arXiv subjects

Alexey Nefediev

Publications and source records attributed to Alexey Nefediev.

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 $\psi(4160)$ and $\psi(4415)$, 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 $\psi(4160)$ and $\psi(4415)$, and final-state interactions in the $Z_c$ channels. We perform simultaneous fits to the BESIII cross sections for $e^+e^-\to J/\psi\pi^+\pi^-$, $h_c\pi^+\pi^-$, $D\bar{D}^*\pi$, $D^*\bar{D}^*\pi$, $J/\psi\eta$, and $\chi_{c0}\omega$, 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.

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arXiv2026

What can we learn from the radiative decays of the $D_{s1}(2460)$ meson?

We study the radiative decays $D_{s1}(2460)\toγD^{*}_{s0}(2317)$ and $D_{s1}(2460)\to γD^0K^+/γD^+K^0$ and argue that their simultaneous experimental measurement, or at least a constraint on the ratio of the corresponding branching fractions, can allow one to probe the nature of the $D^{*}_{s0}(2317)$ and $D_{s1}(2460)$ mesons.

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arXiv2026

Exotic hadrons associated with $b$-quark

Compared to charmonium-like states, exotic hadrons associated with $b$-quark offer distinct advantages for exploring the nature of multiquark phenomena and the dynamics of the strong interaction. Due to the heavier bottom quark mass, theoretical calculations, particularly those based on effective field theories and potential models, tend to be more reliable and under better control in the bottomonium sector. With its clean $e^+e^-$ collision environment and high luminosity, the Belle and Belle II experiments are ideally suited to explore these exotic hadrons associated with $b$-quark, including $Z_b$, $X_b$, and $Y_b$ states, and charmonium-like states in $B$ decays. Utilizing the large proton--proton collision dataset, the LHCb experiment has conducted extensive investigations of heavy-flavor multiquark states through $B$ and $\Lambda_b$ decay channels. The relevant phenomenological interpretations are also reviewed.

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arXiv2026

Radiative decays of hadronic molecules: From confusion to inspiration

Radiative decays of hadronic states provide an essential source of information that can facilitate deciphering their nature and properties. However, a lot of confusion concerning radiative decays of hadronic molecules and their interpretation can be found in the literature. In this paper, we briefly review several types of such decays and pinpoint similarities and essential differences between them. In particular, we emphasise the crucial role played by the hierarchy of the scales relevant to the studied system and the resulting necessity of employing an approach that considers them appropriately. We illustrate the situation with several instructive examples.

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arXiv2025

Chiral symmetry and its breaking

In addition to fundamental symmetries playing a crucial role for establishing the Standard Model of fundamental interactions, approximate symmetries provide essential insight into the respective phenomena and shed light on the underlying physics. Here we give a brief pedagogical introduction to chiral symmetry as an approximate but still rather accurate symmetry of strong interactions and its spontaneous breaking in the vacuum of Quantum Chromodynamics. Special attention is paid to a microscopic picture of this phenomenon and understanding a dual nature of the chiral pion that is the Goldstone boson related to spontaneous breaking of chiral symmetry and the lowest pseudoscalar quark-antiquark state in the spectrum of hadrons simultaneously.

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arXiv2025

Quark models: What can they teach us?

Quark models have a more than 60-year history and through this time they served as a powerful investigation and prediction tool in hadronic physics. In recent years, a lot of new experimental information has been arriving on hadrons that do not qualify as simple quark model states. Yet, quark models remain the cornerstone of the classification scheme for hadrons, provide valuable insights into various phenomena inherent in QCD, and facilitate gaining a clear and physically transparent picture of the underlying physics. In the spotlight of this review is a chiral quark model inspired by quantum field theory approach to confined quarks. The model is well suited for studies of spontaneous breaking of chiral symmetry in the vacuum of QCD as well as its implications in the spectrum of hadrons. It can also be employed to investigate chiral restoration at finite temperatures.

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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.

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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 $X(3872)$.

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arXiv2024

Extraction of nonperturbative parameters for $D^{(*)}$ mesons from lattice data

Recent data for the masses of $D$ and $D^*$ mesons determined using methods of lattice QCD for several values of the charm quark mass different from its physical mass are analysed in Heavy Quark Effective Theory. Nonperturbative parameters are extracted that arise at order ${\cal O}(1/m_c)$ in the heavy-quark mass expansion of a heavy-light meson mass. The determined parameters are used to establish the charm quark masses corresponding to the employed lattice sets.

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arXiv2024

Towards the quark mass dependence of $T_{cc}^+$ from lattice QCD

The $DD^*$ scattering phase shifts in the $T_{cc}^+=cc\bar{u}\bar{d}$ channel are extracted from lattice QCD for five different charm quark masses and a fixed light-quark mass corresponding to $m_π\simeq 280$~MeV. The phase shifts are analysed employing two approaches: effective range expansion and Lippmann--Schwinger equation derived in the effective field theory. In the latter case, the results imply an attraction at short range parametrised by contact terms and a slight repulsion at long range mediated by one-pion exchange with $m_π>m_{D^*}-m_D$. The poles in the amplitude across the complex energy plane are extracted and their trajectories are discussed as the charm quark mass is varied. Two complex conjugate poles corresponding to a resonance below threshold are found for $m_c$ close to the physical value. They turn into a pair of virtual states at the largest $m_c$ studied. With further increasing $m_c$, one virtual pole representing $T_{cc}^+$ is expected to move towards the two-body threshold and turn into a bound state. The light-quark mass dependence of the $T_{cc}^+$ pole is briefly discussed using the data on $DD^*$ scattering from other lattice collaborations.

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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 $ψ(2S)γ$.

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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.

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arXiv2023

Chromopolarizabilities of fully-heavy baryons

We compute the chromopolarizabilities of the fully heavy baryons $Ω_{QQQ'}$ ($Q,Q'=b,c$) in the framework of potential nonrelativistic quantum chromodynamics. At leading order, the fully heavy hadrons are considered as ground chromo-Coulombic bound states. We find that the chromopolarizability $β_Ω$ of a fully heavy baryon $QQQ$ is 2.6 times the chromopolarizability $β_ψ$ of the quarkonium $\bar{Q}Q$ with the same heavy quark flavor $Q$. This result is accurate up to the correction of the order 0.3 for $Q=b$ and provides an order-of-magnitude estimate for $Q=c$. We discuss the dependence of the ratio $β_Ω/β_ψ$ on the heavy quark mass $m_Q$ and the strong coupling constant $α_s$ as well as on the ratio of the masses $m_{Q'}/m_Q$, in the case not all quarks in the baryon are identical. Since the chromopolarizability of heavy hadrons defines the strength of their interaction at low energies mediated by soft gluons, which at long range hadronize into pairs of pions and kaons, our findings argue in favor of the existence of near-threshold states composed of pairs of fully heavy baryons.

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arXiv2022

Do near-threshold molecular states mix with neighbouring $\bar QQ$ states?

The last two decades are marked by a renaissance in hadronic spectroscopy caused by the arrival of vast experimental information on exotic states in the spectrum of charmonium and bottomonium. Most of such states have properties at odds with the predictions of the quark model and reside very close to strong hadronic thresholds. Prominent examples are provided by the glorious $X(3872)$ charmonium-like state and the doubly charmed tetraquark $T_{cc}^+$ with the masses within less than 1 MeV from the $D\bar{D}^*$ and $DD^*$ open-charm thresholds, respectively. The universality of this feature hints towards the existence of a general pattern for such exotic states. In this work we discuss a possible generic mechanism for the formation of near-threshold molecular states as a result of the strong coupling of compact quark states with a hadronic continuum channel. The compact states that survive the strong coupling limit decouple from the continuum channel and therefore also from the formed hadronic molecule - if realised this scenario would provide a justification to treat hadronic molecules isolated, ignoring the possible influence from surrounding, compact quark-model states. We confront the phenomenology of the $D_{s1}(2460)$ and $D_{s1}(2536)$ with this picture and find consistency, although other explanations remain possible for those states.

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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.

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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.

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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.

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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.

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