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Evidence for the neutron-rich hypernucleus 6{Lambda}H is presented from the FINUDA experiment at DA{Phi}NE, Frascati, studying ({pi}+, {pi}-) pairs in coincidence from the K- +6Li rightarrow 6 H+{pi}+ production reaction followed by 6{Lambda}H rightarrow 6He + {pi}- weak decay. The production rate of 6{Lambda}H undergoing this two-body {pi}- decay is determined to be (2.9pm2.0)cdot10-6/K-. Its binding energy, evaluated jointly from production and decay, is B{Lambda}(6{Lambda}H) = (4.0pm1.1) MeV with respect to 5H+{Lambda}. A systematic difference of (0.98 pm 0.74) MeV between B{Lambda} values derived separately from decay and from production is tentatively assigned to the 6{Lambda}H 0+g.s. rightarrow 1+ excitation.
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Three candidate events of the neutron-rich hypernucleus 6{Lambda}H were uniquely identified in the FINUDA experiment at DA{Phi}NE, Frascati, by observing {pi}+ mesons from the (K-stop,{pi}+) production reaction on 6Li targets, in coincidence with {pi}-mesons from 6{Lambda}H rightarrow 6He+{pi}- weak decay. Details of the experiment and the analysis of its data are reported, leading to an estimate of (2.9pm2.0)cdot10-6/K- stop for the 6{Lambda}H production rate times the two-body {pi}- weak decay branching ratio. The 6{Lambda}H binding energy with respect to 5H + {Lambda} was determined jointly from production and decay to be B{Lambda} = (4.0 pm 1.1) MeV, assuming that 5H is unbound with respect to 3H + 2n by 1.7 MeV. The binding energy determined from production is higher, in each one of the three events, than that determined from decay, with a difference of (0.98 pm 0.74) MeV here assigned to the 0+g.s. rightarrow 1+ excitation. The consequences of this assignment to {Lambda} hypernuclear dynamics are briefly discussed.
We searched for the bound state of the neutron-rich $Lambda$-hypernucleus ${}^{6}_{Lambda}$H, using the ${}^{6}$Li($pi^{-}, K^{+}$)X double charge-exchange reaction at a $pi^{-}$ beam momentum of 1.2 GeV/c at J-PARC. A total of $1.4 times 10^{12}$ $pi^{-}$ was driven onto a ${}^{6}$Li target of 3.5-g/$rm cm^2$ thickness. No event was observed below the bound threshold, i.e., the mass of ${}^{4}_{Lambda}$H + 2n, in the missing-mass spectrum of the ${}^{6}$Li($pi^{-}, K^{+}$)X reaction in the $2^{circ}$ < $theta_{pi K}$ < $20^{circ}$ angular range. Furthermore, no event was found up to 2.8 MeV/$c^2$ above the bound threshold. We obtained the the double-differential cross section spectra of the ${}^{6}$Li($pi^{-}, K^{+}$)X reaction in the angular range of $2^{circ}$ < $theta_{pi K}$ < $14^{circ}$. An upper limit of 0.56 nb/sr (90% C.L.) was obtained for the production cross section of the ${}^{6}_{Lambda}$H hypernucleus bound state. In addition, not only the bound state region, but also the $Lambda$ continuum region and part of the $Sigma^{-}$ quasi-free production region of the ${}^{6}$Li($pi^{-}, K^{+}$)X reaction, were obtained with high statistics. The present missing-mass spectrum will facilitate the investigation of the $Sigma^{-}$-nucleus optical potential for $Sigma^{-}$-${}^{5}$He through spectrum shape analysis.
The extremely neutron-rich system $^{6}$H was studied in the direct $^2text{H}(^8text{He},{^4text{He}})^{6}$H transfer reaction with a 26 $A$ MeV secondary $^{8}$He beam. The measured missing mass spectrum shows a broad bump at $sim 4-8$ MeV energy relative to the $^3$H+$3n$ decay threshold. This bump can be interpreted as a broad resonant state in $^{6}$H at $6.8(5)$ MeV. The population cross section of such a presumably $p$-wave state (or may be few overlapping states) in the energy range from 4 to 8 MeV is $dsigma/dOmega_{text{c.m.}} simeq 190(40)$ $mu$b/sr in the angular range $5^{circ}<theta_{text{c.m.}}<16^{circ}$. The obtained missing mass spectrum is practically free of the $^{6}$H events below 3.5 MeV ($dsigma/dOmega_{text{c.m.}} lesssim 5$ $mu$b/sr in the same angular range). The steep rise of the $^{6}$H missing mass spectrum at $sim 3$ MeV allows to derive the lower limit for the possible resonant state energy in $^{6}$H of $4.5(3)$ MeV. According to the paring energy estimates, such a $4.5(3)$ MeV resonance is a realistic candidate for the $^{6}$H ground state (g.s.). The obtained results confirm that the decay mechanism of the $^{7}$H g.s. (located at 2.2 MeV above the $^{3}$H+$4n$ threshold) is the true (or simultaneous) $4n$ emission. The resonance energy profiles and the momentum distributions of fragments of the sequential $^{6}$H$ ,rightarrow , ^5$H(g.s.)+$n, rightarrow , ^3$H+$3n$ decay were analyzed by the theoretically-updated direct four-body-decay and sequential-emission mechanisms. The measured momentum distributions of the $^{3}$H fragments in the $^{6}$H rest frame indicate very strong dineutron-type correlations in the $^{5}$H ground state decay.
The production of neutron rich $Lambda$-hypernuclei via the ($K^-_stop$,$pi^+$) reaction has been studied using data collected with the FINUDA spectrometer at the DA$Phi$NE $phi$-factory (LNF). The analysis of the inclusive $pi^+$ momentum spectra is presented and an upper limit for the production of $^6_Lambda$H and $^7_Lambda$H from $^6$Li and $^7$Li, is assessed for the first time.
The nuclear shell structure, which originates in the nearly independent motion of nucleons in an average potential, provides an important guide for our understanding of nuclear structure and the underlying nuclear forces. Its most remarkable fingerprint is the existence of the so-called `magic numbers of protons and neutrons associated with extra stability. Although the introduction of a phenomenological spin-orbit (SO) coupling force in 1949 helped explain the nuclear magic numbers, its origins are still open questions. Here, we present experimental evidence for the smallest SO-originated magic number (subshell closure) at the proton number 6 in 13-20C obtained from systematic analysis of point-proton distribution radii, electromagnetic transition rates and atomic masses of light nuclei. Performing ab initio calculations on 14,15C, we show that the observed proton distribution radii and subshell closure can be explained by the state-of-the-art nuclear theory with chiral nucleon-nucleon and three-nucleon forces, which are rooted in the quantum chromodynamics.
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