arXiv subjects

Zhen-Ni Xu

Publications and source records attributed to Zhen-Ni Xu.

9 recordsLinked to original sources

arXiv2026

Distribution amplitudes of vector and axial-vector mesons in a nonperturbatively improved symmetry-preserving framework

Using continuum Schwinger-function methods with a nonperturbatively improved, symmetry-preserving kernel, we deliver predictions for the leading-twist light-front distribution amplitudes (DAs) of the $ρ$, $K^\ast$, $a_1(1260)$, $b_1(1235)$, and the unmixed strange partners of the $1^{++}$ and $1^{+-}$ axial-vector (AV) channels, reconstructed from Mellin moments of the associated Bethe--Salpeter wave functions. For vector mesons, polarisation barely affects longitudinal momentum sharing: the longitudinal and transverse DAs are nearly degenerate, and both narrower than the asymptotic distribution in the second-moment sense. The AV sector is different in kind. Charge conjugation compels one projection -- interchanged between the $1^{++}$ and $1^{+-}$ channels -- to vanish at $x=1/2$ and change sign; breaking $SU_F(3)$ symmetry removes this protection, whereupon the zeroth moments become nonzero and the nodes shift from the midpoint. Under a common weighted normalisation, the $1^{+-}$ zeroth moment is $1.63$ times that of the $1^{++}$ channel, and the profile distortion follows the same pattern. A coupling forbidden by charge conjugation in the symmetric limit, $f_{b_1}=0$, becomes $f_{K_1^{+-}}=0.019\,$GeV in the strange channel: an independent measure of the same symmetry breaking, obtained from a current matrix element rather than from the DA reconstruction. What distinguishes the two sectors is thus a symmetry-enforced zero, not the size of the flavour asymmetry.

hep-ph↗

arXiv2026

Quest for an Understanding of Pion and Kaon Structure

The emergence of massless (Nambu-Goldstone) bosons in association with a dynamically global broken symmetry is a long known and widespread phenomenon in physics. However, practically nothing is known about the expressions of Nambu--Goldstone boson character on the internal structure of these bound states. Indeed, their structure is often ignored. In strong interactions, pions and kaons are the (would-be) Nambu-Goldstone bosons and experiments underway or planned at existing or anticipated high-energy, high-luminosity facilities will gather data that it is hoped will enable maps to be drawn of their internal structure. Meanwhile, theory and phenomenology find themselves in something of a quagmire. Herein, we provide a snapshot of the current status, highlighting issues under debate and identifying areas that deserve greater attention so that best use can be made of what is likely to be a huge volume of data delivered in the next decade or so.

hep-ph↗

arXiv2026

Pseudoscalar charmonium and bottomonium: light-front wave functions, distribution amplitudes and distribution functions

Light-front wave functions play a central role in the program of understanding the structure of hadrons as QCD bound states. Using continuum Schwinger methods, based on Dyson-Schwinger and Bethe-Salpeter equations, they can be computed directly within a framework connected to QCD. For light pseudoscalar mesons, previous studies revealed an approximate separability of longitudinal and transverse lightcone momentum dependences in the LFWFs, leading to a simple relation between distribution functions and amplitudes. In this work, we extend those previous studies to the case of pseudoscalar charmonium and bottomonium, using the fictitious $π_s$ meson as a benchmark. Motivated by the observed deviations, we propose a modified non-separable LFWF ansatz that successfully reproduces the properties of heavy pseudoscalar quarkonia and allows the calculation of zero-skewness generalised parton distribution functions, electromagnetic and gravitational form factors, and transverse charge and mass distributions.

hep-ph↗

arXiv2026

Pion structure from its light-front wave function

Understanding the structural properties of the pion is essential for elucidating the mechanisms of mass generation within the Standard Model and their role in the emergence and properties of the hadronic matter. Light-front wave functions encode extensive information about the internal structure of these systems and provide the link to measurable quantities such as generalized parton distributions and transverse-momentum-dependent distributions. Guided by recent progress in continuum Schwinger methods, we derive well-founded and practical representations of these quantities, enabling the exploration of several facets of the pion structure, including distribution amplitudes and distribution functions, elastic and gravitational form factors, and the associated momentum and spatial distributions. The results presented here are consistent with expectations and can be tested at modern experimental facilities, including the new generation of electron-ion colliders.

hep-ph↗

arXiv2026

Symmetry-preserving calculation of pion light-front wave functions

Poincaré-covariant Bethe-Salpeter wave functions are used to calculate light-front wave functions (LFWFs) of the pion, $π$, and an analogue state, $π_{s\bar s}$. The current masses of the degenerate valence constituents in the $π_{s\bar s}$ are around $25$-times larger than those of the pion's valence constituents. Both valence spin-antialigned ($\mathcal L=0$) and valence spin-aligned ($\mathcal L=1$) components are obtained and combined to produce the complete LFWF for each system. Comparing predictions delivered by two distinct Bethe-Salpeter kernels, the impact of nonperturbative dynamical effects contained in the more sophisticated (bRL) kernel are seen to be significant; and contrasts between $π$, $π_{s \bar s}$ results reveal the interplay between emergent hadron mass and mass effects owing to Higgs-boson couplings. Amongst the results, one finds that for $π$, $π_{s\bar s}$, the LFWFs can be approximated by a separable form, with that representation being pointwise reliable in the bRL cases. Moreover, the $\mathcal L=1$ component is important; so a LFWF obtained after omission of this piece is typically a poor representation of the system. These features are naturally expressed in $π$, $π_{s\bar s}$ transverse momentum dependent parton distribution functions (TMDs). In this connection, it is found that a Gaussian \textit{Ansatz} can only provide a rough guide to TMD pointwise behaviour: magnitude deviations between \textit{Ansatz} and prediction exceed a factor of two on $k_\perp^2 \gtrsim 0.55\,$GeV$^2$. One should therefore be cautious in interpreting conclusions drawn from phenomenological analyses based upon Gaussian \textit{Ansätze}.

hep-ph↗

arXiv2024

Kaon Distribution Functions from Empirical Information

Using available information from Drell-Yan data on pion and kaon structure functions, an approach is described which enables the development of pointwise profiles for all pion and kaon parton distribution functions (DFs) without reference to theories of hadron structure. The key steps are construction of structure-function-constrained probability-weighted ensembles of valence DF replicas and use of an evolution scheme for parton DFs that is all-orders exact. The DFs obtained express qualitatively sound features of light-meson structure, e.g., the effects of Higgs boson couplings into QCD and the size of heavy-quark momentum fractions in light hadrons. In order to improve the results, additional and more precise data on the $u$-quark-in-kaon, $u^K$, to $u$-quark-in-pion, $u^π$, DF ratio would be necessary. Of greater value would be extraction of $u^K$ alone, thereby avoiding inference from the ratio: currently, the data-based form of $u^K$ is materially influenced by results for $u^π$.

hep-ph↗

arXiv2023

Bethe-Salpeter kernel and properties of strange-quark mesons

Focusing on the continuum meson bound-state problem, a novel method is used to calculate closed-form Bethe-Salpeter kernels that are symmetry consistent with any reasonable gluon-quark vertex, $Γ_ν$, and therewith deliver a Poincaré-invariant treatment of the spectrum and decay constants of the ground- and first-excited states of $u$, $d$, $s$ mesons. The predictions include masses of as-yet unseen states and many unmeasured decay constants. The analysis reveals that a realistic, unified description of meson properties (including level orderings and mass splittings) requires a sound expression of emergent hadron mass in bound-state kernels; alternatively, that such properties may reveal much about the emergence of mass in the standard model.

hep-ph↗

arXiv2022

Heavy + heavy and heavy + light pseudoscalar to vector semileptonic transitions

Using a symmetry-preserving regularisation of a vector$\times$vector contact interaction (SCI), we complete a systematic treatment of twelve semileptonic transitions with vector meson final states: $D\to ρ$, $D_{(s)}\to K^\ast$, $D_s\to ϕ$, $B\to ρ$, $B_s\to K^\ast$, $B_{(s)}\to D_{(s)}^\ast$, $B_c \to B_{(s)}^\ast, J/ψ, D^\ast$; and thereby finalise a unified analysis of semileptonic decays of heavy+heavy and heavy+light pseudoscalar mesons to both pseudoscalar and vector meson final states. The analysis is marked by algebraic simplicity, few parameters, and the ability to consistently describe systems from Nambu-Goldstone modes to heavy+heavy mesons. Regarding the behaviour of the transition form factors, the SCI results compare well wherever sound experimental or independent theory analyses are available; hence, the SCI branching fraction predictions should be a reasonable guide. Considering the ratios $R(D_{(s)}^{(\ast)})$, $R(J/ψ)$, $R(η_c)$, whose values are key tests of lepton universality in weak interactions, the SCI values agree with Standard Model predictions. The $B_{(s)}\to D_{(s)}^\ast$ transitions are used to predict the precursor functions that evolve into the universal Isgur-Wise function in the heavy-quark limit, with results that conform with those from other sources where such are available. The study also exposes effects on the transition form factors that flow from interference between emergent hadron mass from the strong interaction and Higgs boson couplings via current-quark masses, including flavour symmetry violation.

hep-ph↗

arXiv2021

Heavy+light pseudoscalar meson semileptonic transitions

A symmetry-preserving regularisation of a vector$\times$vector contact interaction (SCI) is used to deliver a unified treatment of semileptonic transitions involving $π$, $K$, $D_{(s)}$, $B_{(s,c)}$ initial states. The framework is characterised by algebraic simplicity, few parameters, and the ability to simultaneously treat systems from Nambu-Goldstone modes to heavy+heavy mesons. Although the SCI form factors are typically somewhat stiff, the results are comparable with experiment and rigorous theory results. Hence, predictions for the five unmeasured $B_{s,c}$ branching fractions should be a reasonable guide. The analysis provides insights into the effects of Higgs boson couplings via current-quark masses on the transition form factors; and results on $B_{(s)}\to D_{(s)}$ transitions yield a prediction for the Isgur-Wise function in fair agreement with contemporary data.

hep-ph↗