arXiv subjects

Siqi Xu

Publications and source records attributed to Siqi Xu.

At least 19 recordsLinked to original sources

arXiv2026

Exclusive $J/\psi$ photoproduction in photon-proton diffractive scattering: A light-front Hamiltonian approach

We investigate the cross-section for exclusive $J/\psi$ production in photon-proton diffractive scattering within the Basis Light-Front Quantization (BLFQ) framework. The leading-order contribution to this process is well approximated by the charge conjugation-even two-gluon (``pomeron") exchange mechanism in the dipole model, which factorizes the total amplitude into the dipole scattering amplitude and the convolution of the $J/\psi$ and photon light-front wave functions (LFWFs). We express the dipole scattering amplitude as the matrix element of gluon field operators inserted between proton states, with the element being sensitive to the proton LFWFs and the Bjorken scaling variable, $x$. The proton and $J/\psi$ LFWFs are obtained by diagonalizing their respective light-front Hamiltonians within the BLFQ approach, while the virtual photon LFWFs are employed from perturbative QCD. Our results provide initial conditions for the Balitsky-Kovchegov (BK) equation, which can be used to probe the proton structure at smaller Bjorken scales. This work offers valuable theoretical insights for future electron-ion collider experiments.

hep-ph↗

arXiv2026

Spin-orbit correlation of quarks within quarkonium

The spin-orbit correlation (SOC) provides a unique probe into the internal spin structure of hadrons. Defined via the parity-odd (P-odd) energy-momentum tensor (EMT), this observable can remain non-vanishing even in systems where the total angular momentum is zero. In this study, we connect the formal field-theoretical definition of the SOC to a non-perturbative quantum many-body framework utilizing light-front dynamics. Furthermore, we show how the SOC can be extracted directly from the hadronic matrix elements of the P-odd EMT, establishing a pathway to access this observable within the partonic picture. As a practical application, we compute the transverse and longitudinal SOC distributions for charmonium and $B_c$ mesons. While our findings align with rough estimates based on the Clebsch-Gordan decomposition, we demonstrate that these observables yield rich, non-trivial information regarding partonic dynamics.

hep-ph↗

arXiv2026

Generalized parton distributions of valence, sea, and gluon components of the proton

We compute the generalized parton distributions (GPDs) of valence quarks, sea quarks, and gluons in the proton using light-front wave functions obtained within the basis light-front quantization (BLFQ) framework, providing a realistic description of the nucleon at a low resolution scale. The wave functions are derived from a light-front QCD Hamiltonian without an explicit confining potential and include the three-quark, three-quark-gluon, and three-quark-quark-antiquark Fock sectors. For the first time within BLFQ, we evaluate quark GPDs at nonzero skewness in both the DGLAP and ERBL regions, while gluon GPDs are computed in the DGLAP region. The resulting GPDs exhibit qualitative features similar to, but smaller than the GUMP1.0 global extraction of GPDs based on experimental and lattice QCD data at next-to-leading order accuracy. We further compute the associated Compton form factors and obtain results consistent with the global analysis.

hep-ph↗

arXiv2026

Transverse force tomography inside a proton from Basis Light-front Quantization

The twist-3 transverse spin--dependent nucleon structure function $g_2$ arises in high-energy processes involving a transversely polarized nucleon. Its connection to quark--gluon correlations allows for an interpretation in terms of the average transverse color Lorentz force acting on unpolarized quarks inside a transversely polarized nucleon. In this work, we investigate this force using light-front wave functions obtained by diagonalizing the light-front Hamiltonian with quantum chromodynamics inputs within the Basis Light-front Quantization approach. We evolve our results to a common scale of $5~\mathrm{GeV}^2$ and present the corresponding form factors in momentum space as well as the transverse force components in impact-parameter space. These distributions provide a complementary perspective on the Sivers asymmetry in transversely polarized deep-inelastic scattering. In the forward limit, we extract the twist-3 reduced matrix element $d_2$, and our results are found to be comparable with those from other theoretical calculations and experimental determinations.

hep-ph↗

arXiv2026

Profound impacts of interlayer interactions in bilayer altermagnetic V2S2O

Two-dimensional altermagnets exhibit exceptional potential for low-power spintronics via nonrelativistic spin splitting and zero net magnetization. Here, we systematically investigate the influence of interlayer interactions on the electronic, magnetic and quantum transport properties of bilayer vanadium oxysulfide (V2S2O), a prototypical layered altermagnet, using DFT and NEGF calculations. Our results reveal that interlayer interactions predominantly modulate the p-orbital derived top valence bands, inducing a profound competitive valence band maximum position between Gamma-point pz and X/Y-point pxy orbitals, with an energy difference as small as 9 meV. Furthermore, interlayer interactions suppress the piezomagnetic effect and impose additional requirements on the type of strain for the bilayer system, compared to its monolayer counterpart. Out-of-plane external electric fields effectively weaken interlayer coupling by enlarging the energy difference of Gamma/X-Y top valence bands to 170 meV. Quantum transport simulations on a bilayer Au/V2S2O/Au two-probe device demonstrate the presence of pronounced spin current. Interlayer interactions reduce the transmission spin polarization from nearly 100% (monolayer) to 60% (bilayer) for energies above the Fermi level. Notably, gate-voltage modulation exhibits significant asymmetry in controlling charge-to-spin current conversion efficiency, originating from the out-of-plane symmetry breaking induced by the electrode geometry. Specifically, a positive gate voltage markedly enhances the contribution of the bottom layer to the overall spin polarization, while a negative gate voltage induces a marginal reduction of transmission spin polarization, attributed to the inherently weak polarization contribution of the bottom layer. These findings provide essential insights for the design and optimization of multilayer altermagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2026

Spin entanglement signatures of proton from a light-front Hamiltonian

Quantum entanglement provides a quantitative probe of the internal structure of hadrons and offers a sensitive means to study the quantum correlation in the hadron wave functions. For baryons, the spin state of the three valence quarks forms a tripartite qubit system, whose entanglement structure can be characterized by the four classes of three-qubit states. In this work, we compare the proton spin entanglement obtained from Basis Light-Front Quantization (BLFQ) with that from a quark-diquark model. By analyzing both bipartite and tripartite entanglement, we find that the quark-diquark model yields a substantially more entangled spin state than the BLFQ wave function in the valence Fock sector. This difference mainly originates from the larger W-type and Bell-type entanglement in the quark-diquark model. Within BLFQ, larger stronger coupling constant and smaller quark mass drive the spin correlation among the valence quarks towards an effective quark-diquark configuration with an active $d$ quark and a correlated $uu$ pair.

hep-ph↗

arXiv2026

Simultaneously accounting for the winner's curse and sample structure in Mendelian randomization: bivariate rerandomized inverse variance weighted estimator

The recently developed rerandomized inverse variance weighted (RIVW) estimator provides a simple and efficient framework to break the winner's curse in two-sample Mendelian randomization (MR). However, this method does not account for sample structure (e.g., residual population stratification and sample overlap), a common source of confounding in MR studies. Sample structure can not only distort SNP-exposure and SNP-outcome association estimates but also induce correlation between them, leading exposure-side instrument selection to propagate bias to the outcome side. To address this challenge, we propose the bivariate RIVW (BRIVW) estimator to simultaneously account for the winner's curse and sample structure. The BRIVW estimator extends the RIVW framework by modeling the joint distribution of SNP-exposure and SNP-outcome association estimates, first adjusting their covariance matrix via linkage disequilibrium score regression to account for sample structure and then applying randomized instrument selection and bivariate Rao-Blackwellization to obtain unbiased post-selection association estimates together with an estimator of their covariance matrix. Under mild conditions, we show that the BRIVW estimator is consistent and asymptotically normal. The finite-sample performance of the proposed estimator is evaluated through extensive simulations and real data analyses.

stat.ME↗

arXiv2026

Non-perturbative flavor asymmetry in the nucleon and deuteron: The light-front Hamiltonian effective field theory approach

We investigate non-perturbative multi-pion contributions to nucleon flavor asymmetry within the framework of Light-Front Hamiltonian Effective Field Theory (LFHEFT). Utilizing a Fock sector expansion, we systematically incorporate pionic degrees of freedom, with the nucleon-pion interactions governed by a scalar variant of chiral effective field theory. Our results demonstrate that the non-perturbatively calculated longitudinal momentum distributions exhibit significant deviations from leading-order perturbative predictions, emphasizing the importance of higher-order Fock components in describing the proton's sea quark structure. Furthermore, we demonstrate the feasibility of extending this framework to investigate nuclear effects in light nuclei, such as the deuteron. This unified approach provides a consistent basis for analyzing the interplay between intrinsic nucleon structure and nuclear modifications, potentially offering new insights into the flavor asymmetry observed in fixed-target and collider experiments.

hep-ph↗

arXiv2026

Dynamical gluon effects in twist-3 generalized parton distributions of the proton

Within the Basis Light-Front Quantization framework, we systematically investigate the subleading-twist (twist-3) generalized parton distributions (GPDs) of the proton's valence quarks beyond the Wandzura--Wilczek (WW) approximation. The twist-3 GPDs are not independent; through the equations of motion they decompose into a non-genuine contribution and a genuine twist-3 term. The latter encodes quark--quark--gluon correlations and involves interference between the light-front Fock sectors |qqq> and |qqqg>, which are typically neglected in the WW approximation. Using light-front wave functions obtained from diagonalizing the proton light-front Hamiltonian for its |qqq> and |qqqg> Fock components, we compute these GPDs via their overlap representations. To further explore their physical implications, we also evaluate several twist-3--related quantities, including the quark orbital angular momentum, the total quark spin contribution, and the quark spin--orbit correlation. Our results provide new nonperturbative input on higher-twist dynamics particularly multi-parton interference effects relevant for future measurements at the EicC and the EIC.

hep-ph↗

arXiv2025

Spatiotemporal topological phase transitions in photonic spacetime crystals

Topological phase transitions, characterized by the closing and reopening of band gaps and a concomitant change in topological invariants, have played a central role in topological physics. However, such transitions have so far been restricted to spatial crystals, relying solely on energy band gaps and spatial interfaces. Here, we transcend this conventional framework and report, for the first time, spatiotemporal topological phase transition in photonic spacetime crystals - structures that are periodically modulated in both space and time. Using a dynamically modulated transmission line metamaterial, we theoretically propose and experimentally demonstrate complete spatiotemporal topological phase transitions characterized by the closing and reopening of both energy and momentum band gaps, alongside changes in spatiotemporal topological invariants and topological phases. Furthermore, in a genuine photonic spacetime crystal that possesses a complete energy-momentum band gap, we directly observe a space-time topological event that localizes in both space and time, exhibiting relativistic-causality-governed excitation and robustness against spatiotemporal disorders. Our findings reveal the interplay among space, time, and topology, establishing a unified framework that provides a comprehensive picture of the emerging topological space-time physics and opening new avenues for robust spatiotemporal topological wave manipulations.

physics.optics↗

arXiv2025

Nucleon Structure from Basis Light-Front Quantization : Status and Prospects

We review recent advancements in understanding nucleon structure within the Basis Light-Front Quantization (BLFQ) framework--a fully relativistic, nonperturbative approach to solving quantum field theories. In its initial phase, we start with the leading Fock sector $|qqq\rangle$ and an effective light-front Hamiltonian incorporating confinement and one-gluon exchange within which BLFQ can already successfully describe key nucleon observables. The framework has since been extended to include the next-to-leading Fock sector $|qqqg\rangle$, enabling studies of gluonic contributions to the nucleon's internal structure, including gluon helicity, orbital angular momentum, and three-dimensional imaging through generalized and transverse momentum dependent parton distributions (GPDs and TMDs). Most recently, BLFQ has achieved a significant milestone by computing nucleon light-front wavefunctions as eigenstates of the QCD Hamiltonian without an explicit confining potential. These calculations, including Fock sectors up to $|qqqq\bar{q}\rangle$, further develop the path to first-principles predictions of quark and gluon matter densities, helicity and transversity distributions, and spin observables, showing qualitative agreement with experimental and phenomenological results. Together, these developments highlight BLFQ's growing capacity to provide an increasingly complete and realistic picture of nucleon structure grounded in QCD.

hep-ph↗

arXiv2025

Basis light-front quantization approach to $Λ$ and $Λ_c$ and their isospin triplet baryons

We obtain the masses, the electromagnetic properties, and the parton distribution functions (PDFs) of $Λ$, $Λ_c$, and their isospin triplet baryons, i.e, $Σ^0$, $Σ^+$, $Σ^-$ and $Σ_c^0$, $Σ_c^+$, $Σ_c^{++}$ from a light-front effective Hamiltonian in the leading Fock sector in the basis light-front quantization framework. The light-front wave functions of these baryons are given by the eigenstates of the effective Hamiltonian consisting of a three-dimensional confinement potential and a one-gluon exchange interaction with fixed coupling. The masses of these baryons in our approach are in the experimental range while isospin-dependent mass differences are too small. Meanwhile, the electromagnetic properties are in agreement with the available experimental data, the lattice QCD simulations, and the other theoretical calculations. We also present the gluon and the sea quark PDFs, which we generate dynamically from the QCD evolution of the valence quark distributions.

hep-ph↗

arXiv2025

Double parton distributions of the proton from basis light-front quantization

Within the basis light-front quantization framework, we systematically investigate the unpolarized and longitudinally polarized double parton distributions (DPDs) of quarks inside the proton. We utilize the light-front wave functions of the proton derived in the valence sector from a Hamiltonian quantized on the light-front. The interaction terms of the Hamiltonian consist of a one-gluon exchange interaction at fixed coupling and a three-dimensional confinement potential. Our current analysis yields significant correlations of the quarks' longitudinal momenta with their transverse separation. We also demonstrate that our calculations do not support the commonly used $x-\vec{k}_\perp$ factorization of the DPDs in $x$ and $k_\perp$. Our results are qualitatively consistent with those of other phenomenological models.

hep-ph↗

arXiv2025

Proton Gravitational Structure and Mass Decomposition on the Light Front

Gravitational form factors (GFFs) of hadrons encode essential information about the internal distributions of mass, spin, pressure, and shear among their quark and gluon constituents. We compute the quark and gluon GFFs of the proton using a fully relativistic, nonperturbative framework based on a light-front quantized Hamiltonian with quantum chromodynamics (QCD) input. This allows us to quantify the impact of a dynamical gluon on the proton's mechanical properties, such as pressure and shear distributions. Our predictions agree well with recent lattice QCD results and experimental extractions. We also determine the proton's mass and mechanical radii and address the long-standing puzzle of its mass decomposition. At the scale $μ^2 = 4~\mathrm{GeV}^2$, we find that quark energy, gluon field energy, the quark condensate, and the QCD trace anomaly contribute $31.5\%$, $34.7\%$, $11.3\%$, and $22.5\%$, respectively, which are consistent with lattice QCD findings.

hep-ph↗

arXiv2025

Basis light-front quantization approach to deuteron

We obtain the deuteron's wave functions as eigenstates of the light-front quantum chromodynamics (QCD) Hamiltonian using a fully relativistic and nonperturbative approach based on light-front quantization, without an explicit confining potential. These eigenstates include six-quark and six-quark--one-gluon components. The deuteron wave function consists of both a singlet-singlet color state and additional hidden color states arising from non-trivial color rearrangements. Our results reveal that while the singlet-singlet state is present, the hidden color states collectively dominate, contributing a larger probability to the deuteron wave function. This highlights the significant role of hidden color components in the QCD description of nuclear structure. Using these wave functions, we investigate the deuteron's electromagnetic properties.

hep-ph↗

arXiv2025

Gravitational form factor $D$ of charmonium from shear stress

Based on our recent analysis of the hadronic matrix element of the stress-energy tensor in covariant light front dynamics, we extract the charmonium gravitational form factor $D(Q^2)$ from shear stress $T^{12}$. This is in contrast to our recent work using the (light-front) energy density $T^{+-}$. Indeed, by comparing these two currents, we identify terms that are responsible for the violation of the current conservation. Numerical results based on basis light-front quantization show that the violation effects are small and the $D$-term extracted from the two currents are close to each other, hence validating our previous work using $T^{+-}$.

hep-ph↗

arXiv2025

Color structure of deuteron on the light front

We investigate the color structure of the deuteron by solving the light-front QCD Hamiltonian for its six-quark and six-quark-one-gluon components using basis light-front quantization. In this framework, the deuteron wavefunction consists of a singlet-singlet color state as well as additional hidden color states arising from non-trivial color rearrangements. Our analysis shows that while the singlet-singlet state is present, the hidden color states collectively dominate, contributing a larger probability to the deuteron wavefunction. These findings provide new insights into the role of hidden color components in the QCD description of nuclear structure.

hep-ph↗

arXiv2025

Basis light-front quantization: Advancing a first principles approach for the nucleon

We present our recent progress in applying the basis light-front quantization approach to investigate the nucleon's structure. We solve its wave functions from the eigenstates of the light-front QCD Hamiltonian using a fully relativistic, nonperturbative approach without an explicit confining potential. These eigenstates are determined for the three-quark, three-quark-gluon, and three-quark-quark-antiquark Fock representations, making them suitable for low-resolution probes. From these wave functions, we compute the nucleon's quark and gluon matter densities, as well as its helicity and transversity distributions, showing qualitative consistency with experimental data. We also determine the quark and gluon helicity contributions to the proton spin and the tensor charges. The resulting light-front wave functions represent a significant step toward a unified description of hadron distribution functions in both longitudinal and transverse momentum space.

hep-ph↗