Vadim Baru

We present an effective field theory (EFT) framework for coupled-channel B ( * ) D ¯ ( * ) scattering, applying it to recent lattice QCD results by Alexandrou []. Two complementary EFT approaches are developed: (1) a low-energy theory near the B D ¯ ( J = 0 ) and B * D ¯ ( J = 1 ) thresholds, where coupled-channel effects are integrated out; (2) 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 D ¯ * and B * 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.

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.

To improve the theoretical understanding of multiquark states like Z b ( 10610 ) and Z b ( 10650 ) , we calculate the heavy-meson heavy-(anti)meson scattering potential up to next-to-leading order, O ( Q 2 ) , within chiral effective field theory ( χ EFT ) employing a power counting scheme that explicitly keeps track with the large momentum scale Q ∼ 2 μ δ (where δ = m V − m P is the vector-pseudoscalar mass difference and μ their reduced mass) introduced by the coupled channel dynamics. We provide expressions for the two-pion exchange (TPE) terms up to O ( Q 2 ) and their partial-wave decomposition. We show that these potentials are well approximated by contact terms at O ( Q 2 ) , with minor residual nonanalytic TPE contributions, supporting χ EFT convergence in the theoretical predictions for Z b ( 10610 ) and Z b ( 10650 ) , as well as their spin partners. These findings are also relevant for D ( * ) D ( * ) scattering, especially for the T c c state, for both physical and lattice quantum chromodynamics (QCD) data with moderately larger pion masses. We further demonstrate that the differences between isovector and isoscalar potentials for heavy mesons are naturally explained by the TPE contributions. Published by the American Physical Society 2025

In a recent paper [], the LHCb data on the di- J / ψ production in proton-proton collisions were analyzed in a coupled-channel framework based on double-vector-charmonium channels. This investigation identified a robust pole near the J / ψ J / 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 dielectron 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 / ψ ψ ( 2 S ) channel is discussed, and a good agreement with the ATLAS data is found.

Experimentally, the doubly charm tetraquark channel c c q ¯ q ¯ with q = u , d features an exotic hadron, T c c , with isospin I = 0 near the D D * 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 ≃ 280 MeV . Finite-volume energies calculated across five charm quark masses consistently feature a positive energy shift with respect to noninteracting energies, indicating repulsive interaction at energies near the threshold. These energies are used to compute the D D * 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 a scattering amplitude that does not feature any poles in the energy region near the D D * threshold, in line with the 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.