International Journal of Modern Physics A

Radiative neutrino mass models and the seesaw models are viewed from the unifying framework of standard model effective operators that explicitly violate lepton number by two units $(\mathrm{\Delta }L=2)$. After some comments on naturalness and leptogenesis in the minimal type 1 seesaw model, a full list of minimal renormalisable models that produce mass dimension-7, $\mathrm{\Delta }L=2$ operators at low energies is presented. By way of example, phenomenological bounds from Run 1 LHC and lepton flavour violation data are then placed on one of these models. A possible connection between radiative neutrino mass models and the current flavour anomalies in $b\to c$ and $b\to s$ transitions is then described.

In view of the observation of a relatively large ${𝜃}_{13}^{}$, one has to break the $\mu$–$\tau$ permutation symmetry properly which would otherwise result in a vanishing ${𝜃}_{13}^{}$ (as well as ${𝜃}_{23}^{}=\pi /4$). Motivated by this, we study in a thorough way the various symmetry-breaking patterns and in particular whether an approximate this symmetry can lead to viable consequences. Furthermore, the symmetry-breaking effects arising from some specific physics (e.g. the renormalization group running effect) are discussed as well.

Sterile neutrinos are among the most attractive extensions of the SM to generate the light neutrino masses observed in neutrino oscillation experiments. When the sterile neutrinos are subject to a protective symmetry, they can have masses around the electroweak scale and potentially large neutrino Yukawa couplings, which makes them testable at planned future particle colliders. We systematically discuss the production and decay channels at electron–positron, proton–proton and electron–proton colliders and provide a complete list of the leading order signatures for sterile neutrino searches. Among other things, we discuss several novel search channels, and present a first look at the possible sensitivities for the active-sterile mixings and the heavy neutrino masses. We compare the performance of the different collider types and discuss their complementarity.

We analytically solved the QED$\otimes$QCD-coupled DGLAP evolution equations at leading order (LO) quantum electrodynamics (QED) and next-to-leading order (NLO) quantum chromodynamics (QCD) approximations, using the Laplace transform method and then computed the proton structure function in terms of the unpolarized parton distribution functions. Our analytical solutions for parton densities are in good agreement with those from CT14QED $(1.295^2<Q^2<10^{10})$ (Ref. 6) global parametrizations and APFEL (A PDF Evolution Library) $(2<Q^2<10^8)$ (Ref. 4). We also compared the proton structure function, $F_2^p(x,Q^2)$, with the experimental data released by the ZEUS and H1 collaborations at HERA. There is a nice agreement between them in the range of low and high $x$ and $Q^2$.

The ratio of elastic to total proton cross-sections is related to the darkness of the spatial profile of inelastic interactions by a single parameter in the framework of a simple analytical model. Their critical values at LHC energies are discussed. Two possible variants of their asymptotical behavior are described.

We consider the $D^8{{ℛ}}^5$ and $D^{10}{{ℛ}}^5$ terms in the low momentum expansion of the five graviton amplitude in type IIB string theory at one loop. They involve integrals of various modular graph functions over the fundamental domain of $SL(2,\mathbb{Z})$. Unlike the graphs which arise in the four graviton amplitude or at lower orders in the momentum expansion of the five graviton amplitude where the links are given by scalar Green functions, there are several graphs for the $D^8{{ℛ}}^5$ and $D^{10}{{ℛ}}^5$ terms where each of these two links are given by a derivative of the Green function. Starting with appropriate auxiliary diagrams, we show that these graphs can be expressed in terms of those which do not involve any derivatives. This results in considerable simplification of the amplitude.

The rare decays $B_{s,d}^{\ast }\to l^{+}{l}^{-}$ are important to probe the flavor sector of the standard model and to search new physics beyond the SM. Unlike pseudoscalar $B$ meson, the leptonic decays of vector $B_{s,d}^{\ast }$ mesons are not chirally suppressed which compensates for their short lifetimes, and results in significant branching ratios. In this paper, we estimate the branching ratios of $B_{s,d}^{\ast }\to l^{+}{l}^{-}$$(l=e,\mu )$ rare decays in $Z^{\prime }$ model which is an extension of the SM with an extra $U{(1)}^{\prime }$ gauge symmetry. We find that the branching ratios are increased from their corresponding standard model values and vary with the mass of $Z^{\prime }$ boson. The lower the mass of $Z^{\prime }$ boson, the higher is the branching ratio.

Yang–Mills theories supplemented by an additional coordinate constraint, which is solved and substituted in the original Lagrangian, provide examples of the so-called Nambu models, in the case where such constraints arise from spontaneous Lorentz symmetry breaking. Some explicit calculations have shown that, after additional conditions are imposed, Nambu models are capable of reproducing the original gauge theories, thus making Lorentz violation unobservable and allowing the interpretation of the corresponding massless gauge bosons as the Goldstone bosons arising from the spontaneous symmetry breaking. A natural question posed by this approach in the realm of gauge theories is to determine under which conditions the recovery of an arbitrary gauge theory from the corresponding Nambu model, defined by a general constraint over the coordinates, becomes possible. We refer to these theories as extended Nambu models (ENM) and emphasize the fact that the defining coordinate constraint is not treated as a standard gauge fixing term. At this level, the mechanism for generating the constraint is irrelevant and the case of spontaneous Lorentz symmetry breaking is taken only as a motivation, which naturally bring this problem under consideration. Using a nonperturbative Hamiltonian analysis we prove that the ENM yields the original gauge theory after we demand current conservation for all time, together with the imposition of the Gauss laws constraints as initial conditions upon the dynamics of the ENM. The Nambu models yielding electrodynamics, Yang–Mills theories and linearized gravity are particular examples of our general approach.

We introduce a finite off-shell hypermultiplet with no off-shell central charge. This requires 192+192 degrees of freedom, all but 8+8 of which are auxiliary or gauge. In the absence of supergravity, the model has a saddle-point vacuum instability implying ghost-like propagators. These are cured by realizing the model superconformally, such that the erstwhile ghosts are realized as compensators. Gauge fixing these links the physical hypermultiplets to supergravity. This evokes the prospect of realizing ${𝒩}=4$ super-Yang–Mills theory off-shell.