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Register a new userConstituent Quark Model Calculation for a possible J^P=0^-,T=0 Dibaryon
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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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The results for the elastic nucleon form factors and the electromagnetic transition amplitudes to the Delta(1232) resonance, obtained with the Hypercentral Constituent Quark Model with the inclusion of a meson cloud correction are briefly presented. The pion cloud effects are explicitly discussed.
${bf Background}$ Knowledge of nucleon structure is today ever more of a precision science, with heightened theoretical and experimental activity expected in coming years. At the same time, a persistent gap lingers between theoretical approaches grounded in Euclidean methods (e.g., lattice QCD, Dyson-Schwinger Equations [DSEs]) as opposed to traditional Minkowski field theories (such as light-front constituent quark models). ${bf Purpose}$ Seeking to bridge these complementary worldviews, we explore the potential of a Euclidean constituent quark model (ECQM). This formalism enables us to study the gluonic dressing of the quark-level axial-vector vertex, which we undertake as a test of the framework. ${bf Method}$ To access its indispensable elements with a minimum of inessential detail, we develop our ECQM using the simplified quark $+$ scalar diquark picture of the nucleon. We construct a hyperspherical formalism involving polynomial expansions of diquark propagators to marry our ECQM with the results of Bethe-Salpeter Equation (BSE) analyses, and constrain model parameters by fitting electromagnetic form factor data. ${bf Results}$ From this formalism, we define and compute a new quantity --- the Euclidean density function (EDF) --- an object that characterizes the nucleons various charge distributions as functions of the quarks Euclidean momentum. Applying this technology and incorporating information from BSE analyses, we find the dressing effect on the protons axial-singlet charge to be small in magnitude and consistent with zero. ${bf Conclusions}$ The scalar quark $+$ diquark ECQM is a step toward a realistic quark model in Euclidean space, and urges additional refinements. The small size we obtain for the impact of the dressed vertex on the axial-singlet charge suggests that models without this effect are on firm ground to neglect it.
The $beta$ decay of the odd-odd nucleus $^{70}$Br has been investigated with the BigRIPS and EURICA setups at the Radioactive Ion Beam Factory (RIBF) of the RIKEN Nishina Center. The $T=0$ ($J^{pi}=9^+$) and $T=1$ ($J^{pi}=0^+$) isomers have both been produced in in-flight fragmentation of $^{78}$Kr with ratios of 41.6(8)% and 58.4(8)%, respectively. A half-life of $t_{1/2}=2157^{+53}_{-49}$ ms has been measured for the $J^{pi}=9^+$ isomer from $gamma$-ray time decay analysis. Based on this result, we provide a new value of the half-life for the $J^{pi}=0^+$ ground state of $^{70}$Br, $t_{1/2}=78.42pm0.51$ ms, which is slightly more precise, and in excellent agreement, with the best measurement reported hitherto in the literature. For this decay, we provide the first estimate of the total branching fraction decaying through the $2^+_1$ state in the daughter nucleus $^{70}$Se, $R(2^+_1)=1.3pm1.1%$. We also report four new low-intensity $gamma$-ray transitions at 661, 1103, 1561, and 1749 keV following the $beta$ decay of the $J^{pi}=9^+$ isomer. Based on their coincidence relationships, we tentatively propose two new excited states at 3945 and 4752 keV in $^{70}$Se with most probable spins and parities of $J^{pi}=(6^+)$ and $(8^+)$, respectively. The observed structure is interpreted with the help of shell-model calculations, which predict a complex interplay between oblate and prolate configurations at low excitation energies.
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.
Polarization properties of strange baryons produced in pp reactions, p + p -> p + Lambda^0 + K^+ and p + p -> p + Sigma^0 + K^$, near thresholds of the final states (p Lambda^0 K^+) and (p Sigma^0 K^+) are analysed relative to polarizations of colliding protons. The cross sections for pp reactions are calculated within the effective Lagrangian approach accounting for strong pp rescattering in the initial state of colliding protons with a dominant contribution of the one-pion exchange and strong final-state interaction of daughter hadrons (Eur. Phys. J. A 9, 425 (2000)).
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