Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Clear indication of a strong I=0 K̄N attraction in the Λ(1405)
region from the CLAS photo-production data∗1,∗2
∗2,∗3
∗1
∗2,∗3
Hassanvand
Y.Akaishi,
Akaishi∗2∗2andT. Yamazaki
M. M.
Hassanvand,Y.
T. Yamazaki

The possible existence of deeply bound kaonic nuclear systems was proposed1) more than a decade ago,
based on an ansatz that Λ∗ ≡ Λ(1405) mass is 1405
MeV/c2 , where Λ∗ is a K̄N quasi-bound state decaying to Σπ. Recently, a large number of data on thephoto-production of Λ(1405) in the γp → K + π 0± Σ0∓
reaction were provided by the CLAS collaboration2) ,and the double-pole structure of Λ∗ has been intensively discussed by chiral dynamics analyses3–5) .

Σ0π0 invariant mass spectrum from CLAS

Fig. 2. Variation of χ2 values with respect to Λ∗ pole position parameter M for the Σ0 π 0 invariant mass spectraat γp total energies, W = 2.0 − 2.8 GeV, from CLAS.
The χ2 minimum appears around 1400 MeV/c2 .

Λ*Outerparameters

319Innerparameters

2 (M*,Γ )Ours0Mai-Meissner

χ299%

Chiral#4NoFixedconfidence
is given by
the CLAS data.

Fig. 1. Analyses of a Σ0 π 0 invariant mass spectrum
from CLAS. Mai-Meissner’s analysis (brown) with chiral theory and Hassanvand-Akaishi-Yamazaki’s analysis(black and blue) are compared.

We have analyzed the CLAS data. Figure 1 compares two CLAS-data analyses. The most essential
question is what the pole position of Λ∗ extracted
from the CLAS data themselves is. To answer it,
we classiﬁed χ2 ﬁtting parameters into “inner” and“outer” ones, where the “inner” indicates the parameters appearing inside the T -matrix which can vary thepole position and width of Λ∗ . The double pole positions recommended in Ref.5) are cases selected using
only “outer” parameters, holding the pole position unchanged in χ2 ﬁtting processes. Therefore, the chiralpole position in Ref.4) , for example, gets no conﬁdence
from the CLAS data. On the other hand, we used theΛ∗ pole position and width as ﬁtting parameters andfound the χ2 minimum at 1405.5 MeV/c2 , rejecting
the chiral result with more than 99% statistical significance.
∗1
∗2
∗3

Department of Physics, Isfahan University of Technology
RIKEN, Nishina Center
Department of Physics, University of Tokyo- 121 -Figure 2 shows the variation of χ2 values with respect to the Λ∗ pole position for all the Σ0 π 0 invariant
mass data from CLAS. The χ2 minimum χ2min (M =Mpole ) appears around Mpole = 1400 MeV/c2 in allcases. Statistical conﬁdence of the Λ∗ pole position Mpole can be obtained incrementing Δχ2 (M ) ≡χ2 (M ) − χ2min (Mpole ): Δχ2 = 2.36, 4.74, 9.23, corresponding to conﬁdence levels of 68.3%, 95% and 99.9%,respectively. Thus, the conﬁdence of the Λ∗ pole position, that is of our main concern, was obtained throughthe “inner” ﬁtting parameters. It is again stressed thatthe “outer” parameters used in Ref.4) (see also Tablein Ref.5) ) cannot get any quantitative conﬁdence aboutthe Λ∗ pole position, in spite of the seemingly beautiful reproduction of all the global neutral and charged
spectra of CLAS.
In summary, the pole position for Λ(1405) extracted
from the CLAS photo-production is not shallow-K̄N binding ones, 1421 ∼ 1434 MeV/c2 5) , but is consistent with the deep-K̄N -binding PDG value 1405.1+1.3−1.0
MeV/c2 .
References
1) Y. Akaishi, T. Yamazaki, Phys. Rev. C 65 (2002)
044005.
2) K. Moriya et al., Phys. Rev. C 87 (2013) 035206.
3) L. Roca, E. Oset, Phys. Rev. C 87 (2013) 055201.
4) M. Mai, U.-G. Meissner, Eur. Phys. J. A 51 (2015) 30.
5) U.-G. Meissner, T. Hyodo, Chin. Phys. C 38 (2014)
090001 and 2015 update.