arXiv subjects

Qin-Tao Song

Publications and source records attributed to Qin-Tao Song.

At least 19 recordsLinked to original sources

arXiv2026

Next-to-next-to-leading order QCD corrections to pion (kaon)-induced exclusive Drell-Yan process

The high-energy pion and kaon beams proposed for future experiments at J-PARC offer a unique opportunity to investigate exclusive Drell-Yan processes induced by pions or kaons, which correspond to inverse deeply virtual meson production with $M=π,K$. To facilitate precise comparisons between theoretical predictions and forthcoming experimental data, we calculate the next-to-next-to-leading order (NNLO) QCD corrections to the processes $π^- p\to γ^*(\to l^+l^-) + n$ and $K^- p\to γ^*(\to l^+l^-) + Λ$. Our calculations are performed within the generalized parton distribution (GPD) factorization framework, accurate to leading twist in the generalized Bjorken limit ($Q^2\gg |t|,\,Λ_{\rm QCD}^2$). We find that the NNLO QCD corrections are substantial and positive; therefore, their inclusion is imperative for reliable theoretical predictions in confrontation with future experiments.

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arXiv2026

Deeply virtual pion production through two-loop order

Deeply virtual meson production (DVMP) is among the most prominent channels to extract the nucleon's generalized parton distributions (GPDs) at $ep$ scattering facilities such as {\tt JLab} and the upcoming {\tt EIC/EicC} experiments, which plays a vital role in unravelling the three-dimensional internal structure of nucleon. In this work we calculate for the first time the next-to-next-to-leading order (NNLO) QCD radiative corrections to the DV$π$P processes $γ_L^* p\to π^+ n$ and $γ_L^* p\to π^0 p$ in the generalized Bjorken limit $Q^2\gg \vert t\vert, Λ_{\text{QCD}}^2$, accurate at the leading twist within collinear factorization framework. The impact of the two-loop QCD corrections appears to be positive and substantial, including which considerably improves the agreement between the perturbative QCD prediction and the available {\tt JLab} data. In addition to the differential longitudinal DV$π$P cross section, we also study the impact of the two-loop QCD corrections on the transverse single-spin asymmetries (TSSA) in some benchmark kinematics at {\tt JLab}, {\tt EIC} and {\tt EicC}.

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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.

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arXiv2026

Accessing baryon-antibaryon generalized distribution amplitudes in $e^{\pm} γ\to e^{\pm} B \bar{B} $

$γ^* γ\to B \bar{B}$ is the golden process to access chiral-even di-baryon generalized distribution amplitudes (GDAs) as deeply virtual Compton scattering has proven to be for the generalized parton distributions. In the framework of colinear QCD factorization where the leading twist amplitude is the convolution of GDAs and a perturbatively calculable coefficient function, we study the scattering amplitude for the baryonic channels where $B$ is a spin $1/2$ baryon such as the nucleon or hyperon $Λ, Σ$. Taking into account the interfering QED amplitude, we calculate the cross section of the $e^\pm γ\to e^\pm B \bar B$ process that can be experimentally studied in $e^- e^+$ as well as in electron-ion facilities. We explore both the final state polarization summed case and the polarization dependent effects. Numerical estimates are presented for $e^- γ\to e^- p \bar{p}$, using motivated models for GDAs. Our results show that a first extraction of baryon-antibaryon GDAs from experimental measurements is feasible at Belle II.

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arXiv2025

Gravitational form factors in the perturbative limit

Generalized distribution amplitudes (GDAs) have attracted significant attention in recent years due to their connection with the energy-momentum tensor (EMT) form factors (FFs). The GDAs can be experimentally accessed through the study of amplitudes in $γ^{\ast} γ\to M_1 M_2$ and $γ^{\ast} \to M_1 M_2 γ$, where $M_1M_2$ is a pseudoscalar meson pair such as $πη$ and $ηη^{\prime}$. In this work, we calculate these amplitudes in the perturbative limit and express the extracted $M_1M_2$ GDAs in terms of meson distribution amplitudes that have been constrained by the previous experiments. Our explicit calculation verifies the existence of a new EMT FF that violates the conservation law of EMT when the hadronic matrix element of the EMT operator is considered separately for each quark flavor. In addition, our result shows that the $M_1M_2$ GDAs are identical in $γ^{\ast} γ\to M_1 M_2$ and $γ^{\ast} \to M_1 M_2 γ$, which confirms the universality of GDAs in the perturbative limit. In the future, the GDAs and the EMT FFs studied in this paper can be probed at Belle II. Our study enhances the accessibility to the $P$-wave GDAs in $γ^{\ast} γ\to M_1 M_2$ and $γ^{\ast} \to M_1 M_2 γ$, and provides a promising approach for searching for exotic hybrid mesons in future experiments.

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arXiv2025

Baryon-antibaryon generalized distribution amplitudes and $e^+ e^- \to B \bar{B} γ$

Baryon-antibaryon generalized distribution amplitudes (GDAs) give an access to timelike gravitational form factors (GFFs) which are complementary to the spacelike ones which can be deduced from the hadronic generalized parton distributions (GPDs) measured in deep exclusive electroproduction processes. They allow to probe the GFFs of unstable baryons in the baryon octet, since the second moments of hadronic generalized distribution amplitudes (GDAs) lead to the timelike GFFs. These GDAs can be measured in the process $e^+ e^- \to B \bar{B} γ$, in the generalized Bjorken regime where the invariant mass of the $B \bar{B}$ pair is near threshold at high energy facilities, such as BESIII, Belle II, and the proposed Super Tau-Charm Facility. In this work, we investigate this process using the QCD collinear factorization framework, where the scattering amplitudes are expressed in terms of the baryon timelike electromagnetic (EM) FFs and Compton FFs. We also provide a numerical estimate of the cross sections with a model for baryon-antibaryon GDAs. Our work provides us a possibility to extract the timelike baryon GFFs from near future experimental measurements, and these GFFs may be further used to study longstanding questions in hadronic physics such as the baryon spin decomposition and D-term.

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arXiv2025

Spin-orbit correlation and spatial distributions for spin-0 hadrons

The spin-orbit correlation in spin-0 hadrons can be investigated through the kinetic energy-momentum tensor form factor $\tilde F^q(t)$. We observe that the latter is also related to a torque about the radial direction, which we interpret as a chiral stress. If we neglect the quark mass contribution, then $\tilde F^q(t)$ is simply proportional to the electromagnetic form factor for spin-0 hadrons, and the spin-orbit correlation is equal to minus half of the valence quark number. Given the extensive studies on the electromagnetic form factor for spin-0 hadrons such as pions, kaons, and the $α$ particle, we present the spatial distributions of chiral stress and kinetic spin-orbit correlation based on current parametrizations of the pion electromagnetic form factor.

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arXiv2025

Twist-3 contribution in the Drell-Yan process with tensor-polarized deuteron

The tensor-polarized structures of the deuteron can be probed through the proton-deuteron Drell-Yan process, where the proton is unpolarized and the deuteron is tensor polarized. This measurement will be conducted at Fermilab in the near future. In this reaction, the twist-3 contribution is not negligible compared to the twist-2 contribution due to the limited invariant mass of the dilepton pair. We calculate the twist-3 contribution for the Drell-Yan cross section with a tensor-polarized deuteron target, preserving the U(1)-gauge invariance of the hadronic tensor. The cross sections and weighted cross sections are expressed in terms of the tensor-polarized parton distribution functions (PDFs), thus one can extract the PDFs $f_{1\scriptscriptstyle{LL}}$, $f_{\scriptscriptstyle{LT}}$, and $f^{(1)}_{\scriptscriptstyle{1LT}}$ from the experimental measurements of Drell-Yan process. Our study should be helpful to solve the puzzle in the tensor-polarized structures of the deuteron.

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arXiv2024

Kinematical higher-twist corrections in $γ^* \to M_1 M_2 γ$: Charged meson production

Generalized distribution amplitudes (GDAs) of mesons can be probed by the reactions $e^- e^+ \to M_1 M_2 γ$, which are accessible at electron-positron colliders such as BESIII and Belle II. After discussing the neutral meson production case in the first paper of this series \cite{Pire:2023kng}, we discuss here the complementary case of the charged meson ($M^+ M^-$) production, where one can extract the complete information on GDAs from the interference of the amplitudes of the two competing processes where the photon is emitted either in the initial or in the final state. Considering the importance of the charged meson production, we present a complete expression for the interference term of the cross section which is experimentally accessible thanks to its charge conjugation specific property. We adopt two types of models for leading twist $ππ$ GDAs to estimate the size of the interference term in the process $e^- e^+ \to π^+ π^- γ$ numerically, namely a model extracted from previous experimental results on $γ^* γ\to π^0 π^0$ at Belle and the asymptotic form predicted by QCD evolution equations. We include in the calculation the kinematical power suppressed (sometimes called kinematical higher-twist) corrections up to $1/Q^2$ for the helicity amplitudes. Both models of GDAs indicate that the kinematical corrections are not negligible for the interference term of the cross section measured at BESIII, thus it is necessary to include them if we try to extract the GDAs precisely. On the other side, the kinematical corrections are very tiny for the measurements at Belle II, and the leading twist-2 formula of the interference term will be good enough to describe the charge conjugation odd part of the differential cross section.

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arXiv2023

Novel relations for twist-3 tensor-polarized fragmentation functions in spin-1 hadrons

There are three types of fragmentation functions (FFs) which are used to describe the twist-3 cross sections of the hard semi-inclusive processes under QCD collinear factorization, and they are called intrinsic, kinematical, and dynamical FFs. In this work, we investigate the theoretical relations among these FFs for a tensor-polarized spin-1 hadron. Three Lorentz-invariance relations are derived by using the identities between the nonlocal quark-quark and quark-gluon-quark operators, which guarantee the frame independence of the twist-3 spin observables. The QCD equation of motion relations are also presented for the tensor-polarized FFs. In addition, we show that the intrinsic and kinematical twist-3 FFs can be decomposed into the contributions of twist-2 FFs and twist-3 three-parton FFs, and the latter are also called dynamical FFs. If one neglects the dynamical FFs, we can obtain relations which are analogous to the Wandzura-Wilczek relation. Then, the intrinsic and kinematical twist-3 FFs are expressed in terms of the leading-twist ones. Since the FFs of a spin-1 hadron can be measured at various experimental facilities in the near future, these theoretical relations will play an important role in the analysis of the collinear tensor-polarized FFs.

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arXiv2023

Gravitational transverse-momentum distributions

We study the energy-momentum tensor of spin-$0$ and spin-$\frac{1}{2}$ hadrons in momentum space. We parametrize this object in terms of so-called gravitational transverse-momentum distributions, and we identify in the quark sector the relations between the latter and the usual transverse-momentum distributions. Focusing on particular components of the energy-momentum tensor, we study momentum densities, flux of inertia and stress distribution in momentum space, revealing part of the wealth of physical information that can be gained from higher-twist transverse-momentum distributions.

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arXiv2023

Kinematical higher-twist corrections in $γ^* \to M_1 M_2 γ$: Neutral meson production

We carry out the calculation of kinematical higher-twist corrections to the cross section of $γ^* \to M_1 M_2 γ$ up to twist 4, where $M_i$ is a scalar or pseudoscalar neutral meson. The three independant helicity amplitudes are presented in terms of the twist-2 generalized distribution amplitudes (GDAs), which are important non-perturbative quantities for understanding the 3D structure of hadrons. Since this process can be measured by BESIII in $e^+ e^-$ collisions, we perform the numerical estimate of the kinematical higher-twist corrections by using the kinematics of BESIII. We adopt the $ππ$ GDA extracted from Belle measurements and the asymptotic $ππ$ GDA to study the size of the kinematical corrections in the case of pion meson pair, and a model $ηη$ GDA is used to see the impact of target mass corrections $\mathcal O(m^2/s)$ for $γ^* \to ηηγ$. Our results show that the kinematical higher-twist corrections account for $\sim 20\%$ of the cross sections at BESIII on the average, and it is necessary to include them if one tries to extract GDAs from experimental measurements precisely. We also comment the case of $π^0 η$ production which is important for the search of hybrid mesons.

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arXiv2022

Kinematical higher-twist corrections in $γ^* γ\to M \bar M $

We estimate kinematical higher-twist (up to twist 4) corrections to the $γ^*(q_1) γ(q_2) \to M(p_1) \bar{M}(p_2)$ amplitudes at large $Q^2=-q_1^2$ and small $s=(q_1+q_2)^2$, where $M$ is a scalar or pseudoscalar meson. This process is known to factorize at leading twist into a perturbatively calculable coefficient function and generalized distribution amplitudes (GDAs). The kinematical higher-twist contributions of order $s/Q^2$ and $m^2/Q^2$ turn out to be important in the cross section, considering the kinematics accessible at Belle and Belle II. We present numerical estimates for the cross section for $γ^* γ\to π^0 π^0$ with the $ππ$ GDA extracted from Belle measurements and with the asymptotic $ππ$ GDA as inputs to study the magnitude of the kinematical corrections. To see how the target mass corrections of order $m^2/Q^2$ affect the cross section, we also perform the calculation for $γ^* γ\to ηη$ by using a model $ηη$ GDA.In the range $s> 1$ GeV$^2$, the kinematical higher-twist corrections account for $\sim 15 \%$ of the total cross section, an effect which is not negligible. Since $ππ$ GDAs are the best way to access the pion energy-momentum tensor (EMT), our study demonstrates that an accurate evaluation of EMT form factors requires the inclusion of kinematical higher-twist contributions.

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arXiv2022

Kinematical higher-twist corrections in $γ^* γ\to ππ$

We apply the Braun-Manashov technique to improve the description of $γ^*(q_1) γ(q_2) \to M(p_1) M(p_2)$ amplitudes at large $Q^2=-q_1^2$ and small $s=(q_1+q_2)^2$. We derive the kinematical higher-twist contributions of order $s/Q^2$ and $m^2/Q^2$ to the helicity amplitudes and estimate their sizes in the kinematics accessible at Belle and Belle II.Since pion GPDs cannot be directly measured by experiment, $ππ$ GDAs are the best way to investigate the energy-momentum tensor form factors for pions.

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arXiv2022

Mass spectra and decay properties of the higher excited $ρ$ mesons

Although there are some experimental hints for the higher excited $ρ$ mesons, our knowledge of their properties is much poor theoretically. Based on our recent work about excited $ρ$ mesons [Phys.Rev.D104(2021)034013], we present the mass spectra and decay properties of the higher excited $ρ$ mesons with the modified Godfrey-Isgur quark model and the $^3P_0$ strong decay model, and compare our predictions with the experimental hints, which should be helpful to search for these higher excited $ρ$ mesons.

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arXiv2022

Equation-of-motion and Lorentz-invariance relations for tensor-polarized parton distribution functions of spin-1 hadrons

Structure functions of polarized spin-1 hadrons will be measured at various accelerator facilities in the near future. Recently, transverse-momentum-dependent and collinear parton distribution functions were theoretically proposed at twist 3 and twist 4 in addition to the twist-2 ones, so that full investigations became possible for structure functions of spin-1 hadrons in the same level with those of the spin-1/2 nucleons. Furthermore, twist-3 tensor-polarized multiparton distribution functions were also recently found for spin-1 hadrons. In this work, we show relations among the collinear parton- and multiparton-distribution functions for spin-1 hadrons by using equation of motion for quarks. These relations are valuable in constraining the distribution functions and learning about multiparton correlations in spin-1 hadrons.

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arXiv2022

Useful relations and sum rules for PDFs and multiparton distribution functions of spin-1 hadrons

There are two types of polarizations in spin-1 hadrons, and they are vector and tensor polarizations. The latter is a unique one since it does not exist in the spin-1/2 proton. The vector-polarized PDFs are the same for both the proton and spin-1 hadrons; therefore, we mainly investigate the unique PDFs in tensor-polarized hadrons. By using the operator product expansion, the twist-3 PDF $f_{LT}$ can be expressed by two terms in the same way with $g_T$ of the proton. The first term is determined by the twist-2 PDF $f_{1LL}$ (or $b_1$) which was measured by an experiment, and the second term is expressed by twist-3 quark-gluon distributions. If we neglect the higher-twist effects, $f_{LT}$ is simply given by $f_{1LL}$, and this relation is similar to the Wandzura-Wilczek relation of $g_T$. Furthermore, a new sum rule is also obtained for $f_{2LT}=2/3 f_{LT}- f_{1LL}$, which is analogous to the Burkhardt-Cottingham sum rule, in the tensor-polarized spin-1 hadrons. In future, these interesting relations could be studied at accelerator facilities, such as the Jefferson Laboratory, the Fermilab, the nuclotron-based ion collider facility in Russia, the proposed electron-ion colliders in US and China.

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arXiv2022

TMDs for spin-1 hadrons

Transverse-momentum-dependent parton distribution functions (TMDs) were investigated at the twists 3 and 4 for spin-1 hadrons in addition to the twist-2 ones. They were found by studying all the possible decomposition of a quark correlation function in a Lorentz-invariant way with the Hermiticity and parity invariance. The time-reversal invariance was not imposed for the TMDs due to an active role of gauge links; however, they were used for collinear parton distribution functions (PDFs) by integrating the TMDs over the transverse momentum. We found that 30 TMDs exist in the tensor-polarized spin-1 hadron at the twists 3 and 4, whereas there are 10 TMDs at the twist 2. We also showed that there are 3 collinear PDFs at the twists 3 and 4. The corresponding TMD fragmentation functions exist sat the twists 2, 3, and 4 simply by changing function names and variables. Since the time-reversal invariance is valid in the collinear PDFs, the integrals of time-reversal-odd TMDs over the transverse momentum should vanish. It leads to the sum rules $ \int d^2 k_T \, h_{1LT} (x, k_T^{\, 2}) = \int d^2 k_T g_{LT} (x, k_T^{\, 2}) = \int d^2 k_T h_{LL} (x, k_T^{\, 2}) = \int d^2 k_T h_{3LL}(x, k_T^{\, 2}) =0$ on the time-reversal odd TMDs at the twists 3 and 4.

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