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Register a new userSystematic twist expansion of $(eta_c,eta_b)togamma^*gamma$ transition form factors in light-front quark model
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The light-front quark model analysis of the meson-photon transition form factor $F_{Pgamma} (Q^2)$ amenable both for the spacelike region ($Q^2 >0$) and the timelike region ($Q^2 <0$) provides a systematic twist expansion of $Q^2 F_{Pgamma} (Q^2)$ for the high $|Q^2|$ region. Investigating $F_{Pgamma} (Q^2) (P = eta_c,eta_b)$ for the entire kinematic regions of $Q^2$, we examine the twist-2 and twist-3 distribution amplitudes of $(eta_c,eta_b)$ mesons in the light-front quark model and quantify their contributions to $Q^2 F_{(eta_c,eta_b)gamma}(Q^2)$. Our numerical results for the normalized transition form factor $F_{(eta_c,eta_b)gamma}(Q^2)/F_{(eta_c,eta_b)gamma}(0)$ and the decay width $Gamma_{(eta_c,eta_b)togammagamma}$ are compared with the available data checking the sensitivity of our model to the variation of the constituent quark masses.
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We report our investigation on the doubly virtual TFFs $F_{{rm P}gamma^*}(Q^2_1,Q^2_2)$ for the ${rm P}togamma^*(q_1)gamma^*(q_2) ;({rm P}=pi^0,eta,eta)$ transitions using the light-front quark model (LFQM). Performing a LF calculation in the exactly solvable manifestly covariant Bethe-Salpeter (BS) model as the first illustration, we used $q^+_1=0$ frame and found that both LF and manifestly covariant calculations produce exactly the same results for $F_{{rm P}gamma^*}(Q^2_1,Q^2_2)$. This confirms the absence of the LF zero mode in the doubly virtual TFFs. We then mapped this covariant BS model to the standard LFQM using the more phenomenologically accessible Gaussian wave function provided by the LFQM analysis of meson mass spectra. For the numerical analyses of $F_{{rm P}gamma^*}(Q^2_1,Q^2_2)$, we compared our LFQM results with the available experimental data and the perturbative QCD (pQCD) and the vector meson dominance (VMD) model predictions. As $(Q^2_1, Q^2_2)toinfty$, our LFQM result for doubly virtual TFF is consistent with the pQCD prediction, i.e. $F_{{rm P}gamma^*}(Q^2_1, Q^2_2)sim 1/(Q^2_1 + Q^2_2)$, while it differs far from the result of VMD model which behaves $F^{rm VMD}_{{rm P}gamma^*}(Q^2_1, Q^2_2)sim 1/(Q^2_1 Q^2_2)$. Our LFQM prediction for $F_{etagamma^*}(Q^2_1,Q^2_2)$ shows an agreement with the very recent experimental data obtained from the BaBar collaboration for the ranges of $2< Q^2_1, Q^2_1 <60$ GeV$^2$.
We investigate the $(pi^0,eta,eta)togamma^*gamma$ transitions both for the spacelike region and the timelike region using the light-front quark model (LFQM). In particular, we present the new direct method to explore the timelike region without resorting to mere analytic continuation from the spacelike region to the timelike region. Our direct calculation in timelike region shows the complete agreement not only with the analytic continuation result from the spacelike region but also with the result from the dispersion relation between the real and imaginary parts of the form factor. For the low energy regime, we compare our LFQM results of the transition form factors (TFFs) for the low timelike momentum transfer region and the slope parameters at $q^2=0$ with the recent experimental data from the Dalitz decays of $(pi^0,eta,eta)$. For the high energy regime, we incorporate the QCD factorization in our LFQM to examine the asymptotic behavior of TFFs both for the spacelike region and the timelike region. We compare our results with the available experimental data.
We present a recent calculation of the gravitational form factors (GFFs) of proton using the light-front quark-diquark model constructed by the soft-wall AdS/QCD. The four GFFs $~A(Q^2)$ , $B(Q^2)$ , $C(Q^2)$ and $bar{C}(Q^2)$ are calculated in this model. We also show the pressure and shear distributions of quarks inside the proton. The GFFs, $A(Q^2)$ and $B(Q^2)$ are found to be consistent with the lattice QCD, while the qualitative behavior of the $D$-term form factor is in agreement with the extracted data from the deeply virtual Compton scattering (DVCS) experiments at JLab, the lattice QCD, and the predictions of different phenomenological models.
We predict the $mathcal{V} to mathcal{P} gamma$ decay widths and the $mathcal{V} to mathcal{P} gamma^{*}$ transition form factors, where $mathcal{V}=(rho, omega, K^*, phi)$ and $mathcal{P}= (pi,K, eta,eta^prime)$, using spin-improved holographic light-front wavefunctions for the mesons. We find excellent agreement with the available data for both the decay widths and the timelike transition form factors extracted from the leptonic conversion decays $mathcal{V} to mathcal{P} l^+ l^-$.
Using the light front wave functions for the nucleons in a quark model in AdS/QCD, we calculate the nucleon electromagnetic form factors. The flavor decompositions of the nucleon form factors are calculated from the GPDs in this model. We show that the nucleon form factors and their flavor decompositions calculated in AdS/QCD are in agreement with experimental data.
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