Behavior Of Form Factors Research Articles

γ*N→N*(1520) form factors in the timelike regime

The covariant spectator quark model, tested before in a variety of electromagnetic baryon excitations, is applied here to the $\gamma^\ast N \to N^\ast(1520)$ reaction in the timelike regime. The transition form factors are first parametrized in the spacelike region in terms of a valence quark core model together with a parametrization of the meson cloud contribution. The form factor behavior in the timelike region is then predicted, as well as the $N^\ast(1520) \to \gamma N$ decay width and the $N^\ast (1520)$ Dalitz decay, $N^\ast (1520) \to e^+ e^- N$. Our results may help in the interpretation of dielectron production from elementary $pp$ collisions and from the new generation of HADES results using a pion beam. In the $q^2=0$--1 GeV$^2$ range we conclude that the {\it QED approximation} (a $q^2$ independent form factor model) underestimates the electromagnetic coupling of the $N^\ast(1520)$ from 1 up to 2 orders of magnitude. We conclude also that the $N^\ast (1520)$ and the $\Delta(1232)$ Dalitz decay widths are comparable.

Incommensurate charge ordered states in the t–t′–J model

We study the incommensurate charge ordered states in the model using the Gutzwiller mean field theory on large systems. In particular, we explore the properties of incommensurate charge modulated states referred to as nodal pair density waves (nPDW) in the literature. nPDW states intertwine site and bond charge order with modulated d-wave pair order, and are characterized by a nonzero amplitude of uniform pairing; they also manifest a dominant intra-unit cell d-density wave form factor. To compare with a recent scanning tunneling microscopy (STM) study (Hamidian et al 2015 Nat. Phys. 12 150) of the cuprate superconductor BSCCO-2212, we compute the continuum local density of states (LDOS) at a typical STM tip height using the Wannier function based approach. By Fourier transforming Cu and O sub-lattice LDOS we also obtain bias-dependent intra-unit cell form factors and spatial phase difference. We find that in the nPDW state the behavior of form factors and spatial phase difference as a function of energy agrees remarkably well with the experiment.This is in contrast to commensurate charge modulated states, which we show do not agree with experiment. We propose that the nPDW states are good candidates for the charge density wave phase observed in the superconducting state of underdoped cuprates.

The longitudinal and transverse distributions of the pion wave function from the present experimental data on the pion–photon transition form factor

It is noted that the low-energy behavior of the pion-photon transition form factor $F_{\pi\gamma}(Q^2)$ is sensitive to the transverse distribution of the pion wavefunction, and its high-energy behavior is sensitive to the longitudinal one. Thus a careful study on $F_{\pi\gamma}(Q^2)$ can provide helpful information on the pion wavefunction precisely. In this paper, we present a combined analysis of the data on $F_{\pi\gamma}(Q^2)$ reported by the CELLO, the CLEO, the BABAR and the BELLE collaborations. It is performed by using the method of least squares. By using the combined measurements of BELLE and CLEO Collaborations, the pion wavefunction longitudinal and transverse behavior can be fixed to a certain degree, i.e. we obtain $\beta \in [0.691,0.757] \rm GeV$ and $B \in [0.00,0.235]$ for $P_{\chi^2} \geq 90\%$, where $\beta$ and $B$ are two parameters of a convenient pion wavefunction model whose distribution amplitude can mimic the various longitudinal behavior under proper choice of parameters. We observe that the CELLO, CLEO and BELLE data are consistent with each other, all of which prefers the asymptotic-like distribution amplitude; while the BABAR data prefers a more broad distribution amplitude, such as the CZ-like one.

Light Front Wave Function for Hadrons with Arbitrary Twist

We present a phenomenological light-front wave function for hadrons with arbitrary twist dimension (mesons, baryons and multiquark states), which gives the correct scaling behavior of structure functions and form factors. Some examples of his uses are presented.

Search for diphoton decays of exotic scalars at BaBar

We report on a search in e+e− annihilations for new π0-like particles produced in association with a τ-lepton pair. These objects, with a similar mass and similar decay modes to π0 mesons, could provide an explanation for the non-asymptotic behavior of the pion-photon transition form factor measured by the BaBar Collaboration. No significant signal is observed, and limits on the production cross section at the level of 73 fb−1 or 370 fb−1, depending on the model parameters, are determined at 90% confidence level. These upper limits lie below the cross section values needed to explain the BaBar form factor data.

Search for newπ0-like particles produced in association with aτ-lepton pair

We report on a search in $e^+e^-$ annihilations for new $\pi^0$-like particles produced in association with a $\tau$-lepton pair. These objects, with a similar mass and similar decay modes to $\pi^0$ mesons, could provide an explanation for the non-asymptotic behavior of the pion-photon transition form factor observed by the Babar Collaboration. No significant signal is observed, and limits on the production cross section at the level of 73 fb or 370 fb, depending on the model parameters, are determined at 90% confidence level. These upper limits lie below the cross section values needed to explain the Babar form factor data.

Comparison between shell model and self-consistent mean field calculations for ground charge density distributions and elastic form factors of 12C and 16O nuclei

The ground charge density distributions, elastic form factors and root mean square radii for 12C and 16O nuclei in 1p-shell nuclei are calculated in shell model using core plus valence and no-core–shell model calculations. The single-particle wave functions of harmonic-oscillators potential are used. For such potential, two harmonic-oscillators size parameters are used: one for neutron and the other for proton. Calculations are compared with the results of self-consistent mean field using SKX Skyrme parameterization and experimental data. The calculated charge density distributions in shell model for 12C and 16O nuclei show good agreement with experimental data. The calculated charge form factors at low q in shell model for 12C and 16O show good agreement with experimental data. At high q, the result for 16O shows no prediction for the position of second diffraction minimum. In general, it is found that enlarging the model space in shell model leads to suppress slightly the behavior of density distribution around central region and also the behavior of form factor at high q, which is good to approach experimental data. On the other side, such increase in model space leads to increase the calculated root mean square radii, which is the same behaviour of the results of Hartree–Fock calculations. The calculated charge form factors in Hartree–Fock calculations show the existence of many diffraction minima, which do not agree with experimental data in contrary to shell model, which predicts less diffraction minima.

The last of the simple remainders

We compute the n-point two-loop form factors of the half-BPS operators Tr(phi_{AB}^n) in N=4 super Yang-Mills for arbitrary n >2 using generalised unitarity and symbols. These form factors are minimal in the sense that the n^{th} power of the scalar field in the operator requires the presence of at least n on-shell legs. Infrared divergences are shown to exponentiate as for amplitudes, reproducing the known cusp and collinear anomalous dimensions at two loops. We define appropriate infrared-finite remainder functions and compute them analytically for all n. The results obtained by using the known expressions of the integral functions involve complicated combinations of Goncharov multiple polylogarithms, but we show that much simpler expressions can in fact be derived using the symbol of transcendental functions. For n=3 we find a very compact remainder function expressed in terms of classical polylogarithms only. For arbitrary n>3 we are able to write all the remainder functions in terms of a single compact building block, expressed as a sum of classical polylogarithms augmented by two multiple polylogarithms. The decomposition of the symbol into specific components is crucial in order to single out a natural combination of multiple polylogarithms. Finally, we analyse in detail the behaviour of these minimal form factors in collinear and soft limits, which deviates from the usual behaviour of amplitudes and non-minimal form factors.

Light-front quark model consistent with Drell-Yan-West duality and quark counting rules

We propose a phenomenological light-front wave function for hadrons with arbitrary twist-dimension (mesons, baryons and multiquark states), which gives the correct scaling behavior of structure functions (parton distributions) and form factors for pions and nucleons. For other hadronic states the proposed wave function produces form factors consistent with quark counting rules, and gives predictions for structure functions (parton distributions). As an application we build a light-front quark model for nucleons based on the proposed wave function.

Electromagnetic structure of the proton within the CP-violation hypothesis

The so-called non-Rosenbluth behavior of the proton electromagnetic form factors can be explained within the hypothesis of CP violation in electromagnetic processes involving composite systems of strongly interacting particles. It is shown that this hypothesis leads to the appearance of an additional, anapole, form factor of the proton. The proton electromagnetic form factors, including the anapole form factor, are estimated on the basis of experimental data on elastic electron-proton scattering.

LONG-DISTANCE PROPERTIES OF BARYONS IN THE SAKAI-SUGIMOTO MODEL

We reconsider the realization of baryons in the Sakai-Sugimoto model. It has previously been shown that, at least with the standard approach to calculations of baryon properties in the theory, one does not reproduce some model-independent predictions for the behavior of baryon electromagnetic form factors connected with long-range pion physics. It is suspected that the pion contributions has been dropped because of the linearization approximations. We study the long-range properties of baryons without relying on the approximations. The baryon solution we obtain gives the correct result for the model-independent ratio of the form factors.

Long-distance properties of baryons in the Sakai–Sugimoto model

We reconsider the realization of baryons in the Sakai--Sugimoto model. It has previously been shown that, at least with the standard approach to calculations of baryon properties in the theory, one does not reproduce some model-independent predictions for the behavior of baryon electromagnetic form factors that are connected with long-range pion physics. In the standard approach, considerations involving the consequences of the ${\ensuremath{\alpha}}^{\ensuremath{'}}$ expansion motivated the use of a linearized approximation. This made it appear that the long-range pion physics of baryons may be hidden in (intractable) ${\ensuremath{\alpha}}^{\ensuremath{'}}$ corrections in the gravity dual. In this paper, we study the long-range properties of baryons in the Sakai--Sugimoto model without relying on the standard linearization approximations that we suspect appeared to drop pion contributions to baryon properties. The baryon solution we obtain gives the correct result for the model-independent ratio of form factors, implying that the model captures the expected infrared properties of baryons without the need to go beyond the leading order in the ${\ensuremath{\alpha}}^{\ensuremath{'}}$ expansion.

Spacelike pion form factor from analytic continuation and the onset of perturbative QCD

The factorization theorem for exclusive processes in perturbative QCD predicts the behavior of the pion electromagnetic form factor $F(t)$ at asymptotic spacelike momenta $t(=-Q^2)<0$. We address the question of the onset energy using a suitable mathematical framework of analytic continuation, which uses as input the phase of the form factor below the first inelastic threshold, known with great precision through the Fermi-Watson theorem from $\pi\pi$ elastic scattering, and the modulus measured from threshold up to 3 GeV by the BaBar Collaboration. The method leads to almost model-independent upper and lower bounds on the spacelike form factor. Further inclusion of the value of the charge radius and the experimental value at $-2.45 \gev^2$ measured at JLab considerably increases the strength of the bounds in the region $ Q^2 \lesssim 10 \gev^2$, excluding the onset of the asymptotic perturbative QCD regime for $Q^2< 7\gev^2$. We also compare the bounds with available experimental data and with several theoretical models proposed for the low and intermediate spacelike region.

Scale-invariant asymptotic (automodel) behavior in quantum field theory

We summarize the results of our 40-year investigations of scale-invariant (automodel) behavior of form factors in hadron-lepton deep-inelastic scattering processes at high energies and large transferred momenta within the Bogoliubov axiomatics of quantum field theory. This approach was conducive to the emergence of the notion of quark color in quantum chromodynamics.

Bridging light-cone and NRQCD approaches: asymptotic behavior of B c electromagnetic form factor

This work aims at illustrating that, for a class of leading-twist hard exclusive reactions involving two heavy quarkonia, the light-cone approach, when equipped with the strategy of refactorization of the light-cone distribution amplitude of quarkonium, can be employed to elegantly reproduce the corresponding predictions made in the nonrelativistic QCD (NRQCD) factorization approach, order by order in perturbative expansion. Taking the electromagnetic form factor of the $B_c$ meson at large momentum transfer, $Q^2$, as a concrete example, we compare the results obtained from both NRQCD-based and light-cone-based calculations through the next-to-leading order (NLO) in $\alpha_s$, while at the leading order (LO) in both velocity and $1/Q^2$ expansion, and explicitly confirm their mutual agreement. As a byproduct, we apply our NLO result to explore certain features about the asymptotic behavior of the heavy-light meson form factor. We also address the major theoretical obstacles that prevent us from establishing an analogous equivalence between these two approaches for the double charmonium production process of phenomenological interest, $e^+e^-\to J/\psi+\eta_c$.

Implications of the new proton charge rms radius from spectroscopy of muon hydrogen atom

It is demonstrated a coincidence of the proton electric rms radius from the global analysis of the JLab proton polarization data together with all other existing nucleon electromagnetic form factor data in the framework of the Unitary & Analytic model of nucleon electromagnetic structure with its value recently determined in a spectroscopy of muon hydrogen atom.The result confirms a non-dipole behavior of the proton electric form factor in the space-like region and strengthen a belief in an existence of the proton electric form factor zero revealed in global analysis of nucleon form factors data.

Pion form factors and decays

In this communication we discuss few topics related with modern experimental data on the physics of light pseudoscalar mesons. It includes the contribution of the pseudoscalar mesons to the muon anomalous magnetic moment (AMM), the rare decays of light pseudoscalar mesons to lepton pair, g –2, the transition form factors of pseudoscalar mesons at large momentum transfer, the pion transversity form factor. Measuring the muon anomalous magnetic moment g –2 and the rare decays of light pseudoscalar mesons into lepton pair P → l + l − serve as important test of the standard model. To reduce the theoretical uncertainty in the standard model predictions the data on the transition form factors of light pseudoscalar mesons play significant role. Recently new data on behavior of these form factors at large momentum transfer was supplied by the BABAR collaboration.

Electromagnetic properties of hyperons

In the framework of unitary and analytic model of nucleon electromagnetic structure, the values of nucleon coupling constants are received from the fit of nucleon data. With the help of SU(3) relations the coupling constants of other hyperons, i.e., Λ, Σ + , Σ 0 , Σ − , Ξ 0 , Ξ − are calculated. The values of hyperon coupling constants are used to predict the behaviour of hyperon electromagnetic form factors and corresponding cross sections e + e − → hyperon–antihyperon.

Etalon for any model space-like region pion electromagnetic form factor behavior

Model independent behavior of the pion electromagnetic form factor in space-like region is achieved transferring to this region the precise experimental information on σ t o t ( e + e − → π + π − ) by means of the form factor analyticity.

On form factors of the conjugated field in the nonlinear Schrödinger model

Izergin-Korepin's lattice discretization of the nonlinear Schrödinger model along with Oota's inverse problem provides one with determinant representations for the form factors of the lattice discretized conjugated field operator. We prove that these form factors converge, in the zero lattice spacing limit, to those of the conjugated field operator in the continuous model. We also compute the large-volume asymptotic behavior of such form factors in the continuous model. These are, in particular, characterized by Fredholm determinants of operators acting on closed contours. We provide a way of defining these Fredholm determinants in the case of generic parameters.