Narrow Results

The theoretical motivation to look for S=+1 resonances is introduced and searches for such states in the literature are reviewed. The motivation for the recent pentaquark searches is discussed and the experimental details are described for several positive results of the S=+1 resonance with a mass of approximately 1.54 GeV, now called the Θ⁺. Finally, some new theoretical proposals to explain the recent experimental results are briefly mentioned, followed by a short discussion of future experimental prospects to learn more about pentaquark states.

The pentaquark state of has been observed to decay with two decay modes: Θ⁺→nK⁺ and Θ⁺→pK⁰. The decay probability ratio of the two decay modes is studied with general symmetry consideration of isospin, spin and parity. We arrive at a result of the ratio , which is valid for the Θ⁺ state to be a pure isoscalar or isovector state, or an isoscalar and isovector mixing state, or an isotensor state with mixture of isoscalar and isovector components with coefficients α and β. The dependence on spin and parity of the pentaquark Θ⁺ state is found to be small due to small difference between the center-of-mass decay momenta k₁ and k₂ of the two decay modes. We also provide an analysis on the constraint of the isospin of Θ⁺ from the absence of a peak in the pK⁺ invariant mass distribution in the γp→pK⁺K^- process. Future experimental results about the decay probability ratio may provide information about the properties of the pentaquark Θ⁺ state.

Using the picture of the flux tube model, we propose that the Θ₅ pentaquark as the first candidate of the three-dimensional non-planar hadron with the extremely stable diamond structure. The up and down quarks lie at the corners of the diamond while the anti-strange quark sits in the center. Various un-excited color flux tubes between the five quarks bind them into a stable and narrow color-singlet. Such a configuration allows the lowest state having the negative parity naturally. The decay of the Θ₅ pentaquark into the nucleon and kaon requires the breakup of the non-planar diamond configuration into two conventional planar hadrons, which involves some kind of structural phase transition as in the condensed matter physics. Hence the width of the Θ⁺ pentaquark should be narrow despite that it lies above the kaon nucleon threshold. We suggest that future lattice QCD calculation adopt non-planar interpolating currents to explore the underlying structure of the Θ₅ pentaquark.

New LHCb Collaboration results on pentaquarks with hidden charm¹ are discussed. These results fit nicely in the hadrocharmonium pentaquark scenario.${}^{2,3,}$ In the new data the old LHCb pentaquark $P_c(4450)$ splits into two states $P_c(4440)$ and $P_c(4457)$. We interpret these two almost degenerated hadrocharmonium states with $J^P=1/2^{-}$ and $J^P=3/2^{-}$, as a result of hyperfine splitting between hadrocharmonium states predicted in Ref. 2. It arises due to QCD multipole interaction between color-singlet hadrocharmonium constituents. We improve the theoretical estimate of hyperfine splitting${}^{2,3}$ that is compatible with the experimental data. The new $P_c(4312)$ state finds a natural explanation as a bound state of ${\chi }_{c0}$ and a nucleon, with $I=1/2$, $J^P=1/2^{+}$ and binding energy 42 MeV. As a bound state of a spin-$0$ meson and a nucleon, hadrocharmonium pentaquark $P_c(4312)$ does not experience hyperfine splitting. We find a series of hadrocharmonium states in the vicinity of the wide $P_c(4380)$ pentaquark that can explain its apparently large decay width. We compare the hadrocharmonium and molecular pentaquark scenarios and discuss their relative advantages and drawbacks.

Since LEPS collaboration reported the first evidence of Θ⁺ pentaquark in early 2003, eleven other experimental groups have confirmed this exotic state while many other groups did not see any signal. If this state is further established by future high statistical experiments, its discovery shall be one of the most important events in hadron physics for the past three decades. This exotic baryon with such a low mass and so narrow a width imposes a big challenge to hadron theorists. Up to now, there have appeared more than two hundred theoretical papers trying to interpret this charming state. I will review some important theoretical developments on pentaquarks based on my biased personal views.

A critical look is taken at both positive and null evidence for the Θ⁺ pentaquark. Potential problems with experiments will be discussed and the question of what conclusion can be drawn from both the positive and the null results is examined. First the question of existence of the Θ⁺ pentaquark is considered, followed by a discussion of new experiments that are either planned or in progress to answer questions about its mass, width and isospin. Finally, indirect evidence for the parity of the Θ⁺ is examined, and suggestions for experiments to measure its parity directly are given.

Meson photoproduction was studied at the CEBAF Large Acceptance Spectrometer (CLAS) utilizing a 5.5 GeV/c photon beam incident on a hydrogen target. Here we report on the partial wave analyzes (PWA) performed on the exclusive final states π⁺π⁺π^-n and K⁺K^-π⁺n. In addition, CLAS has data on the final state π⁺π⁰π^-p which will be reported in the near future. The analysis of the π⁺π⁺π^- system shows clearly the a₂(1320), as well as evidence for the a₁(1260) and the π₂(1670). The π₁(1600), previously observed in π^-p interactions, is seen very weakly here. In the K⁺K^-π⁺n channel there is is evidence for the Θ⁺(1540) S=+1 baryon pentaquark. The PWA of this system gives an excellent description of the background which rules out mesonic reflection as a source of the signal.

Results are reported based on samples of 58 million J/ψ and 14 million ψ(2S) decays obtained by the BESII experiment. Improved branching fraction measurements are determined, including branching fractions for J/ψ→π⁺π^-π⁰, ψ(2S)→π⁰J/ψ, ηJ/ψ, π⁰π⁰J/ψ, anything J/ψ, and ψ(2S)→γχ_c1, γχ_c2→γγJ/ψ. The decay J/ψ→ωπ⁺π^- is studied. At low ππ mass, a large, broad peak due to the σ is observed, and its pole position is determined. Results are presented on ψ(2S) and J/ψ hadronic decays to and final states. No significant Θ(1540) signal, the pentaquark candidate, is observed, and upper limits are set. An enhancement near the m_p+M_Λ mass threshold is observed in the invariant mass spectrum from decays. It can be fit with an S-wave Breit-Wigner resonance with a mass m=2075±12 (stat)±5 (syst) MeV and a width of Γ=90±35 (stat)±9 (syst) MeV.

I first give a brief overview of the status of exotic hadron search so far. Then I review the recent theoretical development on pentaquarks. Finally I emphasize that none of theoretical formalisms predicts the existence of Θ pentaquark very reliably. None of them explains its narrow width in a natural way.

A model for two-pion production in pion-induced reactions including the excitation of baryon resonances is presented and applied to the recent Brookhaven data. The impact of the Θ⁺(1540) resonance on differential and integrated cross sections for kaon reactions on nuclei is analysed using the Jülich kaon-nucleon model. An upper limit for the width of the pentaquark candidate of 1 MeV results.

Using large N_c counting rule, it is argued that tetra-quark resonances do not exist. Also it is pointed out that there exists the violation of exchange degeneracy in the exotic KN scattering channel. It implies either the failure of resonance saturation assumption or it suggests the existence of exotic baryon resonances in such a channel.

A study of the decay is performed using 81 fb^-1 of data collected at the ϒ(4S) with the BABAR detector at PEP-II. The branching fraction of is measured to be (6.7 ± 0.9 ± 0.6) × 10⁶. An upper limit on the branching fraction of , where Θ*⁺⁺ is a narrow state decaying to pK⁺, is set to be 1.5 × 10^-7 for 1.43 < m(Θ*⁺⁺) < 1.85 GeV/c² at 90% confidence level. All results are preliminary.

A spate of remarkable new hadrons reported in 2003 may lead to unequivocal proof of states beyond conventional and qqq structure. Claimed baryonic states Θ⁺, Φ, and would consist of five quarks, and new -states and/or X(3872) might contain four quarks. I review efforts to search for and study this "new" spectroscopy in -collisions with the CDF II detector. Pentaquark searches are negative, and no evidence for exotic analogs of D_sJ-states was found. CDF has confirmed the X(3872). My main focus is the production and decay properties of the X(3872), and its possible interpretations.

A brief review is given of the lattice QCD calculation of the hadron spectrum. The status of current attempts toward inclusion of dynamical up, down and strange quarks is summarized focusing on our own work. Recent work on the possible existence of pentaquark states are assessed. We touch upon the PACS-CS Project for building our next machine for lattice QCD, and conclude with a near-term physics and machine prospects.

A review is given to pentaquark mass predictions in quark models and QCD. It is pointed out that no successful quark model prediction is available for low-lying pentaquark states. Some new results of direct application of QCD, QCD sum rules and lattice QCD, are also presented.

We present results on pentaquark searches from nuclear collisions at RHIC with the STAR detector system. An intriguing peak has been observed in the invariant mass distribution of from 18.6 Million d+Au collision events at . The peak centers at a mass 1528 ± 2 ± 5 MeV/c² and the FWHM ~15 MeV/c² limited by detector responses. The statistical significance of the peak is 4.2σ. Such a state if confirmed is manifestly exotic and implies a family of isospin one states. A weak signal of less statistical significance (~3σ) has been observed in 5.6M Au+Au collision events at 62.4 GeV. Searches in 10.7M Au+Au collision events at 200 GeV yield no significant signal. The Au+Au results neither confirm nor rule out the d+Au observation as a possible state.

The pentaquark width is calculated in QCD sum rules. The higher dimension operators contribution is accounted. It is shown, that Γ_Θ should be very small, less than 1 MeV.

The CLAS Collaboration is operating the CLAS detector at theThomas Jefferson National Laboratory (JLab) in USA. The unique combination of the detector large acceptance and high intensity of the continuous electron beam of CEBAF has opened the way to a comprehensive study of the hadrons structure in a kinematic domain between nuclear and particle physics. Meson spectroscopy plays a central role in the physics program of the Collaboration. Many exclusive channels have been studied with virtual and real photon beams in a wide kinematic domain providing key information about the hadron structure as well as the reactions dynamic. In this contribution, the rich physics program covered by present and future experiments will be reviewed.

It has been ten years since the first report for a positive strangeness pentaquark-like baryon state. However, the existence of the pentaquark state is still controversial. Some contradictions between the experiments are unsolved. In this paper, we review the experimental search for the pentaquark candidates Θ⁺, Θ⁺⁺, Ξ^{- -}, and N* in detail. We review the experiments with positive results and compare the experiments with similar conditions but opposite results.

Relativistic five-quark equations are found in the framework of the dispersion relation technique. The solutions of these equations using the method based on the extraction of the leading singularities of the amplitudes are obtained. The five-quark amplitudes for the low-lying pentaquarks are calculated under the condition that flavor SU(3) symmetry holds. The poles of the five-quark amplitudes determine the masses of the lowest pentaquarks. The mass spectra of pentaquarks which contain only light quarks are calculated. The calculation of pentaquark amplitudes estimates the contributions of three subamplitudes: molecular subamplitude BM, Mqqq subamplitude and subamplitude. The main contributions to the pentaquark amplitude are determined by the subamplitudes, which include the meson states M.