Pentaquark Baryon Research Articles

Hyperons and Θs+ in holographic QCD

We revisit the holographic description of strange baryons in the context of the Sakai-Sugimoto construction, by considering the strange quark mass as heavy. Hyperons are described by a massive $(K,{K}^{*})$ multiplet, bound to a light-flavor instanton in bulk, much in the spirit of the Callan-Klebanov construction. The modular Hamiltonian maps onto the Landau problem, a charged particle in a two-dimensional external magnetic field, induced by the bulk Chern-Simons interaction, plus spin-orbit coupling. The ensuing holographic hyperon spectrum compares fairly with the empirical one. The holographic strange pentaquark baryon ${\mathrm{\ensuremath{\Theta}}}_{s}^{+}$ is shown to be unbound.

Nonstandard heavy mesons and baryons: Experimental evidence

Quantum Chromodynamics (QCD), the generally accepted theory for the strong interactions, describes the interactions between quarks and gluons. The strongly interacting particles that are seen in nature are hadrons, which are composites of quarks and gluons. Since QCD is a strongly coupled theory at distance scales that are characteristic of observable hadrons, there are no rigorous, first-principle methods to derive the spectrum and properties of the hadrons from the QCD Lagrangian, except for Lattice QCD simulations that are not yet able to cope with all aspects of complex and short-lived states. Instead, a variety of "QCD inspired" phenomenological models have been proposed. Common features of these models are predictions for the existence of hadrons with substructures that are more complex than the standard quark-antiquark mesons and the three quark baryons of the original quark model that provides a concise description of most of the low-mass hadrons. Recently, an assortment of candidates for non-standard multi-quark mesons, meson-gluon hybrids and pentaquark baryons that contain heavy (charm or bottom) quarks have been discovered. Here we review the experimental evidence for these states and make some general comparisons of their measured properties with standard quark-model expectations and predictions of various models for non-standard hadrons. We conclude that the spectroscopy of all but simplest hadrons is not yet understood.

The new pentaquarks in the diquark model

Pentaquark baryons are a natural expectation of an extended picture of hadrons where quarks and diquarks are the fundamental units. The parity/mass pattern observed, when compared to that of exotic mesons, appears as the footprint of a compact five-quark structure. What has been learned from the X, Y, Z phenomenology informs about the newly found pentaquark structure and suggests further experimental tests and directions to be explored.

A new hadron spectroscopy

QCD-motivated models for hadrons predict an assortment of "exotic" hadrons that have structures that are more complex than the quark-antiquark mesons and three-quark baryons of the original quark-parton model. These include pentaquark baryons, the six-quark H-dibaryon, and tetraquark, hybrid and glueball mesons. Despite extensive experimental searches, no unambiguous candidates for any of these exotic configurations have been identified. On the other hand, a number of meson states, one that seems to be a proton-antiproton bound state, and others that contain either charmed-anticharmed quark pairs or bottom-antibottom quark pairs, have been recently discovered that neither fit into the quark-antiquark meson picture nor match the expected properties of the QCD-inspired exotics. Here I briefly review results from a recent search for the H-dibaryon, and discuss some properties of the newly discovered states --the proton-antiproton state and the so-called XYZ mesons-- and compare them with expectations for conventional quark-antiquark mesons and the predicted QCD-exotic states.

QCD exotics

QCD-motivated models for hadrons predict an assortment of "exotic" hadrons that have structures that are more complex then the quark-antiquark mesons and three-quark baryons of the original quark-parton model. These include pentaquark baryons, the six-quark H-dibaryon, and tetra-quark, hybrid, and glueball mesons. Despite extensive experimental searches, no unambiguous candidates for any of these exotic configurations have yet to be identified. On the other hand, a number of meson states, one that seems to be a proton-antiproton bound state, and others that contain either charmed-anticharmed quark pairs or bottom-antibottom quark pairs, have been recently discovered that neither fit into the quark-antiquark meson picture nor match the expected properties of the QCD-inspired exotics. Here I briefly review results from a recent search for the H-dibaryon, and discuss some properties of the newly discovered states --the so-called XYZ mesons-- and compare them with expectations for conventional quark-antiquark mesons and the predicted QCD-exotic states.

Observation of a narrow baryon resonance with positive strangeness formed inK+Xe collisions

The charge-exchange reaction ${K}^{+}\mathrm{Xe}\ensuremath{\rightarrow}{K}^{0}p{\mathrm{Xe}}^{\ensuremath{'}}$ is investigated using the data of the DIANA experiment. The distribution of the $p{K}^{0}$ effective mass shows a prominent enhancement near 1538 MeV formed by nearly 80 events above the background, whose width is consistent with being entirely due to the experimental resolution. Under the selections based on a simulation of ${K}^{+}$Xe collisions, the statistical significance of the signal reaches 5.5$\ensuremath{\sigma}$. We interpret this observation as strong evidence for formation of a pentaquark baryon with positive strangeness, ${\ensuremath{\Theta}}^{+}(uudd\overline{s})$, in the charge-exchange reaction ${K}^{+}n\ensuremath{\rightarrow}{K}^{0}p$ on a bound neutron. The mass of the ${\ensuremath{\Theta}}^{+}$ baryon is measured as $m({\ensuremath{\Theta}}^{+})=1538\ifmmode\pm\else\textpm\fi{}2$ MeV. Using the ratio between the numbers of resonant and nonresonant charge-exchange events in the peak region, the intrinsic width of this baryon resonance is determined as $\ensuremath{\Gamma}({\ensuremath{\Theta}}^{+})=0.34\ifmmode\pm\else\textpm\fi{}0.10$ MeV.

New hadron spectroscopies

QCD-motivated models for hadrons predict an assortment of "exotic" hadrons that have structures that are more complex than the quark-antiquark mesons and three-quark baryons of the original quark-parton model. These include pentaquark baryons, the six-quark H-dibaryon, and tetraquark and glueball mesons. Despite extensive experimental searches, no unambiguous candidates for any of these exotic configurations have yet to be identified. On the other hand, a number of meson states, one that seems to be a proton-antiproton bound state, and others that contain either charmed-anticharmed quark pairs or bottom-antibottom quark pairs, have been recently discovered that neither fit into the quark-antiquark meson picture nor match the expected properties of the QCD-inspired exotics. Here I briefly review results from a recent search for the H-dibaryon, and discuss some properties of the newly discovered states –the so-called XYZ mesons– and compare them with expectations for conventional quark-antiquark mesons and the predicted QCD-exotic states.

Search for theΘ+Pentaquark via theπ−p→K−XReaction at1.92 GeV/c

The $\Theta^+$ pentaquark baryon was searched for via the $\pi^-p\to K^-X$ reaction in a missing-mass resolution of 1.4 MeV/$c^2$(FWHM) at J-PARC. $\pi^-$ meson beams were incident on the liquid hydrogen target with the beam momentum of 1.92 GeV/$c$. No peak structure corresponding to the $\Theta^+$ mass was observed. The upper limit of the production cross section averaged over the scattering angle of 2$^{\circ}$ to 15$^{\circ}$ in the laboratory frame was obtained to be 0.26 $\mu$b/sr in the mass region of 1.51$-$1.55 GeV/$c^2$.The upper limit of the $\Theta^+$ decay width using the effective Lagrangian approach was obtained to be 0.72 MeV/$c^2$ and 3.1 MeV/$c^2$ for $J^P_{\Theta}=1/2^+$ and $J^P_{\Theta}=1/2^-$, respectively.

Search for Θ + via the Pion Induced Reaction at J-PARC: J-PARC E19

The Θ+ pentaquark baryon was searched for via the π−p → K−X reaction at J-PARC. π− meson beams were irradiated on the liquid hydrogen target with the beam momentum of 1.92 GeV/c. In the missing mass spectrum, no peak structure corresponding to Θ+ mass was observed. The preliminary upper limit of the differential cross section averaged over the scattering angle of 2° to 15° at the laboratory frame was less than 0.3 μb/sr at the 90 % confidence level in the missing mass region over 1.51 to 1.55 GeV/c2.

η PHOTO-PRODUCTION ON THE DEUTERON AT LNS, TOHOKU UNIVERSITY

The differential and total cross sections were measured for the γp → ηp and γd → ηpn reactions at Eγ ≤ 1150 MeV by using an electro-magnetic (EM) calorimeter SCISSORS II at the Laboratory of Nuclear Science (LNS), Tohoku University. The total cross section on the deuteron shows a bump around Eγ = 1 GeV , while no bump is observed in the same energy region of that on the proton. This bump is attributed to be a nucleon resonance excited from the neutron, and it is a candidate of anti-decuplet penta-quark baryons with hidden strangeness. It was difficult, however, to detect all the γ's coming from η decay since the solid angle of SCISSORS II was only 12.6% in total. Statistics of the detected η produced events is poor and systematic uncertainty of the obtained cross section is not small due to low acceptance. A new EM calorimeter complex called FOREST with a solid angle of about 4π sr has been constructed. The spin and parity of the relevant resonance are expected to be determined by the experiments with FOREST.

Magnetic moments of the proton and of octet baryons in a quasiparticle diquark model

The strange contribution to the proton magnetic moment is investigated assuming a uuds$\overline{s}$ configuration for the proton in the context of a quasiparticle diquark model. The magnetic moment and mass of octet pentaquark baryons are also investigated considering the diquark-diquark-antiquark scheme. The results are found to be in agreement with the existing theoretical and experimental predictions.

Spin-3/2pentaquark in QCD sum rules

The QCD sum rule method is formulated for the strangeness $+1$ pentaquark baryon with isospin $I=0$ and spin-parity ${J}^{\ensuremath{\pi}}={\frac{3}{2}}^{\ifmmode\pm\else\textpm\fi{}}$. The spin-$\frac{3}{2}$ states are considered to be narrower than the spin-$\frac{1}{2}$ ones, and thus may provide a natural explanation for the experimentally observed narrow width of ${\ensuremath{\Theta}}^{+}$. In order to obtain reliable results in QCD sum rule calculations, we stress the importance of establishing a wide Borel window, where convergence of the operator product expansion and sufficient low-mass strength of the spectral function are guaranteed. To this end, we employ the difference of two independent correlators so that the high-energy continuum contribution is suppressed. The stability of the physical quantities against the Borel mass is confirmed within the Borel window. It is found that the sum rule gives positive evidence for the $(I,{J}^{\ensuremath{\pi}})=(0,{\frac{3}{2}}^{+})$ state with a mass of about $1.4\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{ }\mathrm{GeV}$, while we cannot extract any evidence for the $(0,{\frac{3}{2}}^{\ensuremath{-}})$ state.

String Junction Model, Cluster Hypothesis, Penta-Quark Baryon and Tetra-Quark Meson

Thirty years ago we proposed string junction model of hadrons and examined structure and reaction of hadrons including exotic ones. Mass m of exotic hadrons of light quarks is roughly given by m ∼ NJ · mB ,w hereNJ is the total number of junctions and mB ∼ 1 GeV is the ordinary light baryon mass. In this paper we introduce “cluster hypothesis” into the model by which mass of a complex hadron is given by the sum of masses of clusters composing it. The hypothesis guarantees the established picture that mass differences of hadrons of the same string junction structure are due to those of the constituent quarks. A candidate for penta-quark baryon Θ (1530 MeV, S = +1) including a strange antiquark s and that for tetra-quark meson Z + (4430 MeV) recently reported by the Belle collaboration are examined in parallel. Θ is considered to have non-strange partners, which are lighter by the mass difference Δs between strange and non-strange quarks. Mass of such light penta-quark baryons with NJ = 3 is expected to be about 3 GeV. Several parameters of the model are estimated such as mass of junction of mJ ∼ O(10) MeV. While mass of light tetra-quark meson with NJ = 2 is expected to be about 2 GeV, Z + (4430 MeV) containing (u, c, d, c) gives a clue to determine some parameters of the model, e.g., inter-junction string energy mIJ.

VECTOR AND AXIAL-VECTOR STRUCTURES OF THE Θ+

We present in this talk recent results of the vector and axial-vector transitions of the nucleon to the pentaquark baryon Θ+, based on the SU(3) chiral quark-soliton model. The results are summarized as follows: K*NΘ vector and tensor coupling constants turn out to be gK*NΘ ≃ 0.81 and fK*NΘ ≃ 0.84, respectively, and the KNΘ axial-vector coupling constant to be [Formula: see text]. As a result, the total decay width for Θ+ → NK becomes very small: ΓΘ→NK ≃ 0.71 MeV , which is consistent with the DIANA result ΓΘ→NK = 0.36 ± 0.11 MeV .

Vector transition form factors of the [formula omitted] and [formula omitted] in the [formula omitted] chiral quark–soliton model

We investigate the vector transition form factors of the nucleon and vector meson K ∗ to the pentaquark baryon Θ + within the framework of the SU ( 3 ) chiral quark–soliton model. We take into account the rotational 1 / N c and linear m s corrections, assuming isospin symmetry and employing the symmetry-conserving quantization. It turns out that the leading-order contributions to the form factors are almost cancelled by the rotational corrections. Because of this, the flavor SU ( 3 ) symmetry-breaking terms yield sizeable effects on the vector transition form factors. In particular, the main contribution to the electric-like transition form factor comes from the wave-function corrections, which is a consequence of the generalized Ademollo–Gatto theorem derived in the present work. We estimate with the help of the vector meson dominance the K ∗ vector and tensor coupling constants for the Θ + : g K ∗ N Θ = 0.74 – 0.87 and f K ∗ N Θ = 0.53 – 1.16 . We argue that the outcome of the present work is consistent with the null results of the CLAS experiments in the reactions γ n → K − Θ + and γ p → K ¯ 0 Θ + . The results of the present work are also consistent with the recent experiments at KEK. In addition, we present the results of the Σ 10 ¯ → N K ¯ ∗ transition form factors and its K ¯ ∗ N Σ 10 ¯ coupling constants.

Production of the Pentaquark Exotic Baryon $Xi_5$ in $bar{K}N$ Scattering: $bar{K}N o KXi_5$ and $bar{K}N o K^{*}Xi_5$

We investigate the production of the newly found pentaquark exotic baryon Xi_5 in the bar{K} N -> K Xi_5 and the bar{K} N -> K* Xi_5 reactions at the tree level. We consider both positive- and negative parities of the Xi_5. The reactions are dominated by the s- and u-channel processes, and the resulting cross sections depend very much on the parity of Xi_5 and on the type of form factor. We have seen that the cross sections for the positive-parity Xi_5 are generally about a hundred times larger than those of the negative-parity one. This large difference in the cross sections will be useful for further study of pentaquark baryons.

Charmed exotics in heavy ion collisions

Based on the color–spin interaction in diquarks, we argue that charmed multiquark hadrons are likely to exist. Because of the appreciable number of charm quarks produced in central nucleus–nucleus collisions at ultrarelativistic energies, the production of charmed multiquark hadrons is expected to be enhanced in these collisions. Using both the quark coalescence model and the statistical hadronization model, we estimate the yield of charmed tetraquark mesons, Tcc, and pentaquark baryons, Θcs, in heavy ion collisions at RHIC and LHC. We further discuss the decay modes of these charmed exotic hadrons in order to facilitate their detections in experiments.

Quark spin content of the proton, hyperon semileptonic decays, and the decay width of theΘ+pentaquark

Using the exisiting experimental data for hyperon semileptonic decays and the flavor-singlet axial-vector charge ${g}_{A}^{(0)}$ from polarized deep inelastic scattering of the proton, we derive the decay width of the ${\ensuremath{\Theta}}^{+}$ pentaquark baryon. We take into account the effects of flavor SU(3) symmetry breaking within the framework of the chiral quark-soliton model. All dynamical parameters of the model are fixed by using the five experimental hyperon semileptonic decay constants and flavor-singlet axial-vector charge. We obtain the numerical results of the decay width of the ${\ensuremath{\Theta}}^{+}$ pentaquark baryon as a function of the pion-nucleon sigma term ${\ensuremath{\Sigma}}_{\ensuremath{\pi}N}$ and investigate the dependence of the decay width of the ${\ensuremath{\Theta}}^{+}$ on the ${g}_{A}^{(0)}$, varying the ${g}_{A}^{(0)}$ within the range of the experimental uncertainty. We demonstrate that the combined values of all known semileptonic decays with the generally accepted value of ${g}_{A}^{(0)}\ensuremath{\approx}0.3$ for the proton are compatible with a small decay width ${\ensuremath{\Gamma}}_{\ensuremath{\Theta}KN}$ of the ${\ensuremath{\Theta}}^{+}$ pentaquark, i.e. ${\ensuremath{\Gamma}}_{\ensuremath{\Theta}KN}\ensuremath{\le}1\text{ }\text{ }\mathrm{MeV}$.

A Study on the Pentaquark States

We have described the pentaquark baryon based on diquarks in analogy to the quasi-particle picture in many body systems and we come across two diquarks and the strange antiquark as the constituents of Θ + . The former also stand for the building blocks, similar to the excitation states in crystals and the effective mass of the diquarks gets reduced in this quasiparticle approximation. We presume that they do not interact with each other and the effective mass characterizes the dynamic properties of the quasiparticle. The Θ + mass has been estimated to be 1552MeV and it is found to be in good agreement with the experimental data.

Recent Results and Prospects on the Θ+Study at LEPS

The LEPS experiment at SPring-8 has collected data in reactions of 1.5–2.4 GeV photons with a liquid deuterium target. The pentaquark baryon Θ + was searched for in a missing mass spectrum of K − and proton from a deuteron. A peak structure of 4–5σ was observed at 1.53 GeV/c 2 above background spectra which were estimated by two complementary methods. Further data is being collected with a higher intensity beam. Confirmations and systematic studies of the Θ + will continue at new facilities of SPring-8 and J-PARC.