Weak Magnetic Dipole Moments Research Articles

Research on the electromagnetic and weak dipole moments of the tau-lepton at the Bestest Little Higgs Model

In this paper, using the Bestest Little Higgs Model (BLHM) we calculate at the one-loop level the contributions to the Anomalous Magnetic Dipole Moment (AMDM) and Anomalous Weak Magnetic Dipole Moment (AWMDM) of the tau-lepton. The implications from this model are studied, emphasizing the contributions of the new physics induced by the new scalar and vector bosons of the BLHM: S_i=H_0, A_0, phi ^{0}, eta ^{0}, sigma , H^{pm }, phi ^{pm }, eta ^{pm }, and V_i=Z', W'^{pm } because these quantify the new physics. With these new contributions, we estimated bounds on the real and imaginary parts of the AMDM and AWMDM of the tau-lepton. Our study complements other one-loop-level research performed on models beyond the Standard Model.

Weak dipole moments of heavy fermions with flavor violation induced by Z′ gauge bosons

A calculation of weak dipole moments of charged fermions of the standard model at the one-loop level, in the context of a general effective extended neutral current model with flavor changing vertices, is presented. We give numerical predictions for the anomalous weak magnetic dipole moment , and the weak electric dipole moment (WEDM) , for the τ lepton and t quark. For several Z′ gauge bosons considered, we find that, for the τ lepton, the best prediction for the real part of is of the order of 10−9, while the imaginary part is four orders of magnitude below. The highest value for the WEDM, , corresponds to 10−26 e cm, for its real part, and the imaginary part is three orders of magnitude below. Moreover, we found for the top quark, that the best prediction for the real part of is of the order of 10−7 and its imaginary part is of the order of 10−11. We also found that is of the order of 10−26 e cm for its real part, and its imaginary part can be as high as 10−31 e cm.

Colossal enhancement of the magnetic dipole moment by exploiting lattice coupling in metasurfaces

An artificial magnetic response is not only intellectually intriguing but also key to multiple applications. While previously suitably structured metallic particles and high-permittivity dielectric particles have been used for this purpose, here, we highlight the possibility of exploiting lattice effects to significantly enhance an intrinsically weak magnetic dipole moment of a periodically arranged scatterer. We identify the effective magnetic dipole moment as it is modulated by the lattice and coupled to other electromagnetic multipole moments the scatterer can sustain. Besides a more abstract consideration on the base of parametrized Mie coefficients to study the theoretical upper limit, we present an actual particle that shows an enhancement of the magnetic dipole moment by 100 with respect to what is attainable as a maximal value for an isolated particle.

Magnetic properties of NH4Al(SO4)2•12H2O: Evidence of glass behavior based on proton orbitals

The polarity of the magnetic susceptibility χ(T) of ammonium alum NH4Al(SO4)2•12H2O single crystals below its ferroelectric Curie temperature TC is identified to depend only on the cooling rate. To explain this, we consider reorienting distorted NH4+ to carry weak magnetic and electric dipole moments that are linearly coupled. Slow cooling allows the dipole moments to align which enhances χ(T) to become positive. However, due to energy degeneracy of rotational motion and geometric frustration of the NH4+, faster cooling rates render the system to become an orientational glass state causing χ(T) to remain negative. Specific heat measurements also show the presence of excess entropy below a negative anomaly near 53 K. Our findings identify a possible new class of non-equilibrium system with potential applications in non-volatile memory devices.

Constraining EDM and MDM lepton dimension-five interactions in the electroweak sector

We investigate dimension-five Lorentz-violating (LV) nonminimal interactions in the electroweak sector, in connection with the possible generation of electric dipole moment (EDM), weak electric dipole moment (WEDM), magnetic dipole moment (MDM) and weak magnetic dipole moment (WMDM) for leptons. These couplings are composed of the physical fields in the Standard Model and LV tensors of ranks ranging from 1 to 4. The CPT-odd couplings do not generate EDM behavior nor provide the correct MDM signature, while the CPT-even ones are compatible with EDM and MDM behavior, being subject to improved constraining. Tau lepton experimental data is used to constrain the WEDM and WMDM couplings to the level of 10−4(GeV)−1, whereas electron MDM and EDM data are employed to improve constraints to the level of 10−10(GeV)−1 and 10−16(GeV)−1, respectively.

Controlling stability and transport of magnetic microswimmers by an external field

The interplay between external field and fluid-mediated interactions in active suspensions leads to patterns of collective motion that are poorly understood. Here, we study the hydrodynamic stability and transport of microswimmers with weak magnetic dipole moments in an external field using a kinetic theory framework. Combining linear stability analysis and non-linear 3D continuum simulations, we show that for sufficiently high activity and moderate magnetic field strengths, a homogeneous polar steady state is unstable and distinct types of splay and bend instabilities for puller and pusher swimmers emerge. The instabilities arise from the amplification of anisotropic hydrodynamic interactions due to the external alignment and lead to a partial depolarisation and a reduction of the average transport speed of the swimmers in the field direction. Interestingly, at higher field strengths the homogeneous polar state becomes stable and a transport efficiency identical to that of active particles without hydrodynamic interactions is restored.

Weak dipole moments of the tau lepton in models with an extended scalar sector

We consider renormalizable couplings of neutral $\phi$, singly $\phi^\pm$, and doubly charged $\phi^{\pm\pm}$ scalar bosons to leptons and the $Z$ gauge boson and calculate the one-loop contributions to the anomalous weak magnetic dipole moment (AWMDM) $a_\tau^W$ and the weak electric dipole moment (WEDM) $d_\tau^W$ of a charged lepton in a model independent way. The analytic expressions are presented in terms of both parametric integrals and Passarino-Veltman scalar functions. Among the new contributions, there are those arising from the vertices of the type $\phi^\pm W^\mp Z$ and $Z \phi_i\phi_j$ ($i\ne j$), along with contributions from doubly charged scalar bosons. Both $a_\tau$ and $d_\tau^W$ are evaluated in several scenarios , first in a model independent way and then within some popular models, such as two-Higgs doublet models (THDMs), multiple-Higgs doublet models and Higgs triplet models. As far as $a_\tau^W$ is concerned, its real part reaches values as high as $10^{-10}-10^{-9}$ for masses of the new scalar bosons in the 200 GeV range, whereas the imaginary part is one or two orders of magnitude below. On the other hand, the most promising scenario for a nonvanishing WEDM is offered by a $CP$-violating THDM in a scenario where the heavy neutral scalar bosons are a mixture of $CP$ eigenstates. It is found that the real part of $d_\tau^W$ is of the order of $10^{-24}$ ecm and its imaginary part can reach the $10^{-26}$ ecm level for masses of the new scalar bosons of the order of a few hundred of GeVs. Both the tau AWMDM and WEDM decrease dramatically as the scalar boson masses increase.

Anomalous magnetic and weak magnetic dipole moments of the tau lepton in the simplest little Higgs model

We obtain analytical expressions, both in terms of parametric integrals and Passarino–Veltman scalar functions, for the one-loop contributions to the anomalous weak magnetic dipole moment (AWMDM) of a charged lepton in the framework of the simplest little Higgs model (SLHM). Our results are general and can be useful to compute the weak properties of a charged lepton in other extensions of the standard model (SM). As a by-product we obtain generic contributions to the anomalous magnetic dipole moment (AMDM), which agree with previous results. We then study numerically the potential contributions from this model to the tau lepton AMDM and AWMDM for values of the parameter space consistent with current experimental data. It is found that they depend mainly on the energy scale f at which the global symmetry is broken and the t_{beta } parameter, whereas there is little sensitivity to a mild change in the values of other parameters of the model. While the tau AMDM is of the order of 10^{-9}, the real (imaginary) part of its AWMDM is of the order of 10^{-9} (10^{-10}). These values seem to be out of the reach of the expected experimental sensitivity of future experiments.

Dipole moments of charged leptons in the THDM-III with textures

We study the magnetic and weak magnetic dipole moments (WEDMs) of charged leptons in the framework of the two Higgs doublet model type III (THDM-III) with four zero textures. We first analyze the possibility that the discrepancy between the experimental measurements and the theoretical prediction of the muon anomalous magnetic dipole moment () in the standard model can be explained in the context of the model and then find values for the parameters of the THDM-III: neutral and charged Higgs boson masses (mH, ), mixing angles (), CP-violating angles and an additional parameter arising from the mass matrix (), for which a correspondence between theory and experiment is possible. We use and current low energy processes mixing, , , , , , , , and the rare higgs decay in order to determine the allowed parameter space of THDM-III and from these we give a prediction for the magnetic and WEDM of charged leptons, in particular of the τ lepton. The obtained magnetic dipole moment of the τ lepton is of the order of (10−8–10−7) and the weak magnetic is of the order (10−10–10−7).

Probing top-Z dipole moments at the LHC and ILC

We investigate the weak electric and magnetic dipole moments of top quark-Z boson interactions at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). Their vanishingly small magnitude in the Standard Model makes these couplings ideal for probing New Physics interactions and for exploring the role of top quarks in electroweak symmetry breaking. In our analysis, we consider the production of two top quarks in association with a Z boson at the LHC, and top quark pairs mediated by neutral gauge bosons at the ILC. These processes yield direct sensitivity to top quark-Z boson interactions and complement indirect constraints from electroweak precision data. Our computation is accurate to next-to-leading order in QCD, we include the full decay chain of top quarks and the Z boson, and account for theoretical uncertainties in our constraints. We find that LHC experiments will soon be able to probe weak dipole moments for the first time.

Static weak dipole moments of theτlepton via renormalizable scalar leptoquark interactions

The weak dipole moments of elementary fermions are calculated at the one-loop level in the framework of a renormalizable scalar leptoquark model that forbids baryon number violating processes and so is free from the strong constraints arising from experimental data. In this model there are two scalar leptoquarks accommodated in a $S{U}_{L}(2)\ifmmode\times\else\texttimes\fi{}{U}_{Y}(1)$ doublet: One of these leptoquarks is nonchiral and has electric charge of $5/3e$, whereas the other one is chiral and has electric charge $2/3e$. In particular, a nonchiral leptoquark contributes to the weak properties of an up fermion via a chirality-flipping term proportional to the mass of the virtual fermion, and can also induce a nonzero weak electric dipole moment provided that the leptoquark couplings are complex. The numerical analysis is focused on the weak properties of the $\ensuremath{\tau}$ lepton since they offer good prospects for experimental study. The constraints on leptoquark couplings are briefly discussed for a nonchiral leptoquark with nondiagonal couplings to the second and third fermion generations, a third-generation nonchiral leptoquark, and a third-generation chiral leptoquark. It is found that although the chirality-flipping term can enhance the weak properties of the $\ensuremath{\tau}$ lepton via the top quark contribution, such an enhancement would be offset by the strong constraints on the leptoquark couplings. So, the contribution of scalar leptoquarks to the weak magnetic dipole moment of the $\ensuremath{\tau}$ lepton are smaller than the standard model (SM) contributions but can be of similar size to those arising in some SM extensions. A nonchiral leptoquark can also give contributions to the weak electric dipole moment larger than the SM one but well below the experimental limit. We also discuss the case of the off-shell weak dipole moments and, for completeness, analyze the behavior of the $\ensuremath{\tau}$ electromagnetic properties.

MAGNETOSPHERES OF “HOT JUPITERS”: THE IMPORTANCE OF MAGNETODISKS IN SHAPING A MAGNETOSPHERIC OBSTACLE

Weak intrinsic magnetic dipole moments of tidally locked close-in giant exoplanets ( hot Jupiters ) have been shown in previous studies to be unable to provide an efficient magnetospheric protection for their expanding upper atmospheres against the stellar plasma flow, which should lead to significant non-thermal atmosphere mass loss. The present work provides a more complete view of the magnetosphere structure of hot Jupiters, based on a paraboloid magnetospheric model (PMM). Besides the intrinsic planetary magnetic dipole, the PMM considers among the main magnetic field sources also the electric current system of the magnetotail, magnetopause currents, and the ring current of a magnetodisk. Due to the outflow of ionized particles from the hydrodynamically expanding upper atmosphere, hot Jupiters may have extended magnetodisks. The magnetic field produced by magnetodisk ring currents dominates above the contribution of an intrinsic magnetic dipole of a hot Jupiter and finally determines the size and shape of the whole magnetosphere. A slower-than-the-dipole-type decrease of the magnetic field with the distance forms the essential specifics of magnetodisk-dominated magnetospheres of hot Jupiters. This results in their 40%–70% larger scales compared to those traditionally estimated by only the planetary dipole taken into account. Therefore, the formation of magnetodisks has to be included in the studies of the stellar wind plasma interaction with close-in exoplanets, as well as magnetospheric protection for planetary atmospheres against non-thermal escape due to erosion by the stellar plasma flow. Key words: planets and satellites: magnetic fields – planets and satellites: physical evolution – planets and satellites: rings – planet–star interactions

Thin accretion discs around millisecond X-ray pulsars

Millisecond X-ray pulsars have weak magnetic dipole moments of ∼1016 T m3 compared to ordinary X-ray pulsars with dipole moments of 1020 T m3. For this reason a surrounding accretion disc can extend closer to the millisecond X-ray pulsars, and thus reach a higher temperature, at which the opacity is dominated by electron scattering and radiation pressure is strong. We compute the self-similar structure of such a geometrically thin axisymmetric accretion disc with an internal dynamo. Such models produce significantly stronger torques on the neutron star than models without dynamos, and can explain the strong spin variations in some millisecond X-ray pulsars.

Magnetic Assembly of Nonmagnetic Particles into Photonic Crystal Structures

We report the rapid formation of photonic crystal structures by assembly of uniform nonmagnetic colloidal particles in ferrofluids using external magnetic fields. Magnetic manipulation of nonmagnetic particles with size down to a few hundred nanometers, suitable building blocks for producing photonic crystals with band gaps located in the visible regime, has been difficult due to their weak magnetic dipole moment. Increasing the dipole moment of magnetic holes has been limited by the instability of ferrofluids toward aggregation at high concentration or under strong magnetic field. By taking advantage of the superior stability of highly surface-charged magnetite nanocrystal-based ferrofluids, in this paper we have been able to successfully assemble 185 nm nonmagnetic polymer beads into photonic crystal structures, from 1D chains to 3D assemblies as determined by the interplay of magnetic dipole force and packing force. In a strong magnetic field with large field gradient, 3D photonic crystals with high reflectance (83%) in the visible range can be rapidly produced within several minutes, making this general strategy promising for fast creation of large-area photonic crystals using nonmagnetic particles as building blocks.

Rotational dynamics of magnetic silica spheres studied by measuring the complex magneticsusceptibility

The weak permanent magnetic dipole moment of cobalt ferrite-doped colloidal silicaspheres was increased by exposure to a saturating magnetic field. The resulting change ofthe rotational dynamics of the magnetic microspheres in a weak alternating field wasmeasured from low to high volume fraction in ethanol, using a frequency-dependentcomplex magnetic susceptibility setup. At low volume fractions, the increased dipolarattraction slows down Brownian rotation. At higher volume fractions, however, rotation isno longer slowed down as rapidly with increasing concentration, likely due to dipolarcoupling between the particles which accelerates their partial alignment with thealternating field. This explanation is supported by the unexpected finding that saltaddition accelerates particle rotation rather than slowing it down. At the highest volumefraction, colloidal crystals and glasses were formed in which only a small fraction of thespheres exhibit rotational mobility.

Search for anomalous weak dipole moments of the $\tau$ lepton

The anomalous weak dipole moments of the tau lepton are measured in a data sample collected by ALEPHfrom 1990 to 1995 corresponding to an integrated luminosity of 155pb-1. Tau leptons produced in the reaction e+e- -> tau+tau- at energies close to the Z mass are studied using their semileptonic decays to pi, rho, a_1 -> pi 2pi0 or a_1 -> 3 pi. The real and imaginary components of both the anomalous weak magnetic dipole moment and the CP-violating anomalous weak electric dipole moment, Re,mu_tau, Im,mu_tau, Re, d_tau and Im, d_tau, are measured simultaneously by means of a likelihood fit built from the full differential cross section. No evidence of new physics is found. The following bounds are obtained 95% CL: |Re, mu_tau | < 1.14 x 10-3, |Im}, mu_tau | < 2.65 x 10-3, |Re}, d_tau | < 0.91 x 10-3, and |Im, d_tau | < 2.01 x 10-3.

Effects of anomalous charged current dipole moments of the tau on the decay τ−→ντπ−π0

We analyse the process $\tau^-\to\nu_\tau\pi^-\pi^0$ allowing for anomalous weak charged current magnetic and electric dipole moment interactions and determine the effects on the differential and total decay rates. Using recent experimental data we determine the following values for the anomalous magnetic and electric dipole moment parameters, respectively: $\kappa = 0.16 \pm 0.08; and |\tilde\kappa| = 0.88^{+0.25}_{-0.35}.$

τ weak dipole moments from azimuthal asymmetries

We show that transverse and normal single- tau polarization of tau pairs produced at e + e − unpolarized collisions, at the Z peak, are sensitive to weak (magnetic and electric) dipole moments of the tau. We also show how these components of the τ polarization are accessible by measuring appropriate azimuthal asymmetries in the angular distribution of its decay products. Sensitivities of the order of 10 −18 e · cm, for the weak-electric dipole moment, and 10 −4 (10 −3), for the real (imaginary) part of the weak-magnetic dipole moment of τ, may be achieved. Compatible bounds are also presented from spin-spin correlated asymmetries.

Constraints on anomalous charged current couplings, tau neutrino mass and fourth generation mixing from tau leptonic branching fractions

We use recent experimental measurements of tau branching fractions to determine the weak charged current magnetic and electric dipole moments of the tau and the Michel parameter η with unprecedented precision. These results are then used to constrain the tau compositeness scale and the allowed parameter space for Higgs doublet models. We also present new constraints on the mass of the tau neutrino and its mixing with a fourth generation neutrino.

LEP summary on weak dipole moments of the τ lepton

LEP measurements of the weak electric dipole moment d w τ and the weak anomalous magnetic dipole moment a ω τ of the τ lepton are reviewed and the search for CP violation in the production of τ pairs on the Z peak is discussed. Two methods of the measurement of the weak dipole moments based on the analysis of the transverse and normal components of single τ polarisation (azimuthal asymmetries) and spin-spin correlations (optimal observables) are discussed. Averaging the LEP measurements, upper limits on the weak electric dipole moment of the τ lepton are obtained.