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Register a new userHyperon I: Study of the $Lambda(1405)$
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Low-energy data on the three charge states in $gamma p to K^+(Sigmapi)$ from CLAS at JLab, on $K^-pto pi^0pi^0Lambda$ and $pi^0pi^0Sigma$ from the Crystal Ball at BNL, bubble chamber data on $K^-ptopi^-pi^+pi^{pm}Sigma^{mp}$, low-energy total cross sections on $K^-$ induced reactions, and data on the $K^-p$ atom are fitted with the BnGa partial-wave-analysis program. We find that the data can be fitted well with just one isoscalar spin-1/2 negative-parity pole, the $Lambda(1405)$, and background contributions.
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The photo-induced $K^*$ vector meson production is investigated for the study of the $Lambda(1405)$ resonance. This reaction is particularly suited to the isolation of the second pole in the $Lambda(1405)$ region which couples dominantly to the $bar K N$ channel. We obtain the mass distribution of the $Lambda(1405)$ which peaks at 1420 MeV, and differs from the nominal one. Combined with several other reactions, like the $pi^- p to K^0 pi Sigma$ which favours the first pole, this detailed study will reveal a novel structure of the $Lambda(1405)$ state.
We investigate the photoproduction of $K^*$ vector meson for the study of the $Lambda(1405)$ resonance. The invariant mass distribution of $piSigma$ shows a different shape from the nominal one, peaking at 1420 MeV. This is considered as a consequence of the double pole structure of $Lambda(1405)$, predicted in the chiral unitary model. Combined with other reactions, such as $pi^- p to K^0 piSigma$, experimental confirmation of this fact will reveal a novel structure of the $Lambda(1405)$ state.
It appears that there are two resonances with $J^P= 1/2^-$ quantum numbers in the energy region near the $Lambda(1405)$ hyperon. The nature of these states is a topic of current debate. To provide further insight we use Regge phenomenology to access how these two resonances fit the established hyperon spectrum. We find that only one of these resonances is compatible with a three-quark state.
We investigate the photoproduction of Lambda(1405,1/2^-) = Lambda* off the proton target using the effective Lagrangian in the Born approximation. We observed that, depending on the choice of the K* N Lambda* coupling strength, the total cross section becomes 0.1 <~ sigma_Lambda* <~ 0.2 mu b near the threshold and starts to decrease beyond E_gamma ~ 1.6 GeV, and the angular dependence shows a mild enhancement in the forward direction. It turns out that the energy dependence of the total cross section is similar to that shown in the recent LEPS experiment. This suggests that the production mechanism of the Lambda* is dominated by the s-channel contribution.
We discuss several aspects of the Lambda(1405) resonance in relation to the recent theoretical developments in chiral dynamics. We derive an effective single-channel KbarK N interaction based on chiral SU(3) coupled-channel approach, emphasizing the important role of the pi Sigma channel and the structure of the Lambda(1405) in Kbar N phenomenology. In order to clarify the structure of the resonance, we study the behavior with the number of colors (Nc) of the poles associated with the Lambda(1405), and argue the physical meaning of the renormalization procedure.
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