Hunting double hypernuclei in photographic emulsion: J-PARC E07
J. Yoshida1), S. Ashikaga1,2), H. Ekawa1,2), M. Fujita3), T. Hashimoto1), S. Hasegawa1), S. Hayakawa1,4)
Y. Ichikawa1), K. Imai1), K. Nakazawa5), T. Nanamura1,2), M. Naruki1,2), H. Sako1), S. Sato1),
K. Suzuki1,2), T. Takahashi6), H. Tamura1,3), K. Tanida1), M. Yoshimoto5), andthe J-PARC E07 collaboration.
1) : Hadron Nuclear Physics Gr., JAEA, 2) : Kyoto Univ., 3) : Tohoku Univ.,
4) : Osaka Univ., 5) : Gifu Univ., 6) : KEKDouble  hypernuclei and  hypernuclei shown in Fig. 1,
collectively called “double hypernuclei”, have come to play
important roles in hadron nuclear physics as valuable information
sources of baryon-baryon interaction. Baryon-baryon interaction
is essential for our understanding of the hierarchical structure ofmatter from quarks to nuclei. However, up to now - and nucleon interactions still have many unknown aspects. There are
only a few ways to obtain information on - and -nucleon
interactions, and mass measurements of double hypernuclei are
one of them.
The most effective method to investigate double hypernuclei
is an event-by-event analysis with photographic emulsion sheets,so called “emulsion”. Emulsion visualizes tracks of charged
particles with sub-m resolution. The production and decays of
double hypernuclei can be seen under an optical microscope andbe discriminated against other huge background.
To find double hypernuclei effectively, we employ the socalled "Hybrid emulsion method" [1], which is a combination ofelectrical detectors and emulsion. The procedure is as follows.
Firstly, a K- beam provided from an accelerator is irradiated to atarget made of diamond. In the target,  particles are produced
via 'p'(K-, K+) reaction, where 'p' is a proton bound inside a
carbon nucleus. The s go into the emulsion through a SSD(Silicon Strip Detector) which is located between the target andthe emulsion module. Some s travel through the emulsion bylosing kinetic energy and stop, where, finally, some s arecaptured by the surrounding nuclei. A  hypernuclei is formed ifthe  is bound in a deep enough orbit by the -nucleon
interaction. Moreover, a double hypernucleus is formed if the 2s converted from  are bound on the same nucleus. The tracks are searched based on the SSD prediction and followed inemulsion with an advanced image recognition technique.
J-PARC E07, an experiment for double hypernuclei with theHybrid emulsion method, is designed to detect 10 times higher
statistics of double hypernuclei compared to the past experiments[2-4]. So far, only 2 double hypernuclei events, Nagara and Kiso
event, are identified without ambiguity [1,5]. However, a larger
variety of double hypernuclear nuclides are necessary for asystematic validation of the - and -nucleon interactions. For
this purpose, we designed the experiment to obtain one hundred
double hypenuclei and nearly ten clearly identified double
hypernuclear nuclides. Furthermore, another goal of thisexperiment is the first observation of  atom X-ray with a Ge
detector array with surrounding emulsion. We measure the X-ray
energy with about 0.2 keV accuracy and use this information toreconstruct -nucleon interaction. Although the background ishuge, we expect it to be strongly suppressed by selecting  stop
events in emulsion.
Beam exposure of J-PARC E07 has been completed at the K1.8
beamline in the J-PARC hadron facility in 2017. Totally 118
modules, containing 1.5k emulsion sheets, were used.
Photographic developing of all emulsion sheets has been

completed at Gifu Univ., after which they are presently being
analysed with dedicated microscope systems at JAEA and Gifu
Univ. The time to scan one emulsion module is about one weekwith a microscope system. 104  stop events will be obtained
according to our yield. We plan to finish the main part ofemulsion scanning within FY2018.
So far, about 10 double hypernuclear events have been
observed in nearly ten percent of total statistics. Figure 3 showsa typical double  nucleus event. The production and sequential
decay of the double  hypernucleus shown as red points areclearly seen. Even though we were not able to identify this event
uniquely, we expect unambiguous events to be found in nearfuture.

Fig.1 Double  hypernucleus and  hypernucleus. Red and blue circle
represent proton and neutron, respectively.

Fig.2 Experimental apparatus of J-PARC E07.

Fig.3 A typical double  nucleus event found in E07 experiment.

References[1] J. K. Ahn et al., Physical Review C 88, 014003 (2013).
[2] K. Nakazawa, JPS Conf. Proc. 17, 031001 (2017).
[3] H. Ekawa, JPS Conf. Proc. 17, 033002 (2017).
[4] S. Hayakawa, JPS Conf. Proc. 17, 033003 (2017).
[5] K.Nakazawa et al., PTEP 2015, 033D02 (2015).