Near-threshold State Research Articles

On the nature of X(3960)

A near-threshold enhancement in the D^{+}_{s}D^{-}_{s} system, dubbed as X(3960), is observed by the LHCb collaboration recently. A combined analysis on chi _{c0}(3930)~(rightarrow D^+ D^-), X(3960)~(rightarrow D^{+}_{s}D^{-}_{s}), and X(3915)~(rightarrow J/psi omega ) is performed using both a K-matrix approach of D_{(s)}bar{D}_{(s)} four-point contact interactions and a model of Flatté-like parameterizations. The use of the pole counting rule and spectral density function sum rule indicate, under current statistics, that this D^{+}_{s}D^{-}_{s} near-threshold state has probably the mixed nature of a cbar{c} confining state and D^{+}_{s}D^{-}_{s} continuum.

Long-range structure of [formula omitted] state

Tcc is a very near-threshold state (below the D⁎+D0 threshold about 273 keV). Its long-distance structure is of the significance. In the molecular scheme, we relate the coupling constants of Tcc with D⁎+D0 and D⁎0D+ to its binding energy and mixing angle of two components with a coupled-channel effective field theory without cutoff-dependence and prior isospin assignment. We investigate the kinetically allowed strong decays Tcc→D0D0π+, Tcc→D+D0π0 and radiative decays Tcc→D+D0γ and show that the decay width of Tcc→D0D0π+ is the largest one, which is just the experimental observation channel. Our results show that the decay width obtained in the first analysis from LHCb collaboration in Breit-Wigner distribution is over-estimated. Our results were supported by the following experimental analysis from LHCb within unitarized Breit-Wigner formalism.

Axial-Vector D_{1} Hadrons in D^{*}π Scattering from QCD.

We present I=1/2 D^{*}π scattering amplitudes from lattice QCD and determine two low-lying J^{P}=1^{+} axial-vector D_{1} states and a J^{P}=2^{+} tensor D_{2}^{*}. Computing finite-volume spectra at a light-quark mass corresponding to m_{π}≈391 MeV, for the first time, we are able to constrain coupled J^{P}=1^{+} D^{*}π amplitudes with ^{2S+1}ℓ_{J}=^{3}S_{1} and ^{3}D_{1} as well as coupled J^{P}=2^{+} Dπ(^{1}D_{2}} and D^{*}π(^{3}D_{2}} amplitudes via Lüscher's quantization condition. Analyzing the scattering amplitudes for poles we find a near-threshold bound state, producing a broad feature in D^{*}π{^{3}S_{1}}. A narrow bump occurs in D^{*}π{^{3}D_{1}} due to a D_{1} resonance. A single resonance is found in J^{P}=2^{+} coupled to Dπ and D^{*}π. A relatively low mass and large coupling are found for the lightest D_{1}, suggestive of a state that will evolve into a broad resonance as the light-quark mass is reduced. An earlier calculation of the scalar D_{0}^{*} using the same light-quark mass enables comparisons to the heavy-quark limit.

Deep Underground Laboratory Measurement of ^{13}C(α,n)^{16}O in the Gamow Windows of the s and i Processes.

The ^{13}C(α,n)^{16}O reaction is the main neutron source for the slow-neutron-capture process in asymptotic giant branch stars and for the intermediate process. Direct measurements at astrophysical energies in above-ground laboratories are hindered by the extremely small cross sections and vast cosmic-ray-induced background. We performed the first consistent direct measurement in the range of E_{c.m.}=0.24 to 1.9MeV using the accelerators at the China Jinping Underground Laboratory and Sichuan University. Our measurement covers almost the entire intermediate process Gamow window in which the large uncertainty of the previous experiments has been reduced from 60% down to 15%, eliminates the large systematic uncertainty in the extrapolation arising from the inconsistency of existing datasets, and provides a more reliable reaction rate for the studies of the slow-neutron-capture and intermediate processes along with the first direct determination of the alpha strength for the near-threshold state.

Probing the long-range structure of the Tcc+ with the strong and electromagnetic decays

Very recently, the LHCb Collaboration reported the doubly charmed tetraquark state ${T}_{cc}^{+}$ below the ${D}^{*+}{D}^{0}$ threshold about 273 keV. As a very near-threshold state, its long-distance structure is very important. In the molecular scheme, we relate the coupling constants of ${T}_{cc}^{+}$ with ${D}^{*0}{D}^{+}$ and ${D}^{*+}{D}^{0}$ to its binding energy and mixing angle of two components with a coupled-channel effective field theory. With the coupling constants, we investigate the kinetically allowed strong decays ${T}_{cc}^{+}\ensuremath{\rightarrow}{D}^{0}{D}^{0}{\ensuremath{\pi}}^{+}$, ${T}_{cc}^{+}\ensuremath{\rightarrow}{D}^{+}{D}^{0}{\ensuremath{\pi}}^{0}$ and radiative decays ${D}^{+}{D}^{0}\ensuremath{\gamma}$. Our results show that the decay width of ${T}_{cc}^{+}\ensuremath{\rightarrow}{D}^{0}{D}^{0}{\ensuremath{\pi}}^{+}$ is the largest one, which is just the experimental observation channel. Our theoretical total strong and radiative widths are in favor of the ${T}_{cc}^{+}$ as a $|{D}^{*+}{D}^{0}⟩$ dominated bound state. The total strong and radiative width in the single channel limit and isospin singlet limit are given as ${59.7}_{\ensuremath{-}4.4}^{+4.6}\text{ }\text{ }\mathrm{keV}$ and ${46.7}_{\ensuremath{-}2.9}^{+2.7}\text{ }\text{ }\mathrm{keV}$, respectively. Our calculation is cutoff-independent and without prior isospin assignment. The absolute partial widths and ratios of the different decay channels can be used to test the structure of ${T}_{cc}^{+}$ state when the updated experimental results are available.

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 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_{c}η_{c} molecule with a similar binding energy.

Investigating the mapping of chromophore excitations onto the electron detachment spectrum: photodissociation spectroscopy of iodide ion-thiouracil clusters.

Laser photodissociation spectroscopy (3.1-5.7 eV) has been applied to iodide complexes of the non-native nucleobases, 2-thiouracil (2-TU), 4-thiouracil (4-TU) and 2,4-thiouracil (2,4-TU), to probe the excited states and intracluster electron transfer as a function of sulphur atom substitution. Photodepletion is strong for all clusters (I-·2-TU, I-·4-TU and I-·2,4-TU) and is dominated by electron detachment processes. For I-·4-TU and I-·2,4-TU, photodecay is accompanied by formation of the respective molecular anions, 4-TU- and 2,4-TU-, behaviour that is not found for other nucleobases. Notably, the I-·2TU complex does not fragment with formation of its molecular anion. We attribute the novel formation of 4-TU- and 2,4-TU- to the fact that these valence anions are significantly more stable than 2-TU-. We observe further similar behaviour for I-·4-TU and I-·2,4-TU relating to the general profile of their photodepletion spectra, since both strongly resemble the intrinsic absorption spectra of the respective uncomplexed thiouracil molecule. This indicates that the nucleobase chromophore excitations are determining the clusters' spectral profile. In contrast, the I-·2-TU photodepletion spectrum is dominated by the electron detachment profile, with the near-threshold dipole-bound excited state being the only distinct spectral feature. We discuss these observations in the context of differences in the dipole moments of the thionucleobases, and their impact on the coupling of nucleobase-centred transitions onto the electron detachment spectrum.

Extraction of ND scattering lengths from the Λb → π−pD0 decay and properties of the Σc(2800)+

The isovector and isoscalar ND s-wave scattering lengths are extracted by fitting to the LHCb data of the pD0 invariant-mass distribution in the decay Λb→π−pD0, making use of the cusp effect at the nD+ threshold. The analysis is based on a coupled-channel nonrelativistic effective field theory. We find that the real part of the isovector ND scattering length is unnaturally large due to the existence of a near-threshold state with a mass around 2.8 GeV. The state is consistent with the Σc(2800)+ resonance observed at Belle. Our results suggest that it couples strongly to the ND channel in an s-wave, and that its quantum numbers are JP=1/2−. The strong cusp behavior at the nD+ threshold can be verified using updated LHCb data.

Extending the Southern Shore of the Island of Inversion to ^{28}F.

Detailed spectroscopy of the neutron-unbound nucleus ^{28}F has been performed for the first time following proton/neutron removal from ^{29}Ne/^{29}F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the ^{27}F^{(*)}+n and ^{26}F^{(*)}+2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the ^{28}F ground state, with S_{n}(^{28}F)=-199(6) keV, while analysis of the 2n decay channel allowed a considerably improved S_{n}(^{27}F)=1620(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of ^{28}F. Importantly, in the case of the ground state, the reconstructed ^{27}F+n momentum distribution following neutron removal from ^{29}F indicates that it arises mainly from the 1p_{3/2} neutron intruder configuration. This demonstrates that the island of inversion around N=20 includes ^{28}F, and most probably ^{29}F, and suggests that ^{28}O is not doubly magic.

Beta-delayed charged-particle spectroscopy using TexAT

β-delayed charged-particle emission is a sensitive probe of three-body decays in light nuclei. Time Projection Chambers (TPCs) offer a significant advantage over traditional charged-particle spectroscopy techniques due to a low-energy threshold and a high-geometric efficiency (≈4π) which are essential for use with radioactive ion beams where the beam intensities are limited. The technique for high-sensitivity spectroscopy of β-delayed charged-particle emission is shown to be possible using the Texas Active Target (TexAT) TPC in conjunction with the General Electronics for TPCs (GET) system. The benchmark case studied was that of 12Nβ-decay to the first α-unbound state in 12C, the Hoyle state. Half-life and branching ratio measurements are presented and are in good agreement with previous studies. The efficacy of using TPCs to study such a near-threshold state and disentangle the three-body dynamics of the decay products is demonstrated.

Partial wave decomposition in Friedrichs model with self-interacting continua

We consider the nonrelativistic model of coupling bare discrete states with continuum states in which the continuum states can have interactions among themselves. By partial-wave decomposition and constraint to the conserved angular momentum eigenstates, the model can be reduced to Friedrichs-like model with additional interactions between the continua. If a kind of factorizable form factor is chosen, the model can be exactly solvable, that is, the generalized discrete eigenstates including bound states, virtual states, and resonances can all be represented using the original bare states, and so do the in-state and out-state. The exact S matrix is thus obtained. We then discuss the behaviors of the dynamically generated S-wave and P-wave discrete states as the coupling is varying when there is only one self-interacting bare continuum state. We find that even when the potential is repulsive there could also be resonances and virtual states. In the P-wave cases with attractive interaction, we find that when there is a near-threshold bound state, there will always be an accompanying virtual state and we also give a more general argument of this effect.

Linking structure and dynamics in (p,pn) reactions with Borromean nuclei: The 11Li(p,pn)10Li case

One-neutron removal (p,pn) reactions induced by two-neutron Borromean nuclei are studied within a Transfer-to-the-Continuum (TC) reaction framework, which incorporates the three-body character of the incident nucleus. The relative energy distribution of the residual unbound two-body subsystem, which is assumed to retain information on the structure of the original three-body projectile, is computed by evaluating the transition amplitude for different neutron-core final states in the continuum. These transition amplitudes depend on the overlaps between the original three-body ground-state wave function and the two-body continuum states populated in the reaction, thus ensuring a consistent description of the incident and final nuclei. By comparing different 11Li three-body models, it is found that the 11Li(p,pn)10Li relative energy spectrum is very sensitive to the position of the p1/2 and s1/2 states in 10Li and to the partial wave content of these configurations within the 11Li ground-state wave function. The possible presence of a low-lying d5/2 resonance is discussed. The coupling of the single particle configurations with the non-zero spin of the 9Li core, which produces a spin-spin splitting of the states, is also studied. Among the considered models, the best agreement with the available data is obtained with a 11Li model that incorporates the actual spin of the core and contains ∼31% of p1/2-wave content in the n-9Li subsystem, in accord with our previous findings for the 11Li(p,d)10Li transfer reaction, and a near-threshold virtual state.

Coupled-channel Dπ, Dη and D s K ¯ $$ {D}_s\overline{K} $$ scattering from lattice QCD

We present the first lattice QCD study of coupled-channel $D\pi$, $D\eta$ and $D_{s}\bar{K}$ scattering in isospin-1/2 in three partial waves. Using distillation, we compute matrices of correlation functions with bases of operators capable of resolving both meson and meson-meson contributions to the spectrum. These correlation matrices are analysed using a variational approach to extract the finite-volume energy eigenstates. Utilising L\"uscher's method and its extensions, we constrain scattering amplitudes in $S$, $P$ and $D$-wave as a function of energy. By analytically continuing the scattering amplitudes to complex energies, we investigate the $S$-matrix singularities. Working at $m_\pi \approx 391$ MeV, we find a pole corresponding to a $J^{P} = 0^{+}$ near-threshold bound state with a large coupling to $D\pi$. We also find a deeply bound $J^{P} = 1^{-}$ state, and evidence for a $J^{P} = 2^{+}$ narrow resonance coupled predominantly to $D\pi$. Elastic $D\pi$ scattering in the isospin-$3/2$ channel is studied and we find a weakly repulsive interaction in $S$-wave.

Power counting for three-body decays of a near-threshold state

We propose a new power counting for the effective field theory describing a near-threshold state with unstable constituents, such as the X(3872) meson. In this counting, the momenta of the heavy particles, the pion mass and the excitation energy of the unstable constituent -- the D* in the case of the X -- are treated as small scales, of order Q. The difference, delta, between the excitation energy of the D* and the pion mass is smaller than either by a factor ~20. We therefore assign delta an order Q^2 in our counting. This provides a consistent framework for a double expansion in both delta/m_pi and the ratio of m_pi to the high-energy scales in this system. It ensures that amplitudes have the correct behaviour at the three-body threshold. It allows us to derive, within an effective theory, various results which have previously been obtained using physically-motivated approximations.

Excited States in Electron-Transfer Reaction Products: Ultrafast Relaxation Dynamics of an Isolated Acceptor Radical Anion

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.

${\boldsymbol N}\boldsymbol{\bar N}$ exotics and antiprotonic atoms

The BES Collaboration measurements of J/ψ radiative decays into \( p{\bar p}\) indicate a strong enhancement at \( p {\bar p}\) threshold. In a related experiment a peak in the invariant π + π − η′ mass is observed. It is shown that both structures may be related to a broad \( N{\bar N}\) quasi-bound state in the \(^{11}S_{0} \) wave. The existence of this state finds additional support from the antiprotonic atom level widths. It is also explained by a traditional model of \( N \bar{N} \) interactions based on G-parity transformation. The level widths in H, D and He antiprotonic atoms and the radiochemical studies of \( {\bar p}\) capture in nuclei indicate the existence of another near-threshold quasi-bound state in a P wave.

Antiproton-proton channels inJ/ψdecays

The recent measurements by the BES Collaboration of $J/\ensuremath{\psi}$ decays into $\ensuremath{\gamma}p\overline{p}$ indicate a strong enhancement at $p\overline{p}$ threshold not observed in the decays into ${\ensuremath{\pi}}^{0}p\overline{p}$. Is this enhancement due to a $p\overline{p}$ quasibound state or a baryonium? A natural explanation follows from a traditional model of $p\overline{p}$ interactions based on the G-parity transformation. The observed $p\overline{p}$ structure is due to a strong attraction in the ${}^{1}{S}_{0}$ state and possibly to a near-threshold quasibound state in the ${}^{11}{S}_{0}$ wave.

Is Θ+(1540) a kaon–skyrmion resonance?

We reconsider the relationship between the bound state and the SU(3) rigid rotator approaches to strangeness in the Skyrme model. For non-exotic S=−1 baryons the bound state approach matches for small m K onto the rigid rotator approach, and the bound state mode turns into the rotator zero-mode. However, for small m K , we find no S=+1 kaon bound states or resonances in the spectrum, confirming previous work. This suggests that, at least for large N and small m K , the exotic state may be an artifact of the rigid rotator approach to the Skyrme model. An S=+1 near-threshold state comes into existence only for sufficiently large SU(3) breaking. If such a state exists, then it has the expected quantum numbers of Θ +: I=0, J= 1 2 and positive parity. Other exotic states with (I=1,J P= 3 2 +) , (I=1,J P= 1 2 +) , (I=2,J P= 5 2 +) and (I=2,J P= 3 2 +) appear as its SU(2) rotator excitations. As a test of our methods, we also identify a D-wave S=−1 near-threshold resonance that, upon SU(2) collective coordinate quantization, reproduces the mass splittings of the observed states Λ(1520), Σ(1670) and Σ(1775) with good accuracy.

Low-energy electron scattering from C 60 molecules

Electron scattering from C 60 has been studied theoretically using a model interaction potential containing static-exchange and local polarization terms. The partial cross-sections for scattering in the a g and t 1u symmetries at collision energies from 0.001 to 2.0 eV are presented. In this energy range scattering in the a g symmetry dominates at energies below ∼0.03 eV due to a near-threshold virtual state in the s-wave scattering. The results from this model suggest that low energy electron attachment in e–C 60 scattering may occur through s-wave scattering which is enhanced by the presence of a virtual state.

Nucore — A system for nuclear structure calculations with cluster-core models

One-neutron removal (p,pn) reactions induced by two-neutron Borromean nuclei are studied within a Transfer-to-the-Continuum (TC) reaction framework, which incorporates the three-body character of the incident nucleus. The relative energy distribution of the residual unbound two-body subsystem, which is assumed to retain information on the structure of the original three-body projectile, is computed by evaluating the transition amplitude for different neutron-core final states in the continuum. These transition amplitudes depend on the overlaps between the original three-body ground-state wave function and the two-body continuum states populated in the reaction, thus ensuring a consistent description of the incident and final nuclei. By comparing different 11Li three-body models, it is found that the 11Li(p,pn)10Li relative energy spectrum is very sensitive to the position of the p1/2 and s1/2 states in 10Li and to the partial wave content of these configurations within the 11Li ground-state wave function. The possible presence of a low-lying d5/2 resonance is discussed. The coupling of the single particle configurations with the non-zero spin of the 9Li core, which produces a spin-spin splitting of the states, is also studied. Among the considered models, the best agreement with the available data is obtained with a 11Li model that incorporates the actual spin of the core and contains ∼31% of p1/2-wave content in the n-9Li subsystem, in accord with our previous findings for the 11Li(p,d)10Li transfer reaction, and a near-threshold virtual state.