The reaction gamma + p -> K+ + Sigma + pi was used to determine the invariant mass distributions or "line shapes" of the Sigma+ pi-, Sigma- pi+ and Sigma0 pi0 final states, from threshold at 1328 MeV/c^2 through the mass range of the Lambda(1405) and the Lambda(1520). The measurements were made with the CLAS system at Jefferson Lab using tagged real photons, for center-of-mass energies 1.95 < W < 2.85 GeV. The three mass distributions differ strongly in the vicinity of the I=0 Λ(1405), indicating the presence of substantial I=1 strength in the reaction. Background contributions to the data from the Sigma0(1385) and from K^* Sigma production were studied and shown to have negligible influence. To separate the isospin amplitudes, Breit-Wigner model fits were made that included channel-coupling distortions due to the NKbar threshold. A best fit to all the data was obtained after including a phenomenological I=1, J^P = 1/2^- amplitude with a centroid at 1394\pm20 MeV/c^2 and a second I=1 amplitude at 1413\pm10 MeV/c^2. The centroid of the I=0 Lambda(1405) strength was found at the Sigma pi threshold, with the observed shape determined largely by channel-coupling, leading to an apparent overall peak near 1405 MeV/c^2.
We report the exclusive photoproduction cross sections for the Sigma(1385), Lambda(1405), and Lambda(1520) in the reactions gamma + p -> K+ + Y* using the CLAS detector for energies from near the respective production thresholds up to a center-of-mass energy W of 2.85 GeV. The differential cross sections are integrated to give the total exclusive cross sections for each hyperon. Comparisons are made to current theoretical models based on the effective Lagrangian approach and fitted to previous data. The accuracy of these models is seen to vary widely. The cross sections for the Lambda(1405) re…
Measurement results of photoproduced excited hyperon states using the CLAS detector at Jefferson Lab are shown. The invariant mass distribution of the Lambda(1405) has recently been shown to be different for each of the three Sigma pi channels that it decays to, showing that there is prominent interference between the isospin I=0 and I=1 isospin amplitudes. Measurements of the differential and total cross sections of the three hyperons Lambda(1405), Sigma0(1385), and Lambda(1520) are presented and compared. Prospects of future studies using a 12 GeV beam with the GlueX detector are briefly giv…
We study the reaction mechanism of K*Lambda(1116) photoproduction off the proton target near threshold considering the contributions from nucleon-resonances. Employing the effective Lagrangian method at the tree-level Born approximation, we investigate the role of the D13(2080) and the D15(2200). We found that the D13 plays a crucial role to reproduce the experimental data well in the forward scattering region at low energy. We also present theoretical predictions on the single photon-beam asymmetry as well as the energy and angular dependence for the cross sections of this reaction.
We investigate the influence of new GRAAL Sigma-beam asymmetry measurements on the neutron in multipole fits to the single-pion photoproduction database. Results are compared to those found with the addition of a double-polarization quantity associated with the sum rule.
Recent cross section data for the reaction gamma + p \to K^+ + Lambda are examined for evidence of scaling in both the low-t Regge domain and in the high \sqrt{s} and -t domain where constituent counting may apply. It is shown that the reaction does scale in both regimes. At large center of mass angles, s^{-7} scaling appears to hold at essentially all -t, but with angle-dependent oscillations. The scaled data show particularly strong evidence for s-channel resonances for -t below 2 GeV^2 and for W below about 2.3 GeV. The dominant contributions are consistent with an N^* S_{11} resonance at 1…
We develop a model that describes the gamma p -> K^+ pi Sigma reaction in the Lambda(1405) region. The model consists of gauge invariant photo-production mechanisms, and the chiral unitary model that gives the rescattering amplitudes where Lambda(1405) is contained. The model also contains phenomenological parameters, associated with short-range dynamics, to be used in fitting data. We successfully fit recent CLAS data for the pi-Sigma invariant mass distributions (line-shape) in the gamma p -> K^+ pi Sigma reaction for all the charge states. We find that the higher mass pole for Lambda(1405) …
Recent CLAS data for the pi Sigma invariant mass distributions (line-shapes) in the gamma p -> K^+ pi Sigma reaction are theoretically investigated. The line-shapes have peaks associated with the Lambda(1405) excitation. Our model consists of gauge invariant photo-production mechanisms, and the chiral unitary model that gives the rescattering amplitudes where Lambda(1405) is contained. It is found that, while the pi Sigma line-shape data in the Lambda(1405) region are successfully reproduced by our model for all the charge states, the production mechanism is not so simple that we need to intro…
We estimate the valence quark contributions for the gamma* Y -> Lambda* (Y=Lambda, Sigma0) electromagnetic transition form factors. We focus particularly on the case Lambda*=Lambda(1670) as an analog reaction with gamma* N -> N(1535). The results are compared with those obtained from chiral unitary model, where the Lambda* resonance is dynamically generated and thus the electromagnetic structure comes directly from the meson cloud excitation of the baryon ground states. The form factors for the case Y=Sigma0 in particular, depend crucially on the two real phase (sign) combination, a phase betw…
We study the photoproduction of the ^12{_Lambda}B hypernucleus within a fully covariant effective Lagrangian based model, employing Lambda bound state spinors derived from the latest quark-meson coupling model. The kaon production vertex is described via creation, propagation and decay of N*(1650), N*(1710), and N*(1720) intermediate baryonic resonant states in the initial collision of the photon with a target proton in the incident channel. The parameters of the resonance vertices are fixed by describing the total and differential cross section data on the elementary gamma (p, K+) Lambda reac…
We formulated the Lambda(1405) (abbreviated as Lambda^*) --> (Sigma pi)^0 invariant-mass spectra produced in p+p --> p + Lambda^* + K^+ reactions, in which both the incident channel for a quasi-bound K^-p state and its decay process to (Sigma pi)^0 were taken into account realistically. We calculated M(Sigma pi) spectral shapes for various theoretical models for Lambda^*. They are asymmetric and skewed, and were compared with recent experimental data of HADES, yielding M(Lambda^*) = 1405 -9 +11 MeV/c^2 and Gamma = 62 +- 10 MeV, where the interference effects of the Kbar N-Sigma pi resonance wi…
The pi-Lambda-N - pi-Sigma-N coupled-channel system with quantum numbers (Y,I,JP) = (1,3/2,2+) is studied in a relativistic three-body model, using two-body separable interactions in the dominant p-wave pion-baryon and 3S1 YN channels. Three-body equations are solved in the complex energy plane to search for quasibound-state and resonance poles, producing a robust narrow pi-Lambda-N resonance about 10 - 20 MeV below the pi-Sigma-N threshold. Viewed as a dibaryon, it is a 5S2 quasibound state consisting of Sigma(1385)-N and Delta(1232)-Y components. Comparison is made between the present relati…
We report on a theoretical study of the pp->p Lambda K+ and pp->p Sigma0 K+ reactions near threshold using a chiral dynamical approach. The production process is described by single-pion and single-kaon exchange. The final state interactions of nucleon-hyperon, K-hyperon and K-nucleon systems are also taken into account. We show that our model leads to a fair description of the experimental data on the total cross section of the pp->p Lambda K+ and pp->p Sigma0 K+ reactions. We find that the experimental observed strong suppression of Sigma0 production compared to Lambda production at the same…
The $γ+ p \rightarrow K^0 + Σ^+$ photoproduction reaction is investigated in the energy region from threshold to $E_γ= 2250$\,MeV. The differential cross section exhibits increasing forward-peaking with energy, but only up to the $K^*$ threshold. Beyond, it suddenly returns to a flat distribution with the forward cross section dropping by a factor of four. In the total cross section a pronounced structure is observed between the $K^*Λ$ and $K^*Σ$ thresholds. It is speculated whether this signals the turnover of the reaction mechanism from t-channel exchange below the $K^*$ production threshold…
In this Reply, we respond to the above Comment. Our computation [Phys. Rev. D 91 (2015) 074512] only took into account pure QCD effects, arising from quark mass differences, so it is not surprising that there are discrepancies in isospin splittings and in the Sigma - Lambda mixing angle. We expect that these discrepancies will be smaller in a full calculation incorporating QED effects.
We investigate the p(gamma,K+)Lambda and p(gamma,K+)Sigma^0 reactions within a coupled-channels effective-Lagrangian method which is based on the K-matrix approach. The two-body final channels included are pi-N, eta-N, phi-N, rho-N, gamma-N, K-Lambda, and K-Sigma. Non-resonant meson-baryon interactions are included in the model via nucleon intermediate states in the s- and u-channels and meson exchanges in the t-channel amplitude and the u-channel resonances. The nucleon resonances S11 (1535), S11 (1650), S31 (1620), P11 (1440), P11 (1710), P13 (1720), P33 (1232), P33 (1600), D13 (1520), D13 (…
In Search of the f0 (or "sigma") Meson: New data on pi0 pi0 Production by pi- and K- on Hydrogen
本文给出了从阈能到p（pi）=750 MeV/c的过程pi- p -> pi 0 pi 0 n的截面和Dalitz图密度的初步结果，以及在p（K）=520到750 MeV/c的过程K- p -> pi 0 pi 0 Lambda和K- p -> pi 0 pi 0 Sigma 0的截面密度和Dalitz图密度的初步结果。我们发现σ（tot）（pi- p -> pi 0 pi 0 n）~= 2 σ（tot）（K- p -> pi 0 pi 0 λ）. pi 0 pi 0 n Dalitz图是非常不均匀的，特别是对于较高的p（pi），在m（pi n）~= 1.2 GeV和Gamma ~= 0.1 GeV附近的“岛”上有高浓度的事件，在高pi 0 pi 0不变质量处达到峰值。这表明δ 0（1234）（3/2^+）共振在最终态中的主导作用。π 0 π 0 λ Dalitz图与π 0 π 0 n图非常相似，只是岛的能量集中在m（π λ）~= 1.38 GeV，宽度较窄，γ ~= 0.05 GeV。这表明Sigma 0（1385）（3/2+）共振的主导作用。Dalitz图的相似性和总截面的比例性是SU（3）味对称破缺对反应动力学适用性的有力证明。我们测量了sigma（tot）（K-p-> pi 0 pi 0 Lambda）~= 6 sigma（tot）（K-p-> pi 0 pi 0 Sigm…
We extend the study of a JP=2+, I=3/2 pion-Lambda-nucleon quasibound state [Phys. Rev. D 78, 014013 (2008)] by solving nonrelativistic Faddeev equations, using 3S1-3D1, Lambda N - Sigma N coupled channels Chiral Quark Model local interactions, and pion-nucleon and coupled pion-Lambda - pion-Sigma separable interactions fitted to the position and decay parameters of the Delta(1232) and Sigma(1385) resonances, respectively. The results exhibit a strong sensitivity to the p-wave pion-hyperon interaction, with a pion-Lambda-nucleon quasibound state persisting over a wide range of acceptable parame…
Recent experimental evidence presented by the FINUDA Collaboration for a particle-stable Lambda-6H has stirred renewed interest in charting domains of particle-stable neutron-rich Lambda hypernuclei, particularly for unbound nuclear cores. We have studied within a shell-model approach several neutron-rich Lambda hypernuclei in the nuclear p shell that could be formed in (pi-,K+) or in (K-,pi+) reactions on stable nuclear targets. Hypernuclear shell-model input is taken from a theoretically inspired successful fit of gamma-ray transitions in p-shell Lambda hypernuclei which includes also Lambda…
Role of N*(1650) in the near threshold pp --> p Lambda K+ and pp --> p Sigma0 K+ reactions
在有效拉格朗日模型下，研究了p-> p λ K+和p-> p Σ 0 K+反应，其中奇异态的产生是通过N*（1650），N*（1710）和N*（1720）重子共振的激发进行的.结果表明，在阈能附近，$N^*$（1650）共振主导这两种反应.从这个共振的贡献与出射粒子之间的最终状态的相互作用能够解释所观察到的束能量依赖的截面的比例的两个反应在近阈值区域。
The branching ratios of K^- absorption at rest in nuclear matter are theoretically investigated in order to understand the mechanism of K^- absorption into nuclei. For this purpose mesonic and nonmesonic absorption potentials are evaluated as functions of nuclear density, the kaon momentum and energy from one- and two-body K^- self-energy, respectively. By using a chiral unitary approach for the s-wave Kbar N amplitude we find that both the mesonic and nonmesonic absorption potentials are dominated by the Lambda(1405) contributions. The fraction of the mesonic and nonmesonic absorptions are ev…
