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Register a new userQuasifree Lambda, Sigma^0, and Sigma^- electroproduction from 1,2H, 3,4He, and carbon
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Kaon electroproduction from light nuclei and hydrogen, using 1H, 2H, 3He, 4He, and Carbon targets has been measured at Jefferson Laboratory. The quasifree angular distributions of Lambda and Sigma hyperons were determined at Q^2= 0.35(GeV/c)^2 and W= 1.91GeV. Electroproduction on hydrogen was measured at the same kinematics for reference.
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We report measurements of the exclusive electroproduction of $K^+Lambda$ and $K^+Sigma^0$ final states from an unpolarized proton target using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The separated structure functions $sigma_U$, $sigma_{LT}$, $sigma_{TT}$, and $sigma_{LT}$ were extracted from the $Phi$-dependent differential cross sections acquired with a longitudinally polarized 5.499 GeV electron beam. The data span a broad range of momentum transfers $Q^2$ from 1.4 to 3.9 GeV$^2$, invariant energy $W$ from threshold to 2.6 GeV, and nearly the full center-of-mass angular range of the kaon. The separated structure functions provide an unprecedented data sample, which in conjunction with other meson photo- and electroproduction data, will help to constrain the higher-level analyses being performed to search for missing baryon resonances.
An instrument of central importance for the strangeness photo- and electroproduction at the 1.5-GeV electron beam of the MAMI accelerator at the Institut fur Kernphysik in Mainz, Germany, is the newly installed magnetic spectrometer Kaos that is operated by the A1 collaboration in $(e,eK)$ reactions on the proton or light nuclei. Its compact design and its capability to detect negative and positive charged particles simultaneously complements the existing spectrometers. The strangeness program performed with Kaos in 2008-9 is addressing some important issues in the field of elementary kaon photo- and electroproduction reactions. Although recent measurements have been performed at Jefferson Lab, there are still a number of open problems in the interpretation of the data and the description of the elementary process using phenomenological models. With the identification of $Lambda$ and $Sigma^0$ hyperons in the missing mass spectra from kaon production off a liquid hydrogen target it is demonstrated that the extended facility at MAMI is capable to perform strangeness electroproduction spectroscopy at low momentum transfers $Q^2$ < 0.5 (GeV/c)$^2$. The covered kinematics and systematic uncertainties in the cross-section extraction from the data are discussed.
This paper reports results on simultaneous measurements of the reaction channels pp -> pK+Lambda and pp -> pK+Sigma^0 at excess energies of 204, 239, and 284 MeV (Lambda) and 127, 162, and 207 MeV (Sigma^0). Total and differential cross sections are given for both reactions. It is concluded from the measured total cross sections that the high energy limit of the cross section ratio is almost reached at an excess energy of only about 200 MeV. From the differential distributions observed in the overall CMS as well as in the Jackson and helicity frames, a significant contribution of interfering nucleon resonances to the Lambda production mechanism is concluded while resonant Sigma^0-production seems to be of lesser importance and takes place only through specific partial waves of the entrance channel. The data also indicate that kaon exchange plays a minor role in the case of Lambda- but an important role for Sigma^0-production. Thus the peculiar energy dependence of the Lambda-to-Sigma^0 cross section ratio appears in a new light as its explanation requires more than mere differences between the pLambda and the pSigma^0 final state interaction. The data provide a benchmark for theoretical models already available or yet to come.
Threshold measurements of the associated strangeness production reactions pp --> p K(+) Lambda and pp --> p K(+) Sigma(0) are presented. Although slight differences in the shapes of the excitation functions are observed, the most remarkable feature of the data is that at the same excess energy the total cross section for the Sigma(0) production appears to be about a factor of 28 smaller than the one for the Lambda particle. It is concluded that strong Sigma(0)-p final state interactions, and in particular the Sigma-N --> Lambda-p conversion reaction, are the likely cause of the depletion for the yield in the Sigma signal. This hypothesis is in line with other experimental evidence in the literature.
Properties of hypernuclei are studied in the context of a chiral Lagrangian which successfully describes ordinary nuclei. Lambda-Sigma^0 mixing arises from nondiagonal vertices in flavor space induced by the vector mesons and by the electromagnetic field. The set of Dirac equations for the coupled hyperon system is discussed. Results are presented for energy spectra and electromagnetic properties of hyperons in the nuclear environment. Simple estimates suggest that flavor mixing can lead to sizable changes in the lifetimes of Lambda hypernuclei.
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