I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Surface localization of the dineutron in 11 Li†

A unique aspect of nuclei with respect to otherfermionic many-body systems is the emergence ofa spatially compact two-neutron pair, dineutron,1)
which is completely diﬀerent from the Bardeen-CooperSchrieﬀer-(BCS)-like pairings that appear in momentum space. The dineutron correlation is presumed to
be important for elucidating the stabilities and exoticstructures of neutron drip-line nuclei, as well as the inﬁnite nuclear matter. Studies on the dineutron formationand the density dependence of 11 Li are crucial because
it has a halo structure: the matter density graduallyvaries from the saturated core to the very low-density
tail where only valence neutrons exist. It allows thestudy of the density-dependent properties of the dineutron correlation.
The quasifree (p, pn) reaction was employed to probe
the entire volume of 11 Li with the least eﬀect of absorption. The measurement was performed at RIBF using
the SAMURAI spectrometer,2) combined with the 15cm-thick liquid hydrogen target MINOS3) and dedicated(p, pn) setup.
The strength of the dineutron in 11 Li was evaluatedby using the correlation angle θnf , which is the angle
between the momentum vectors of two valence neutrons.
The spatially compact dineutron should have an angle
larger than 90◦ . Figure 1 shows that the mean value ofCondensed from the article in Phys. Rev. Lett. 125, 252501(2020)∗1 RIKEN Nishina Center∗2 Center for Nuclear Study, University of Tokyo∗3 Département de Physique Nucléaire, IRFU, CEA, UniversitéParis-Saclay∗4 Department of Physics, Technische Universität Darmstadt∗5 Department of Physics, Peking University∗6 IPN Orsay, Université Paris Sud∗7 LPC Caen, ENSICAEN, Université de Caen Normandie∗8 Department of Physics, Tohoku University∗9 Department of Applied Physics, University of Miyazaki∗10 Tokuyama College, National Institute of Technology∗11 Department of Physics, Osaka City University∗12 Department of Physics, University of Tokyo∗13 Department of Physics, Tokyo Institute of Technology∗14 Center for Exotic Nuclear Studies, Institute for Basic Science∗15 Institute for Nuclear Research, Hungarian Academy of Sciences∗16 Department of Physics, Kyoto University∗17 Department of Physics, Kyushu University∗18 Research Center for Nuclear Physics, Osaka University∗19 Department of Physics, Technische Universität München†

Correlation angle 〈θ nf 〉 (deg)Y. Kubota,∗1,∗2 A. Corsi,∗3 G. Authelet,∗3 H. Baba,∗1 C. Caesar,∗4 D. Calvet,∗3 A. Delbart,∗3 M. Dozono,∗2J. Feng,∗5 F. Flavigny,∗6 J. -M. Gheller,∗3 J. Gibelin,∗7 A. Giganon,∗3 A. Gillibert,∗3 K. Hasegawa,∗8 T. Isobe,∗1Y. Kanaya,∗9 S. Kawakami,∗9 D. Kim,∗14 Y. Kikuchi,∗10,∗1,∗11 Y. Kiyokawa,∗2 M. Kobayashi,∗2 N. Kobayashi,∗12T. Kobayashi,∗8 Y. Kondo,∗13 Z. Korkulu,∗14,∗15 S. Koyama,∗12 V. Lapoux,∗3 Y. Maeda,∗9 F. M. Marqués,∗7T. Motobayashi,∗1 T. Miyazaki,∗12 T. Nakamura,∗13 N. Nakatsuka,∗16 Y. Nishio,∗17 A. Obertelli,∗3K. Ogata,∗18,∗11 A. Ohkura,∗17 N. A. Orr,∗7 S. Ota,∗2 H. Otsu,∗1 T. Ozaki,∗13 V. Panin,∗1 S. Paschalis,∗4∗3
E. C. Pollacco,∗3 S. Reichert,∗19 J. -Y. Roussé,∗3 A. T. Saito,∗13 S. Sakaguchi,∗17 M. Sako,∗1 C. Santamaria,Thu Jan 21 22:40:29 2021
∗1∗1∗13∗1∗17∗14,∗1∗8
M. Sasano, H. Sato, M. Shikata,Y. Shimizu, Y. Shindo,
L. Stuhl,
T. Sumikama, Y. L. Sun,∗3
∗17
∗13
∗13
∗1
∗17
M. Tabata,
Y. Togano,
J. Tsubota,
Z. H. Yang, J. Yasuda,
K. Yoneda,∗1 J. Zenihiro,∗1 T. Uesaka∗1100
9Li95

θnfK'kn2
n1

90
85
80

This work
Quasifree model

00.5
1
1.5
Missing momentum k (fm -1)

Fig. 1. Mean values of the correlation angle. Black dashedline shows the expected ⟨θnf ⟩ value for the two uncorrelated neutrons. Inset shows a schematic of θnf in 11 Li.

θnf clearly depends on the missing momentum k, whichis the measure of the radial position of the two neutronsin 11 Li. The peak structure of ⟨θnf ⟩ at k ∼ 0.3 fm−1
can be interpreted as the localization of the dineutron,
which is maximized at r ∼ 3.6 fm from the center of
the 9 Li core. The quasi-free model4) well reproduces theexperimental data.
The result implies that the dineutron correlation isprominent only around the 9 Li core surface where thedensity is 10−3 ≲ ρ/ρ0 ≲ 10−2 , and it becomes weakerat the tail of the halo, where the density is extremely low.
It is consistent with the Hartree-Fock-Bogoliubov calculation5) for inﬁnite nuclear matter. If this is a universal
characteristic of the dineutron correlation, it should appear at the low-density surface of any neutron-rich nuclei. Future (p, pn) experiments should investigate thenature of the dineutron correlation in nuclei of interest,
such as 6 He, 16 C, and 24 O.
References1) A. B. Migdal, Soviet J. Nucl. Phys. 16, 238 (1973).
2) T. Kobayashi et al., Nucl. Instrum. Methods Phys. Res.
B 317, 294 (2013).
3) A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).
4) Y. Kikuchi et al., Prog. Theor. Exp. Phys. 2016, 103D03(2016).
5) M. Matsuo, Phys. Rev. C 73, 044309 (2006).

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