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Register a new userThe d-Dibaryon in the Nonrelativistic Quark Model
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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.
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We calculate the mass and structure of a J^P=0^-, T=0 six-quark system using a colored diquark-tetraquark cluster wave function and a nonrelativistic quark model Hamiltonian. The calculated mass is some 350 MeV above the empirical value if the same confinement strength as in the nucleon is used. If the effective two-body confinement strength is weaker in a compound six-quark system than in a single baryon, as expected from a simple harmonic oscillator model, one obtains M_d = 2092 MeV close to experiment.
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 value if the same two-body confinement strength as in the nucleon and Delta is used. This paper also examines the validity of the usual assumption of a universal two-quark confinement strength. We propose that the effective two-body confinement strength in an exotic six-quark system, such as the d, could be weaker than in a single baryon. The weaker confinement hypothesis leads to a d mass of M=2092 MeV and a d radius of r=1.53 fm.
Charmed dibaryon states with the spin-parity $J^{pi}=0^+$, $1^+$, and $2^+$are predicted for the two-body $Y_cN$ ($=Lambda_c$, $Sigma_c$, or $Sigma^*_c$) systems. We employ the complex scaling method for the coupled channel Hamiltonian with the $Y_cN$-CTNN potentials, which were proposed in our previous study. We find four sharp resonance states near the $Sigma_c N$ and $Sigma^*_c N$ thresholds. From the analysis of the binding energies of partial channel systems, we conclude that these resonance states are Feshbach resonances. We compare the results with the $Y_c N$ resonance states in the heavy quark limit, where the $Sigma_c N$ and $Sigma^*_c N$ thresholds are degenerate, and find that they form two pairs of the heavy-quark doublets in agreement with the heavy quark spin symmetry.
We look for $DeltaDelta$ and $NDelta$ resonances by calculating $NN$ scattering phase shifts of two interacting baryon clusters of quarks with explicit coupling to these dibaryon channels. Two phenomenological nonrelativistic chiral quark models giving similar low-energy $NN$ properties are found to give significantly different dibaryon resonance structures. In the chiral quark model (ChQM), the dibaryon system does not resonate in the $NN$ $S$-waves, in agreement with the experimental SP07 $NN$ partial-wave scattering amplitudes. In the quark delocalization and color screening model (QDCSM), the $S$-wave NN resonances disappear when the nucleon size $b$ falls below 0.53 fm. Both quark models give an $IJ^P = 03^+$ $DeltaDelta$ resonance. At $b=0.52 $fm, the value favored by baryon spectrum, the resonance mass is 2390 (2420) MeV for the ChQM with quadratic (linear) confinement, and 2360 MeV for the QDCSM. Accessible from the $^3D_3^{NN}$ channel, this resonance is a promising candidate for the known isoscalar ABC structure seen more clearly in the $pn$$to $$dpipi$ production cross section at 2410 MeV in the recent preliminary data reported by the CELSIUS-WASA Collaboration. In the isovector dibaryon sector, our quark models give a bound or almost bound $^5S_2^{DeltaDelta}$ state that can give rise to a $^1D_2^{NN}$ resonance. None of the quark models used has bound $NDelta$ $P$-states that might generate odd-parity resonances.
There exists experimental evidence that a dibaryon resonance d with quantum numbers J^P=0^-,T=0 and mass 2065 MeV could be the origin of the narrow peak in the (pi^+ ,pi^- ) double charge exchange cross--sections on nuclei. We investigate the six--quark system with these quantum--numbers within the constituent quark model, with linear confinement, effective one--gluon exchange at short range and chiral interactions between quarks (pi and sigma exchange). We classify all possible six quark states with J^P=0^-,T=0, and with N=1 and N=3 harmonic oscillator excitations, using different reduction chains. The six--quark Hamiltonian is diagonalized in the basis including the unique N=1 state and the 10 most important states from the N=3 shell. We find, that with most of the possible sets of parameters, the mass of such a dibaryon lies above the N(939)+N^ast(1535) threshold. The only possibility to describe the supposed d(2065) in the present context is to reduce the confinement strength to very small values, however at the expense of describing the negative parity resonances N^ast. We also analyze the J^P=0^-,T=2,N=1 six--quark state.
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