We study the two step sequential one pion production mechanism, $np(I=0)\to π^-pp$, followed by the fusion reaction $pp\to π^+d$, in order to describe the $np\to π^+π^-d$ reaction with $π^+π^-$ in $I=0$, where a narrow peak, so far identified with a "$d(2380)$" dibaryon, has been observed. We find that the second step $pp\to π^+d$ is driven by a triangle singularity that determines the position of the peak of the reaction and the large strength of the cross section. The combined cross section of these two mechanisms produce a narrow peak with the position, width and strength compatible with the experimental observation within the approximations done. This novel interpretation of the peak without invoking a dibaryon explains why the peak is not observed in other reactions where it has been searched for.
In a comment [arXiv:2106.00494] to our paper on ``Sequential single-pion production explaining the dibaryon $d^*(2380)$ peak'' the authors provide arguments that apparently invalidate our claims and present what they call ``proofs'' of the dibaryon explanation of the $np\to π^0π^0d$ fusion reaction. In this reply we refute the arguments of the comment and show that no existing experiment proves the dibaryon nature of the peak of the fusion reaction.
The isoscalar single-pion production exhibits a broad bump in the energy dependence of the total cross section, which does not correspond to the usual opening of the $N^*(1440)$ production channel with subsequent pion decay. In arxiv:2102.05575 it was interpreted as a narrow Breit-Wigner structure, which leads in a sequential single-pion production process to a possible explanation of the $d^*(2380)$ resonance. We demonstrate that such an attempt fails already, when confronted with the data base for isoscalar single-pion production. We investigate whether the observed bump structure rather poi…
It is demonstrated within the dibaryon-induced model for $NN$ interaction that $pp$ scattering in $P$ waves is governed mainly by the production of the intermediate dibaryon resonances. Two dibaryon resonances with a mass of about 2200 MeV discovered recently by the ANKE-COSY Collaboration are shown to determine both elastic $pp$ phase shifts and inelasticities in the $^3P_0$ and $^3P_2$-$^3F_2$ channels from zero energy up to $T_p=0.7$-$0.9$ GeV. It is also demonstrated clearly that the $^3P_0$ dibaryon plays a decisive role in near-threshold neutral pion production in $pp$ collisions which i…
The status of $NΔ$ and $ΔΔ$ dibaryons introduced by Dyson and Xuong in 1964 is briefly reviewed with focus on the d*(2380), tentatively assigned as a $ΔΔ$ dibaryon resonance. It is argued that the apparently small value of width, $Γ_{d^\ast}$$\approx$70 MeV, favors hadronic structure for the d*(2380) dibaryon rather than a six-quark structure.
We discuss a role of dibaryon fields in an effective field theory to study pion cloud effect around the unitary limit of two nucleon systems at low energies.
Experimental evidence for $I(J^P)=0(3^+)$ $ΔΔ$ dibaryon ${\cal D}_{03}(2370)$ has been presented recently by the WASA-at-COSY Collaboration. Here I review new hadronic-basis Faddeev calculations of $L=0$ nonstrange pion-assisted $NΔ$ and $ΔΔ$ dibaryon candidates. These calculations are so far the only ones to reproduce the relatively small ${\cal D}_{03}(2370)$ width of 70--80 MeV. Predictions are also given for the location and width of ${\cal D}_{30}$, the $I(J^P)=3(0^+)$ exotic partner of ${\cal D}_{03}(2370)$. Extensions to strangeness ${\cal S}=-1$ dibaryons are briefly discussed.
Processes of one- and two-pion production in $NN$ collisions are considered in connection with excitation of intermediate dibaryon resonances. In particular, relative contributions of the conventional meson-exchange and dibaryon excitation mechanisms in the reaction $pp \to d π^+$ are investigated in detail. Inclusion of the intermediate isovector dibaryon resonances is shown to essentially improve the description of experimental data for this reaction, provided the soft meson-baryon form factors consistent with $πN$ elastic scattering are used. Manifestation of the intermediate isoscalar and …
We discuss a new type of L=0 positive-parity dibaryons, pion-B-B', where the dominant binding mechanism is provided by resonating p-wave pion-baryon interactions. Recent calculations of such pion assisted dibaryons are reviewed with special emphasis placed on the non-strange I(JP)=1(2+) N-Delta dibaryon D_{12}(2150) studied recently at JLab, and on the 0(3+) Delta-Delta dibaryon D_{03}(2380) discovered recently by the WASA-at-COSY Collaboration. We review recent searches by the HADES Collaboration at GSI and by the E15 and E27 Experiments at J-PARC for a strangeness S=-1 I(JP)=1/2(0-) K-pp dib…
In arxiv: 2102.05575 a two-step process $pn \to (pp) π^- \to (ΔN) π^- \to (d π^+) π^-$ was calculated by using experimental total cross sections for the single-pion production processes $pn \to pp π^-(I=0)$ and $pp \to d π^+$. As a result the authors obtain a resonance-like structure for the total $pn \to dπ^+π^-$ cross section of about the right size and width of the observed $d^*(2380)$ peak at an energy about 40 MeV below the $d^*(2380)$ mass. We object both the results of the sequential process calculation and its presentation as an alternative to the dibaryon interpretation.
The state of the art in dibaryons with strangeness is reviewed, including the K^{-}pp dibaryon which signals the onset of antikaon-nuclear binding. A new type of strange dibaryons is highlighted, where the primary binding mechanism is provided by strong p-wave pion interactions, as demonstrated by a quasibound (I=3/2,JP=2+) pion-hyperon-nucleon dibaryon calculation.
The Lambda_c(2286)-N system is studied in a chiral constituent quark model and the resulting s-wave interaction is used in separable form within three-body models of the pi-Lambda_c-N system with quantum numbers (C,I,JP)=(+1,3/2,2+). Separable interactions are also used for the dominant p-wave pion-baryon channels dominated by the Delta(1232) and Sigma_c(2520) resonances. Faddeev equations with relativistic kinematics are solved on the real axis to search for bound states and in the complex plane to search for three-body resonances. Some of the models considered generate a very narrow bound st…
We study breakup of the deuteron induced by neutrinos in the neutral $νd\to νnp$, $\barν d\to \barν np$ and the charged $\barν d\to e^+ n n$, $νd\to e^- pp$ processes. Pionless effective field theory with dibaryon fields is used to calculate the total cross sections for neutrino energies $E_ν$ from threshold to 20 MeV. Amplitudes are expanded up to next-to-leading order, and the partial wave is truncated at $P$-waves. The Coulomb interaction between two protons is included nonperturbatively in the reaction amplitudes, and an analytic expression of the amplitudes is obtained. The contribution o…
N-Delta and Delta-Delta dibaryon candidates are discussed and related quark-based calculations are reviewed. New hadronic calculations of L=0 nonstrange dibaryon candidates are reported. For N-Delta, I(JP)=1(2+) and 2(1+) S-matrix poles slightly below threshold are found by solving pi-N-N Faddeev equations with relativistic kinematics, and for Delta-Delta several S-matrix poles below threshold are found by solving pi-N-Delta Faddeev equations with relativistic kinematics in which the N-Delta interaction is dominated by the 1(2+) and 2(1+) resonating channels. In particular, the I(JP)=0(3+) Del…
We give reasons for the existence of the NN decoupled dibaryon resonance $d^*_1$(1956). Strong evidence for its presence has first been found in the energy spectrum of coincident photons emitted at $\pm 90^0$ from the $pp \to γγX$ process at 216 MeV measured by the DIB2$γ$ collaboration at JINR. As further experimental indications of the $d^*_1$(1956) existence we present those found in the available photon energy spectra of $npγ$, $pdγ$, and $pAγ$ reactions below the pion threshold. It is noted that serious discrepancies between the $npγ$ and $pdγ$ experimental cross sections and theoretical …
The D(03) dibaryon is generated dynamically as a resonance pole in a pion-nucleon-Delta' three-body model, where Delta' is a stable Delta baryon. Using separable interactions dominated by the Delta(1232) isobar for pion-nucleon and by the D(12) isobar for nucleon-Delta', where D(12) is the nucleon-Delta 2150 MeV dibaryon deduced in and constrained by 1D2 pp scattering, the model reduces to an effective two-body problem for Delta-Delta' which is solved. The mass and width of D(03) are found close to those of the I(JP)=0(3+) resonance peak observed by WASA@COSY in pion-production pn collisions a…
The energy spectrum for high energy $γ$-rays ($E_γ\geq 10$ MeV) from the process $pp \to γγX$ emitted at $90^0$ in the laboratory frame has been measured at an energy below the pion production threshold, namely, at 216 MeV. The resulting photon energy spectrum extracted from $γ-γ$ coincidence events consists of a narrow peak at a photon energy of about 24 MeV and a relatively broad peak in the energy range of (50 - 70) MeV. The statistical significances for the narrow and broad peaks are 5.3$σ$ and 3.5$σ$, respectively. This behavior of the photon energy spectrum is interpreted as a signature …
We present evidence for the existence of a bound H-dibaryon, an I=0, J=0, s=-2 state with valence quark structure uuddss, at a pion mass of m_pi ~ 389 MeV. Using the results of Lattice QCD calculations performed on four ensembles of anisotropic clover gauge-field configurations, with spatial extents of L ~ 2.0, 2.5, 3.0 and 3.9 fm at a spatial lattice spacing of b ~ 0.123 fm, we find an H-dibaryon bound by B = 16.6 +- 2.1 +- 4.6 MeV at a pion mass of m_pi ~ 389 MeV.
We propose a low energy effective field theory of QCD at the scale of pion mass for the $N_B=2$ sector, $N_B$ being the baryon number, which contains two dibaryon fields in addition to the nucleons and pions. It has a well defined counting, is renormalizable and the nucleon-nucleon scattering amplitudes are manifestly unitary at leading order. We work out a lower energy effective theory for nucleons with energy much lower than the pion mass and three momentum comparable to it, which also has a well defined counting and is renormalizable. The dibaryon fields must also be kept as explicit degree…
The possibility of observation of supernarrow dibaryons, decay of which into two nucleons forbidden by the Pauli principle, produced in processes of charged pion photoproduction by polarized photons from the deuteron is analyzed. It is shown that the expectable dibaryon yield may exceed the background by a factor of 10--100.
The $ΔΔ$ dibaryon resonance $d^\ast (2380)$ with $(J^P, I)=(3^+, 0)$ is studied theoretically on the basis of the 3-flavor lattice QCD simulation with heavy pion masses ($m_π=679, 841$ and $1018$ MeV). By using the HAL QCD method, the central $Δ$-$Δ$ potential in the ${}^7S_3$ channel is obtained from the lattice data with the lattice spacing $a\simeq 0.121$ fm and the lattice size $L\simeq 3.87$ fm. The resultant potential shows a strong short-range attraction, so that a quasi-bound state corresponding to $d^\ast (2380)$ is formed with the binding energy $25$-$40$ MeV below the $ΔΔ$ threshold…
The current constraints from lattice QCD on the existence of the H-dibaryon are discussed. With only two significant lattice QCD calculations of the H-dibaryon binding energy at approximately the same lattice spacing, the forms of the chiral and continuum extrapolations to the physical point are not determined. In this brief report, we consider the constraints on the H-dibaryon imposed by two simple chiral extrapolations. In both instances, the extrapolation to the physical pion mass allows for a bound H-dibaryon or a near-threshold scattering state. Further lattice QCD calculations are requir…
A method to investigate possible loosely bound dibaryon states is suggested. The momentum dependence of a two-pion correlation function is shown to depend on the (non)existence of a bound state.
The first attempt is made to provide a quantitative theoretical interpretation of the WASA-at-COSY experimental data on the basic double-pion production reactions $pn \to d π^0π^0$ and $pn \to d π^+π^-$ in the energy region $T_p =1$ - $1.3$ GeV [P. Adlarson et al., Phys. Lett. B 721, 229 (2013)]. The data are analyzed within a model based on production and decay of an intermediate $I(J^P)=0(3^+)$ dibaryon resonance $\mathcal{D}_{03}$ (denoted also as $d^*(2380)$). The observed decrease of the near-threshold enhancement (the so-called ABC effect) in the reaction $pn \to d π^+π^-$ in comparison …
The narrow peak recently found in various pionic double charge exchange (DCX) cross sections can be explained by the assumption of a universal resonance at 2065 MeV, called d'. We calculate the mass of a six-quark system with J^P=0^-, T=0 quantum numbers employing a cluster model and a shell model basis to diagonalize the nonrelativistic quark model Hamiltonian.
Phase shifts and inelasticity parameters for $NN$ scattering in the partial-wave channels ${}^3S_1$--${}^3D_1$ and ${}^1S_0$ at energies $T_{\rm lab}$ from zero to about 1 GeV are described within a unified $NN$ potential model assuming the formation of isoscalar and isovector dibaryon resonances near the $NN^*(1440)$ threshold. Evidence for these near-threshold resonances is actually found in the recent WASA experiments on single- and double-pion production in $NN$ collisions. There, the excitation of the Roper resonance $N^*(1440)$ exhibits a structure in the energy dependence of the total c…
We develop a new hadronic model for GEANT4 that is specialized for the disintegration of the deuteron by photons, $dγ\to n p$. For the description of two-nucleon interactions, we employ a pionless effective field theory with dibaryon fields (dEFT). We apply the new model of GEANT4 (G4dEFT) to the calculations of the total and the differential cross sections in $dγ\to n p$ and compare the results with empirical data. As an application of the new model, we calculate the neutron yield from the $γ+ {\rm CD}_2$ process. G4dEFT predicts peaks for the neutron yield, but the existing model of GEANT4 d…
Under certain circumstances, a three-body decay width can be approximated by an integral involving a product of two off-shell two-body decay widths. This ``angle-average'' approximation is used to calculate the $πNN$ decay width of the $d^*(J^π=3^+, T=0)$ dibaryon in a simple $Δ^2$ model for the most important Feynman diagrams describing pion emissions with baryon-baryon recoil and meson retardation. The decay width is found to be about 0.006 (0.07, 0.5) MeV at the $d^*$ mass of 2065 (2100, 2150) MeV for input dynamics derived from the Full Bonn potential. The smallness of this width is qualit…
The mass and wave function of a six-quark system with quantum numbers J^P=0^-, T=0, called d', are calculated. We use a colored diquark-tetraquark cluster model for the six-quark wave function. A constituent quark model Hamiltonian with a two-body confinement potential, and residual one-gluon, one-pion, and one-sigma exchange interactions is used. The complications due to the quark exchange interactions between tetraquark and diquark clusters (Pauli principle) are taken into account within the framework of the Resonating Group Method. The calculated d' mass is some 350 MeV above the empirical …
The production of d' dibaryons in heavy ion collisions due to the elementary process NN -> d' + pion is considered. The cross section NN -> d' + pion is estimated using the vacuum d' width = 0.5 MeV extracted from data on the double charge exchange reactions on nuclei. The d' production rate per single collision of heavy ions is estimated at an incident beam energy of 1 A GeV within the framework of the Quantum Molecular Dynamics transport model. We suggest to analyse the invariant mass spectrum of the NN + pion system in order to search for an abundance of events with the invariant mass of th…
We present a lattice QCD spectroscopy study in the isospin singlet, strangeness $-2$ sectors relevant for the conjectured $H$ dibaryon. We employ both local and bilocal interpolating operators to isolate the ground state in the rest frame and in moving frames. Calculations are performed using two flavors of O($a$)-improved Wilson fermions and a quenched strange quark. Our initial point-source method for constructing correlators does not allow for bilocal operators at the source; nevertheless, results from using these operators at the sink indicate that they provide an improved overlap onto the…
We here concentrate on available $π^\pm$ and $K^\pm$ ALICE preliminary $R_{AA}$ data in central 2.76 TeV Pb+Pb collisions at LHC. These data show an interesting fine resolution hierarchy, i.e. the measured $K^\pm$ data have consistently lower suppression compared to $π^\pm$ measurements. We here ask whether theoretical predictions based on energy loss in dynamical QCD medium can quantitatively and qualitatively explain such fine resolution. While our suppression calculations agree well with the data, we find that qualitatively explaining the fine hierarchy critically depends on the choice of f…
The scattering of pions on a dibaryon configuration is analyzed within the $SU(2)$ Skyrme model. It is shown that this model leads to a low-lying $(J^P,I)= (0^-,2)$ resonance. The possibility that this resonance corresponds to one proposed recently in the context of double charge exchange pion scattering on nuclei is discussed. Given the setup used in those experiments we also show that a resonance with isospin assignment $I=0$ cannot be excited according to the description presented here.
We present first results for two-baryon correlation functions, computed using $N_f=2$ flavours of O($a$) improved Wilson quarks, with the aim of explaining potential dibaryon bound states, specifically the H-dibaryon. In particular, we use a GEVP to isolate the groundstate using two-baryon (hyperon-hyperon) correlation functions $\big(\langle C_{XY}(t)C_{XY}(0) \rangle$, where $XY=ΛΛ, ΣΣ, NΞ, \cdots\big)$, each of which has an overlap with the H-dibaryon. We employ a `blocking' algorithm to handle the large number of contractions, which may easily be extended to N-baryon correlation functions.…
It is demonstrated that both elastic and inelastic $NN$ scattering at laboratory energies up to 600--800 MeV, at least in some partial waves characterized by a large inelasticity, can be described by a superposition of the conventional long-range one-pion exchange and a specific short-range interaction induced by the $s$-channel dibaryon exchange. For the $^3P_2$, $^1D_2$ and $^3F_3$ partial waves, the pole parameters giving the best fit of the real and imaginary parts of the $NN$ phase shifts are consistent with the parameters of the respective isovector dibaryon resonances found experimental…
A hypothesis is proposed herein, suggesting that a pion-nuclear resonance may be observed in the $α+d\to{}^6\mathrm{Li}(3.563)+π^0$ reaction. The resonance has a $πNNα$ structure, containing $αNN$ and $πNN$ subsystems. The former corresponds to the $A=6$ isotriplet ($^6\mathrm{He}_\text{g.s.}$, $^6\mathrm{Li}(3.563)$, $^6\mathrm{Be}_\text{g.s.}$), whereas the latter is a hypothetical $NN$-decoupled dibaryon. We propose an experiment to search for this resonance using the $^7\mathrm{Li}(p,d)$ reaction.
Results of a high-statistics, multi-volume Lattice QCD exploration of the deuteron, the di-neutron, the H-dibaryon, and the Xi-Xi- system at a pion mass of m ~ 390 MeV are presented. Calculations were performed with an anisotropic n_f = 2+1 Clover discretization in four lattice volumes of spatial extent L ~ 2.0, 2.5, 3.0 and 4.0 fm, with a lattice spacing of b_s ~ 0.123 fm in the spatial-direction, and b_t ~ b_s/3.5 in the time-direction. The Xi-Xi- is found to be bound by B_{Xi-Xi-} = 14.0(1.4)(6.7) MeV, consistent with expectations based upon phenomenological models and low-energy effective …
With the use of pionless effective field theory including dibaryon fields, we study the $γd \to \vec{n} p$ reaction for the laboratory photon energy $E_γ^{lab}$ ranging from threshold to 30 MeV. Our main goal is to calculate the neutron polarization $P_{y'}$ defined as $P_{y'} = (σ_+ - σ_-)/(σ_+ + σ_-)$, where $σ_+$ and $σ_-$ are the differential cross sections for the spin-up and spin-down neutrons, respectively, along the axis perpendicular to the reaction plane. We also calculate the total cross section as well as the differential cross section $σ(θ)$, where $θ$ is the colatitude angle. Alt…
The reaction $pp \to pp2γ$, proposed earlier to probe for the NN--decoupled dibaryon resonances, has been studied by the DIB$2γ$ Collaboration (JINR) and their preliminary data seem to give evidence for the resonance effect at about 1920 MeV. We discuss some other two-photon processes: the double radiative capture reaction in pionic deuterium, photoabsorption sum rules and the nuclear Compton scattering, which we consider to be especially feasible for further test and invesigation of possible low-lying, exotic six-quark states.
In the framework of a Chiral effective theory with dibaryon fields, we calculate the pion mass dependence of the inverse scattering length of the nucleon-nucleon system in the $^3S_1$ channel at order ${(m_π^3 / Λ_χ^2)}\times (m_π^{1/2}m_N^{3/2}/8πf_π^2)^n$ for all $n\ge 0$. We show that certain sets of potentially large higher order contributions vanish. We discuss the difficulties of extending the proof to the $^1S_0$ channel. We apply our results to chiral extrapolations of current lattice data.
