Search for a 𝑲! 𝒑𝒑 Bound State by Using the 𝝅! + 𝒅 → 𝑲! + 𝑿 Reaction

Y. Ichikawa1), T. Nagae2), H. Fujioka2), H. Bhang3), S. Bufalino4), H. Ekawa1, 2), P. Evtoukhovitch5) ,A. Feliciello4), S. Hasegawa1), S. Hayakawa1, 6), R. Honda6), K. Hosomi1), K. Imai1), S. Ishimoto7) ,C. Joo3), S. Kanatsuki2), R. Kiuchi8), T. Koike9), H. Kumawat10), Y. Matsumoto9), K. Miwa9) ,M. Moritsu6), M. Naruki2), M. Niiyama2), Y. Nozawa2), R. Ota6), A. Sakaguchi6), H. Sako1) ,V. Samoilov5), S. Sato1), K. Shirotori11), H. Sugimura1), S. Suzuki7), T. Takahashi7), T.N. Takahashi11),
H. Tamura9), T. Tanaka6), K. Tanida1), A.O. Tokiyasu12), Z. Tsamalaidze5), B. Roy10), M. Ukai9),
T.O. Yamamoto9), and S. Yang3)
1)
ASRC, JAEA 2) Kyoto Univ. 3) Seoul Univ. 4) INFN 5) JINR 6) Osaka Univ.
7)
KEK 8) Gifu Univ. 9) Tohoku Univ. 10) BARC 11) RCNP 12) ELPH

The strange dibaryons (strangeness 𝑆 = －1 and baryon
number 𝐵 = 2) such as 𝐾𝑁𝑁 system, in which a bound state of𝐾 ! 𝑝𝑝 is expected, are experimentally not well revealed yet. We
have searched for a 𝐾 ! 𝑝𝑝 bound state by using the π! + 𝑑 →𝐾 ! + 𝑋  reaction at pion incident momentum of 1.69 GeV/c inthe laboratory angles between 2° and 16°.
The present experiment, J-PARC E27, has been carried out at
the K1.8 beam line of the hadron experimental hall at J-PARC.
The K1.8 beam line and the SKS spectrometer have been usedwith a good momentum resolution of Δ𝑝 𝑝 ∼ 10!! to measure
π! and 𝐾 ! momenta. Since the SKS has a wide acceptance(about 100 msr) and good momentum resolution, we could
measure the missing mass spectrum with the good energy
resolution of 3 MeV (FWHM) and wide missing-mass region
from the production of a quasi-free Λ to a quasi-free hyperon
resonances, Λ(1405) and Σ(1385). The elementary process ofπ! + 𝑝 𝑛 → 𝐾 ! 𝑋 reaction has already been measured byusing the hydrogen bubble chamber. Thus, we can explore
exotic states in the wide energy range.
In this reaction Λ (1405) hyperon resonance, which isconsidered as the bound state of 𝐾𝑁 system, is expected to beproduced as a doorway to form the 𝐾 ! 𝑝𝑝 bound state throughthe Λ 1405 + 𝑝 → 𝐾 ! 𝑝𝑝 process. However, most of theproduced Λ (1405)’s would escape from deuterons without
secondary reactions. Therefore, coincidence of high-momentum( > 250  MeV/𝑐 ) proton(s) in large emission angles ( 39° <θ!"# < 122° ) was required to enhance the signal-to-noise ratio.
We have measured the inclusive missing-mass spectra at thismomentum in high statistics and high energy resolution for thefirst time [1]. The overall structure was understood with asimple quasi-free picture based on the known elementary
processes. However, we have observed two distinct deviations
from this picture. One is the Σ𝑁 cusp, of which mass was foundto be 2130.5 ± 0.4 stat. ± 0.9 syst.   MeV/𝑐 ! with the decay!!.!
width of Γ = 5.3!!.!
!!.! stat. !!.! syst.   MeV. The peak position
is consistent within the error of the previous measurements.
Since this result is the first measurement of the Σ𝑁 cusp
structure in the inclusive spectrum of the  π! + 𝑑 → 𝐾 ! + 𝑋
reaction, further detailed theoretical studies including thepresent data would reveal the information on the Σ𝑁 - Λ𝑁
coupling strength and the pole position. The other notablefeature is that the centroid of the broad bump structure in theregion of hyperon resonance production was significantly
shifted to the low-mass side as compared with a simple quasifree simulation. The observed “shift” values are 32.4 ±!!.!
!
0.5 stat. !!.!
!!.! (syst. ) (22.4 ± 0.4 stat. !!.! syst. )MeV/𝑐 ,which are the calculated missing mass in the kinematics of a
deuteron (proton) at rest as a target. In order to clarify the originof the peak “shift”, further theoretical and experimental studies
to measure the invariant mass spectra of the decay particle arenecessary.
We have measured a mass distribution of a “𝐾 ! 𝑝𝑝”-likestructure as Fig. 1 in the π! + 𝑑 → 𝐾 ! + “𝐾 ! 𝑝𝑝”, “𝐾 ! 𝑝𝑝”→
Σ ! + 𝑝 mode in the scattering angles between 2° and 14° for the
first time [2]. Here, the final state of Σ ! 𝑝 was identified using
the missing–mass square 𝑀!! distribution of π! + 𝑑 → 𝐾 ! 𝑝𝑝 +𝑋 reaction by measuring the two protons in the Range Counter
Array (RCA). A broad enhancement has been observed in themissing mass spectrum around 2.27 GeV/c2. By fitting with therelativistic Breit-Wigner function, the mass and decay width of“ 𝐾 ! 𝑝𝑝 ”-like structure have been evaluated to be!!"2275  !!"
MeV 𝑐 !
and!!" stat.   !!" syst.   
!!"!!!
Γ = 162  !!" stat.   !!" syst.
MeV , respectively. Theseobtained mass and width are not inconsistent with the values ofthe past experiments (FINUDA [3] and DISTO [4] experiments)
within the errors, although our statistical and systematic errors
are large. If the observed structure originates from the 𝐾 ! 𝑝𝑝!!"bound state, its binding energy is 95!!"!!" stat. !!" syst.   MeV.
Such a large binding energy together with a large width isdifficult to reproduce in the present theoretical models. Thus,
the observed enhancement might not be the simple 𝐾 ! 𝑝𝑝 boundstate but the other probabilities such as a 𝜋Λ𝑁-  𝜋Σ𝑁 dibaryon[5] and a lower pole of the 𝐾 ! 𝑝𝑝 [6].

Fig. 1 Missing mass spectrum of the 𝜋 ! + 𝑑 → 𝐾 ! + 𝑋 reaction fortwo-proton coincidence and the Σ ! 𝑝 decay branch events [2].

References[1] Y. Ichikawa et al., Prog. Theor. Exp. Phys. 2014, 101D03(2014).
[2] Y. Ichikawa et al., Prog. Theor. Exp. Phys. 2015, 021D01(2015).
[3] M. Agnelo et al., Phys. Rev. Lett. 94, 212305 (2005).
[4] T. Yamazaki et al., Phys. Rev. Lett. 104, 132502 (2010).
[5] H. Garcilazo and A. Gal, Nucl. Phys. A 897, 167 (2013).
[6] A. Dote et al., Prog. Theor. Exp. Phys. 2015, 043D02 (2015).