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Register a new userEvidence of Xi hypernuclear production in the ^{12}C(K^-,K^+)^{12}_{Xi}Be reaction
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The E885 collaboration utilized the 1.8 GeV/c K^- beam line at the AGS to accumulate 3 x 10^5 (K^-,K^+) events. Xi hypernuclear states are expected to be produced through the reaction K^- + ^{12}C -> K^+ + ^{12}_{Xi}Be. The measured missing-mass spectrum indicates the existence of a signal below the threshold for free Xi production. Although the resolution was not sufficient to resolve discrete hypernuclear states, the excess of events in the region of missing mass, kinematically inaccessible in free Xi production, is compared to theoretical prediction for ^{12}_{Xi}Be production.
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We study the production of $Xi^-$-hypernuclei, $^{12}_{Xi^{-}}$Be and $^{28}_{Xi^{-}}$Mg, via the ($K^-,K^+$) reaction within a covariant effective Lagrangian model, employing the bound $Xi^-$ and proton spinors calculated by the latest quark-meson coupling model. The present treatment yields the $0^circ$ differential cross sections for the formation of simple s-state $Xi^-$ particle-hole states peak at a beam momentum around 1.0 GeV/c with a value in excess of 1 $mu$b.
The $(K^-,K^+)$ $Xi^-$ production inclusive spectrum is reinvestigated in view of the very weak $Xi$-nucleus potential predicted by microscopic calculations with the SU$_6$ quark-model baryon-baryon interaction. The inclusive spectrum is evaluated by the semiclassical distorted wave (SCDW) method. The explicit comparison of the strength function with that of the Green-function method demonstrates the quantitative reliability of the SCDW approximation. It is presumed that the presently available data at the $Xi$ production threshold region does not necessarily imply the attractive strength of about 15 MeV for the $Xi$-nucleus potential in a conventional Woods-Saxon form. Instead, an almost zero potential is preferable.
The missing mass spectroscopy of $Xi^{-}$ hypernuclei with the $(K^{-},K^{+})$ reaction is planned to be performed at the J-PARC K1.8 beam line by using a new magnetic spectrometer, Strangeness $-2$ Spectrometer (S-2S). A $v{C}$cerenkov detector with a radiation medium of pure water (refractive index of 1.33) is designed to be used for on-line proton rejection for a momentum range of 1.2 to 1.6 GeV/$c$ in S-2S. Prototype water $v{C}$erenkov detectors were developed and tested with positron beams and cosmic rays to estimate their proton-rejection capability. We achieved an average number of photoelectrons of greater than 200 with the latest prototype for cosmic rays, which was stable during an expected beam time of one month. The performance of the prototype in the cosmic-ray test was well reproduced with a Monte Carlo simulation in which some input parameters were adjusted. Based on the Monte Carlo simulation, we expect to achieve $>90%$ proton-rejection efficiency while maintaining $>95%$ $K^{+}$ survival ratio in the whole S-2S acceptance. The performance satisfies the requirements to conduct the spectroscopic study of $Xi^{-}$ hypernuclei at J-PARC.
Carbon and oxygen burning reactions, in particular, $^{12}$C+$^{12}$C fusion, are important for the understanding and interpretation of the late phases of stellar evolution as well as the ignition and nucleosynthesis in cataclysmic binary systems such as type Ia supernovae and x-ray superbursts. A new measurement of this reaction has been performed at the University of Notre Dame using particle-$gamma$ coincidence techniques with SAND (a silicon detector array) at the high-intensity 5U Pelletron accelerator. New results for $^{12}$C+$^{12}$C fusion at low energies relevant to nuclear astrophysics are reported. They show strong disagreement with a recent measurement using the indirect Trojan Horse method. The impact on the carbon burning process under astrophysical scenarios will be discussed.
The $bar{K} + N to K + Xi$ reaction is studied for center-of-momentum energies ranging from threshold to 3 GeV in an effective Lagrangian approach that includes the hyperon $s$- and $u$-channel contributions as well as a phenomenological contact amplitude. The latter accounts for the rescattering term in the scattering equation and possible short-range dynamics not included explicitly in the model. Existing data are well reproduced and three above-the-threshold resonances were found to be required to describe the data, namely, the $Lambda(1890)$, $Sigma(2030)$, and $Sigma(2250)$. For the latter resonance we have assumed the spin-parity of $J^P=5/2^-$ and a mass of 2265 MeV. The $Sigma(2030)$ resonance is crucial in achieving a good reproduction of not only the measured total and differential cross sections, but also the recoil polarization asymmetry. More precise data are required before a more definitive statement can be made about the other two resonances, in particular, about the $Sigma(2250)$ resonance that is introduced to describe a small bump structure observed in the total cross section of $K^- + p to K^+ + Xi^-$. The present analysis also reveals a peculiar behavior of the total cross section data in the threshold energy region in $K^- + p to K^+ + Xi^-$, where the $P$- and $D$-waves dominate instead of the usual $S$-wave. Predictions for the target-recoil asymmetries of the $bar{K} + N to K + Xi$ reaction are also presented.
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