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The ratio of the radiation energy of the plasma relativistic microwave amplifier at a frequency of the input signal to the noise energy is measured. In the absence of the input signal, the noise radiation is detected. It is shown that, under certain conditions, the amplifier noise energy sharply decreases, as compared to the noise energy measured in the absence of the input signal.

A supersonic flow around a cylinder with a flat end is studied for the case in which a single particle is launched from the end surface towards the incident flow. Numerical modeling is carried out with allowance for the gas-dynamic interaction of the particle with the shock layer. High-resolution, adaptive, rectangular grids are used to obtain detailed spatial and temporal flow patterns. The calculation results are compared with experimental data. It is shown that a single, large particle moving along the axis of symmetry against the incident flow and crossing the bow shock wave causes a significant restructuring of the flow and the formation of complex shock-wave and vortex structures. An important role in the formation of the flow structure is played by the formation of a toroidal vortex, which leads to an “inviscid” separation of the incident flow from the axis of symmetry. A distinctive feature of the flow around a cylinder with a flat end is the oscillatory flow regime caused by alternating stages of growth and decay of a toroidal vortex. The patterns of the oscillating flow and the oscillation frequency obtained via numerical simulation are in good agreement with the experimental data.

The paper is devoted to the numerical study of the shock layer–particle interaction in a supersonic flow past bodies. The shock wave associated with a moving particle and the flow in the particle wake is taken into consideration. Distinctive features of the numerical technique are the adaptive Cartesian sliding grids of high resolution, a ghost cells immersed boundary method for the realization of conditions on curvilinear boundaries, parallelization of computations on graphics processors. The results of computational experiment obtained allows to study the characteristic shock and vortex structures formed during the passage of a particle rebounding from the surface through the bow shock wave. The oscillations observed for a flow over the fla-tended cylinder are discussed.

We develop methods to study the scalar sector of multi-Higgs models with large discrete symmetry groups that are softly broken. While in the exact symmetry limit, the model has very few parameters and can be studied analytically, proliferation of quadratic couplings in the most general softly broken case makes the analysis cumbersome. We identify two sets of soft breaking terms which play different roles: those which preserve the symmetric vacuum expectation value alignment, and the remaining terms which shift it. Focusing on alignment preserving terms, we check which structural features of the symmetric parent model are conserved and which are modified. We find remarkable examples of structural features which are inherited from the parent symmetric model and which persist even when no exact symmetry is left. The general procedure is illustrated with the example of the three-Higgs-doublet model with the softly broken symmetry group $\Sigma(36)$., Comment: 21 pages, 2 figures; v2: minor updates, extra appendix and references, matches the published version

The paper aims to study an issues of ensuring the safe launch of spacecraft. The phenomenon of a turbulent boundary layer separation from the rocket engine nozzle wall intensively studied in the overexpansion mode that is realized at the start–up stage at sea–level. The emergence of the so–called restricted flow separation leads to the side loads on the wall of the nozzle, the shaking the structure and the destruction of the engine. The conditions for the realization of restricted or free shock separation in the thrust–optimized contoured nozzles of rocket engines are studied numerically. A hysteresis was obtained when changing the type of separation at the stages of increasing and decreasing pressure at the nozzle inlet with the same ambient pressure. Two ways of controlling the transition from one type of separation to another one were studied numerically. The numerical simulation of separated flow in a model nozzle with a back cone shaped extension was carried out. The numerical results confirm that in the nozzle with the large back cone the restricted shock separation can arise and this result appears to be in agreement with experiment.

The spectral composition of high-power microwave radiation of the plasma relativistic oscillator in the range of 1−4 GHz was studied experimentally as a function of the plasma density and the cavity length and configuration. The duration of the generated microwave pulse was in the range of 250−400 ns, and its total energy reached 15 J. The main features of the spectral composition for different cavity configurations are explained.

Transonic and supersonic flows near the cones were visualized with two types of discharges. Pulse volume discharge with pre-ionization by plasma sheets was used to visualize the flow with discontinuities with time resolution in nanosecond range. Bow shock angle was measured by the glow images, and flow Mach number calculated. Stationary discharge was used to record a non-changing weakly luminous micro-nozzle jet flow spreading into a high vacuum region and interacting with a conical metal skimmer, which is essential for gas cluster equipment. A fusiform Mach barrel at the nozzle exit and the bow shock at the obstacle are visualized. The computational flow dynamics model based on Navier–Stokes–Fourier equations was verified by comparison with the experimental images and showed a good agreement with the observed flow structure.

The characteristics of a distributed sliding surface discharge with a duration of ∼300 ns (plasma sheet) in a non-uniform supersonic airflow with a vortex zone behind a thin wedge have been studied in a shock tube channel. The spatial distribution of the discharge radiation, the spectra of the discharge radiation, and the discharge current are analyzed in the flows behind plane shock waves with Mach numbers 2.4–3.5 (the Mach numbers of flows are 1.16–1.47 and the density is 0.02–0.20 kg/m3). It is shown that in an airflow with a vortex zone, the surface discharge develops as a 1–3-mm-wide channel located in the region of low density behind the wedge. The calculated electron concentration in the discharge channel is an order of magnitude higher than the electron concentration when a discharge is initiated in a homogeneous medium.

Collisions of twisted particles --- that is, non-plane-wave states of photons, electrons, or any other particle, equipped with a non-zero orbital angular momentum (OAM) with respect to its propagation direction --- offer novel ways to probe particle structure and interactions. In the recent paper \cite{Ivanov:2019vxe}, we argued that resonance production in twisted photon collisions or twisted $e^+e^-$ annihilation gives access to parity- and spin-sensitive observables in inclusive cross sections, even when the initial particles are unpolarized. Here, we explore these features in detail, providing a qualitative picture and illustrating it with numerical examples. We show how one can detect parity-violating effects in collisions of unpolarized twisted photons and how one can produce almost $100\%$ polarized vector mesons in unpolarized twisted $e^+e^-$ annihilation. These examples highlight the unprecedented level of control over polarization offered by twisted particles, impossible in the usual plane wave collisions., 26 pages, 9 figures

The scalar potential of the $N$-Higgs-doublet model (NHDM) is best analyzed not in the space of $N$ complex doublets ${\ensuremath{\phi}}_{a}$ but in the ${N}^{2}$-dimensional space of real-valued bilinears constructed of ${\ensuremath{\phi}}_{a}^{\ifmmode\dagger\else\textdagger\fi{}}{\ensuremath{\phi}}_{b}$. In particular, many insights have been gained into $CP$ violation in the 2HDM and 3HDM by studying how generalized $CP$ transformations (GCPs) act in this bilinear space. These insights relied on the fact that GCPs, which involved an odd number of mirror reflection, could be clearly distinguished from Higgs family transformations by the sign of the determinant of the transformation matrix. It was recently pointed out that this criterion fails starting from 4HDM, where the reflection/rotation dichotomy does not exist anymore. In this paper, we restore intuition by finding a different quantity which faithfully discriminates between GCPs and Higgs family transformations in the bilinear space for any number of Higgs doublets. We also establish the necessary and sufficient conditions for an orthogonal transformation in the bilinear space to represent a viable transformation back in the space of $N$ doublets, which is helpful if one prefers to build an NHDM directly in the bilinear space.

The results of numerical modeling of the gasdynamic interaction between a highly inertial particle and the shock layer are presented. The evolution of the shock-wave and vortex flow pattern that appears when a particle reflected from a streamlined surface passes through a shock wave is analyzed. It is shown that an essential part is played in the formation of a wave flow pattern by a toroidal vortex, which results in the “nonviscous” detachment of the near-axis incident flow from the symmetry axis and its further interaction with the outer flow and the body surface. It is indicated that an intensive pressure wave passes along the streamlined surface, thus creating the conditions for the intensification of convective heat transfer.

Generalized CP symmetry of order 4 (CP4) is surprisingly powerful in shaping scalar and quark sectors of multi-Higgs models. Here, we extend this framework to the neutrino sector. We build two simple Majorana neutrino mass models with unbroken CP4, which are analogous to Ma's scotogenic model. Both models use three Higgs doublets and two or three right-handed (RH) neutrinos. The minimal CP4 symmetric scotogenic model uses only two RH neutrinos, leads to three non-zero light neutrino masses, and contains a built-in mechanism to further suppress them via phase alignment. With three RH neutrinos, one generates a type I seesaw mass matrix of rank 1, which is then corrected by the same scotogenic mechanism, naturally leading to two neutrino mass scales with mild hierarchy. These minimal CP4-based constructions emerge as a primer for introducing additional symmetry structures and exploring their phenomenological consequences., Comment: 13 pages, 2 figures; v2: extra comments and references, matches the published version

We present the results of experimental studies of THz emission from argon cluster nanoplasma switched by the intense femtosecond laser pulses. We have performed measurements of the angular distribution of the THz radiation emitted from the clustered nanoplasma with two polarization states of the linearly polarized laser beam. We have found that polarization state of the laser radiation may influence on the efficiency of THz generation in cluster beam.

Establishing if multi-Higgs potentials are bounded from below (BFB) can be rather challenging, and it may impede efficient investigation of all phenomenological consequences of such models. In this paper, we find the necessary and sufficient BFB conditions for the three-Higgs-doublet model (3HDM) with the global symmetry group $U(1)\ifmmode\times\else\texttimes\fi{}U(1)$. We observed an important role played by charge-breaking directions in the Higgs space, even for situations when a good-looking neutral minimum exists. This remark is not limited to the particular model we consider but represents a rather general feature of elaborate multi-Higgs potentials which must be carefully dealt with. Also, applying this method to Weinberg's model (the ${\mathbb{Z}}_{2}\ifmmode\times\else\texttimes\fi{}{\mathbb{Z}}_{2}$ symmetric 3HDM) turned out to be more challenging than was believed in the literature. In particular, we have found that the approach taken in a paper from 2009 does not lead to the necessary and sufficient BFB conditions for this case.

We present a detailed experimental and theoretical study on the relativistic non-dipole effects in strong-field atomic ionisation by near-infrared linearly-polarised few-cycle laser pulses in the intensity range 1014 -1015 W/cm2. We record high-resolution photoelectron momentum distributions of argon using a reaction microscope and compare our measurements with a truly ab-initio fully relativistic 3D model based on the time-dependent Dirac equation. We observe counter-intuitive peak shifts of the transverse electron momentum distribution in the direction opposite to that of laser propagation as a function of laser intensity and demonstrate an excellent agreement between experimental results and theoretical predictions., 6 pages, 6 figures, Manuscript prepared for submission to Physical Review Letters

Physical observables cannot depend on the basis one chooses to describe fields. Therefore, all physically relevant properties of a model are, in principle, expressible in terms of basis-invariant combinations of the parameters. However, in many cases it becomes prohibitively difficult to establish key physical features exclusively in terms of basis invariants. Here, we advocate an alternative route in such cases: the formulation of basis-invariant statements in terms of basis-covariant objects. We give several examples where the basis-covariant path is superior to the traditional approach in terms of basis invariants. In particular, this includes the formulation of necessary and sufficient basis-invariant conditions for various physically distinct forms of CP conservation in two- and three-Higgs-doublet models., Comment: 20 pages, no figures

Three-Higgs-doublet models (3HDM) allow for a novel, physically distinct form of CP invariance: CP symmetry of order 4 (CP4). Due to the large basis change freedom in 3HDM, it is imperative to recognize the presence of a possibly hidden CP4 in a basis-invariant way. In the present work, we solve this problem and establish basis-invariant necessary and sufficient conditions for a 3HDM to possess a CP4 symmetry. We also derive a basis-invariant criterion to decide whether or not a CP4 symmetric 3HDM possesses any additional CP symmetry, as well as a criterion to decide whether or not CP4 is spontaneously broken.

"Twisted particles" refer to non-plane-wave states of photons, electrons, hadrons, or any other particle which carry non-zero, adjustable orbital angular momentum with respect to their average propagation direction. Twisted photons and electrons have already been experimentally demonstrated, and one can expect creation of twisted states of other particles in future. Such states can be brought in collisions, offering a completely new degree of freedom in collider experiments and, especially, a novel tool for hadronic physics. We recently showed that $2 \to 1$ processes with two twisted particles such as resonance production in twisted $e^+e^-$ annihilation give access to observables which are difficult or impossible to probe in the usual plane-wave collisions. In this paper, we discuss in detail surprising kinematic features of this process, focusing on spinless particle annihilation. They include (1) a new dimension in the final momentum space available in twisted annihilation, (2) interference fringes emerging in the cross section as a function of the total energy, and (3) the built-in mass spectrometric capability of this process, that is, simultaneous production and automatic angular separation of several resonances with different masses in monochromatic twisted particle annihilation experiment running at fixed energy. All these features cannot be obtained in the usual plane wave collision setting., Comment: 14 pages, 8 figures

Twisted, or vortex, particles refer to freely propagating non-plane-wave states with helicoidal wave fronts. In this state, the particle possesses a non-zero orbital angular momentum with respect to its average propagation direction. Twisted photons and electrons have been experimentally demonstrated, and creation of other particles in twisted states can be anticipated. If brought in collisions, twisted states offer a new degree of freedom to particle physics, and it is timely to analyze what new insights may follow. Here, we theoretically investigate resonance production in twisted photon collisions and twisted $e^+e^-$ annihilation and show that these processes emerge as a completely novel probe of spin and parity-sensitive observables in fully inclusive cross sections with unpolarized initial particles. This is possible because the initial state with a non-zero angular momentum explicitly breaks the left-right symmetry even when averaging over helicities. In particular, we show how one can produce almost $100\%$ polarized vector mesons in unpolarized twisted $e^+e^-$ annihilation and how to control its polarization state., Comment: 5+2 pages, 2 figures

In this chapter we introduce our recent studies on terahertz (THz) and X-ray emission generated in an Ar gas-cluster jet under irradiation with high-intense ultrashort laser pulses. We carried out a numerical simulation of cluster formation processes in a supersonic jet produced under adiabatic extension of gaseous Ar into vacuum, which shows that the concentration ratios among non-clusterized Ar monomers, small-size Ar clusters and large-size Ar clusters significantly vary along the jet. The chapter presents experimental results on THz and X-ray emission generated by irradiation of the jet with intense femtosecond laser pulses at various downstream distances measured from the nozzle outlet along the axis of symmetry of the nozzle. It is shown that THz and X-ray emission from the jet is a useful tool for the study of clustering dynamics in the course of free expansion of gas through a nozzle into vacuum.

Higgs doublets may come in three generations. The scalar sector of the resulting three-Higgs-doublet model (3HDM) may be constrained by global symmetry groups $G$ leading to characteristic phenomenology. There exists the full list of symmetry groups $G$ realizable in the 3HDM scalar sector and the expressions for $G$-symmetric scalar potentials written in special bases where the generators of $G$ take simple form. However recognizing the presence of a symmetry in a generic basis remains a major technical challenge, which impedes efficient exploration of the 3HDM parameter space. In this paper, we solve this problem using the recently proposed approach, in which basis-independent conditions are formulated as relations among basis-covariant objects. We develop the formalism and derive basis-independent necessary and sufficient conditions for the 3HDM scalar sector to be invariant under each of the realizable symmetry group. We also comment on phenomenological consequences of these results., Comment: 39 pages

Multi-Higgs models equipped with global symmetries produce scalar dark matter (DM) candidates stabilized by the unbroken symmetry. It is remarkable that a conserved CP symmetry can also stabilize DM candidates, provided it is a CP symmetry of order higher than two. CP4 3HDM, the three-Higgs-doublet model with CP symmetry of order 4, is the simplest example of this kind. It contains two mass-degenerate scalar DM candidates $\varphi$ and $\bar\varphi$, each of them being a CP4 eigenstate and, therefore, its own antiparticle. A novel phenomenological feature of this model is the presence of $\varphi\varphi \leftrightarrow \bar\varphi\bar\varphi$ conversion process, which conserves CP. It offers a rare example of DM models in which self-interaction in the dark sector can significantly affect cosmological and astrophysical observables. Here, we explore the thermal evolution of these DM species in the asymmetric regime. We assume that a mechanism external to CP4 3HDM produces an initial imbalance of the densities of $\varphi$ and $\bar\varphi$. As the Universe cools down, we track the evolution of the asymmetry through different stages, and determine how the final asymmetry depends on the interplay between the conversion and annihilation $\varphi\bar\varphi \to $ SM and on the initial conditions. We begin with the analytic treatment of Boltzmann equations, present a detailed qualitative description of the process, and then corroborate it with numerical results obtained using a dedicated computer code. Finally, we check if the model can produce an observable indirect detection signal., Comment: 17 pages, 4 figures

An experimental study was made of the dynamics of shock waves developing from a localized channel of a nanosecond surface sliding discharge in supersonic airflows past a wedge in the shock tube with Mach numbers 1.16–1.47 at a density of 0.02 kg/m3–0.20 kg/m3. It is shown that a semi-cylindrical explosive-type shock wave is formed from the discharge channel in the low-density vortex zone behind the wedge. Numerical simulations of the resulting gas-dynamic flow are performed. The shadowgraphy images of shock waves after the discharge were compared with simulations of the flow field after an energy input near the wall in a supersonic flow. The energy input region in the simulations was set in accordance with the experimental results on the geometry of the discharge channel. Based on a comparison of experimental results and simulations, it is shown that shock waves are formed when a heat energy of 0.07 ± 0.04 J is released in the channel of localized discharge in supersonic airflows.

Checking that a scalar potential is bounded from below (BFB) is an ubiquitous and notoriously difficult task in many models with extended scalar sectors. Exact analytic BFB conditions are known only in simple cases. In this work, we present a novel approach to algorithmically establish the BFB conditions for any polynomial scalar potential. The method relies on elements of multivariate algebra, in particular, on resultants and on the spectral theory of tensors, which is being developed by the mathematical community. We give first a pedagogical introduction to this approach, illustrate it with elementary examples, and then present the working Mathematica implementation publicly available at GitHub. Due to the rapidly increasing complexity of the problem, we have not yet produced ready-to-use analytical BFB conditions for new multi-scalar cases. But we are confident that the present implementation can be dramatically improved and may eventually lead to such results., 27 pages, 2 figures; v2: added references

When building CP-symmetric models beyond the Standard Model, one can impose CP-symmetry of higher order. This means that one needs to apply the CP-transformation more than two times to get the identity transformation, but still the model is perfectly CP-conserving. A multi-Higgs-doublet model based on CP-symmetry of order 4, dubbed CP4, was recently proposed and its phenomenology is being explored. Here, we show that the construction does not stop at CP4. We build examples of renormalizable multi-Higgs-doublet potentials which are symmetric under CP8 or CP16, without leading to any accidental symmetry. If the vacuum conserves CP-symmetry of order 2k, then the neutral scalars become CP-eigenstates, which are characterized not by CP-parities but by CP-charges defined modulo 2k. One or more lightest states can be the dark matter candidates, which are protected against decay not by the internal symmetry but by the exotic CP. We briefly discuss their mass spectra and interaction patterns for CP8 and CP16., 13 pages; v2: extra comments and references; v3: extra clarifications, matches published version

The inert Two Higgs Doublet Model (i2HDM) is a theoretically well-motivated example of a minimal consistent Dark Matter(DM) model which provides mono-jet, mono-Z, mono-Higgs and Vector-Boson-Fusion+Missing Transverse Momentum signatures at the LHC, complemented by signals in direct and indirect DM search experiments. In this paper we have performed a detailed analysis of the constraints in the full 5D parameter space of the i2HDM, coming from perturbativity, unitarity, electroweak precision data, Higgs data from LHC, DM relic density, direct/indirect DM detection and LHC mono-jet analysis, as well as implications of experimental LHC studies on disappearing charged tracks relevant to high DM mass region. We demonstrate the complementarity of the above constraints and present projections for future LHC data and direct DM detection experiments to probe further i2HDM parameter space. The model is implemented into the CalcHEP and micrOMEGAs packages, which are publicly available at the HEPMDB database, and is ready for a further exploration in the context of the LHC, relic density and DM direct detection., Comment: Text and figures are updated to be consistent with with published PRD version

Both classical and quantum waves can form vortices: with helical phase fronts and azimuthal current densities. These features determine the intrinsic orbital angular momentum carried by localized vortex states. In the past 25 years, optical vortex beams have become an inherent part of modern optics, with many remarkable achievements and applications. In the past decade, it has been realized and demonstrated that such vortex beams or wavepackets can also appear in free electron waves, in particular, in electron microscopy. Interest in free-electron vortex states quickly spread over different areas of physics: from basic aspects of quantum mechanics, via applications for fine probing of matter (including individual atoms), to high-energy particle collision and radiation processes. Here we provide a comprehensive review of theoretical and experimental studies in this emerging field of research. We describe the main properties of electron vortex states, experimental achievements and possible applications within transmission electron microscopy, as well as the possible role of vortex electrons in relativistic and high-energy processes. We aim to provide a balanced description including a pedagogical introduction, solid theoretical basis, and a wide range of practical details. Special attention is paid to translate theoretical insights into suggestions for future experiments, in electron microscopy and beyond, in any situation where free electrons occur., 87 pages, 34 figures

We explore the origin and Yukawa interactions of the scalars with peculiar $CP$ properties which were recently found in a multi-Higgs model based on an order-4 $CP$ symmetry. We relate the existence of such scalars to the enhanced freedom of defining $CP$, even beyond the well-known generalized $CP$ symmetries, which arises in models with several zero-charge scalar fields. We also show that despite possessing exotic $CP$ quantum numbers, these scalars do not have to be inert: they can have $CP$-conserving Yukawa interactions provided the $CP$ acts on fermions by also mixing generations. This paper focuses on formal aspects---exposed in a pedagogical manner---and includes a brief discussion of possible phenomenological consequences.

Models with non-minimal Higgs sectors represent a mainstream direction in theoretical exploration of physics opportunities beyond the Standard Model. Extended scalar sectors help alleviate difficulties of the Standard Model and lead to a rich spectrum of characteristic collider signatures and astroparticle consequences. In this review, we introduce the reader to the world of extended Higgs sectors. Not pretending to exhaustively cover the entire body of literature, we walk through a selection of the most popular examples: the two- and multi-Higgs-doublet models, as well as singlet and triplet extensions. We will show how one typically builds models with extended Higgs sectors, describe the main goals and the challenges which arise on the way, and mention some methods to overcome them. We will also describe how such models can be tested, what are the key observables one focuses on, and illustrate the general strategy with a subjective selection of results., 57 pages + 32-page bibliography, 10 figures; invited review for Progress in Particle and Nuclear Physics; v2: minor modifications, extra references

An experimental study of the plasma-gas dynamic fluid formed after pulse ionization of the gas flow with a plane shock wave with Mach number 2.2–4.8 is carried out. Nanosecond volume discharge with UV preionization was switched on when the shock moved in a tube channel test section. Energy input occurs in the low-pressure gas volume separated by the shock surface within a time less than 200–300 ns; a single shock wave breaks into three discontinuities in accordance with the 1D Riemann problem solution. The initial (plasma-dynamic) stage of the flow in the nanosecond time range is visualized by glow recording; the supersonic gas processes in the microsecond time range are recorded using high-speed shadow imaging. Quantitative information about the dynamics of the shocks and contact surface (plots of horizontal distance) was obtained within time up to 25 µs. A region with an increased gas-discharge plasma glow intensity, after the discharge electric current termination, was recorded in the time interval from 0.3 to 1.5 µs; it was explained by a jump in gas temperature and density between the new shock wave and the contact discontinuity.An experimental study of the plasma-gas dynamic fluid formed after pulse ionization of the gas flow with a plane shock wave with Mach number 2.2–4.8 is carried out. Nanosecond volume discharge with UV preionization was switched on when the shock moved in a tube channel test section. Energy input occurs in the low-pressure gas volume separated by the shock surface within a time less than 200–300 ns; a single shock wave breaks into three discontinuities in accordance with the 1D Riemann problem solution. The initial (plasma-dynamic) stage of the flow in the nanosecond time range is visualized by glow recording; the supersonic gas processes in the microsecond time range are recorded using high-speed shadow imaging. Quantitative information about the dynamics of the shocks and contact surface (plots of horizontal distance) was obtained within time up to 25 µs. A region with an increased gas-discharge plasma glow intensity, after the discharge electric current termination, was recorded in the time interval fro...

Vortex electron beams are freely propagating electron waves carrying adjustable orbital angular momentum with respect to the propagation direction. Such beams were experimentally realized just a few years ago and are now used to probe various electromagnetic processes. So far, these experiments used the single vortex electron beams, either propagating in external fields or impacting a target. Here, we investigate the elastic scattering of two such aligned vortex electron beams and demonstrate that this process allows one to experimentally measure features which are impossible to detect in the usual plane-wave scattering. The scattering amplitude of this process is well approximated by two plane-wave scattering amplitudes with different momentum transfers, which interfere and give direct experimental access to the Coulomb phase. This phase (shift) affects the scattering of all charged particles and has thus received significant theoretical attention but was never probed experimentally. We show that a properly defined azimuthal asymmetry, which has no counterpart in plane-wave scattering, allows one to directly measure the Coulomb phase as function of the scattering angle., 23 pages, 6 figures

Models beyond the Standard Model (bSM) often involve elaborate Higgs sectors, which can be a source of CP-violation. It brings up the question of recognizing in an efficient way whether a model is CP-violating. There is a diffuse belief that the issue of explicit CP invariance can be linked to the existence of a basis in which all coefficients are real; with even a theorem proposed a decade ago claiming that the scalar sector of any multi-Higgs doublet model is explicitly CP-conserving if and only if all of its coefficients can be made real by a basis change. This is compounded by the fact that in all specific multi Higgs models considered so far, the calculations complied with this claim. Here, we present the first counterexample to this statement: a CP-conserving three-Higgs-doublet model for which no real basis exists. We outline the phenomenological consequences of this model, and notice that the extra neutral Higgs bosons are neither CP-even nor CP-odd but are "half-odd" under the generalized CP-symmetry of the model.

We present the detailed quantum electrodynamical description of Vavilov-Cherenkov radiation emitted by a relativistic twisted electron in the transparent medium. Simple expressions for the spectral and spectral-angular distributions as well as for the polarization properties of the emitted radiation are obtained. Unlike the plane-wave case, the twisted electron produces radiation within the annular angular region, with enhancement towards its boundaries. Additionally, the emitted photons can have linear polarization not only in the scattering plane but also in the orthogonal direction. We find that the Vavilov-Cherenkov radiation emitted by an electron in a superposition of two vortex states exhibits a strong azimuthal asymmetry. Thus, the Vavilov-Cherenkov radiation offers itself as a convenient diagnostic tool of such electrons and complements the traditional microscopic imaging., 23 pages, 6 figures

It has been known since decades that imposing a symmetry group G on the scalar sector of multi-Higgs-doublet models has consequences for CP-violation. In all examples of two- and three-Higgs-doublet models equipped with symmetries, one observes the following intriguing property: if G prevents explicit CP-violation (CPV), at least in the neutral Higgs sector, then it also prevents spontaneous CPV, and if G allows explicit CPV, then it allows for spontaneous CPV. One is led to conjecture that this is a general phenomenon. In this paper, we prove this conjecture for any rephasing symmetry group G and any number of doublets., 16 pages

Young's classic double-slit experiment demonstrates the reality of interference when waves and particles travel simultaneously along two different spatial paths. Here, we propose a double-slit experiment in momentum space, realized in the free-space elastic scattering of vortex electrons. We show that this process proceeds along two paths in momentum space, which are well localized and well separated from each other. For such vortex beams, the (plane-wave) amplitudes along the two paths acquire adjustable phase shifts and produce interference fringes in the final angular distribution. We argue that this experiment can be realized with the present day technology. We show that it gives experimental access to the Coulomb phase, a quantity which plays an important role in all charged particle scattering but which usual scattering experiments are insensitive to., Comment: 6 pages, 3 figures; v2: extra clarifications, matches published versoin

Temporal characterization of a laser pulse is an essential task in many applications. Temporal characterization methods that are currently available support only a limited spectral bandwidth without information on the carrier-envelope phase (CEP) of the laser pulse or require complicated equipment in a vacuum environment. Here we demonstrate that an arbitrary time-dependent laser field can be directly sampled using subcycle tunneling ionization in a gaseous medium or in air. The subcycle ionization is used as a fast temporal gate for the direct sampling of the laser field. This unique approach enables the complete temporal characterization of the laser field, including its CEP, for a broad spectral range in ambient air, providing a universal tool for the precise measurement of the laser field.

The numerical simulation of influence of viscous effects on flow inside shock tubes are described. We considered three shock tubes with different length-to-height ratio: 1 to 1 (Daru & Tenaud [6] case), 8 to 1 (extended Daru & Tenaud case) and 116 to 1 (HAST-1 [10-11] tube). Some details of different stages of the shock wave-boundary layer interaction are discussed.

We establish the full list of flavour symmetry groups which may be enforced, without producing any further accidental symmetry, on the Yukawa-coupling matrices of an SO(10) Grand Unified Theory with arbitrary numbers of scalar multiplets in the 10, $\overline{126}$, and 120 representations of SO(10). For each of the possible discrete non-Abelian symmetry groups, we present examples of minimal models which do not run into obvious contradiction with the phenomenological fermion masses and mixings., Comment: 22 pages

Extended Higgs sectors offer rich opportunities for various forms of CP-violation. Here, we describe a new form of CP-conservation and discuss its consequences. We give a concrete example of a three-Higgs-doublet model dubbed CP4-3HDM with a CP-symmetry of order 4 and no other other accidental symmetries. If the vacuum conserves this symmetry, the model is CP-conserving with pairwise mass-degenerate extra neutral Higgs bosons. These fields cannot be classified as CP-even or CP-odd but they can be combined into complex physical fields which are CP-half-odd, that is, they pick up the $i$ factor upon CP transformation. These CP-half-odd scalars can be Yukawa-coupled to the fermion bilinears in a CP-conserving way. We discuss fundamental and phenomenological features of the model, and stress a peculiar clash between the CP-symmetry and any convention for the particle-antiparticle assignment., Comment: 9 pages, talk given at DISCRETE 2016: Fifth Symposium on Prospects in the Physics of Discrete Symmetries, Warsaw, 28 Nov - 3 Dec, 2016

In the two Higgs doublet model, there is the possibility that the vacuum where the universe resides in is metastable. We present the tree-level bounds on the scalar potential parameters which have to be obeyed to prevent that situation. Analytical expressions for those bounds are shown for the most used potential, that with a softly broken $Z_2$ symmetry. The impact of those bounds on the model's phenomenology is discussed in detail, as well as the importance of the current Large Hadron Collider results in determining whether the vacuum we live in is or is not stable. We demonstrate how the vacuum stability bounds can be obtained for the most generic CP-conserving potential, and provide a simple method to implement them., Comment: 22 pages, 10 figures

In the Standard Model, the Higgs potential allows only one minimum at tree-level. But the open possibility that there might be two scalar doublets enriches the vacuum structure, allowing for the risk that we might now be in a metastable state, which we dub the panic vacuum. Current experiments at the LHC are probing the Higgs particle predicted as a result of the spontaneous symmetry breaking. Remarkably, in the two Higgs model with a softly broken U(1) symmetry, the LHC experiments already preclude panic vacuum solutions., 7 pages, 4 figures

We explore the phenomenology of a unique three-Higgs-doublet model based on the single CP symmetry of order 4 (CP4) without any accidental symmetries. The CP4 symmetry is imposed on the scalar potential and Yukawa interactions, strongly shaping both sectors of the model and leading to a very characteristic phenomenology. The scalar sector is analyzed in detail, and in the Yukawa sector we list all possible CP4-symmetric structures which do not run into immediate conflict with experiment, namely, do not lead to massless or mass-degenerate quarks nor to insufficient mixing or CP-violation in the CKM matrix. We show that the parameter space of the model, although very constrained by CP4, is large enough to comply with the electroweak precision data and the LHC results for the 125 GeV Higgs boson phenomenology, as well as to perfectly reproduce all fermion masses, mixing, and CP violation. Despite the presence of flavor changing neutral currents mediated by heavy Higgs scalars, we find through a parameter space scan many points which accurately reproduce the kaon CP-violating parameter $\epsilon_K$ as well as oscillation parameters in K and $B_{(s)}$ mesons. Thus, CP4 offers a novel minimalistic framework for building models with very few assumptions, sufficient predictive power, and rich phenomenology yet to be explored., Comment: 39 pages, 8 figures, 1 table; v2: expanded discussion, extra references, matches published version