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Register a new userObservation of Coulomb-assisted nuclear bound state of $Xi^-$-$^{14}$N system
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S.H. Hayakawa
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In an emulsion-counter hybrid experiment performed at J-PARC, a $Xi^-$ absorption event was observed which decayed into twin single-$Lambda$ hypernuclei. Kinematic calculations enabled a unique identification of the reaction process as $Xi^{-} + ^{14}$N$ rightarrow ^{10}_Lambda$Be + $^5_Lambda$He. For the binding energy of the $Xi^{-}$ hyperon in the $Xi^-$-$^{14}$N system a value of $1.27 pm 0.21$ MeV was deduced. The energy level of $Xi^-$ is likely a nuclear $1p$ state which indicates a weak ${Xi}N$-$LambdaLambda$ coupling.
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Bound-systems of $Xi^-$--$^{14}_{}{rm N}$ are studied via $Xi^-$ capture at rest followed by emission of a twin single-$Lambda$ hypernucleus in the emulsion detectors. Two events forming extremely deep $Xi^-$ bound states were obtained by analysis of a hybrid method in the E07 experiment at J-PARC and reanalysis of the E373 experiment at KEK-PS. The decay mode of one event was assigned as $Xi^-+^{14}_{}{rm N}to^{5}_{Lambda}{rm He}$+$^{5}_{Lambda}{rm He}$+$^{4}_{}{rm He}$+n. Since there are no excited states for daughter particles, the binding energy of the $Xi^-$ hyperon, $B_{Xi^-}$, in $^{14}_{}{rm N}$ nucleus was uniquely determined to be 6.27 $pm$ 0.27 MeV. Another $Xi^-$--$^{14}_{}{rm N}$ system via the decay $^{9}_{Lambda}{rm Be}$ + $^{5}_{Lambda}{rm He}$ + n brings a $B_{Xi^-}$ value, 8.00 $pm$ 0.77 MeV or 4.96 $pm$ 0.77 MeV, where the two possible values of $B_{Xi^-}$ correspond to the ground and the excited states of the daughter $^{9}_{Lambda}{rm Be}$ nucleus, respectively. Because the $B_{Xi^-}$ values are larger than those of the previously reported events (KISO and IBUKI), which are both interpreted as the nuclear $1p$ state of the $Xi^-$--$^{14}_{}{rm N}$ system, these new events give the first indication of the nuclear $1s$ state of the $Xi$ hypernucleus, $^{15}_{Xi}{rm C}$.
We study the coupled $LambdaLambda nn-Xi^- pnn$ system to check whether the inclusion of channel coupling is able to bind the $LambdaLambda nn$ system. We use a separable potential three-body model of the coupled $LambdaLambda nn - Xi^- pnn$ system as well as a variational four-body calculation with realistic interactions. Our results exclude the possibility of a $LambdaLambda nn$ bound state by a large margin. However, we have found a $Xi^- t$ quasibound state above the $LambdaLambda nn$ threshold.
We have performed an exclusive measurement of the $K^{-}+! ~^{3}{rm He} to Lambda pn$ reaction at an incident kaon momentum of $1 {rm GeV}/c$.In the $Lambda p$ invariant mass spectrum, a clear peak was observed below the mass threshold of $bar{K}!+!N!+!N$, as a signal of the kaonic nuclear bound state, $bar{K}NN$.The binding energy, decay width, and $S$-wave Gaussian reaction form-factor of this state were observed to be $B_{K} = 42pm3({rm stat.})^{+3}_{-4}({rm syst.}) {rm MeV}$, $Gamma_{K} = 100pm7({rm stat.})^{+19}_{-9}({rm syst.}) {rm MeV}$, and $Q_{K} = 383pm11({rm stat.})^{+4}_{-1}({rm syst.}) {rm MeV}/c$, respectively. The total production cross-section of $bar{K}NN$, determined by its $Lambda p$ decay mode, was $sigma^{tot}_{K} cdot BR_{Lambda p} = 9.3pm0.8({rm stat.})^{+1.4}_{-1.0}({rm syst.}) mu{rm b}$.We estimated the branching ratio of the $bar{K}NN$ state to the $Lambda p$ and $Sigma^{0}p$ decay modes as $BR_{Lambda p}/BR_{Sigma^{0}p} sim 1.7$, by assuming that the physical processes leading to the $Sigma N!N$ final states are analogous to those of $Lambda pn$.
Neutron-rich light nuclei and their reactions play an important role for the creation of chemical elements. Here, data from a Coulomb dissociation experiment on $^{20,21}$N are reported. Relativistic $^{20,21}$N ions impinged on a lead target and the Coulomb dissociation cross section was determined in a kinematically complete experiment. Using the detailed balance theorem, the $^{19}mathrm{N}(mathrm{n},gamma)^{20}mathrm{N}$ and $^{20}mathrm{N}(mathrm{n},gamma)^{21}mathrm{N}$ excitation functions and thermonuclear reaction rates have been determined. The $^{19}mathrm{N}(mathrm{n},gamma)^{20}mathrm{N}$ rate is up to a factor of 5 higher at $T<1$,GK with respect to previous theoretical calculations, leading to a 10,% decrease in the predicted fluorine abundance.
The first-excited state $g$~factor of $^{26}$Mg has been measured relative to the $g$ factor of the $^{24}$Mg($2^+_1$) state using the high-velocity transient-field technique, giving $g=+0.86pm0.10$. This new measurement is in strong disagreement with the currently adopted value, but in agreement with the $sd$-shell model using the USDB interaction. The newly measured $g$ factor, along with $E(2^+_1)$ and $B(E2)$ systematics, signal the closure of the $ u d_{5/2}$ subshell at $N=14$. The possibility that precise $g$-factor measurements may indicate the onset of neutron $pf$ admixtures in first-excited state even-even magnesium isotopes below $^{32}$Mg is discussed and the importance of precise excited-state $g$-factor measurements on $sd$~shell nuclei with $N eq Z$ to test shell-model wavefunctions is noted.
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