Doublet-Triplet Fermionic Dark Matter

We extend the Standard Model (SM) by adding a pair of fermionic SU(2)-doublets with opposite hypercharge and a fermionic SU(2)-triplet with zero hypercharge. We impose a discrete Z_2-symmetry that distinguishes the SM fermions from the new ones. Then, gauge invariance allows for two renormalizable Yukawa couplings between the new fermions and the SM Higgs field, as well as for direct masses for the doublet (M_D) and the triplet (M_T). After electroweak symmetry breaking, this model contains, in addition to SM particles, two charged Dirac fermions and a set of three neutral Majorana fermions, the lightest of which contributes to Dark Matter (DM). We consider a case where the lightest neutral fermion is an equal admixture of the two doublets with mass M_D close to the Z-boson mass. This state remains stable under radiative corrections thanks to a custodial SU(2)-symmetry and is consistent with the experimental data from oblique electroweak corrections. Moreover, the amplitudes relevant to spin-dependent or independent nucleus-DM particle scattering cross section both vanish at tree level. They arise at one loop at a level that may be observed in near future DM direct detection experiments. For Yukawa couplings comparable to the top-quark, the DM particle relic abundance is consistent with observation, not relying on co-annihilation or resonant effects and has a mass at the electroweak scale. Furthermore, the heavier fermions decay to the DM particle and to electroweak gauge bosons making this model easily testable at the LHC. In the regime of interest, the charged fermions suppress the Higgs decays to diphoton by 45-75 % relative to SM prediction.Comment: 40 pages, v2: discussion and references on dark matter direct detection expanded, matches published version, v3: formulae in Appendix A correcte

Radiative Light Dark Matter

We present a Peccei-Quinn (PQ)-symmetric two-Higgs doublet model that naturally predicts a fermionic singlet dark matter in the mass range 10 keV-1 GeV. The origin of the smallness of the mass of this light singlet fermion arises predominantly at the one-loop level, upon soft or spontaneous breakdown of the PQ symmetry via a complex scalar field in a fashion similar to the so-called Dine-Fischler-Sredniki-Zhitnitsky axion model. The mass generation of this fermionic Radiative Light Dark Matter (RLDM) requires the existence of two heavy vector-like SU(2) isodoublets, which are not charged under the PQ symmetry. We show how the RLDM can be produced via the freeze-in mechanism, thus accounting for the missing matter in the Universe. Finally, we briefly discuss possible theoretical and phenomenological implications of the RLDM model for the strong CP problem and the CERN Large Hadron Collider (LHC).Comment: 17 pages, v2: typos corrected, matches published versio

NSC++: Non-Standard Cosmologies in C++

We introduce NSC++, a header-only C++ library that simulates the evolution of the plasma and a decaying fluid in the early Universe. NSC++ can be used in C++ programs or called directly from python scripts without significant overhead. There is no special installation process or external dependencies. Furthermore, there are example programs that can be modified to handle several cases.Comment: 18 pages; 3 figures; 4 tables; The stable version of the library can be found at https://github.com/dkaramit/NSCpp/tree/stable. v2: Added reference; Minor corrections to text. v3: Minor corrections, matches published versio

Modelling of microstructure sensitive short crack growth in Ni single crystals

Ni single crystal turbine blades have been extensively used in aerospace applications due to their high resistance to fatigue fracture under extreme loading and temperature conditions. The key to the high resistance of Ni single crystals at high temperature is the γ-γ' microstructure and the hardening of the γ' precipitates with temperature increase. However, research done so far has not fully addressed the effect of the microstructure to the fatigue life and predicted fatigue life across length scales. For this reason, microstructurally sensitive short crack growth has been investigated in Ni-based superalloy single crystals using an energy-based method to predict the crack path and growth rate across length scales. Short crack growth studied in homogenised γ-γ' Ni-based superalloy single crystal has achieved a good correlation of the predicted crack growth path and rate with experimental crack growth data. The broad applicability of this physics-based methodology was investigated at microstructural scale by developing Ni-based superalloy single crystal models that explicitly represent the γ and γ' phases. This enabled further research on the nature of crack path in γ-γ' microstructures and the role of crystallography, microstructural properties and the loading conditions on the crack path and the crack growth rate, with the crack paths and growth rates predicted at microstructural scale to achieve a good comparison with crack growth experiments at nanoscale. Therefore, the ability of modelling crack growth in γ-γ' microstructures has driven fundamental research on the physics of microstructurally short crack growth in Ni single crystals, the computation of Ni single crystal fatigue failure limits based on the stored energy per cycle at macroscale defining a critical stored energy (Gc) required for failure and the prediction of both short and long crack growth over thousands of loading cycles compared to extensive experimental data.Open Acces

Towards a Localised S-Matrix Theory

We formulate an S-matrix theory in which localisation effects of the particle interactions involved in a scattering process are consistently taken into account. In the limit of an infinite spread of all interactions, the S-matrix assumes its standard form. To better understand the significance of the emerging quantum phenomena in this formalism, we consider a solvable field-theoretic model with spatial Gaussian spreads at the interaction vertices. This solvable model, which was previously introduced in the literature, enables accurate descriptions of detection regions that are either close to or far from the source. In close analogy with light diffraction in classical optics, we call these two regions near-field and far-field zones, or the Fresnel and Fraunhofer regions. We revisit the question whether mixed mediators produce an oscillating pattern if their detection occurs in the Fresnel region. Besides corroborating certain earlier findings of the S-matrix amplitude in the forward Fresnel and Fraunhofer regimes, we observe several novel features with respect to its angular dependence which have not been accounted before in the literature. In particular, we obtain a ``quantum obliquity factor'' that suppresses particle propagation in the backwards direction, thereby providing an explicit quantum field-theoretic description for its origin in diffractive optics. Present and future colliders, as well as both short and long baseline neutrino experiments, would greatly benefit from the many predictions that can be offered from such a holistic localised S-matrix theory.Comment: 34 pages, 7 figures; v2: changed title to more accurately reflect the content, added extensive discussion on possible experimental probes, additional references included; v3: added comments to clarify some points in the paper, matches published version in PR

Varying Entropy Degrees of Freedom Effects in Low-Scale Leptogenesis

We analyse in detail the effect of varying entropy degrees of freedom on low-scale leptogenesis models. As an archetypal model, we consider the Tri-Resonant Leptogensis${}$ (TRL) scenario introduced recently by the authors, where the neutrino-Yukawa coupling matrix is dictated by an approximate $\mathbb{Z}_n$ discrete symmetry (with $n=3,6$). TRL models exhibit no preferred direction in the leptonic flavour space and have the remarkable feature that leptogenesis can successfully take place even if all light neutrinos are strictly massless up to one-loop order. Most interestingly, for TRL scenarios with heavy Majorana neutrinos lighter than 100 GeV, temperature varying degrees of freedom associated with the entropy of the plasma have a dramatic impact on the predictions of the Baryon Asymmetry in the Universe (BAU), and may sensitively depend on the freeze-out sphaleron temperature $T_{\rm sph}$. We find that this is a generic feature of most freeze-out low-scale leptogenesis models discussed in the literature. In the same context, we consider heavy-neutrino scenarios realising dynamics related to critical unstable qudits in the thermal plasma and assess their significance in generating the BAU. The phenomenological implications of TRL scenarios at the intensity and high-energy frontiers are analysed.Comment: 46 pages, 10 figures, further analyses added, to appear in Physical Review