Magnetic Dipole Form Factors Research Articles

Deuteron electromagnetic form factors and tensor polarization observables in the framework of the hard-wall AdS/QCD model

We study the electromagnetic form factors and tensor polarization observables of the deuteron in the framework of the hard-wall AdS/QCD model. We find a profile function for the bulk twist vector field, which describes the deuteron on the boundary and fix the infrared boundary cut-off of AdS space in accordance with the ground state mass of the deuteron. We obtain the deuteron charge monopole, quadrupole, and magnetic dipole form factors and tensor polarization observables from the bulk Lagrangians for the deuteron and photon field interactions. We plot the momentum transfer dependence of the form factors and tensor polarization observables and compare our numerical results with those in the soft-wall model and experimental data.

Transition form factors: γ*+p→Δ(1232) , Δ(1600)

Electroproduction form factors describing the ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Delta}}}^{+}(1232)$, ${\mathrm{\ensuremath{\Delta}}}^{+}(1600)$ transitions are computed using a fully dynamical diquark-quark approximation to the Poincar\'e-covariant three-body bound-state problem in relativistic quantum field theory. In this approach, the $\mathrm{\ensuremath{\Delta}}(1600)$ is an analogue of the Roper resonance in the nucleon sector, appearing as the simplest radial excitation of the $\mathrm{\ensuremath{\Delta}}(1232)$. Precise measurements of the ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Delta}}}^{+}(1232)$ transition already exist on $0\ensuremath{\le}{Q}^{2}\ensuremath{\lesssim}8\text{ }\text{ }{\mathrm{GeV}}^{2}$, and the calculated results compare favorably with the data outside the meson-cloud domain. The predictions for the ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Delta}}}^{+}(1600)$ magnetic dipole and electric quadrupole transition form factors are consistent with the empirical values at the real photon point, and extend to ${Q}^{2}\ensuremath{\approx}6{m}_{p}^{2}$, enabling a meaningful direct comparison with experiment once analysis of existing data is completed. In both cases, the electric quadrupole form factor is particularly sensitive to deformation of the $\mathrm{\ensuremath{\Delta}}$-baryons. Interestingly, while the ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Delta}}}^{+}(1232)$ transition form factors are larger in magnitude than those for ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Delta}}}^{+}(1600)$ in some neighborhood of the real photon point, this ordering is reversed on ${Q}^{2}\ensuremath{\gtrsim}2{m}_{p}^{2}$, suggesting that the ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Delta}}}^{+}(1600)$ transition is more localized in configuration space.

Proton's electromagnetic form factors from a non-power confinement potential

The electric-charge, and magnetic-dipole form factors of the proton are calculated from an underlying constituent quark picture of hadron structure based on a potential shaped after a cotangent function, which has the properties of being both conformally symmetric and color confining, finding adequate reproduction of a variety of related data.

$$N^*$$ N ∗ form Factors Based on a Covariant Quark Model

We discuss the results of the covariant spectator quark model for the several $\gamma^\ast N \to N^\ast$ transitions, where $N$ is the nucleon and $N^\ast$ a nucleon excitation. More specifically, we present predictions for the form factors and transition amplitudes associated with the resonances $N(1440)1/2^+$, $N(1535)1/2^-$, $N(1520)3/2^-$, $\Delta(1620)1/2^-$ and $N(1650)1/2^-$. The estimates based on valence quark degrees of freedom are compared with the available data, particularly with the recent Jefferson Lab data at low and large momentum transfer ($Q^2$). In general, the estimates are in good agreement with the empirical data for $Q^2 >2$ GeV$^2$, with a few exceptions. The results are discussed in terms of the role of the valence quarks and the meson cloud excitations for the different resonances $N^\ast$. We also review our results for the resonance $\Delta(1232)3/2^+$ and discuss the relevance of the pion cloud component for the magnetic dipole form factor and the electric and Coulomb quadrupole form factors.

Magnetic moments and electromagnetic form factors of the decuplet baryons in chiral perturbation theory

We have systematically investigated the magnetic moments and magnetic form factors of the decuplet baryons to the next-to-next-leading order in the framework of the heavy baryon chiral perturbation theory. Our calculation includes the contributions from both the intermediate decuplet and octet baryon states in the loops. We also calculate the charge and magnetic dipole form factors of the decuplet baryons. Our results may be useful to the chiral extrapolation of the lattice simulations of the decuplet electromagnetic properties.

Role of the pion electromagnetic form factor in theΔ(1232)→γ*Ntimelike transition

The $\Delta(1232) \to \gamma^\ast N$ magnetic dipole form factor ($G_M^\ast$) is described here within a new covariant model that combines the valence quark core together with the pion cloud contributions. The pion cloud term is parameterized by two terms: one connected to the pion electromagnetic form factor, the other to the photon interaction with intermediate baryon states. The model can be used in studies of pp and heavy ion collisions. In the timelike region this new model improves the results obtained with a constant form factor model fixed at its value at zero momentum transfer. At the same time, and in contrast to the Iachello model, this new model predicts a peak for the transition form factor at the expected position, i.e. at the $\rho$ mass pole. We calculate the decay of the $\Delta \to \gamma N$ transition, the Dalitz decay ($\Delta \to e^+ e^- N$), and the $\Delta$ mass distribution function. The impact of the model on dilepton spectra in pp collisions is also discussed.

Electroexcitation of theΔ(1232)32+andΔ(1600)32+in a light-front relativistic quark model

The magnetic-dipole form factor and the ratios R_EM and R_SM for the gamma* N -> Delta(1232)3/2}+ transition are predicted within light-front relativistic quark model up to photon virtuality Q2=12 GeV2. We also predict the helicity amplitudes of the gamma* N -> Delta(1600)3/2+ transition assuming the Delta(1600)3/2+ is the first radial excitation of the ground state Delta(1232)3/2+.

Formula omitted] transition in lattice QCD

We study the electromagnetic Ωcγ→Ωc⁎ transition in 2+1 flavor lattice QCD, which gives access to the dominant decay mode of Ωc⁎ baryon. The magnetic dipole and the electric quadrupole transition form factors are computed. The magnetic dipole form factor is found to be mainly determined by the strange quark and the electric quadrupole form factor to be negligibly small, in consistency with the quark model. We also evaluate the helicity amplitudes and the decay rate.

Electromagnetic transitions among octet and decuplet baryons in QCD

The magnetic dipole ${G}_{M}({Q}^{2})$, electric quadrupole ${G}_{E}({Q}^{2})$, and Coulomb quadrupole ${G}_{C}({Q}^{2})$ form factors, describing the spin-$3/2$ to spin-$1/2$ electromagnetic transitions, are investigated within the light cone QCD sum rules. The ${Q}^{2}$ dependence of these form factors, as well as ratios of electric quadrupole and Coulomb quadrupole form factors to the magnetic dipole form factors, are studied. We also compare our results on the magnetic dipole form factor with the prediction of the covariant spectator quark model.

Octet to decuplet electromagnetic transition in a relativistic quark model

We study the octet to decuplet baryon electromagnetic transitions using the covariant spectator quark model, and predict the transition magnetic dipole form factors for those involving the strange baryons. Utilizing SU(3) symmetry, the valence quark contributions are supplemented by the pion cloud dressing based on the one estimated in the $\gamma^\ast N \to \Delta$ reaction. Although the valence quark contributions are dominant in general, the pion cloud effects turn out to be very important to describe the experimental data. We also show that, other mesons besides the pion in particular the kaon, may be relevant for some reactions such as $\gamma^\ast \Sigma^+ \to \Sigma^{*+}$, based on our analysis for the radiative decay widths of the strange decuplet baryons.

Electromagnetic form factors of octet baryons in QCD

The electromagnetic form factors of octet baryons are estimated within light cone QCD sum rules method, using the most general form of the interpolating current for baryons. A comparison of our predictions on the magnetic dipole and electric form factors with the results of other approaches is performed.

Magnetic moments of the ground-stateJP=32+baryon decuplet

The magnetic moment - a function of the electric charge form factor $F_{1}(q^{2})$ and the magnetic dipole form factor $F_{2}(q^{2})$ at zero four-momentum transfer $q^{2}$-of the ground-state $J^{P}=(3/2)^{+}$ baryon decuplet magnetic moments have been studied for many years with limited success. At present, only the magnetic moment of the $\Omega^{-}$ has been accurately determined. We calculate nonperturbatively the magnetic moments of the \emph{physical baryon decuplet $J^{P}=(3/2)^{+}$} members and in particular, we obtain $\mu_{\Delta^{++}}= (+3.67 \pm 0.07) \mu_{N}$, $\mu_{\Delta^{+}}= (+1.83 \pm 0.04) \mu_{N}$, $\mu_{\Delta^{0}}= (0) \mu_{N}$, and the magnetic moments of their $U$-Spin partners in terms of $\Omega^{-}$ magnetic moment data.

Electromagnetic nucleon-to-delta transition in holographic QCD

We study nucleon-to-delta electromagnetic transition form factors and relations between them within the framework of the holographic dual model of QCD proposed by Sakai and Sugimoto. In this setup, baryons appear as topological solitons of the five-dimensional holographic gauge theory that describes a tower of mesons and their interactions. We find a relativistic extension of the nucleon-delta-vector meson interaction vertices and use these to calculate transition form factors from holographic QCD. We observe that at low momentum transfer, magnetic dipole, electric and Coulomb quadrupole form factors, and their ratios follow the patterns expected in the large ${N}_{c}$ limit. Our results at this approximation are in reasonable agreement with experiment.

D-state effects in the electromagneticNΔtransition

We consider here a manifestly covariant quark model of the nucleon and the $\ensuremath{\Delta}$, where one quark is off shell, and the other two quarks form an on-shell diquark pair. Using this model, we have shown previously that the nucleon form factors and the dominant form factor for the $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\Delta}$ transition (the magnetic dipole (M1) form factor) can be well described by nucleon and $\ensuremath{\Delta}$ wave functions with S-state components only. In this paper, we show that nonvanishing results for the small electric (E2) and Coulomb (C2) quadrupole form factors can be obtained if D-state components are added to the $\ensuremath{\Delta}$ valence quark wave function. We present a covariant definition of these components and compute their contributions to the form factors. We find that these components cannot, by themselves, describe the data. Explicit pion cloud contributions must also be added, and these contributions dominate both the E2 and the C2 form factors. By parametrizing the pion cloud contribution for the transition electric and Coulomb form factors in terms of the neutron electric form factor, we estimate that the contributions of the $\ensuremath{\Delta}$ D-state coupled to quark core spin of $3/2$ is of the order of 1%, and the contributions of the $\ensuremath{\Delta}$ D-state coupled to quark core spin $1/2$ is of the order of 4%.

Signatures of chiral dynamics in the nucleon to Delta transition⋆

Utilizing the methods of chiral effective field theory we present an analysis of the electromagnetic NΔ-transition current in the framework of the non-relativistic “small scale expansion” (SSE) to leading-one-loop order. We discuss the momentum dependence of the magnetic dipole, electric quadrupole and Coulomb quadrupole transition form factors up to a momentum transfer of Q2 < 0.3GeV^2. Particular emphasis is put on the identification of the role of chiral dynamics in this transition. Our analysis indicates that there is indeed non-trivial momentum dependence in the two quadrupole form factors at small Q2 < 0.15GeV^2 arising from long-distance pion physics, leading, for example, to negative radii in the (real part of the) quadrupole transition form factors. We compare our results with the EMR(Q2) and CMR(Q2) multipole ratios from pion-electroproduction experiments and find a remarkable agreement up to four-momentum transfer of Q2 ≈ 0.3GeV^2. Finally, we discuss the chiral extrapolation of the three transition form factors at Q2 = 0, identifying rapid changes in the (real part of the) quark mass dependence of the quadrupole transition moments for pion masses below 200MeV, which arise again from long-distance pion dynamics. Our findings indicate that dipole extrapolation methods currently used in lattice QCD analyses of baryon form factors are not applicable for the chiral extrapolation of NΔ quadrupole transition form factors.

Electromagnetic form factors of a massive neutrino

Electromagnetic form factors of a massive neutrino are studied in a minimally extended standard model in an arbitrary R ξ gauge and taking into account the dependence on the masses of all interacting particles. The contribution from all Feynman diagrams to the electric, magnetic, and anapole form factors, in which the dependence of the masses of all particles as well as on gauge parameters is accounted for exactly, are obtained for the first time in explicit form. The asymptotic behavior of the magnetic form factor for large negative squares of the momentum of an external photon is analyzed and the expression for the anapole moment of a massive neutrino is derived. The results are generalized to the case of mixing between various flavors of the neutrino. Explicit expressions are obtained for the electric, magnetic, and electric dipole and anapole transitional form factors as well as for the transitional electric dipole moment.

Gluon and gluino penguin diagrams and the charmless decays of thebquark

Gluon mediated exclusive hadronic decays of b quarks are studied within the standard model (SM) and the constrained minimally supersymmetric standard model (MSSM). For all allowed regions of the MSSM parameter space (A, tan beta, m_0, m_{1/2}) the penguin magnetic dipole form factor F^R_2 is dominant over the electric dipole and can be larger than the magnetic dipole form factor of the SM. However, overall the SM electric dipole decay amplitude F^L_1 dominates the decay rate. The MSSM penguin contributions to the free quark decay rate approach the 10% level for those regions of parameter space close to the highest allowed values of tan beta (~55) for which the gluino is light (m_{\tilde{g}} \approx 360 GeV) and lies within the range of the six d-squark masses. In these regions the supersymmetric box amplitudes are negligible. The MSSM phases change very little over the allowed parameter space and can lead to significant interference with the SM amplitudes.

Measurement of the tau anomalous magnetic moment

We have analysed radiative e + e − → ττ(γ) events using 84 pb −1 of data collected by the L3 experiment during the 1992–1994 LEP runs at the Z resonance. The energy spectrum of the photons is compared to the predictions of QED including additional magnetic and electric dipole terms. No deviations from the standard model are observed so upper limits are set on the magnetic and electric dipole form factors of the tau.

Baryon-baryon interaction in a chiral quark model

Nucleon-nucleon scattering phase shifts and deuteron properties are studied in the non-relativistic quark cluster model. The quark cluster model is modified to include chiral symmetry. This reduces the number of parameters. The σ meson is exchanged between quarks and not as in earlier versions between nucleons. The charge monopole, quadrupole and magnetic-dipole form factors of the deuteron in this microscopic meson-quark cluster model are calculated. The deuteron wave function is derived from a microscopic 6-quark Hamiltonian which, in addition to a quadratic confinement potential, includes the one-pion and the one-gluon exchange potentials between quarks. The electromagnetic current operators are constructed on the quark level, i.e., the photon is coupled directly to the quarks. Aside from the one-body impulse current, pionic and gluonic exchange current corrections are included. Due to the Pauli principle on the quark level, new quark interchange terms arise in the one-body and two-body current matrix elements, that are not present on the nucleon level. While these additional quark exchange currents are small for low momentum transfers, we find that they appreciably influence the electromagnetic structure of the deuteron beyond a momentum transfer of q = 5 fm −1.

ONE-LOOP CORRECTIONS TO WWγ AND WWZ VERTICES IN THE THDM

The one-loop corrections to the three-boson vertices WWγ and WWZ are studied in the context of the minimal standard model (MSM) as well as the two-Higgs-doublet model (THDM). Analytic expressions of the magnetic dipole and electric quadrupole form factors are given and discussed as functions of various parameters.