Isolating the Λ(1405) in lattice QCD

The odd-parity ground state of the Λ baryon lies surprisingly low in mass. At 1405 MeV, it lies lower than the odd-parity ground-state nucleon, even though it has a valence strange quark. Using the PACS-CS (2+1)-flavor full-QCD ensembles, we employ a variational analysis using source and sink smearing to isolate this elusive state. For the first time we reproduce the correct level ordering with respect to nearby scattering thresholds. With a partially quenched strange quark to produce the appropriate kaon mass, we find a low-lying, odd-parity mass trend consistent with the experimental value.     

Coupled-Channels Faddeev Equations for the K ??+?d Break-Up Reaction and the Shape of the ��(1405) Resonance

Coupled-channels Faddeev equations are derived for the K ^? + d break-up reaction with inclusion of the isospin breaking effect of the ${\bar{K}^0-K^-}$ mass difference.     

Can the Λ(1405) resonance Be seen in neutron spectra from the K − + d reaction?

Neutron spectra from the low-energy reaction K ^? + d → π +Σ+n were calculated using coupled-channel Faddeev equations. The possibility of tracing the signature of the Λ(1405) resonance in the spectra was investigated for four different phenomenological models describing K? N-πΣ interactions. We found, that while in the direct spectra kinematic effects mask completely the peak corresponding to the resonance, the deviation spectrum method [J. Esmaili et al., Phys. Rev. C 83, 055207 (2011)] is able to eliminate kinematics and differentiate between different models of Λ(1405).     

Hyperons Analogous to the Λ(1405)

The low mass of the Λ(1405) hyperon with j ^P =  1/2^?, which is higher than the ground state Λ(1116) mass by 290 MeV, is difficult to understand in quark models. We analyze the hyperon spectrum in the bound state approach of the Skyrme model that successfully describes both the Λ(1116) and the Λ(1405). This model predicts that several hyperon resonances of the same spin but with opposite parity form parity doublets that have a mass difference of around 300 MeV, which is indeed realized in the observed hyperon spectrum. Furthermore, the existence of the ${\Xi(1620)}$ and the ${\Xi(1690)}$ of j ^P  = 1/2^? is predicted by this model. Comments on the Ω baryons and heavy quark baryons are made as well.     

The K−d → πΣn reaction revisited

The appearance of some papers dealing with the K ^? d → πΣn reaction, with some discrepancies in the results and a proposal to measure the reaction at forward n angles at J-PARC justifies to retake the theoretical study of this reaction. We do this in the present paper showing results using the Watson approach and the truncated Faddeev approach. We argue that the Watson approach is more suitable to study the reaction because it takes into account the potential energy of the nucleons forming the deuteron, which is neglected in the truncated Faddeev approach. The paper shows the strength and limitations of both approaches and we perform calculations using four different approximations. Comparison of the results shows that the truncated Faddeev approach produces a strong asymmetry between the energy of the two nucleons of the deuteron, while in the Watson approach this energy is equally shared. From the experimental point of view the results are very valuable since they show that the different approximations share the feature that the peak of the πΣ mass distribution is drastically shifted in the presence of the Λ(1405). Additionally, we also show that in the angle-integrated cross section the threshold cusp effects are basically washed away and all approximations show a clear shape of the Λ(1405). In this sense, measurements of all these magnitudes in different K ^? energies are bound to bring new information that sheds new light on the properties and nature of the Λ(1405) resonance.     

Simulation of the d + 3 H →3 He + 2 n; 2 n → n + n Reaction

Results of a simulation of setups intended to search for excited states of few-nucleon systems decaying into two clusters are presented.     

The σK coupling in the chiral unitary approach and the isoscalar ¯N , ¯A interaction

We evaluate the “σ " exchange contribution to the ˉN → ˉN scattering within a chiral unitary approach. We show that the chiral transition potentials for ππ → Kˉ in the t -channel lead to a “σ " contribution that vanishes in the ˉ forward direction and, hence, would produce a null “σ " exchange contribution to the K^- optical potential in nuclear matter in a simple impulse approximation. This is a consequence of the fact that the leading-order chiral Lagrangian gives an I = 0 ππ → Kˉ amplitude proportional to the squared momentum transfer, q². This finding poses questions on the meaning or the origin of “σ " exchange potentials used in relativistic mean-field approaches to the K^- nuclear self-energy. This elementary “σ ” exchange potential in ˉN → ˉN is compared to the Weinberg-Tomozawa term and is found to be smaller than the present theoretical uncertainties but will be relevant in the future when aiming at fitting increasingly more accurate data.     

Study of the Λ(1405) Resonance Through its Neutral and Charged Decay Channels by AMADEUS at DAΦNE

The AMADEUS collaboration has the goal to perform unprecedented measurements in the field of the low-energy charged kaons-nuclei interactions, by implementing the existing KLOE detector with a dedicated setup in the inner region. The KLOE detector at DAΦNE represents a unique opportunity to perform a complete study of the Λ(1405) resonance through all its three Σπ decay channels. The importance of these measurements relies on the possibility to compare the different results for the various decay channels, investigating their contributions to the final resonance shape. In particular, one of the main differences between the neutral and charged Σπ channels is the presence of a second resonance, the Σ(1385), whose contribution to the final Λ(1405) spectra has to be evaluated and subtracted. Very promising preliminary results have been already obtained for the Σ ⁰π⁰ decay channel to which, for isospin selection rules, the Σ(1385) resonance can not decay, and for the Σ ⁺π^? decay channel analysis, and will be presented in this work.     

The role of the P11(1710) in the NN → NΣK reaction

Using the resonance model, which was successfully applied for the study of the pp → pΛK⁺ reaction, we investigate NN → NΣK reactions that are expected to provide cleaner information about resonance excitations and meson exchange contributions. For this purpose we demonstrate that the invariant mass distribution for the ΣK system, as well as the Dalitz plot for the NN → NΣK reaction, provide direct information about the ΣK production mechanism, which can be tested in the near future by experiments at COSY.     

The Λ(1405) state in a chiral unitary approach with off-shell corrections to dimensional regularized loop functions

The Bethe-Salpeter equation is solved in the framework of the unitary coupled-channel approximation by using the pseudoscalar meson-baryon octet interaction. The loop function of the intermediate meson and baryon is deduced in a dimensional regularization scheme, where the relativistic kinetic effect and off-shell corrections are taken into account. Based on the experimental data at the K~-p threshold, the subtraction constants in the loop function are determined. The squared amplitude is suppressed strongly and only one Λ(1405) state is generated dynamically in the strangeness S =-1 and isospin I =0 sector.     

The ̅K N → π Σ decay threshold effect in 3He(in-flight K –, n) reaction spectrum

The formation of a deeply-bound K?? ^–? pp state by the ³He(in-flight K? ^–, n) reaction is investigated theoretically in a distorted-wave impulse approximation using the Green’s function method. The expected inclusive and semi-exclusive spectra at $p_{K^-} = 1.0$ GeV/c and $\theta_n = 0^{\circ}$ are calculated for the forthcoming J-PARC E15 experiment. We discuss these spectra with some K? ^–-“pp” optical potentials, taking into account the energy-dependence of the imaginary part of the potentials by a phase space suppression factor.     

Effect of the Λ n interaction on the cross section of the reaction γd → K+Λ n

Taking into account the n interaction, a nonrelativistic graphical technique is used to calculate the differential cross section of the reaction d K⁺ n as a function of photon energy, K⁺-momentum, and angle of emission of the K⁺. The kinematic region in which variation of the n-scattering parameters gives the most noticeable effect is separated out.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 104–106, August, 1978.The author thanks G. M. Radutskii and V. A. Filimonov for many useful discussions.     

Revisiting the Γ(K→eν)/Γ(K→μν) ratio in supersymmetric unified models

We present a generator for single top-quark production via flavour-changing neutral currents. The MEtop event generator allows for Next-to-Leading-Order direct top production pp??t and Leading-Order production of several other single top processes. A?few packages with definite sets of dimension six operators are available. We discuss how to improve the bounds on the effective operators and how well new physics can be probed with each set of independent dimension six operators.     

Role of the Λ(1600) in the K^-p→Λπ^0π^0 reaction

Role of theΛ(1600)is studied in the K^-p→Λπ^0π^0 reaction by using the effective Lagrangian approach near the threshold.We perform a calculation for the total and differential cross sections by considering the contributions from theΛ(1600)andΛ(1670)intermediate resonances decaying intoπ^0∑^*0(1385)with∑^*0(1385)decaying intoπ^0Λ.Additionally,the non-resonance process from u-channel nucleon pole is also taken into account.With our model parameters,the current experimental data on the total cross sections of the K^-p→Λπ^0π^0 reaction can be well reproduced.It is shown that we really need the contribution from the A(1600)with spin-parity J^P=1/2^+,and that these measurements can be used to determine some of the properties of theΛ(1600)resonance.Furthermore,we also plot theπ0Λinvariant mass distributions which could be tested by the future experimental measurements.     

Study of γ d → π + π − d Reaction in an Energy Region of 0.67 ≤ E γ ≤ 1.08 GeV

Double charged pion photoproductions from the deuteron have been studied at Research Center for Electron Photon Science (ELPH), Tohoku University. Tagged photon beams in an energy range of 0.67 ≤ E _γ ≤ 1.08 GeV were impinged on the liquid deuteron target. Produced charged particles were observed with the Neutral Kaon Spectrometer2 (NKS2). The γ d → π ⁺ π ^? d process was clearly separated among the three charged tracks. We obtained the photon energy dependence of the normalized yields of this process.