Light-front Model Research Articles

Light-front model for the transition distribution amplitudes for backward timelike Compton scattering

To access information on the internal structure of the nucleon, data from a variety of scattering experiments can be analyzed in regimes where the information factorizes from an otherwise known scattering amplitude. A recent development, promising new insight, is the study of exclusive reactions in the backward kinematical region, where the information can be encoded in transition distribution amplitudes. We model the photon-to-nucleon transition distribution amplitudes, entering the factorized description of backward timelike Compton scattering, using techniques of light-front dynamics to integrate information from a quark model for the photon and the nucleon. We include the results of numerical predictions that could inform further experiments at Jefferson Lab and the future Electron-Ion Collider. Published by the American Physical Society 2024

Longitudinal dynamics for mesons on the light cone

We survey a set of proposed light-front models for confinement in the valence quark sector of the mesons and portray similarities as well as differences. We present the spectroscopies for the light mesons that result from a selection of longitudinal confinement forms. We note that the Sturm-Liouville theory provides a unifying framework for many elements of this comparison.

Proton image and momentum distributions from light-front dynamics

We apply a dynamical three-constituent quark light-front model to study the proton. The dynamics is based on the notion of a diquark (bound or virtual) as the dominant interaction channel, which paramaterizes a contact interaction between the quarks in order to build the three-body Faddeev Bethe-Salpeter equations for the valence state, and we focus on the totally symmetric part of the wave function. The Dirac electromagnetic form factor is used to fix the model parameters, and the valence wave function is obtained. From that we investigate its Ioffe-time image, non-polarized longitudinal and transverse momentum distributions, and the double momentum distribution.

Light-front holographic \u03c1-meson distributions in the momentum space

We present the leading-twist quark transverse momentum-dependent parton distribution functions (TMDs) for the spin-1 target, such as the ρ-meson, in the light-front framework. Specifically, we predict the TMDs in the light-front holographic model and compare with the light-front quark model predictions. We obtain the TMDs using the overlap of the light-front wave functions. We evaluate the k⊥ moments upto second order and compare with the available theoretical predictions. Further, we analyze the leading-twist parton distribution functions (PDFs) of the ρ-meson in the light-front holographic model which are found to be in accord with the Nambu-Jona-Lasinio (NJL) model and the light-front quark model predictions. We further study the QCD evolution of the PDFs. The positivity bounds on the TMDs and the PDFs are also discussed. We also present the quark spin densities in the transverse momentum plane for different polarization configurations of the quark and the ρ-meson target.

The twist-three distribution eq(x,k⊥) in a light-front model

We discuss the twist-three, unpolarized, chiral-odd, transverse momentum dependent parton distribution (TMD) eq(x,k⊥) within a light-front model. We review a model-independent decomposition of this TMD, which follows from the QCD equations of motion and is given in terms of a mass term, a pure interaction-dependent contribution, and singular terms. The mass-term and pure twist-three contributions are represented in terms of overlap of light-front wave functions (LFWFs), taking into account the Fock states with three valence quark (3q) and three-quark plus one gluon (3q+g). The 3q and 3q+g LFWFs with total orbital angular momentum zero are modeled using a parametrization derived from the conformal expansion of the proton distribution amplitudes, with parameters fitted to reproduce available phenomenological information on the unpolarized leading-twist quark and gluon collinear parton distributions. Numerical predictions for both the quark TMD eq(x,k⊥) and the collinear parton distribution eq(x) are presented, discussing the role of the quark–gluon correlations in the proton.

Estimating decay rate of X^{{pm }}(5568)rightarrow B_spi ^{{pm }} while assuming them to be molecular states

Discovery of X(5568) brings up a tremendous interest because it is very special, i.e. made of four different flavors. The D0 collaboration claimed that they observed this resonance through portal X(5568)rightarrow B_spi , but unfortunately, later the LHCb, CMS, CDF and ATLAS collaborations’ reports indicate that no such state was found. Almost on the Eve of 2017, the D0 collaboration reconfirmed existence of X(5568) via the semileptonic decay of B_s. To further reveal the discrepancy, supposing X(5568) as a molecular state, we calculate the decay rate of X(5568)rightarrow B_spi ^+ in an extended light front model. Numerically, the theoretically predicted decay width of Gamma (X(5568)rightarrow B_spi ^+) is 20.28 MeV which is consistent with the result of the D0 collaboration (Gamma =18.6^{+7.9}_{-6.1}({textit{stat}})^{+3.5}_{-3.8}({textit{syst}}) MeV). Since the resonance is narrow, signals might be drowned in a messy background. In analog, two open-charm molecular states DK and BD named as X_a and X_b, could be in the same situation. The rates of X_arightarrow D_spi ^0 and X_brightarrow B_cpi ^0 are estimated as about 30 and 20 MeV respectively. We suggest the experimental collaborations round the world to search for these two modes and accurate measurements may provide us with valuable information.

Bayesian analysis of light-front models and the nucleon’s charmed sigma term

We present the results of a recent analysis to study the nucleon's charm sigma term, $\sigma_{c\overline{c}}$. We construct a minimal model in terms of light-front variables and constrain the range of possibilities using extant knowledge from deeply inelastic scattering (DIS) and Bayesian parameter estimation, ultimately computing $\sigma_{c\overline{c}}$ in an explicitly covariant manner. We find a close correlation between a possible nonperturbative component of the charm structure function, $F^{c\overline{c}}_{2,\, \mathrm{IC}}$, and $\sigma_{c\overline{c}}$. Independent of prescription for the covariant relativistic quark-nucleon vertex, we determine $\sigma_{c\overline{c}}$ under several different scenarios for the magnitude of intrinsic charm (IC) in DIS, namely $\langle x \rangle_{c+\overline{c}} = 0.1\%$, $0.35\%$, and $1\%$, obtaining for these $\sigma_{c\overline{c}} = 4 \pm 4$, $12 \pm 13$, and $32 \pm 34$ MeV, respectively. These results imply the existence of a reciprocity between the IC parton distribution function (PDF) and $\sigma_{c\overline{c}}$ such that new information from either DIS or improved determinations of $\sigma_{c \overline{c}}$ could significantly impact constraints to the charm sector of the proton wave function.

In-medium pion valence distributions in a light-front model

Pion valence distributions in nuclear medium and vacuum are studied in a light-front constituent quark model. The in-medium input for studying the pion properties is calculated by the quark-meson coupling model. We find that the in-medium pion valence distribution, as well as the in-medium pion valence wave function, are substantially modified at normal nuclear matter density, due to the reduction in the pion decay constant.

Study on decays of $$Z_c(4020)$$ Z c ( 4020 ) and $$Z_c(3900)$$ Z c ( 3900 ) into $$h_c+\pi $$ h c + π

At the invariant mass spectrum of $h_c\pi^\pm$ a new resonance $Z_c(4020)$ has been observed, however the previously confirmed $Z_c(3900)$ does not show up at this channel. In this paper we assume that $Z_c(3900)$ and $Z_c(4020)$ are molecular states of $D\bar D^*(D^{*} \bar D)$ and $D^*\bar D^*$ respectively, then we calculate the transition rates of $Z_c(3900)\to h_c+\pi$ and $Z_c(4020)\to h_c+\pi$ in the light front model. Our results show that the partial width of $Z_c(3900)\to h_c+\pi$ is only three times smaller than that of $Z_c(4020)\to h_c+\pi$. $Z_c(4020)$ seems to be a molecular state, so if $Z_c(3900)$ is also a molecular state it should be observed in the portal $e^+e^-\to h_c\pi^\pm$ as long as the database is sufficiently large, by contrary if the future more precise measurements still cannot find $Z_c(3900)$ at $h_c\pi^\pm$ channel, the molecular assignment to $Z_c(3900)$ should be ruled out.

Double Parton Scattering and 3D Proton Structure: A Light-Front Analysis

Double parton scattering, occurring in high energy hadron-hadron collisions, e.g. at the LHC, is usually investigated through model dependent analyses of the so called effective cross section $\sigma_{eff}$. We present a dynamic approach to this fundamental quantity making use of a Light-Front model treatment. Within such a framework $\sigma_{eff}$ is initially evaluated at low energy scale using the model and then, through QCD evolution, at high energy scale to reach the experimental conditions. Our numerical outcomes are consistent with the present experimental analyses of data in kinematical region we investigate. An important result of the present work is the $x_i$ dependence of $\sigma_{eff}$, a feature directly connected to double parton correlations and which could unveil new information on the three dimensional structure of the proton.

LIGHT-CONE 2014: Theory and Experiment for Hadrons on the Light-Front

The 2014 edition of the Light-Cone meeting took place at the North Carolina State University (NCSU) in Raleigh, North Carolina on 26–30 May http://www.physics.ncsu.edu/LC2014/). The local organizing committee consisted of myself as the chair of the committee, J. Blondin (NCSU), H. Gao (Duke), D. Lee (NCSU), W. Melnitchouk (Jefferson Lab), T. Schaefer (NCSU), M. Unsal (NCSU), with L. Cochran and R. Bennett providing administrative support. This series of meetings is held under the auspices of the International Light Cone Advisory Committee (ILCAC), Inc. (http://www.ilcacinc.org). The Workshop was supported in part by generous contributions from Jefferson Lab, the College of Sciences at NCSU, and the NCSU Department of Physics. In particular, the support from NCSU allowed ILCAC to award this year’s McCartor Fellowships to four young physicists, enabling them to attend the conference and present the results of their research. LIGHT-CONE 2014 had over 70 participants, who presented a total of 65 talks and 4 posters. An emphasis on the interface between theory and experiment in hadron physics was a feature of this year’s meeting, with several experimental talks from Jefferson Lab (R. McKeown, L. Elouadrhiri) and COMPASS at CERN (A. Ferrero, I. Choi), discussing recent results and future perspectives of hadron physics at their facilities. The workshop covered many topics in hadron phenomenology related to the 12 GeV upgrade at Jefferson Lab, including nucleon generalized parton distributions (GPDs) (F. Wang, Y. Zhao, S. Liuti) and deeplyvirtual Compton scattering (B. Pasquini), Compton form factors (B. Bakker), transverse momentum dependent distributions (TMDs) (D. Boer, A. Metz), and parton distribution functions (P. Jimenez-Delgado). There was discussion of flavor asymmetries in the nucleon sea (J.-C. Peng, W. Melnitchouk), the deuteron b1 structure function (G.Miller), the 3He spectral function (S. Scopetta) and the pion distribution amplitude (A.Radyushkin, N. Stefanis). A variety of other, more formal theoretical topics in QCD and QED were also presented, including chiral symmetry breaking and confinement (P. Hoyer, I. Cloet, M. Schindler), Bethe-Salpeter and Dyson-Schwinger equations (P. Tandy, A. Bashir, M. Viviani, V. Karmanov), light-front models (G. Salme, T. Pena, H. Choi), and issues involving bound states and zero-modes (W. Polyzou, T. Frederico, A. Ilderton). Recent progress in light-front holography and AdS/QCD (R. Sandapen), as well as computational developments in many-body dynamics (J. Vary) were also highlighted. Some particularly stimulating presentations were on issues related to the proton spin (C. Lorce), the proton charge radius (S. Glazek), GPDs and TMDs, as well as light-front chiral sum rules (S. Beane). The closing presentation was given by S. Brodsky. A half-day excursion wasmade to several local museums (Art, History, Natural Sciences) andwas followed by theWorkshop dinner. During the dinner proceedings, as the Chair of LC2014 and ILCAC, I had the privilege to present awards to the four McCartor Fellowship recipients: Tim Hobbs (Indiana University), Yang Li (Iowa

New limits on intrinsic charm in the nucleon from global analysis of parton distributions.

We present a new global QCD analysis of parton distribution functions, allowing for possible intrinsic charm (IC) contributions in the nucleon inspired by light-front models. The analysis makes use of the full range of available high-energy scattering data for Q^{2}≳1 GeV^{2} and W^{2}≳3.5 GeV^{2}, including fixed-target proton and deuteron cross sections at lower energies that were excluded in previous global analyses. The expanded data set places more stringent constraints on the momentum carried by IC, with ⟨x⟩_{IC} at most 0.5% (corresponding to an IC normalization of ∼1%) at the 4σ level for Δχ^{2}=1. We also critically assess the impact of older EMC measurements of F_{2}^{c} at large x, which favor a nonzero IC, but with very large χ^{2} values.

Light-Front Spin-1 Model: Parameters Dependence

We study the structure of the ρ-meson within a light-front model with constituent quark degrees of freedom. We calculate electroweak static observables: magnetic and quadrupole moments, decay constant and charge radius. The prescription used to compute the electroweak quantities is free of zero modes, which makes the calculation implicitly covariant. We compare the results of our model with other ones found in the literature. Our model parameters give a decay constant close to the experimental one.

Phenomenological Implications of the Nucleon’s Meson Cloud

The long-distance structure of the interacting nucleon receives important contributions from its couplings to light hadronic degrees of freedom -- a light meson cloud -- while an analogous nonperturbative mechanism is expected to generate an intrinsic charm (IC) component to the proton wavefunction. We investigate both possibilities, keeping for the former a special eye to improving the theoretical understanding of the pion-nucleon vertex in light of proposed measurements. Regarding the latter possibility of IC, we highlight recent results obtained by a global QCD analysis of the light-front model proposed in Hobbs et al. (Phys Rev D 89:074008, 2014).

The Photon–Pion Transition Form Factor: Incompatible Data or Incompatible Models?

The elastic and $\gamma \to \pi$ transition form factors of the pion along with its usual static observables are calculated within a light-front field approach to the constituent quark model. The focus of this exercise in a simple model is on a unified description of all observables with one singly parametrized light-front wave function to detect possible discrepancies in experimental data, in particular the contentious large momentum-squared data on the transition factor as reported by BaBar and Belle. We also discuss the relation of a small to vanishing pion charge radius with an almost constant pion distribution amplitude and compare our results with those obtained in a holographic light-front model.

Is Z c (3900) a molecular state?

Assuming the newly observed $Z_c(3900)$ to be a molecular state of $D\bar D^*(D^{*} \bar D)$, we calculate the partial widths of $Z_c(3900)\to J/\psi+\pi;\; \psi'+\pi;\; \eta_c+\rho$ and $D\bar D^*$ within the light front model (LFM). $Z_c(3900)\to J/\psi+\pi$ is the channel by which $Z_c(3900)$ was observed, our calculation indicates that it is indeed one of the dominant modes whose width can be in the range of a few MeV depending on the model parameters. Similar to $Z_b$ and $Z_b'$, Voloshin suggested that there should be a resonance $Z_c'$ at 4030 MeV which can be a molecular state of $D^*\bar D^*$. Then we go on calculating its decay rates to all the aforementioned final states and as well the $D^*\bar D^*$. It is found that if $Z_c(3900)$ is a molecular state of ${1\over\sqrt 2}(D\bar D^*+D^*\bar D)$, the partial width of $Z_c(3900)\to D\bar D^*$ is rather small, but the rate of $Z_c(3900)\to\psi(2s)\pi$ is even larger than $Z_c(3900)\to J/\psi\pi$. The implications are discussed and it is indicated that with the luminosity of BES and BELLE, the experiments may finally determine if $Z_c(3900)$ is a molecular state or a tetraquark.

A Combined Study of the Pion’s Static Properties and form Factors

We study consistently the pion’s static observables and the elastic and γ* γ → π 0 transition form factors within a light-front model. Consistency requires that all calculations are performed within a given model with the same and single adjusted length or mass-scale parameter of the associated pion bound-state wave function. Our results agree well with all extent data including recent Belle data on the γ* γ → π 0 form factor at large q 2, yet the BaBar data on this transition form factor resists a sensible comparison. We relax the initial constraint on the bound-state wave function and show the BaBar data can partially be accommodated. This, however, comes at the cost of a hard elastic form factor not in agreement with experiment. Moreover, the pion charge radius is about 40 % smaller than its experimentally determined value. It is argued that a decreasing charge radius produces an ever harder form factor with a bound-state amplitude difficultly reconcilable with soft QCD. We also discuss why vector dominance type models for the photon-quark vertex, based on analyticity and crossing symmetry, are unlikely to reproduce the litigious transition form factor data.

Pion and kaon elastic form factors in a refined light-front model

Within the framework of light-front field theory, we reassess the electromagnetic form factors of the pion and kaon. Comparison with experiment is made for the full range of momentum transfer, q^2<0, including recent data. The light-front model's single regulator mass, m_R, of the \bar qq bound-state vertex function is initially adjusted to reproduce the weak decay constants, f_\pi\ and f_K, and both meson's charge radii, r_\pi\ and r_K. We study the behavior of these observables under variation of the quark masses and find an optimized parameter set, m_u=m_d, m_s and m_R, for which they are in sensibly better agreement with experiment than in a previous analysis; a feature also observed for the elastic form factors, in particular at small q^2. This model refinement is important in view of an extension to vector and heavy-light mesons.

HadronicDdecays involving even-parity light mesons

We study the hadronic D meson decays into a pseudoscalar meson $P$ and an even-parity meson $M$, where $M$ represents a scalar meson $S$, an axial-vector meson $A$, or a tensor meson $T$. These decays are first analyzed in the flavor-diagram approach. Fits to the $SP$ modes with $S$ being a non-strange scalar meson show that neither the simple $q\bar q$ picture nor the $q^2\bar q^2$ scheme is favored by data. Current measurements on the $AP$ decays are insufficient for a meaningful analysis. Some $TP$ data are inconsistent with the others. In certain cases, the $W$-annihilation diagrams indicated by the data are unexpectedly large. As a comparison, we also compute their decay rates in the factorization approach using form factors extracted from the covariant light-front model. We find that factorization works well for Cabibbo-allowed $D^+ \to SP,AP$ decays free of the weak annihilation contributions ($W$-exchange or $W$-annihilation). For the other $SP$ and $AP$ modes, it is necessary to include weak annihilation contributions to account for the data. However, factoriztion fails for $D\to TP$ decays for some unknown reason; the predicted rates are in general too small by at least two orders of magnitude compared to experiment. We also examine the finite width effects of resonances. Some decay modes which are kinematically forbidden become physically allowed due to the finite width of the resonance. We show that the branching fraction of $D^+\to\sigma\pi^+$ extracted from three-body decays is enhanced by a factor of 2, whereas $\B(D^0\to f_2(1270)\ov K^0)$ is reduced by a factor of 4 by finite width effects.

Erratum: Covariant light-front approach forB→K*γ,K1γ,K2γdecays [Phys. Rev. D69, 094007 (2004)

Exclusive radiative B decays, B to K* gamma, K_1(1270) gamma, K_1(1400) gamma and K*_2(1430) gamma, are studied in the framework of a covariant light-front quark model. The tensor form factor T_1(q^2) at q^2=0, which is relevant to the decay B to K* gamma, is found to be 0.24, substantially smaller than what expected from the conventional light-front model or light-cone sum rules. Taking into account the sizable next-to-leading order (NLO) corrections, the calculated branching ratio of B to K* gamma agrees with experiment, while most of the existing models predict too large B to K* gamma compared to the data. The relative strength of B to K_1(1270) gamma and B to K_1(1400) gamma rates is very sensitive to the sign of the K_1(1270)-K_1(1400) mixing angle. Contrary to the other models in which K_1(1270) gamma and K_1(1400) gamma rates are predicted to be comparable, it is found that one of them is strongly suppressed owing to a large cancellation between two different form factor terms. The calculated branching ratio of B to K*_2 gamma is in a good agreement with experiment and this may imply the smallness of NLO corrections to this radiative decay mode.