A search for the K − pp bound state in the 3 He ( K − in − flight , n ) reaction at J-PARC

We have collected the first physics data of an experimental search for the simplest kaonic nuclear bound state, “K−pp”, by the 3He(K−, n) reaction at J-PARC. We confirmed that our spectrometer system works as designed and observed clear peak structure ae-mail: hashimoto@nucl.phys.s.u-tokyo.ac.jp bSpokesperson DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2014 , / 09008 (2014) 201 66 epjconf EPJ Web of Conferences 46609008 This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, 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/20146609008 composed of the quasi-elastic K−“n” → K−n and the charge-exchange K−“p” → K̄0n reactions in the forward neutron spectrum.


Introduction
Recent theoretical studies show that an anti-kaon can be bound in nuclei to form so-called kaonic nuclei as a result of strongly attractive KN interaction in I = 0 channels [1].Among such a new form of nuclear matter, "K − pp" [2] is now attracting great interest as the simplest one.Many theoretical calculations have been progressed for the K − pp system, resulting in various binding energy and width predictions.Experimentally, however, only a small amount of information is available [3] [4], which is not sufficient to discriminate between a variety of conflicting interpretations.
In this situation, we are carrying out an experimental search for the K − pp bound state at J-PARC (J-PARC E15 [5]).The most important key of our experiment is the (K − , n) reaction at 1 GeV/c.In this reaction, neutron backgrounds from non-mesonic two-nucleon absorptions or hyperon decays are expected to be substantially suppressed and kinematically separated.In addition, by using a liquid 3 He target and a large acceptance detector surrounding it, we can detect decay particles from "K − pp" to fully reconstruct the reaction kinematics.
In this paper, we describe our spectrometer system, its performance and a very preliminary result of the forward neutron spectrum obtained in our first physics data taking.

The K1.8BR spectrometer system at J-PARC
A new spectrometer system has been designed and constructed at the secondary beam line K1.8BR in the hadron hall of J-PARC [6]. Figure 1 shows the schematic view of the spectrometer system.

K1.8BR beam line and beam spectrometer
The K1.8BR beam line delivers secondary beams of charged particles with momenta up to 1.2 GeV/c, which are purified by an electro static separator in combination with two correction magnets and two vertical slits.Kaons are identified by using an Aerogel Cherenkov counter at a trigger level.The momentum of the secondary beam is analyzed by a beam line spectrometer with a momentum resolution of 2.2×10 −4 at 1 GeV/c.The measured 1 GeV/c kaon yield at 24 kW 1 primary beam power was 1.5 × 10 4 per spill with a K − /π − ratio of 0.45.

Cylindrical detector system
Figure 2 shows a cylindrical detector system (CDS), which mainly consists of a solenoid magnet operated at 0.7 T, a 15-layer cylindrical drift chamber (CDC) and a 36-segmented cylindrical detector hodoscope (CDH).The CDS covers 60% of solid angle from the target and has good capability of particle identification as shown in Fig. 3.The K 0 s → π + π − (Fig. 4) and Λ → pπ − decays are successfully reconstructed with the designed performance, which corresponds to 10 MeV/c 2 invariant mass resolution for the expected "K − pp" → Λp → π − pp decay channel.

Forward counters
One of the most unique features of our spectrometer system is a large-acceptance high-resolution neutron counter (NC) with a flight length of ∼ 15 m.The geometrical acceptance is ∼20 msr and the detection efficiency for a 1 GeV/c neutron is estimated to be ∼ 35%.A beam sweeping magnet and two sets of veto counter arrays remove charged particles from the NC.We also measure the 3 He(K − , p) reactions with a proton counter array to investigate the isospin dependence of the KN interaction.

E15 first physics run
The first physics run of the J-PARC E15 experiment was carried out in March and May, 2013.About 1% statistics of the full proposal was collected in this period with an integrated beam power of 15 kW × week, which corresponds to 5 ×10 9 kaons on the helium-3 target.

Neutron spectrum at forward angle
Figure 5 and Fig. 6 show the 1/β spectrum of neutral particles and the neutron missing mass spectrum measured by the NC with the data took in May ( ∼ 80% of the full statistics ), respectively.In both spectra, we requested at least one charged track in the CDS to reconstruct the reaction vertex and then we required the vertex to be roughly in the helium-3 target region.The TOF resolution of the system is evaluated to be ∼ 180 ps from the γ peak in Fig. 5, which is equivalent to ∼ 10 MeV/c 2 missing mass resolution for the neutrons in the region of interest ( neutron momentum around 1.2 GeV/c ).A clear gap between the γ peak width and the broad neutron distribution is showing that the accidental background is very small, even without any optimization of offline threshold for the energy deposit  on NC.In Fig. 6, a peak structure due to the quasi-elastic K − "n" → K − n and the charge-exchange K − "p" → K0 n reactions is seen just above the "K − pp" binding threshold (2.37 GeV/c 2 ).From the K 0 s tagged spectrum overlaid in Fig. 6, the accuracy of the missing mass scale in the region of interest is evaluated to be better than 10 MeV/c 2 at the current analysis stage.

Figure 4 .
Figure 4. Invariant mass distribution of π + π − pairs reconstructed by the CDS.The spectrum is fitted with a Gaussian and a background curve.The centroid and resolution of K 0 s are well reproduced by a detailed detector simulation.

Figure 5 .
Figure 5. 1/β spectrum of neutral particles, obtained by TOF analysis using a time-zero counter (T0) and the NC.The dotted line shows an accidental background contribution estimated from the left shoulder of the γ peak.

Figure 6 .
Figure 6.Missing mass spectrum of neutrons at forward angle.At least one charged track in CDS is requested to reconstruct the reaction vertex.A spectrum with K 0 s tag by the CDS is overlaid with a scale factor of 10.The dotted line shows the accidental background obtained in Fig. 5.The "K − pp" binding threshold is indicated by a solid line.