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

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

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

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

Quantum Coherence of Critical Unstable Two-Level Systems

We study in detail the dynamics of unstable two-level quantum systems by adopting the Bloch-sphere formalism of qubits. By employing the Bloch-vector representation for such unstable qubit systems, we identify a novel class of critical scenarios in which the so-called energy-level and decay-width vectors, ${\bf E}$ and ${\bf\Gamma}$, are orthogonal to one another, and the parameter $r = |{\bf \Gamma}|/(2|{\bf E}|)$ is less than 1. Most remarkably, we find that critical unstable qubit systems exhibit atypical behaviours like coherence--decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. In the same frame, a unit Bloch vector ${\bf b}$ describing a pure critical qubit will sweep out unequal areas during equal intervals of time, while rotating about the vector ${\bf E}$. These phenomena emerge beyond the usual oscillatory pattern due to the energy-level difference of the two-level quantum system. Interestingly enough, we observe that these new features will persist even for quasi-critical scenarios, in which the vectors ${\bf E}$ and ${\bf\Gamma}$ are not perfectly orthogonal to each other. Applications of our results to quantum information and to unstable meson--antimeson and other systems are discussed.Comment: 31 pages, 10 figure