Search for the K − pp bound state via the in-flight 3 He ( K − , n ) reaction

In the J-PARC E15 experiment, a K−pp search was performed via the 3He(K−, n) reaction at 1.0 GeV/c. A forward-going neutron is detected by a neutron counter with 15 m flight length, and decay particles from K−pp are simultaneously measured by a cylindrical detector system that surrounds a liquid 3He target system. In March and May, 2013, we carried out the first physics data-taking with 5x109 incident ae-mail: sada@rcnp.osaka-u.ac.jp DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2014 / 0201 (2014) 201 epjconf EPJ Web of Conferences 4 0201 81 81 , 6 6 This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20148102016 kaons on the 3He target, and we have obtained a preliminary exclusive analysis result of 3He(K−,Λp)n reaction.


Introduction
TheKN interaction is one of the keys to understanding meson-baryon interactions in low-energy quantum chromodynamics (QCD) incorporating three flavors in the nuclear system. In particular, the investigation of deeply bound kaonic nuclei have been progressed in both the theoretical [1]- [12] and experimental [13], [14] sides in the last decade. There are various theoretical predictions of the K − pp state at present, thus it is quite important to compare the theoretical calculations with experimental studies. However, the experimental situation is also controversial; The FINUDA [13] and the DISTO [14] collaborations have claimed observations of the deeply-bound K − pp state, while LEPS [15] group recently reported null results of the K − pp population. Therefore, we need to investigate the K − pp state in different reactions and to understand background processes, such as multinucleon absorption processes of K − The E15 experiment was proposed to search for the simplest kaonic nuclear bound states, namely K − pp, via the in-flight 3 He(K − , N) reaction using 1.0 GeV/c K − beam [16]. This experiment has an advantage that an exclusive measurement can be performed by a simultaneous measurement of missing mass spectrum using the primary nucleon and invariant mass spectroscopy via the decay particles from K − pp. We have obtained the missing-mass spectrum of 3 He(K − , n) as well as the exclusive result of 3 He(K − , Λp)n reaction. a detail of the missing-mass analysis of 3 He(K − , n) is reported in [17]. By the reconstructing the exclusive 3 He(K − , Λp)n channel, we have also examined not only the expected K − pp → Λp decay but also multi-nucleon absorption processes of in-flight K − . There is no data about in-flight K − multi-nucleon absorption on 3 He. Thus our data might become important hints for further understanding of theKN interaction.

Experiment overview
The E15 experiment is performed at the K1.8BR beam-line in the Hadron hall at the J-PARC. The spectrometer consists of high precision beam-line spectrometer, a liquid 3 He target system and a cylindrical detector system (CDS) which surrounds the target to detect the decay particles from the target region, and forward time-of-flight (TOF) counters to detect neutron or proton ( whose flight length is about 15 m). Details of other detectors in K1.8BR are discribed in [18].

Beam-line spectrometer
The beam-line spectrometer is composed of beam line magnets, trigger counters, beam trackers, and a kaon identification counter. The beam trigger is generated by a coincidence signal of a beam hodoscope detector (BHD) and a time-zero counter (T0); the flight length between the BHD and T0 is 7.7 m. The kaon beam with momentum around 1.0 GeV/c is identified by using an aerogel Cherenkov counter (AC) with a refractive index of 1.05. The kaon beam is tracked with two beam trackers (BLC1 and BLC2) and the momentum of the kaon is analyzed with this tracking information together with the beam optics of a dipole magnet in between BLC1 and BLC2.

Cylindrical Detector System (CDS)
The CDS consists of the solenoid magnet, the Cylindrical Drift Chamber (CDC), and the Cylindrical Detector Hodoscope (CDH). The decay particles from the target are reconstructed by the CDC which operates in a magnetic field of 0.7 T. The CDH is used for particle identification and as a charged particle trigger. We checked the tracking efficiency of the CDS with kaon beam data. We estimated the tracking efficiency as 93.1 ±1.0%.

Analysis of 3 He(K − , Λp)n reaction
In March and May, 2013, we carried out the first physics data-taking with 5×10 9 incident K − 's on the 3 He target. To analyze K − pp which decays to Λp and multi-nucleon absorption, we selected the 3 He(K − , Λp)n reaction. The Λp are detected with the CDS, and a missing neutron is identified from missing mass spectrum of 3 He(K − , Λp)X. In Fig. 1(a), we show a missing-mass of the 3 He(K − , Λp)X. A clear missing-neutron peak is seen. The cut condition for a missing neutron is defined as the red colored region in Fig. 1(a). Then we obtained data of the 3 He(K − , Λp)n reaction, which is the first measurement of in-flight K − multi-nucleon absorption. The number of events of the 3 He(K − , Λp)n reaction is about 200. To check phase space distribution, we plotted three body energy correlation of final state, which is Dalitz plot with kinematic energy (Fig. 1(b)). Vertical axis of Fig. 1(b) is kinetic energy of neutron in the 3 He(K − , Λp)n reaction. Spectator neutron in two-nucleon absorption processes has only Fermi momentum. Therefore if two-nucleon absorption processes which spectator is neutron are dominant, points in Dalitz plot are distributed in region in which kinetic energies of neutron are low (orange circle area in Fig. 1(b)). But our data is distributed widely in Dalitz plot. It suggests that three-nucleon absorption processes are dominant in the 3 He(K − , Λp)n reaction. When uniform phase-space distribution is assumed, total cross section of this reaction is a few hundred μb.

Conclusion
Our expriment, J-PARC E15 experiment is a search for the kaonic nucleus K − pp via the 3 He(K − , n) reaction at 1.0 GeV/c. In the J-PARC E15 experiment, we are able to simultaneously measure formation reaction and decay of K − pp. In March and May, 2013, we carried out the first physics datataking with 5x10 9 incident kaons on the 3 He target, and we have obtained an exclusive analysis result of 3 He(K − , Λp)n reaction, which is the first measurement of in-flight K − multi-nucleon absorption. Our data suggest that three nucleon absorption processes are dominant in the 3 He(K − , Λp)n reaction. When uniform phase-space distributions are assumed, total cross section of this reaction is a few hundred μb. To understand existence of K − pp in this reaction, we should consider all reactions and global fitting of the 3 He(K − , Λp)n reaction is needed. We will report soon final result about exclusive 3 He(K − , Λp)n reaction.