First observation of a nuclear $s$-state of $Ξ$ hypernucleus, $^{15}_Ξ{\rm C}$

Abstract

In the E07 experiment at J-PARC, we observed an extremely deep Ξ − -14 N bound-system via Ξ − capture at rest followed by a decay into a twin single-Λ hypernucleus in nuclear emulsion detectors. The decay mode of the system was determined as 5 Λ He + 5 Λ He + 4 He + n. Since there are no excited state for daughter particles, the binding energy of Ξ − hyperon, B Ξ − , in 14 N nucleus was uniquely determined to be 6.27 ± 0.27 MeV for the one event. By reanalysis of the past E373 experiment at KEK-PS, we obtained another Ξ − -14 N system having a B Ξ − value, 8.00 ± 0.77 MeV or 4.96 ± 0.77 MeV, via the decay 9 Λ Be + 5 Λ He + n, where the two possible values of B Ξ − correspond to the ground and the excited state of the daughter 9 Λ Be nucleus, respectively. Since the B Ξ − values are larger than those of the previously reported events (KISO and IBUKI) interpreted as the nuclear 1p state of Ξ − -14 N system, these two events give the first indication of the nuclear 1s state of Ξ hypernucleus, 15 Ξ C.

Momentum dependence of near-threshold photoproduction of $Ξ^-$ hyperons off nuclei and their properties in the nuclear medium

“…An information inferred in Refs. [4] and [5][6][7][8][9] from old and new emulsion hypernuclear data, from missing-mass measurements [10][11][12][13][14] in the inclusive (K − ,K + ) reactions on nuclear targets at incident momenta of 1.6-1.8 GeV/c, from the analysis the results of these measurements in the Ξ − bound and quasi-free regions in Refs. [15][16][17] using the DWIA method as well as the theoretical predictions [18][19][20][21][22][23][24][25][26][27][28][29][30][31] indicate that the Ξ − hyperon feels only a moderately attractive low-energy potential in nuclei, the depth of which is about of - (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) MeV at saturation nuclear density ρ 0 .…”

“…(where p F = 1.35 fm −1 =0.2673 GeV/c), we calculated, with accounting for Eqs. ( 4)- (7) as well as by multiplying the vector nucleon potential in Eq. ( 4) by a factor α of α =1.068 [42] to get for Λ hyperon potential V SEP ΛA at zero momentum and at density ρ 0 an experimental value of -(32±2) MeV [47], the momentum dependences of potentials V SEP Ξ − A and U Ξ − at saturation density .…”

“…These values were deduced very recently in Refs. [8] and [9], correspondingly, from the analysis of the Ξ − capture events in light nuclear emulsion identified in KEK and J-PARC experiments [5][6][7] within the quark mean-field model and in the framework of a tρ optical potential method. These potentials are depicted in Fig.…”

“…At the considered initial photon energies Ξ − hyperons are mainly produced in γN interactions at small angles in the laboratory frame 7) . Therefore, we will calculate the Ξ − momentum differential distributions from 12 C and 184 W target nuclei for the laboratory solid angle ∆Ω Ξ − =0 • ≤ θ Ξ − ≤ 45 • , and 0 ≤ ϕ Ξ − ≤ 2π.…”

“…6) The author thanks I. Strakovsky for sending this information from CLAS g11 experiment to him. 7) For instance, the maximum angle of Ξ − hyperon production off a free proton at rest in reaction ( 1) is about 20.3 • at the photon energy of 3.0 GeV.…”