Proposal for the J-PARC 30-GeV Proton Synchrotron

Proposal for the E80 Phase-I Experiment:
Investigation of the K̄N N N Bound StateFocusing on the Λd Decay
submitted on December 20, 2021
H. Asano K. Itahashi, M. Iwasaki, Y. Ma, R. Murayama, H. Outa, F. Sakuma∗ ,T. Yamaga
RIKEN Cluster for Pioneering Research, RIKEN, Saitama, 351-0198, Japan

K. Inoue, S. Kawasaki, H. Noumi, K. Shirotori
Research Center for Nuclear Physics (RCNP), Osaka University, Osaka, 567-0047, Japan

H. Ohnishi, Y. Sada, C. Yoshida
Research Center for Electron Photon Science (ELPH), Tohoku University, Sendai,982-0826, Japan

T. Hashimoto
Japan Atomic Energy Agency (JAEA), Ibaraki 319-1195, Japan

M. Iio, S. Ishimoto, K. Ozawa, S. Suzuki
High Energy Accelerator Research Organization (KEK), Ibaraki, 305-0801, JapanT. Akaishi
Department of Physics, Osaka University, Osaka, 560-0043, Japan

T. Nagae
Department of Physics, Kyoto University, Kyoto, 606-8502, Japan

H. Fujioka
Department of Physics, Tokyo Institute of Technology, Tokyo, 152-8551, Japan

M. Bazzi, A. Clozza, C. Curceanu, C. Guaraldo, M. Iliescu, M. Miliucci, A. Scordo,
D. Sirghi, F. Sirghi
Laboratori Nazionali di Frascati dell’ INFN, I-00044 Frascati, Italy

P. Buehler, E. Widmann, J. Zmeskal
Stefan-Meyer-Institut für subatomare Physik, A-1090 Vienna, Austria∗

Spokesperson, E-mail: sakuma@ribf.riken.jp

1

Abstract
We have recently observed a peak structure below the K − +p+p+n mass
threshold in the Λd invariant-mass spectrum of 4 He(K − , Λd)n using the T77
data. The observed structure would be the signal of the K̄N N N bound state.
To urgently confirm the observed structure at higher statistics with the present
setup, we propose an experiment focusing on the K −4 He → Λdn measurement
as the E80 Phase-I experiment.

Summary of the E80 Phase-I ExperimentBeam Line:
Primary Beam:
Secondary Beam:
Beam Intensity:
Reaction:
Detectors:
Target:Beam Time:
Estimated Yield:

K1.8BR
30 GeV, 90 kW (4.2 seconds repetition cycle)
1.0 GeV/c K −1.9 × 105 on target per pulse
in-flight (K − , N )present K1.8BR beam-line spectrometer andcylindrical detector systemliquid H2 and 4 He
1 day for calibration run with LH2 target, and13 days for the physics run with L4 He target
6 × 103 Λdn final state

Contents
1 J-PARC E80 Experiment

32 Analysis of K−4 He → Λdn Reaction at T77

33 Staging Strategy of E80

44 Beam-Time Request for the Phase-I Experiment62

1 J-PARC E80 Experiment
The E80 experiment is aiming to establish the K̄N N N bound system as a first step
toward the comprehensive study of the light kaonic nuclei [1]. We will perform anexclusive measurement of the K̄N N N state using the following reactions:
4He(K− , n) reaction: search for the K̄N N N bound state with charge +1 (symbolically denoted as K − ppn) through Λd and Λpn decays,

4He(K− , p) reaction: search for the K̄N N N bound state with charge 0 (symbolicallydenoted as K − pnn) through Λnn decay.

The E80 experiment will firstly provide the mass number dependence of the binding
energy, decay width, and system size of the kaonic nuclei, by comparing them fromK̄N (Λ(1405)) to K̄N N N . The mass number dependence will clearly reveal the K̄Ninteraction below the mass threshold. The E80 experiment was proposed at the 30thJ-PARC PAC meeting (July, 2020) and approved as a stage-1 experiment at the 31th
PAC (January, 2021).
Starting with E80, we have planned a series of experimental programs using the(K − , N/d) reaction on light nuclear targets. The programs will enable a detailed studyof a range of nuclei from K̄N (Λ(1405)) to K̄N N N N using the world’s highest intensity
low-momentum kaon beam at J-PARC. The programs comprise:• [K̄N(Λ(1405)] Precise measurements of the Λ(1405) state in a large momentum
transfer region via the d(K − , n) reaction, to experimentally clarify whether it isa baryonic state or a K̄N molecular state,• [K̄NN] Investigations of the spin and parity of the K̄N N states via 3 He(K − , N )reactions (proposed as P89 [2]),• [K̄NNN] A search for K̄N N N states via 4 He(K − , N ) reactions, as a bridge toaccess heavier systems (E80 [1] and this revised proposal),• [K̄NNNN] An advanced search for K̄N N N N states via the 6 Li(K − , d) reaction,and• [K̄K̄NN] Future plan of searching for K̄ K̄N N states via p̄3 He annihilation toaccess the S = −2 kaonic nuclei (Letter of Intent [3]).

2 Analysis of K−4He → Λdn Reaction at T77
We have conducted a prompt analysis on Λdn final state of K −4 He reaction at 1 GeV/c,and found a kinematical anomaly below the mass threshold of M (K − ppn) having quite
similar structure to that of Λpn final state of K −3 He reaction. For the analysis, weutilized 4 He data of the T77 experiment for hypernuclear lifetime measurement of3

4Λ H [4]. The T77 experiment used the present cylindrical detector system (CDS). TheT77 data was accumulated for 66 hours under 51 kW MR beam power corresponding
to 140 kW*days.
At the experiment, incident K − and scattered charged particles were measured with
the K1.8BR beam-line spectrometer and the CDS, respectively [5]. The K − beam wasidentified by time-of-flight measurement with the beam-line spectrometer, and thenthe beam momentum was determined with the final beam-line dipole spectrometer
magnet. Particle identification and momentum determination of scattered charged
particles were performed by the CDS operating in a uniform ∼ 0.7 T magnetic fieldprovided by a solenoid magnet.
We selected the event in which Λd are detected in CDS and missing neutron isidentified by the missing mass method to identify the Λdn final state in the K −4 Hereaction. The event consistency was examined by distance of closest approach (DCA)
between the π − p pair for Λ decay, the kaon beam and the reconstructed Λ track, thekaon beam and the deuteron, and the Λ and the deuteron.
For the Λdn-selected events, we evaluated the invariant mass of the Λd system(IM(Λd)) and the momentum transfer to the Λd (qΛd ), a synthesis momentum of Λand d. Figure 1(left) shows the 2D event distribution on the IM(Λd) and qΛd plane.
A strong event concentration can be seen below the K − + p + p + n mass threshold
M (K − ppn) at lower qΛd , which is quite similar to that of the Λpn final state in theK −3 He reaction shown as Fig. 1(right) [6, 7]. The Λd invariant-mass spectrum, i.e.,the x-axis projection of Fig. 1, is also shown in Fig. 2(left) in which a peak structure
below M (K − ppn) can be seen clearly. As in the case of the K̄N N bound state in theΛpn final state shown as Fig. 2(right), the observed structure in the Λdn final state
would be the signal of the K − ppn bound state.

3 Staging Strategy of E80
It is urgent to confirm the observed structure focusing on the Λd decay mode at higher
statistics with the present setup. At the proposal stage of E80, we assumed the crosssection of the K − ppn → Λd decay branch by analogy with that of the K̄N N → Λpmeasured in E15. With the T77 data analysis, now we have demonstrated the highfeasibility of the K − ppn → Λd observation at E80. Thus, we revise our E80 proposal
to conduct that in a following staging plan.
Phase-I experiment:
By focusing on the K − ppn → Λd decay channel with the present setup, we will
confirm the existence of the K − ppn bound state. The binding energy, decay
width, and reaction form-factor parameter will be deduced at the same data
quality as it is measured in E15 for the K̄N N bound state.
Phase-II experiment:
We need to wait for a new 4π detector system to conduct more comprehensive
4

-

1

1

0.5

counts15
10preliminary
4He(K ,Λd)n3q

Λp

0.50

M(K pp)

E15[GeV/c]qΛ d [GeV/c]-M(K ppn)

T77

counts20-

He(K ,Λp)n35003.2 3.4 3.6 3.8
IM(Λd) [GeV/c2]2

100

2.2 2.4 2.6 2.8IM(Λp) [GeV/c2]

Figure 1: (left) 2D plot on the IM(Λd) and qΛd plane for the Λdn final state obtained
from 4 He(K − , Λd)n at T77. The vertical red dotted line shows M (K − ppn), and theblack dotted line is the kinematical limit of the reaction. (right) 2D plot on the IM(Λp)
and qΛp plane for the Λpn final state obtained from 3 He(K − , Λp)n at E15 [6, 7]. Lines
are similar to the left figure but for Λpn.
-T77M(K ppn)
4--He(K ,Λd)npreliminary

counts per 20 MeV/c2counts per 40 MeV/c2

100E15-He(K ,Λp)n3DataKNN
QFKA
BG
Sum30020050M(K pp)10033.2 3.4 3.6 3.8IM(Λd) [GeV/c2]

22.2 2.4 2.6 2.8
IM(Λp) [GeV/c2]

Figure 2: (left) Invariant-mass spectrum of Λd (x-axis projection of the Fig. 1(left)).
(right) Invariant-mass spectrum of Λp (x-axis projection of the Fig. 1(right)) together
with the decompositions of the fit result by considering the three physics processes:
the K̄N N bound state, the non-mesonic quasi-free kaon absorption process (QFK̄A ),and a broad distribution covering the whole kinematically allowed region of the Λpn
final state (BG) [6, 7].

5

study on multi-particle decay mode such as K − ppn → Λpn and K − pnn → Λnn,
what we described in the original E80 proposal. Based on results of the Phase-Iexperiment, we will submit the TDR in which detailed design of the new 4π
detector system and revised beam-time plan will be presented.

4 Beam-Time Request for the Phase-I ExperimentTo establish the K − ppn state, we need ∼ 6,000 events of the K −4 He → Λdn reaction,
which is the same statistical quality as it is measured in E15 for the K −3 He → Λpnreaction. We have observed ∼ 800 events of Λdn final state with 140 kW*days beamtime at T77, so that we need ∼ 1,050 kW*days beam time for the E80 Phase-I experiment. When we assume 90 kW primary beam power and 90 % up-time ratio ofaccelerator, it corresponds to the beam time of 13 days.
We would like to perform the E80 Phase-I experiment after the MR long shutdown
conducted in 2021-22. Apparatus for the Phase-I has already been ready and wellcommissioned so far. We will also perform H2 run for a day with liquid H2 target toconfirm the spectrometer performance under the high-intensity operation.

References[1] Proposal for J-PARC, 2020, ”Systematic investigation of the light kaonic nuclei:
via the in-flight 4 He(K − , N ) reactions”
http://j-parc.jp/researcher/Hadron/en/Proposal_e.html .
[2] Proposal for J-PARC, 2021, ”Investigation of fundamental properties of theK̄N N state” http://j-parc.jp/researcher/Hadron/en/Proposal_e.html .
[3] Letter of Intent for J-PARC, 2009, ”Double Anti-kaon Production in Nuclei by
Stopped Anti-proton Annihilation”
http://j-parc.jp/researcher/Hadron/en/Proposal_e.html .
[4] Test Beam Proposal for J-PARC, 2020, ”Feasibility study for 3Λ H mesonic weak
decay lifetime measurement with 4 He(K , π 0 )4Λ H reaction”
http://j-parc.jp/researcher/Hadron/en/Proposal_e.html .
[5] K. Agari et al. Prog. Theor. Exp. Phys. 2012 (2012) 02B011.
[6] S. Ajimura et al. Phys. Lett. B789 (2019) 620.
[7] T. Yamaga et al. Phys. Rev. C 102 (2020) 044002.

6