Pentaquark Baryon Research Articles

Effects of instanton induced interactions on pentaquarks

Roles of instanton induced interactions (III) in the masses of pentaquark baryons, ${\ensuremath{\Theta}}^{+}$ ($J=1/2$ and $3/2$) and ${\ensuremath{\Xi}}^{--}$, and a dibaryon, $H$, are discussed using the MIT bag model. It is shown that the two-body terms in III give a strong attraction mainly due to the increase of the number of flavor antisymmetric quark pairs in multiquark systems. In contrast, the three-body $u$-$d$-$s$ interaction is repulsive. It is found that III lowers the mass of negative-parity ${\ensuremath{\Theta}}^{+}$ as much as 100 MeV from the mass predicted by the bag model without III. We also consider possible quark model configurations for positive-parity ${\ensuremath{\Theta}}^{+}$ under III.

HIGHLIGHTS OF RECENT RESULTS WITH CLAS

Recent results on the study of the electromagnetic structure of nucleon resonances, the spin structure of protons and neutrons at small and intermediate photon virtuality, and the search for exotic pentaquark baryons are presented.

RECENT RESULTS FROM LEPS

The photon beam at the LEPS facility is produced by backward-Compton scattering of laser photons from 8 GeV electrons at the SPring-8. The status and prospects of the experimental study on the Θ+ at LEPS are reported. Further evidences for pentaquark baryons and counter-evidences are also discussed.

Electromagnetic excitations of baryon resonances

I will discuss recent results on the electromagnetic excitation of non-strange baryon resonances obtained with the CEBAF Large Acceptance Spectrometer (CLAS). Preliminary data on the N Δ transition multipoles at small and at large photon virtualities are presented, showing the importance of the nucleon pion cloud. In the second nucleon resonance region, new results on the transition amplitudes for the Roper P 11 ( 1440 ) , the S 11 ( 1535 ) , and the D 13 ( 1520 ) states are presented. The transverse Roper photocoupling amplitude shows a strong Q 2 dependence that is best described in terms of large meson cloud contributions. The helicity amplitudes for the S 11 ( 1535 ) are extracted from both single pion production as well as eta production, giving consistent results. In contrast to the Roper resonance, a nearly Q 2 -independent behavior is observed at small Q 2 values supporting the interpretation of this state as a nearly pure three-quark resonance. The D 13 ( 1520 ) amplitudes show the rapid change of the helicity structure predicted by non-relativistic quark models. Searches for new baryon states in two-pion photoproduction and electroproduction show consistently a need for a new state near 1.72 GeV with J P = 3 2 + . Finally, the search for the Θ + ( 1540 ) and other pentaquark baryon states with CLAS is presented.

Search for Θ+(1540) in exclusive proton-induced reaction $$p + C(N) \to \Theta ^ + \bar \kappa ^0 + C(N)$$ at the energy of 70 GeV

A search for narrow Θ+(1540), a candidate for a pentaquark baryon with positive strangeness, has been performed in an exclusive proton-induced reaction \(p + C(N) \to \Theta ^ + \bar \kappa ^0 + C(N)\) on carbon nuclei or quasifree nucleons at \(E_{beam} = 70GeV(\sqrt s = 11.5GeV)\) studying nK+, pKS0, and pKL0 decay channels of Θ+(1540) in four different final states of the \(\Theta ^ + \bar K^0 \) system. In order to assess the quality of the identification of the final states with neutron or KL0, we reconstructed Λ(1520) → nKS0 and ϕ → KL0KS0 decays in the calibration reactions p + C(N) → Λ (1520)K++C(N) and p+C(N) → pϕ+C(N). We found no evidence for a narrow pentaquark peak in any of the studied final states and decay channels. Assuming that the production characteristics of the \(\Theta ^ + \bar K^0 \) system are not drastically different from those of the Λ(1520)K+ and pϕ systems, we established upper limits on the cross-section ratios \(\sigma (\Theta ^ + \bar K^0 )/\sigma (\Lambda (1520)K^ + ) < 0.02\) and \(\sigma (\Theta ^ + \bar K^0 )/\sigma (p\phi ) < 0.15\) at 90% C.L. and a preliminary upper limit for the forward-hemisphere cross section \(\sigma (\Theta ^ + \bar K^0 )\) nb/nucleon.

Chiral perturbation theory for pentaquark baryons and its applications

We construct a chiral Lagrangian for pentaquark baryons assuming that the recently found Θ+(1540) state belongs to an antidecuplet of SU(3) flavor symmetry with JP=12±. We derive the Gell-Mann–Okubo formulae for the antidecuplet baryon masses, and a possible mixing between the antidecuplet and the pentaquark octet. Then we calculate the cross sections for π−p→K−Θ+ and γn→K−Θ+ using our chiral Lagrangian. The resulting amplitudes respect the underlying chiral symmetry of QCD correctly. We also describe how to include the light vector mesons in the chiral Lagrangian.

Traces of theΘ+Pentaquark inK+-Nucleus Dynamics

Long-standing anomalies in K^+ nucleus integral cross sections could be resolved by extending the impulse-approximation t*rho optical-potential framework to incorporate K^+ absorption on pairs of nucleons. Substantially improved fits to the data at p(lab)=500-700 MeV/c are obtained. An upper bound of 3.5 mb for the absorption cross section of K^+ per nucleon is derived. We conjecture that the underlying microscopic absorption process is K^+ n N --> Theta^+ N, where Theta^+(1540) is the newly discovered exotic Y=2, I=0, Z=1 pentaquark baryon, and estimate that the cross section for K^+ d --> Theta^+ p is a fraction of millibarn. Comments are made on Theta^+ production reactions on nuclei.

Evidence for formation of [formula omitted] resonances with masses near 1532, 1578, and 1659 MeV in neutrino interactions with nuclei

Three baryon resonances with masses of 1532.2 ± 1.3 , 1577.7 ± 1.9 , and 1658.6 ± 4.4 MeV are observed in invariant mass of the K S 0 p system formed in neutrino and antineutrino collisions with deuterons and neon nuclei. Observed widths of the 1532-MeV and 1578-MeV resonances are consistent with being entirely due to apparatus smearing, and their intrinsic widths are restricted to Γ 12 and 23 MeV, respectively. For the 1659-MeV resonance, the data suggest a nonvanishing intrinsic width of Γ ∼ 20 MeV . Significance levels of the three signals are near 7.1 σ , 5.0 σ , and 4.5 σ , respectively. The Σ + hypothesis for either of these three resonances is disfavored by the data on associated Λ hyperons and on formation of the Λ π system. These resonant states are tentatively interpreted as the recently discovered pentaquark baryon Θ + ( 1530 ) and its spin/isospin partners. The analysis is based on neutrino data collected by past bubble-chamber experiments.

Two-hadron-irreducible QCD sum rule for pentaquark baryon

We point out that naive pentaquark correlations function include two-hadron-reducible contributions, which are given by convolution of baryon and meson correlation functions and have nothing to do with pentaquark. We show that the two-hadron-reducible contributions are large in the operator product expansion of the correlation functions of three existing works on the pentaquark. Therefore, it is dangerous to draw a conclusion from the sum rules using naive pentaquark correlation functions with naive ansatz for the spectral function under the dispersion integral. Instead, we propose to use the two-hadron-irreducible correlation function, which is obtained by subtracting the two-hadron-reducible contribution from the naive correlation function. Taking one of the works as an example we demonstrate how drastically the results can change if we remove the two-hadron-reducible part from the naive correlation function. We obtain the result opposite to the original work for the parity of the pentaquark.

Pentaquark baryon in anisotropic lattice QCD

The penta-quark(5Q) baryon is studied in anisotropic quenched lattice QCD with renormalized anisotropy a_s/a_t=4 for a high-precision mass measurement. The standard Wilson action at beta=5.75 and the O(a) improved Wilson quark action with kappa=0.1210(0.0010)0.1240 are employed on a 12^3 \times 96 lattice. Contribution of excited states is suppressed by using a smeared source. We investigate both the positive- and negative-parity 5Q baryons with I=0 and spin J=1/2 using a non-NK-type interpolating field. After chiral extrapolation, the lowest positive-parity state is found to have a mass, m_{Theta}=2.25 GeV, which is much heavier than the experimentally observed Theta^+(1540). The lowest negative-parity 5Q appears at m_{Theta}=1.75 GeV, which is near the s-wave NK threshold. To distinguish spatially-localized 5Q resonances from NK scattering states, we propose a new general method imposing a ``Hybrid Boundary Condition (HBC)'', where the NK threshold is artificially raised without affecting compact five-quark states. The study using the HBC method shows that the negative-parity state observed on the lattice is not a compact 5Q but an s-wave NK-scattering state.

Relativistic mean field approximation to baryons

We stress the importance of the spontaneous chiral symmetry breaking for understanding the low-energy structure of baryons. The Mean Field Approximation to baryons is formulated, which solves several outstanding paradoxes of the naive quark models, and which allows to compute parton distributions at low virtuality in a consistent way. We explain why this approach to baryons leads to the prediction of relatively light exotic pentaquark baryons, in contrast to the constituent models which do not take seriously the importance of chiral symmetry breaking. We briefly discuss why, to our mind, it is easier to produce exotic pentaquarks at low than at high energies. PACS: 12.38.-t – 12.39.-x – 12.39.Dc – 14.20-c

EVIDENCE FOR A NARROW BARYON STATE WITH |S|=1 AT THE HERMES EXPERIMENT

A narrow baryon state has been found in the channel [Formula: see text] of quasi-real photoproduction on a deuterium target at the HERMES experiment. Its mass is 1528±2.6±2.1 MeV and its width is somewhat larger than the experimental resolution of 4.3-6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Θ+ pentaquark baryon. No signal for a hypothetical Θ++ baryon was observed, indicating that the Θ+ is an isoscalar particle.

A study of [formula omitted] in lattice QCD

We investigate the mass spectrum of the pentaquark baryon ( udud s ¯ ) in quenched lattice QCD with exact chiral symmetry. Using 3 different interpolating operators, we measure the 3 × 3 correlation matrix and obtain the eigenvalues A ± ( t ) with ± parity. For odd parity states, A − ( t ) deviates from pure exponential decay even at large t , which implies that it cannot be a resonance with narrow decay width, thus is ruled out as a candidate of Θ + ( 1540 ) . For even parity states, they behave like usual resonances seen in quenched lattice QCD, and the mass of the lowest lying J P = 1 / 2 + state is determined to be 1583 ± 121 MeV.

Constituent quark models and pentaquark baryons

We discuss certain general features of the pentaquark picture for the θ, its 1̄0̄F partner, Ξ3/2, and possible heavy quark analogues. Models employing spin-dependent interactions based on either effective Goldstone boson exchange or efective color magnetic exchange are also used to shed light on possible corrections to the Jaffe-Wilczek and Karliner-Lipkin scenarios. Some model-dependent features of the pentaquark picture (splitting patterns and relative decay couplings) are also discussed in the context of these models.

Pentaquark baryons in the SU(3) quark model

We study the SU(3) group structure of pentaquark baryons which are made of four quarks and one antiquark. The pentaquark baryons form {1}, {8}, {10}, {10}-bar, {27}, and {35} multiplets in SU(3) quark model. First, the flavor wave functions of all the pentaquark baryons are constructed in SU(3) quark model and then the flavor SU(3) symmetry relations for the interactions of the pentaquarks with three-quark baryons and pentaquark baryons are obtained.

Light-front approach for heavy pentaquark transitions

Assuming the two diquark structure for the pentaquark state as advocated in the Jaffe-Wilczek model, there exist exotic parity-even anti-sextet and parity-odd triplet heavy pentaquark baryons. The theoretical estimate of charmed and bottom pentaquark masses is quite controversial and it is not clear whether the ground-state heavy pentaquark lies above or below the strong-decay threshold. We study the weak transitions of heavy pentaquark states using the light-front quark model. In the heavy quark limit, heavy-to-heavy pentaquark transition form factors can be expressed in terms of three Isgur-Wise functions: two of them are found to be normalized to unity at zero recoil, while the third one is equal to 1/2 at the maximum momentum transfer, in accordance with the prediction of the large-Nc approach or the quark model. Therefore, the light-front model calculations are consistent with the requirement of heavy quark symmetry. Numerical results for form factors and Isgur-Wise functions are presented. Decay rates of the weak decays Theta_b+ to Theta_c0 pi+ (rho+), Theta_c0 to Theta+ pi- (rho-), Sigma'_{5b}+ to Sigma'_{5c}0 pi+ (rho+) and Sigma'_{5c}0 to N_8+ pi- (rho-) with Theta_Q, Sigma'_{5Q} and N_8 being the heavy anti-sextet, heavy triplet and light octet pentaquarks, respectively, are obtained. For weakly decaying Theta_b+ and Theta_c0, the branching ratios of Theta_b+ to Theta_c0 pi+, Theta_c0 to Theta+ pi- are estimated to be at the level of 10^{-3} and a few percents, respectively.

Pentaquark Baryons in String Theory

The recently observed pentaquark baryons $\Theta^{+}$ and $\Xi^{--}$ are studied in the dual gravity theory of QCD. By developing a general formulation useful for studying the branched string web in a curved space, simple mass formulae of the pentaquark baryons are derived in the Maldacena prototype model for supersymmetric QCD and a more realistic model for ordinary QCD. Even though the approximations adopted here in deriving these formulae are extremely naive, the predicted values of the masses do not differ significantly from their experimental values. We thus conclude that this approach is promising for the purpose of obtaining a description of the observed masses of pentaquarks and of their extremely narrow decay widths. With the aim of constructing more reliable mass formulae, a preliminary sketch is given of how spin, the hyperfine interaction and parity are considered in the string picture.

Search for ${\rm\Theta}(1540)^ + $ in the exclusive proton-induced reaction ${\rm p + {C(N)\to \Theta^ + \bar{K}^0 + {C}(N)}}$ at the energy of 70 GeV

A search for narrow Theta(1540)^+, a candidate for pentaquark baryon with positive strangeness, has been performed in an exclusive proton-induced reaction p+C(N) \to Theta^+ \bar{K}^0 + C(N) on carbon nuclei or quasifree nucleons at E_{beam}=70 GeV (sqrt{s} = 11.5 GeV) studying nK^+, pK_S and pK_L decay channels of Theta(1540)^+ in four different final states of the Theta^+ \bar{K}^0 system. In order to assess the quality of the identification of the final states with neutron or K_L we reconstructed Lambda(1520)\to nK_S and phi\to K_LK_S decays in the calibration reactions p+C(N)\to Lambda(1520)K^+ + C(N) and p+C(N)\to p\phi + C(N). We found no evidence for narrow pentaquark peak in any of the studied final states and decay channels. Assuming that the production characteristics of the Theta^+ \bar{K^0} system are not drastically different from those of the Lambda(1520)K^+ and p\phi systems, we established upper limits on the cross section ratios sigma(Theta^+\bar{K}^0)/sigma(Lambda(1520)K^+) < 0.02 and sigma(Theta^+\bar{K}^0)/sigma(p\phi) < 0.15 at 90% CL and a preliminary upper limit for the forward hemisphere cross section sigma(Theta^+\bar{K}^0) < 30 nb/nucleon.

Theoretical Overview of the Pentaquark Baryons

A review is given of the theoretical ideas concerning the mysterious pentaquark baryons proposed during the first year after its discovery. We focus on the difficulties involved with the constituent quark models and the discrepancy between the QCD predictions and most of the phenomenological models.

Where are the missing members of the baryon antidecuplet?

We analyze the consequences of the observation of exotic pentaquark baryons on the location of the nonexotic baryons belonging to the antidecuplet. We suggest that there must be a new nucleon state at 1650--1690 MeV and a new $\ensuremath{\Sigma}$ baryon at 1760--1810 MeV.