D-foam) may induce oscillations and mixing among flavoured particles. As a result, rather than the mass-eigenstate vacuum, the correct ground state to describe the underlying dynamics is the flavour vacuum, proposed some time ago by Blasone and Vitiello as a description of quantum field theories with mixing. At the microscopic level, the breaking of target-space supersymmetry is induced in our space-time foam model by the relative transverse motion of brane defects. Motivated by these results, we show that the flavour vacuum, introduced through an inequivalent representation of the canonical (anti-) commutation relations, provides a vehicle for the breaking of supersymmetry (SUSY) at a low-energy effective field theory level; on considering the flavour-vacuum expectation value of the energy-momentum tensor and comparing with the form of a perfect relativistic fluid, it is found that the bosonic sector contributes as dark energy while the fermion contribution is like dust. This indicates a strong and novel breaking of SUSY, of a non-perturbative nature, which may characterize the low energy field theory of certain quantum gravity models.Comment: Discussion added in sections II and IV on quantum-gravity induced flavour mixing, references added, conclusions unchange

Atomic responses to general dark matter-electron interactions

In the leading paradigm of modern cosmology, about 80% of our Universe's matter content is in the form of hypothetical, as yet undetected particles. These do not emit or absorb radiation at any observable wavelengths, and therefore constitute the so-called Dark Matter (DM) component of the Universe. Detecting the particles forming the Milky Way DM component is one of the main challenges for astroparticle physics and basic science in general. One promising way to achieve this goal is to search for rare DM-electron interactions in low-background deep underground detectors. Key to the interpretation of this search is the response of detectors' materials to elementary DM-electron interactions defined in terms of electron wave functions' overlap integrals. In this work, we compute the response of atomic argon and xenon targets used in operating DM search experiments to general, so far unexplored DM-electron interactions. We find that the rate at which atoms can be ionized via DM-electron scattering can in general be expressed in terms of four independent atomic responses, three of which we identify here for the first time. We find our new atomic responses to be numerically important in a variety of cases, which we identify and investigate thoroughly using effective theory methods. We then use our atomic responses to set 90% confidence level (C.L.) exclusion limits on the strength of a wide range of DM-electron interactions from the null result of DM search experiments using argon and xenon targets.Comment: 30 pages, 10 figures. Code available at https://github.com/temken/DarkARC and https://doi.org/10.5281/zenodo.3581334 . v2: matches published versio

Multiscale Modelling of Resistive Switching in Gold Nanogranular Films*

Metallic nanogranular films display a complex dynamical response to a constant bias, typically showing up as a resistive switching mechanism which, in turn, could be used to create electrical components for neuromorphic applications. To model such a phenomenon we use a multiscale computational approach blending together (i) an ab initio treatment of the electric current at the nanoscale, (ii) a molecular dynamics approach dictating structural rearrangements, and (iii) a finite-element solution of the heat equation for heat propagation in the sample. We also consider structural changes due to electromigration which are modelled on the basis of experimental observations on similar systems. Within such an approach, we manage to describe some distinctive features of the resistive switching occurring in a nanogranular film and provide a physical interpretation at the microscopic level

Dark matter-electron interactions in materials beyond the dark photon model

The search for sub-GeV dark matter (DM) particles via electronic transitions in underground detectors attracted much theoretical and experimental interest in the past few years. A still open question in this field is whether experimental results can in general be interpreted in a framework where the response of detector materials to an external DM probe is described by a single ionisation or crystal form factor, as expected for the so-called dark photon model. Here, ionisation and crystal form factors are examples of material response functions: interaction-specific integrals of the initial and final state electron wave functions. In this work, we address this question through a systematic classification of the material response functions induced by a wide range of models for spin-0, spin-1/2 and spin-1 DM. We find several examples for which an accurate description of the electronic transition rate at DM direct detection experiments requires material response functions that go beyond those expected for the dark photon model. This concretely illustrates the limitations of a framework that is entirely based on the standard ionisation and crystal form factors, and points towards the need for the general response-function-based formalism we pushed forward recently [1,2]. For the models that require non-standard atomic and crystal response functions, we use the response functions of [1,2] to calculate the DM-induced electronic transition rate in atomic and crystal detectors, and to present 90% confidence level exclusion limits on the strength of the DM-electron interaction from the null results reported by XENON10, XENON1T, EDELWEISS and SENSEI