Dependence Of Form Factors Research Articles

Frame dependence of transition form factors in light-front dynamics

We calculate the transition form factor between vector and pseudoscalar quarkonia in both the timelike and the spacelike region using light-front dynamics. We investigate the frame dependence of the form factors for heavy quarkonia with light-front wavefunctions calculated from the valence Fock sector. This dependence could serve as a measure for the Lorentz symmetry violation arising from the Fock-space truncation. We suggest using frames with minimal longitudinal momentum transfer for calculations in the valence Fock sector, namely the Drell-Yan frame for the space-like region and a specific longitudinal frame for the timelike region; at $q^2=0$ these two frames give the same result. We also use the transition form factor in the timelike region to investigate the electromagnetic Dalitz decay $\psi_A\to \psi_B l^+l^-$ ($l = e,\mu$) and predict the effective mass spectrum of the lepton pair.

Mesons in a soft-wall AdS-Schwarzschild approach at low temperature

We derive a holographic soft-wall approach in five dimensional AdS-Schwarzschild space for the description of mesons at finite temperature. In this first application we consider the small temperature limit and derive analytical expression for the mass spectrum of mesons with adjustable quantum numbers $n$ (radial number), $L$ (angular orbital momentum) and $J$ (total angular momentum). We explicitly separate the contribution at zero temperature and the leading order temperature correction. The temperature corrections arise from the temperature dependence of the dilaton parameter (which is the parameter of spontaneous breaking of chiral symmetry related to the pseudoscalar meson decay constant) and the warping of the AdS metric due to temperature. We extend our results to any hadron with integer spin (tetraquarks, dibaryons, etc.). We present numerical analysis for the temperature dependence of meson masses and form factors.

Study of electromagnetic Dalitz decays χcJ→μ+μ−J/ψ

Using $4.48 \times 10^{8}$ $\psi(3686)$ events collected with the BESIII detector, we search for the decays $\chi_{cJ} \rightarrow \mu^{+}\mu^{-}J/\psi$ through the radiative decays $\psi(3686) \rightarrow \gamma\chi_{cJ}$, where $J=0,1,2$. The decays $\chi_{c1,2} \rightarrow \mu^{+}\mu^{-}J/\psi$ are observed, and the corresponding branching fractions are measured to be $\mathcal{B}(\chi_{c1} \rightarrow \mu^{+}\mu^{-}J/\psi) = (2.51 \pm 0.18 \pm 0.20)\times10^{-4}$ and $\mathcal{B}(\chi_{c2} \rightarrow \mu^{+}\mu^{-}J/\psi) = (2.33 \pm 0.18 \pm 0.29)\times10^{-4}$, where the first uncertainty is statistical and the second one systematic. No significant $\chi_{c0} \rightarrow \mu^{+}\mu^{-}J/\psi$ decay is observed, and the upper limit on the branching fraction is determined to be $2.0\times10^{-5}$ at 90% confidence level. Also, we present a study of di-muon invariant mass dependent transition form factor for the decays $\chi_{c1,2} \rightarrow \mu^{+}\mu^{-}J/\psi$.

Mass dependence of pseudoscalar meson elastic form factors

A continuum approach to quark-antiquark bound-states is used to determine the electromagnetic form factors of pionlike mesons with masses ${m}_{{0}^{\ensuremath{-}}}/\mathrm{GeV}=0.14$, 0.47, 0.69, 0.83 on a spacelike domain that extends to ${Q}^{2}\ensuremath{\lesssim}10\text{ }\text{ }{\mathrm{GeV}}^{2}$. The results enable direct comparisons with contemporary lattice-QCD calculations of heavy-pion form factors at large values of momentum transfer and aid in understanding them. They also reveal, inter alia, that the form factor of the physical pion provides the best opportunity for verification of the factorized hard-scattering formula relevant to this class of exclusive processes and that this capacity diminishes steadily as the meson mass increases.

Higgs couplings at high scales

We study the off-shell production of the Higgs boson at the LHC to probe Higgs physics at higher energy scales utilizing the process $g g \rightarrow h^{*} \rightarrow ZZ$. We focus on the energy scale dependence of the off-shell Higgs propagation, and of the top quark Yukawa coupling, $y_t (Q^2)$. Extending our recent study in arXiv:1710.02149, we first discuss threshold effects in the Higgs propagator due to the existence of new states, such as a gauge singlet scalar portal, and a possible continuum of states in a conformal limit, both of which would be difficult to discover in other traditional searches. We then examine the modification of $y_t (Q^2)$ from its Standard Model (SM) prediction in terms of the renormalization group running of the top Yukawa, which could be significant in the presence of large flat extra-dimensions. Finally, we explore possible strongly coupled new physics in the top-Higgs sector that can lead to the appearance of a non-local $Q^2$-dependent form factor in the effective top-Higgs vertex. We find that considerable deviations compared to the SM prediction in the invariant mass distribution of the $Z$-boson pair can be conceivable, and may be probed at a $2\sigma$-level at the high-luminosity 14 TeV HL-LHC for a new physics scale up to $\mathcal{O}(1 {~\rm TeV})$, and at the upgraded 27 TeV HE-LHC for a scale up to $\mathcal{O}(3 {~\rm TeV})$. For a few favorable scenarios, $5\sigma$-level observation may be possible at the HE-LHC for a scale of about $\mathcal{O}(1 {~\rm TeV})$.

Extraction of |V_{cd}| and |V_{cs}| from experimental decay rates using lattice QCD D rightarrow pi (K) ell nu form factors

We present a determination of the Cabibbo–Kobayashi–Maskawa matrix elements |V_{cd}| and |V_{cs}| obtained by combining the momentum dependence of the semileptonic vector form factors f_+^{D rightarrow pi }(q^2) and f_+^{D rightarrow K}(q^2), recently determined from lattice QCD simulations, with the differential rates measured for the semileptonic D rightarrow pi ell nu and D rightarrow K ell nu decays. Our analysis is based on the results for the semileptonic form factors produced by the European Twisted Mass Collaboration with N_f = 2 + 1 + 1 flavors of dynamical quarks in the whole range of values of the squared 4-momentum transfer accessible in the experiments. The statistical and systematic correlations between the lattice data as well as those present in the experimental data are properly taken into account. With respect to the standard procedure based on the use of only the vector form factor at zero 4-momentum transfer, we obtain more precise and consistent results: |V_{cd} |= 0.2341 ~ (74) and |V_{cs} |= 0.970 ~ (33). The second-row CKM unitarity is fulfilled within the current uncertainties: |V_{cd}|^2 + |V_{cs}|^2 + |V_{cb}|^2 = 0.996 ~ (64). Moreover, using for the first time hadronic inputs determined from first principles, we have calculated the ratio of the semileptonic D rightarrow pi (K) decay rates into muons and electrons, which represent a test of lepton universality within the SM, obtaining in the isospin-symmetric limit of QCD: mathcal{{R}}_{LU}^{Dpi } = 0.985~(2) and mathcal{{R}}_{LU}^{DK} = 0.975~(1).

Meson impact parameter distributions**Supported by National Natural Science Foundation of China (11475192), the fund provided to the Sino-German CRC 110 “Symmetries and the Emergence of Structure in QCD” project by the NSFC (11621131001) and the Key Research Program of Frontier Sciences, CAS (Y7292610K1)

In this paper, the meson impact parameter dependent parton distributions and the impact parameter dependent form factors are introduced and discussed. By employing a Gaussian form wave packet, we calculate the impact parameter distributions of the meson based on a light-cone constituent quark model.

Frame dependence of form factors in light-front dynamics

In light-front dynamics, form factors are traditionally computed with the "good current" $J^+$ within the Drell-Yan frame $q^+=0$. Due to truncations imposed in practical calculations, the from factor may acquire frame dependence, which is often neglected. In this work, we explore the form factors in more general frames, preserving the boost covariance. We find the frame dependence of the elastic form factors for mesons is small in basis light-front holography and related models with two-body Fock space truncation. We suggest to use the difference between form factor results from Drell-Yan frame and the "longitudinal frame" as a metric for the violation of the Lorentz symmetry due to Fock space truncation.

Coincidence charged-current neutrino-induced deuteron disintegration for H22O16

Semi-inclusive charge-changing neutrino reactions on targets of heavy water are investigated with the goal of determining the relative contributions to the total cross section of deuterium and oxygen in kinematics chosen to emphasize the former. The study is undertaken for conditions where the typical neutrino beam energies are in the few GeV region, and hence relativistic modeling is essential. For this, the previous relativistic approach for the deuteron is employed, together with a spectral function approach for the case of oxygen. Upon optimizing the kinematics of the final-state particles assumed to be detected (typically a muon and a proton) it is shown that the oxygen contribution to the total cross section is suppressed by roughly an order of magnitude compared with the deuterium cross section, thereby confirming that CC$\nu$ studies of heavy water can effectively yield the cross sections for deuterium, with acceptable backgrounds from oxygen. This opens the possibility of using deuterium to determine the incident neutrino flux distribution, to have it serve as a target for which the nuclear structure issues are minimal, and possibly to use deuterium to provide improved knowledge of specific aspects of hadronic structure, such as to explore the momentum transfer dependence of the isovector axial-vector form factor of the nucleon.

Nucleon form factors in dispersively improved chiral effective field theory: Scalar form factor

We propose a method for calculating the nucleon form factors (FFs) of $G$-parity-even operators by combining Chiral Effective Field Theory ($\chi$EFT) and dispersion analysis. The FFs are expressed as dispersive integrals over the two-pion cut at $t > 4 M_\pi^2$. The spectral functions are obtained from the elastic unitarity condition and expressed as products of the complex $\pi\pi \rightarrow N\bar N$ partial-wave amplitudes and the timelike pion FF. $\chi$EFT is used to calculate the ratio of the partial-wave amplitudes and the pion FF, which is real and free of $\pi\pi$ rescattering in the $t$-channel ($N/D$ method). The rescattering effects are then incorporated by multiplying with the squared modulus of the empirical pion FF. The procedure results in a marked improvement compared to conventional $\chi$EFT calculations of the spectral functions. We apply the method to the nucleon scalar FF and compute the scalar spectral function, the scalar radius, the $t$-dependent FF, and the Cheng-Dashen discrepancy. Higher-order chiral corrections are estimated through the $\pi N$ low-energy constants. Results are in excellent agreement with dispersion-theoretical calculations. We elaborate several other interesting aspects of our method. The results show proper scaling behavior in the large-$N_c$ limit of QCD because the $\chi$EFT calculation includes $N$ and $\Delta$ intermediate states. The squared modulus of the timelike pion FF required by our method can be extracted from Lattice QCD calculations of vacuum correlation functions of the operator at large Euclidean distances. Our method can be applied to the nucleon FFs of other operators of interest, such as the isovector-vector current, the energy-momentum tensor, and twist-2 QCD operators (moments of generalized parton distributions).

Bs0\u2009\u2192\u2009\u2113+\u2113\u2212\u03b3 as a test of lepton flavor universality

We discuss a number of strategies to reduce the ℬ(Bs0 → ℓ+ℓ−γ) theoretical error, and make such a measurement a new probe of the interactions that are interesting in the light of present-day flavor discrepancies. In particular, for low di-lepton invariant mass we propose to exploit the close parenthood between ℬ(Bs0 → ℓ+ℓ−γ) and the measured ℬ(Bs0 → ϕ(→K+K−)γ). For high q2, conversely, we exploit the fact that the decay is dominated by two form-factor combinations, plus contributions from broad charmonium that we model accordingly. We construct the ratio Rγ , akin to RK and likewise sensitive to lepton-universality violation. Provided the two rates in this ratio are integrated in a suitable region that minimises bremsstrahlung contributions while maximising statistics, the ratio is very close to unity and the form-factor dependence cancels to an extent that makes it a new valuable probe of lepton-universality violating contributions in the effective Hamiltonian. We finally speculate on additional ideas to extract short-distance information from resonance regions, which are theoretically interesting but statistically limited at present.

Инженерный расчет и оптимизация параметров полуэкранированного диэлектрического волновода в неизлучающем режиме

The parameters of half-shielded dielectric waveguides optimized in terms of their frequency bandwidth are investigated as a supplement to the procedure for designing waveguide transitions. The half-shielded dielectric waveguide is a rectangular dielectric rod placed between two parallel metal plates. Such a waveguide is in its own way a "hybrid" of open and closed transmission lines. It simultaneously has the properties of both the types of transmission lines; these properties may exhibit themselves to a greater or lesser extent depending on the design. Under certain conditions, a so called non-radiating effect is observed in this waveguide, a phenomenon consisting in that the electromagnetic energy is held in the dielectric rod when it is bent or deformed (provided, however, that the plane symmetry is retained). The ratio of the distance between the metal plates to the dielectric rod width is called the rod form-factor. For a non-radiating mode, there exists a certain optimal form-factor, at which the maximum frequency bandwidth is reached. The frequency bandwidth as a function of the waveguide form-factor is presented. Dependences characterizing the optimum form factor (according to the largest frequency bandwidth criterion) versus the medium parameters are investigated, and empirical formulas for evaluating them are proposed. An engineering method is proposed for calculating the physical parameters of non-radiating dielectric waveguides, which is one of the stages in calculating waveguide transitions. Empirical formulas for estimating the dependence of optimal form-factor on the parameters of medium are investigated and proposed. Dependences of the optimal form-factor and the frequency bandwidth coefficient on the ratio of dielectric constants of the waveguide rod and external medium are presented. An engineering method for calculating the physical parameters of non-radiating half-shielded dielectric waveguides is suggested. It is found that the non-radiating dielectric waveguide's frequency ratio lies in the range from 24 to 30%. It is shown that materials with a dielectric constant higher than 5 are best suited for achieving the largest frequency bandwidth. A formula for approximating the dependence of the optimal form-factor on the rod dielectric constant in the range from 1.5 to 30 is proposed.

Electric and magnetic energy at axion haloscopes

We review a recent letter published in Phys. Rev. Lett. $\textbf{116}$, 161804 (2016) of which the main argument is that the mode dependent magnetic form factors at axion haloscopes depend on the position of the cavity inside the solenoid while the corresponding electric form factors do not. We, however, find no such dependence, which is also equivalent to the statement that the electric and corresponding magnetic energy stored in the cavity modes are the same regardless of the position of the cavity inside the solenoid. Furthermore, we extend the statement to the cases satisfying $\vec{\nabla}\times\vec{B}_{\rm external}=0$, where $\vec{B}_{\rm external}$ is a static magnetic field provided by a magnet at an axion haloscope. Two typical magnets, solenoidal and toroidal, satisfy $\vec{\nabla}\times\vec{B}_{\rm external}=0$, thus the electric and corresponding magnetic energy stored in the cavity modes are always the same in both cases. The energy, however, is independent of the position of the cavity in axion haloscopes with a solenoid, but depends on those with a toroidal magnet.

The Dirac form factor predicts the Pauli form factor in the Endpoint Model

We compute the momentum-transfer dependence of the proton Pauli form factor $F_{2}$ in the endpoint overlap model. We find the model correctly reproduces the scaling of the ratio of $F_{2}$ with the Dirac Form factor $F_{1}$ observed at the Jefferson Laboratory. The calculation uses the leading-power, leading twist Dirac structure of the quark light-cone wave function, and the same endpoint dependence previously determined from the Dirac form factor $F_{1}$. There are no parameters and no adjustable functions in the endpoint model's prediction for $F_{2}$. The model's predicted ratio $F_{2}(Q^{2})/F_{1}(Q^{2})$ is quite insensitive to the endpoint wave function, which explains why the observed ratio scales like $1/Q$ down to rather low momentum transfers. The endpoint model appears to be the only comprehensive model consistent with all form factor information as well as reproducing fixed-angle proton-proton scattering at large momentum transfer. Any one of the processes is capable of predicting the others.

On Gravitational Form Factors and Transverse Spin Sum Rule

Using the light front wave functions of the scalar quark–diquark model for nucleon predicted by the soft-wall AdS/QCD, we calculate the flavor dependent gravitational form factors. We evaluate the matrix element of Pauli–Lubanski operator in this model and show that the intrinsic spin sum rule involves the higher twist form factor \({\bar{C}}\). The longitudinal momentum densities in the transverse impact parameter space are also discussed for both unpolarized and transversely polarized nucleons.

The finite probe size effect in fluctuation measurements; application to dusty plasmas

The effect of the finite probe size in plasma fluctuation measurements is revisited for dusty plasmas, where it has been argued that dust leads to a significant low-frequency enhancement in the spectral densities of ion density fluctuations, which can constitute the physical basis of a dust diagnostic technique. Theoretical predictions for the spectral modifications are presented and the dust acoustic mode contribution is analysed. The finite probe size effect is treated within the volume average approach, which introduces geometry dependent form factors that are calculated for spherical and cylindrical probes. The volume average approach is compared with the typically employed cutoff wavenumber approximation for various dust and plasma parameters. The contribution of temperature fluctuations to the spectral density of current fluctuations is also evaluated.

Final-state interaction correction to the electromagnetic nucleon form factors in the time-like region

We study the strong energy dependence of the proton electromagnetic form factors in the time-like region, taking into account the one-pion-exchange final-state interaction in a covariant way. This effect is quantified in terms of the corrected Dirac F 1 and Pauli F 2 form factors and in the commonly used electric G E and magnetic G M ones. Our results on the ratio |G E /G M depend only on the values of two free parameters and allow significant comparisons with the BaBar data.

Determination of transition metal ion distribution in cubic spinel Co1.5Fe1.5O4 using anomalous x-ray diffraction

We report anomalous x-ray diffraction studies on Co ferrite with composition Co1.5Fe1.5O4 to obtain the distribution of transition metal ions in tetrahedral and octahedral sites. We synthesize spinel oxide (Co1.5Fe1.5O4) through co-precipitation and subsequent annealing route. The imaginary part (absorption) of the energy dependent anomalous form factor is measured and the real part is calculated theoretically through Kramers–Krönig transformation to analyze anomalous x-ray diffraction peak intensities. Fe and Co K-edge x-ray absorption near edge structure (XANES) spectra are used to estimate charge states of transition metals. Our analysis, within experimental errors, suggests 44% of the tetrahedral sites contain Co in +2 oxidation state and the rest 56% sites contain Fe in +2 and +3 oxidation states. Similarly, 47% of the octahedral sites contain Fe in +3 oxidation states, whereas, the rest of the sites contain Co in +2 and +3 oxidation states. While a distinct pre-edge feature in the Fe K-edge XANES is observed, Co pre-edge remains featureless. Implications of these results to magnetism are briefly discussed.

Density dependence of parity violation in electron quasi-elastic scattering

Within the framework of a relativistic single-particle model, we investigate the effect of the density dependent form factor on parity violating electron scattering from a heavy nucleus in the quasielastic region through the leading order of the interference of the electromagnetic current and the electro-weak neutral current. 208Pb is used as a target nucleus in the incident electron energy range up to 1 GeV. The density-dependent form factors are obtained from a quark-meson coupling model. The sensitivities of the density-dependence are studied separately for each Dirac (F 1), Pauli (F 2), and axial (G A ) form factors. The density enhances the parity violation asymmetry, in particular, in the forward angle region, and the density dependence turns out to be most sensitive to the Pauli form factor.

Joint Resummation for TMD Wave Function of Pion

QCD corrections to transverse-momentum-dependent pion wave function develop the mixed double logarithm [Formula: see text], when the gluon emission is collinear to the energetic pion. The fist scheme-independent kT factorization formula for γ*π → γ transition form factor is achieved by resumming all the enhanced logarithms for both pion wave function and short-distance coefficient function. High-order QCD corrections and transfer momentum [Formula: see text] dependence of pion form factor are found to be distinct from those predicted by the conventional resummation approach.