Pentaquark and Tetraquark states

Classical nuclear theory deals with a many-body system of neutrons and protons interacting nonrelativistically through two-body potentials. It has, of course, long been realized that there must be corrections to this simple picture—for example, the meson exchange effects which preclude a simple interpretation of the magnetic moment of the deuteron in terms of d-state probability. Nevertheless, the availability of beams of pions, and the consequent ability to study the excitation of real isobars in nuclei, has been critical in the realization that for many problems one must develop a theoretical model which explicitly includes pion and isobar degrees of freedom (see, for example, the proceedings of recent topical conferences Cat+ 82, MT 80).

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Chiral Symmetry and the Bag Model: A New Starting Point for Nuclear Physics

http://www.pas.rochester.edu/~ksmcf/rein-sehgal/rein-sehgal.pdf

Neutrino Excitation of Baryon Resonances and Single Pion Production

The past seventeen years have witnessed tremendous progress on the experimental and theoretical explorations of the multiquark states. The hidden-charm and hidden-bottom multiquark systems were reviewed extensively in [Phys. Rept. 639 (2016) 1-121]. In this article, we shall update the experimental and theoretical efforts on the hidden heavy flavor multiquark systems in the past three years. Especially the LHCb collaboration not only confirmed the existence of the hidden-charm pentaquarks but also provided strong evidence of the molecular picture. Besides the well-known $XYZ$ and $P_c$ states, we shall discuss more interesting tetraquark and pentaquark systems either with one, two, three or even four heavy quarks. Some very intriguing states include the fully heavy exotic tetraquark states $QQ\bar Q\bar Q$ and doubly heavy tetraquark states $QQ\bar q \bar q$, where $Q$ is a heavy quark. The $QQ\bar Q\bar Q$ states may be produced at LHC while the $QQ\bar q \bar q$ system may be searched for at BelleII and LHCb. Moreover, we shall pay special attention to various theoretical schemes. We shall emphasize the model-independent predictions of various models which are truly/closely related to Quantum Chromodynamics (QCD). There have also accumulated many lattice QCD simulations through multiple channel scattering on the lattice in recent years, which provide deep insights into the underlying structure/dynamics of the $XYZ$ states. In terms of the recent $P_c$ states, the lattice simulations of the charmed baryon and anti-charmed meson scattering are badly needed. We shall also discuss some important states which may be searched for at BESIII, BelleII and LHCb in the coming years.

About 120 baryons and baryon resonances are known, from the abundant nucleon with $u$ and $d$ light-quark constituents up to the recently discovered $\Omega_b^-=bss$, and the $\Xi_b^-=bsd$ which contains one quark of each generation In spite of this impressively large number of states, the underlying mechanisms leading to the excitation spectrum are not yet understood Heavy-quark baryons suffer from a lack of known spin-parities In the light-quark sector, quark-model calculations have met with considerable success in explaining the low-mass excitations spectrum but some important aspects like the mass degeneracy of positive-parity and negative-parity baryon excitations are not yet satisfactorily understood At high masses, above 18 GeV, quark models predict a very high density of resonances per mass interval which is not observed In this review, issues are identified discriminating between different views of the resonance spectrum; prospects are discussed how open questions in baryon spectroscopy may find answers from photo- and electro-production experiments which are presently carried out in various laboratories

Baryon spectroscopy

We report pion-nucleon partial-wave amplitudes obtained from analysis of scattering data at pion laboratory momenta from 0.42 to 2.0 GeV/c. These partial-wave amplitudes have been analyzed using modified Breit-Wigner, coupled-channel parametrizations. The resulting resonances and their parameters are tabulated.

Pion-nucleon partial-wave amplitudes

We have carried out a partial-wave analysis of amalgamated pion-nucleon scattering data at 35 momenta in the range 0.429-1.995 GeV/c. At each momentum, a large set of fits was obtained, with the amplitude represented as the sum of a fixed background and the fitted terms of an accelerated convergence expansion. The effects of discrete and continuum ambiguities were separated by special statistical and numerical techniques which sort the fits into clusters. Dispersion relations on selected curves within the kinematical region for physical scattering were used to resolve the ambiguities and to derive constraints. These constraints, which ensure that the fitted amplitudes have a relatively smooth energy dependence consistent with analyticity, were used iteratively to arrive at a unique set of amplitudes which fit the scattering data throughout the energy region studied.

Pion - nucleon partial wave analysis

Review of Particle Properties, 1982-1983

We report results of a $\ensuremath{\pi}N$ partial-wave analysis for $I=\frac{3}{2}$ resonances observed in the 1900-MeV region: ${F}_{35}(1885)$, ${F}_{37}(1945)$, ${P}_{31}(1940)$, and ${D}_{35}(1925)$. The ${D}_{35}(1925)$ is difficult to accommodate in conventional baryon models; we discuss an alternative model having such a state.

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R. E. Hendrick

Papers

Pion-nucleon partial-wave amplitudes

Pion - nucleon partial wave analysis

Review of Particle Properties, 1982-1983

I = 3 2 π N Resonances in the 1900-MeV Region