arXiv subjects

Vadim Baru

Publications and source records attributed to Vadim Baru.

At least 19 recordsLinked to original sources

arXiv2026

Vector charmonium(-like) states in the energy range of 4.1-4.6 GeV

The spectrum of vector charmonium(-like) states in the 4.1\dash4.6~GeV energy region exhibits a long-standing tension between inclusive and exclusive measurements. While the inclusive $R$-value indicates only conventional vector charmonia such as $ψ(4160)$ and $ψ(4415)$, exclusive $e^+e^-$ cross sections reveal additional structures whose parameters strongly depend on the observed final states when fitted with Breit--Wigner functions. This puzzling pattern suggests that coupled-channel and threshold effects play an essential role. In this work, we develop a unified coupled-channel framework for the $1^{--}$ resonances in this energy region. The framework incorporates the $S$-wave open-charm channels $D\bar{D}_1$, $D^*\bar{D}_1$, and $D^*\bar{D}_2^*$ constrained by heavy-quark spin symmetry, optional bare poles associated with $ψ(4160)$ and $ψ(4415)$, and final-state interactions in the $Z_c$ channels. We perform simultaneous fits to the BESIII cross sections for $e^+e^-\to J/ψπ^+π^-$, $h_cπ^+π^-$, $D\bar{D}^*π$, $D^*\bar{D}^*π$, $J/ψη$, and $χ_{c0}ω$, together with invariant-mass distributions exhibiting the $Z_c(3900)$ and $Z_c(4020)$ structures. The benchmark models differ in the number of bare seed states and the fitting strategy. We show that even the purely dynamical scheme without bare charmonia captures the gross features of the analyzed distributions. The inclusion of bare compact states improves the fit quality but does not change the conclusion that the measured line shapes can be understood in terms of strong coupled-channel effects with dynamically generated poles. We also discuss possible heavy-quark spin partners of the exotic $1^{--}$ states.

hep-ph↗

arXiv2026

Shallow $T_{bc}$ states from an EFT analysis of $B^{()} \bar D^{()}$ scattering on the lattice

We present an effective field theory (EFT) framework for coupled-channel $B^{(*)}\bar D^{(*)}$ scattering, applying it to recent lattice QCD results by Alexandrou et al. [Phys. Rev. Lett. 132, 151902 (2024)]. Two complementary EFT approaches are developed: (1) A low-energy theory near the $B \bar D$ ($J=0$) and $B^* \bar D$ ($J=1$) thresholds, where coupled-channel effects are integrated out; (2) A coupled-channel formulation, where all relevant momentum scales are treated as soft, incorporating contact interactions and one-pion exchange (OPE). Importantly, OPE contributes to the lowest channels only through off-diagonal transitions, thus resulting in the appearance of the left-hand cut from two-pion exchange. The two approaches yield mutually consistent results, supporting the existence of shallow bound states in both channels, in agreement with the lattice findings. The finite-volume spectra and extracted pole positions show a near-degeneracy in $J=0$ and $J=1$ channels, consistent with heavy-quark spin symmetry (HQSS). Using HQSS, we predict additional shallow bound states near the $B \bar{D}^*$ and $B^* \bar{D}^*$ thresholds, which are accessible to future lattice simulations. The effect of OPE on the finite volume spectra is found to be small, with only moderate impact on HQSS partners.

hep-ph↗

arXiv2026

Ab initio charge form factors and radii of light isoscalar nuclei: Role of the two-body charge density

We make \textit{ab initio} predictions of charge form factors (FFs) and radii for the isoscalar nuclei $^6$Li and $^8$Be using the Jacobi-coordinate No-Core Shell Model. The calculations employ chiral semilocal momentum-space regularized two- and three-nucleon interactions, together with consistently regularized one- and two-nucleon electromagnetic charge operators. With the short-range charge density fixed to the $^4$He charge radius, the predicted FFs and the $^6$Li radius show good agreement with available experimental data. We find that two-nucleon charge density contributions are essential for describing the FFs, particularly at intermediate and large momentum transfers. Although their influence on the charge radii is limited, these contributions remain crucial for attaining accurate predictions. The present results highlight the importance of two-nucleon charge operators in addressing the long-standing underestimation of nuclear charge radii in \textit{ab initio} calculations based on modern chiral interactions.

nucl-th↗

arXiv2025

Left-hand cut problem in lattice QCD and an EFT-based solution

Lattice QCD has become an essential tool for studying the hadron-hadron interaction from the first principles. However, when extracting infinite-volume scattering parameters from finite-volume energy levels, the traditional Lüscher formula encounters limitations due to the left-hand cut induced by long-range interactions such as the one-pion exchange. In this work, we propose an alternative approach based on chiral effective field theory combined with a Hamiltonian method in the plane wave basis. By solving a Schrödinger-like equation in the finite volume, our method connects the finite-volume energy spectrum with infinite-volume observables, while systematically incorporating the long-range physics and solving the left-hand cut problem. The use of the plane wave basis mitigates issues related to partial wave mixing. Our numerical results for $DD^*$ scattering at $m_π\approx$ 280 MeV demonstrate that this approach overcomes the limitations of the Lüscher method and points towards a resonance interpretation of the $T_{cc}(3875)$ state, as opposed to the virtual state predicted by traditional analyses.

hep-lat↗

arXiv2025

Doubly charm tetraquark channel with isospin $1$ from lattice QCD

Experimentally, the doubly charm tetraquark channel $cc\bar q\bar q$ with $q\!=\!u,d$ features an exotic hadron, $T_{cc}$, with isospin $I\!=\!0$ near the $DD^*$ threshold, while no peak was observed for $I\!=\!1$. We present a lattice QCD study of this channel with $I\!=\!1$, $J^P\!=\!1^+$ and $m_π\simeq 280~$MeV. Finite-volume energies calculated across five charm quark masses consistently feature a positive energy shift with respect to non-interacting energies, indicating repulsive interaction at energies near threshold. These energies are used to compute the $DD^*$ scattering amplitude using both the standard Lüscher method and the recently proposed effective-field-theory-based approach in the plane-wave basis, which incorporates the long-range interactions and the left-hand cut. Both analyses render a small negative scattering length and the scattering amplitude that does not feature any poles in the energy region near the $DD^*$ threshold, in line with LHCb results. We identify that the Wick contraction resembling $t$-channel isovector-vector meson exchanges between $D$ and $D^*$ plays a key role in distinguishing between the $I=0$ and $I=1$ channels, leading to repulsion in the $I=1$ and attraction in the $I=0$ channel.

hep-lat↗

arXiv2025

Towards a precision determination of the $X(6200)$ parameters from data

In a recent paper, Phys. Rev. Lett. {\bf 126}, 132001 (2021), the LHCb data on the di-$J/ψ$ production in proton-proton collisions were analysed in a coupled-channel framework based on double-vector-charmonium channels. This investigation identified a robust pole near the $J/ψJ/ψ$ threshold, tagged $X(6200)$, suggesting it as a new state. The present work extends that investigation by incorporating recent di-$J/ψ$ production data from the CMS and ATLAS Collaborations and performing a combined analysis of all three datasets. This study confirms the existence of the $X(6200)$, and its pole position is now determined with a higher precision than in the previous study, where only a single dataset was employed. The pole corresponding to the $X(6900)$ is also extracted, though its actual position and residue depend on a particular coupled-channel model employed and deviate from values reported in the experimental investigations. Moreover, we demonstrate that the currently available data do not allow one to determine whether there is an additional pole in the studied mass range. The di-electron width of the $X(6200)$ is estimated under different conjectures on its quantum numbers, and the number of such states that can be annually produced at the future Super $τ$-Charm Facility is estimated. The line shape in the complimentary $J/ψψ(2S)$ channel is discussed, and a good agreement with the ATLAS data is found.

hep-ph↗

arXiv2025

Internal structure of the $T_{cc}(3875)^+$ from its light-quark mass dependence

We employ a chiral effective field theory-based approach to connect $DD^*$ scattering observables at the physical and variable pion masses accessible in lattice QCD simulations. We incorporate all relevant scales associated with three-body $DDπ$ dynamics and the left-hand cut induced by the one-pion exchange for pion masses higher than the physical one, as required by analyticity and unitarity. By adjusting the contact interactions to match experimental data at the physical pion mass and lattice finite-volume energy levels at $m_π = 280$ MeV, we predict the trajectory of the $T_{cc}$ pole as a function of the pion mass, finding it consistent with the hadronic-molecule scenario. In particular, we find that the explicit treatment of the one-pion exchange has a pronounced effect on the pole trajectory for $m_π\gtrsim 230$ MeV by pushing it into the complex energy plane.

hep-ph↗

arXiv2024

How many vector charmonium(-like) states sit in the energy range from $4.2$ to $4.35$ GeV?

In recent years many vector charmonium(-like) states were reported by different electron-positron collider experiments above $4.2$ GeV. However, so far, there not only exists sizable tension in the parameters of those states, but there is also no consensus on the number of the vector states in this energy range. To some extend, this might be caused by the fact that the experimental data were typically analyzed in single channel analyses employing overlapping Breit-Wigner functions, in particular ignoring the effect of opening thresholds. In this study, we focus on the mass range between $4.2$ and $4.35$ GeV, conducting a comprehensive analysis of eight different final states in $e^+ e^-$ annihilation. Our findings demonstrate that, within this mass range, a single vector charmonium-like state, exhibiting properties consistent with a $D_1\bar D$ molecular structure and characterized by a pole location $\sqrt{s_\text{pole}^{Y(4230)}}=\left( 4227{\pm} 3 {-} \frac{i}{2}(50^{+10}_{-5}) \right) \text{MeV}$, can effectively describe all the collected data. This is made possible by allowing for an interference with the well-established vector chamonium $ψ(4160)$ along with the inclusion of the $D_1\bar D$ threshold effect. Moreover, in contrast to experimental analyses, our study reveals that the highly asymmetric total cross sections for $e^+e^-\to J/ψππ$ and $e^+e^-\to J/ψK\bar K$ around 4230 MeV stem from the same physics, rooted in the approximate SU(3) flavor symmetry of QCD.

hep-ph↗

arXiv2024

How does the $X(3872)$ show up in $e^+e^-$ collisions: dip versus peak

We demonstrate that the dip observed near the total energy of 3872 MeV in the recent cross section data from the BESIII Collaboration for $e^+e^-\to J/ψπ^+π^- $ admits a natural explanation as a coupled-channel effect: it is a consequence of unitarity and a strong $S$-wave $D\bar D^*$ attraction that generates the state $X(3872)$. We anticipate the appearance of a similar dip in the $e^+e^-\to J/ψπ^+π^-π^0$ final state near the $D^*\bar{D}^*$ threshold driven by the same general mechanism, then to be interpreted as a signature of the predicted spin-two partner of the $X(3872)$.

hep-ph↗

arXiv2024

Solving the left-hand cut problem in lattice QCD: $T_{cc}(3875)^+$ from finite volume energy levels

We discuss a novel effective-field-theory-based approach for extracting two-body scattering information from finite volume energies, serving as an alternative to Lüscher's method. By explicitly incorporating one-pion exchange, we overcome the challenging left-hand cut problem in Lüscher's method and can handle finite volume energy levels both below and above the left-hand cut. Applied to the lattice data for $DD^*$ scattering at a pion mass of 280 MeV, as an illustrative example, our results reveal the significant impact of the one-pion exchange on P-wave and S-wave phase shifts. The pole position of the $T_{cc}(3875)^+$ state, extracted from the finite-volume energy levels at this pion mass while taking into account left-hand cut effects, range corrections and partial-wave mixing, is consistent with a near-threshold resonance. This study demonstrates, for the first time, that two-body scattering information can be reliably extracted from lattice spectra including the left-hand cut.

hep-lat↗

arXiv2024

Production of the $X(4014)$ as the spin-2 partner of $X(3872)$ in $e^+e^-$ collisions

In 2021, the Belle collaboration reported the first observation of a new structure in the $ψ(2S) γ$ final state produced in the two-photon fusion process. In the hadronic molecule picture, this new structure can be associated with the shallow isoscalar $D^*\bar{D}^*$ bound state and as such is an excellent candidate for the spin-2 partner of the $X(3872)$ with the quantum numbers $J^{PC}=2^{++}$ conventionally named $X_2$. In this work we evaluate the electronic width of this new state and argue that its nature is sensitive to its total width, the experimental measurement currently available being unable to distinguish between different options. Our estimates demonstrate that the planned Super $τ$-Charm Facility offers a promising opportunity to search for and study this new state in the invariant mass distributions for the final states $J/ψγ$ and $ψ(2S)γ$.

hep-ph↗

arXiv2023

The $Y(4230)$ as a $D_1 \bar{D}$ molecule

We show that the currently available data are consistent with $Y(4230)$ being a $D_1 \bar{D}$ hadronic molecule. By a simultaneous fit to data from $e^+ e^- \rightarrow D^0 D^{* -} π^+,\: J/ψπ^+ π^-,\: J/ψK^+ K^-,\: h_c π^+ π^-,\: J/ψη,\: χ_{c0} ω,\: χ_{c1}(3872) γ$ and $μ^+ μ^-$, we demonstrate that this single state can explain the experimental signals in the mass range from $4.2$ to $4.35\, \rm{GeV}.$

hep-ph↗

arXiv2023

Role of left-hand cut contributions on pole extractions from lattice data: Case study for $T_{cc}(3875)^+$

We discuss recent lattice data for the $T_{cc}(3875)^+$ state to stress, for the first time, a potentially strong impact of left-hand cuts from the one-pion exchange on the pole extraction for near-threshold exotic states. In particular, if the left-hand cut is located close to the two-particle threshold, which happens naturally in the $DD^*$ system for the pion mass exceeding its physical value, the effective-range expansion is valid only in a very limited energy range up to the cut and as such is of little use to reliably extract the poles. Then, an accurate extraction of the pole locations requires the one-pion exchange to be implemented explicitly into the scattering amplitudes. Our findings are general and potentially relevant for a wide class of hadronic near-threshold states.

hep-ph↗

arXiv2022

Effective range expansion for narrow near-threshold resonances

We discuss some general features of the effective range expansion, the content of its parameters with respect to the nature of the pertinent near-threshold states and the necessary modifications in the presence of coupled channels, isospin violations and unstable constituents. As illustrative examples, we analyse the properties of the $χ_{c1}(3872)$ and $T_{cc}^+$ states supporting the claim that these exotic states have a predominantly molecular nature.

hep-ph↗

arXiv2022

Coupled-channel approach to $T_{cc}^+$ including three-body effects

A coupled-channel approach is applied to the charged tetraquark state $T_{cc}^+$ recently discovered by the LHCb Collaboration. The parameters of the interaction are fixed by a fit to the observed line shape in the three-body $D^0D^0π^+$ channel. Special attention is paid to the three-body dynamics in the $T_{cc}^+$ due to the finite life time of the $D^*$. An approach to the $T_{cc}^+$ is argued to be self-consistent only if both manifestations of the three-body dynamics, the pion exchange between the $D$ and $D^*$ mesons and the finite $D^*$ width, are taken into account simultaneously to ensure that three-body unitarity is preserved. This is especially important to precisely extract the pole position in the complex energy plane whose imaginary part is very sensitive to the details of the coupled-channel scheme employed. The $D^0D^0$ and $D^0D^+$ invariant mass distributions, predicted based on this analysis, are in good agreement with the LHCb data. The low-energy expansion of the $D^*D$ scattering amplitude is performed and the low-energy constants (the scattering length and effective range) are extracted. The compositeness parameter of the $T_{cc}^+$ is found to be close to unity, which implies that the $T_{cc}^+$ is a hadronic molecule generated by the interactions in the $D^{*+}D^0$ and $D^{*0}D^+$ channels. Employing heavy-quark spin symmetry, an isoscalar $D^*D^*$ molecular partner of the $T_{cc}^+$ with $J^P=1^+$ is predicted under the assumption that the $ DD^*$-$D^*D^*$ coupled-channel effects can be neglected.

hep-ph↗

arXiv2021

Is the existence of a $J/ψJ/ψ$ bound state plausible?

In a recent measurement LHCb reported pronounced structures in the $J/ψJ/ψ$ spectrum. One of the various possible explanations of those is that they emerge from non-perturbative interactions of vector charmonia. It is thus important to understand whether it is possible to form a bound state of two charmonia interacting through the exchange of gluons, which hadronise into two pions at the longest distance. In this paper, we demonstrate that, given our current understanding of hadron-hadron interactions, the exchange of correlated light mesons (pions and kaons) is able to provide sizeable attraction to the di-$J/ψ$ system, and it is possible for two $J/ψ$ mesons to form a bound state. As a side result we find from an analysis of the data for the $ψ(2S)\to J/ψππ$ transition including both $ππ$ and $K\bar K$ final state interactions an improved value for the $ψ(2S)\to J/ψ$ transition chromo-electric polarisability: $|α_{ψ(2S)J/ψ}|= (1.8\pm 0.1)~\mbox{GeV}^{-3}$, where the uncertainty also includes the one induced by the final state interactions.

hep-ph↗

arXiv2021

Coupled-channel interpretation of the LHCb double-$J/ψ$ spectrum and hints of a new state near the $J/ψJ/ψ$ threshold

Recently, the LHCb Collaboration reported pronounced structures in the invariant mass spectrum of $J/ψ$-pairs produced in proton-proton collisions at the Large Hadron Collider. In this Letter, we argue that the data can be very well described within two variants of a coupled-channel approach employing $T$-matrices consistent with unitarity: (i) with just two channels, $J/ψJ/ψ$ and $ψ(2S)J/ψ$, as long as energy-dependent interactions in these channels are allowed, or (ii) with three channels $J/ψJ/ψ$, $ψ(2S)J/ψ$ and $ψ(3770)J/ψ$ with just constant contact interactions. Both formulations hint at the existence of a near-threshold state in the $J/ψJ/ψ$ system with the quantum numbers $J^{PC}=0^{++}$ or $2^{++}$, which we refer to as $X(6200)$. We suggest experimental tests to check the existence of this state and discuss what additional channels need to be studied experimentally to allow for distinctive tests between the two mechanisms proposed. If the molecular nature of the $X(6200)$, as hinted by the three-channel approach, is confirmed, many other double-quarkonium states should exist driven by the same binding mechanism. In particular, there should be an $η_cη_c$ molecule with a similar binding energy.

hep-ph↗

arXiv2021

Revisiting the nature of the $P_c$ pentaquarks

The nature of the three narrow $P_c$ states, i.e., $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$, is under intense discussion since their discovery from the updated analysis by LHCb. In this work we extend our previous coupled-channel approach [Phys. Rev. Lett. \bf{124}, 072001 (2020)] by including the $Λ_c\bar{D}^{(*)}$ and $η_cp$ as explicitly in addition to the $J/ψp$, as required by unitarity and heavy quark spin symmetry (HQSS). Since inelastic parameters are very badly constrained by the current data, three schemes are considered: (a) scheme I with pure contact interactions and without the $Λ_c\bar D^{(*)}$ interactions, (b) scheme II, where the one-pion exchange is added to scheme I, and (c) scheme III, where the $Λ_c \bar D^{(*)}$ are included in addition. It is shown that to obtain cutoff independent results, OPE in the multichannel system is to be supplemented with $S$-$D$ mixing contact terms. We demonstrate that the experimental data for the $J/ψp$ invariant mass distribution are consistent with the interpretation of the $P_c(4312)$ and $P_c(4440)/P_c(4457)$ as $Σ_c\bar{D}$ and $Σ_c \bar{D}^{*}$ hadronic molecules, respectively, and that the data show clear evidence for a new narrow $P_c(4380)$, as a $Σ_c^*\bar D$ molecule, which should exist as a consequence of HQSS. While two equally good solutions are found in scheme I, only one of these solutions with the quantum numbers of the $P_c(4440)$ and $P_c(4457)$ being $J^P=3/2^-$ and $1/2^-$, respectively, survives the requirement of regulator independence once the OPE is included. Moreover, we predict the line shapes in the elastic and inelastic channels and demonstrate that those related to the $P_c(4440)$ and the $P_c(4457)$ in the corresponding $Σ_c^{(*)}\bar{D}$ and $η_cp$ mass distributions allow one to confirm the quantum numbers given above, once the data are available.

hep-ph↗