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arxiv: 2606.18398 · v1 · pith:GWHVHJVZnew · submitted 2026-06-16 · ⚛️ nucl-ex · nucl-th

First Measurement of the K^- Escape Cross Section in the {}¹²{rm C}(K⁻,p) Reaction

Fumiya Oura , Yudai Ichikawa , Junko Yamagata-Sekihara , Jung Keun Ahn , Sung Wook Choi , Manami Fujita , Takeshi Harada , Shoichi Hasegawa
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Shuhei Hayakawa Kenneth Hicks Satoru Hirenzaki Sang Hoon Hwang Kenichi Imai Yuji Ishikawa Woo Seung Jung Shunsuke Kajikawa Kento Kamada Byung Min Kang Shin Hyung Kim Tomomasa Kitaoka Jaeyong Lee Jong Won Lee Koji Miwa Taito Morino Tamao Sakao Hiroyuki Sako Masayoshi Saito Susumu Sato Toshiyuki Takahashi Kiyoshi Tanida Hirokazu Tamura Mifuyu Ukai Shunsuke Wada Takeshi O. Yamamoto Seongbae Yang
This is my paper

Pith reviewed 2026-06-26 21:31 UTC · model grok-4.3

classification ⚛️ nucl-ex nucl-th
keywords K- nucleus interactionoptical potentialK- escape cross sectioncarbon-12multi-nucleon absorptionJ-PARC
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The pith

The K^- escape cross section in carbon-12 is measured at 436 μb/sr, giving an imaginary optical potential of -100 MeV stronger than one-nucleon models predict.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The experiment at J-PARC measured both the inclusive ^{12}C(K^-,p) reaction and the exclusive K^- escape channel at 1.8 GeV/c beam momentum. By isolating the escape events through coincident detection of the proton and the outgoing K^-, the analysis extracts the imaginary part of the K^- optical potential for the first time via a simultaneous fit. The resulting imaginary strength exceeds predictions from models limited to single-nucleon absorption, indicating that multi-nucleon processes contribute substantially to K^- absorption inside the nucleus.

Core claim

The differential cross section for the K^-escape reaction was determined to be 436 ± 6 (stat.) ± 44 (syst.) μb/sr. A simultaneous likelihood fit yielded real and imaginary potential strengths of V0 = -72^{+3}_{-5} (stat.) ^{+0}_{-8} (syst.) MeV and W0 = -100^{+7}_{-1} (stat.) ^{+0}_{-16} (syst.) MeV at the nuclear center, respectively. The derived W0 is significantly stronger than that predicted by theoretical models based on one-nucleon processes, suggesting possible contribution of multi-nucleon involving processes.

What carries the argument

The K^- escape reaction identified by detecting both the outgoing proton and the escaped K^-, which separates the imaginary part of the optical potential in a simultaneous likelihood fit to inclusive and exclusive data.

If this is right

  • The imaginary part of the K^- optical potential receives substantial contributions from multi-nucleon absorption processes.
  • One-nucleon absorption models underestimate the strength of the imaginary potential by a large margin.
  • The real part of the potential at the nuclear center is -72 MeV.
  • Models of K^- nucleus interactions must incorporate multi-nucleon processes to match the data.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If multi-nucleon absorption is the dominant source, it would alter expected formation probabilities for deeply bound kaonic states or hypernuclei in similar reactions.
  • Applying the same simultaneous-fit method to heavier or lighter targets could reveal how the relative strength of multi-nucleon contributions changes with nuclear size.

Load-bearing premise

The chosen optical potential form and reaction model correctly isolate the imaginary part without significant contamination from unmodeled multi-nucleon absorption channels or detector acceptance effects.

What would settle it

An independent experiment or calculation that reproduces the measured escape cross section of 436 μb/sr using only one-nucleon absorption models without extra imaginary strength.

Figures

Figures reproduced from arXiv: 2606.18398 by Byung Min Kang, Fumiya Oura, Hirokazu Tamura, Hiroyuki Sako, Jaeyong Lee, Jong Won Lee, Jung Keun Ahn, Junko Yamagata-Sekihara, Kenichi Imai, Kenneth Hicks, Kento Kamada, Kiyoshi Tanida, Koji Miwa, Manami Fujita, Masayoshi Saito, Mifuyu Ukai, Sang Hoon Hwang, Satoru Hirenzaki, Seongbae Yang, Shin Hyung Kim, Shoichi Hasegawa, Shuhei Hayakawa, Shunsuke Kajikawa, Shunsuke Wada, Sung Wook Choi, Susumu Sato, Taito Morino, Takeshi Harada, Takeshi O. Yamamoto, Tamao Sakao, Tomomasa Kitaoka, Toshiyuki Takahashi, Woo Seung Jung, Yudai Ichikawa, Yuji Ishikawa.

Figure 1
Figure 1. Figure 1: FIG. 1. Particle identification for [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. ( [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Double differential cross sections of [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Likelihood distribution for [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
read the original abstract

We investigated the $\bar{K}$-nucleus interaction through the simultaneous measurement of the inclusive $^{12}{\rm C}(K^-, p)$ and exclusive $K^-$-escape $^{12}{\rm C}(K^-, p K^-_{esc})$ reactions at $1.8$ GeV/$c$ at J-PARC. The present measurement explicitly focuses on the $K^-$ escape process for the first time, successfully accomplishing a direct experimental determination of the imaginary part of the $K^-$ optical potential. The differential cross section for the $K^-$-escape reaction was determined to be $436 \pm 6\:(\text{stat.}) \pm 44\:(\text{syst.})~\mu\text{b/sr}$. A simultaneous likelihood fit yielded real and imaginary potential strengths of $V_0 = -72\:^{+3}_{-5}\:(\text{stat.})\:^{+0}_{-8}\:(\text{syst.})~\text{MeV}$ and $W_0 = -100\:^{+7}_{-1}\:(\text{stat.})\:^{+0}_{-16}\:(\text{syst.})~\text{MeV}$ at the nuclear center, respectively. The derived $W_0$ is significantly stronger than that predicted by theoretical models based on one-nucleon processes, suggesting possible contribution of multi-nucleon involving processes.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript reports the first measurement of the K^- escape cross section in the ^{12}C(K^-, p) reaction at 1.8 GeV/c, determining the differential cross section to be 436 ± 6 (stat.) ± 44 (syst.) μb/sr. A simultaneous likelihood fit to inclusive and exclusive data extracts central optical potential strengths V0 = -72^{+3}_{-5} (stat.)^{+0}_{-8} (syst.) MeV and W0 = -100^{+7}_{-1} (stat.)^{+0}_{-16} (syst.) MeV, with the latter significantly exceeding one-nucleon absorption model predictions and interpreted as evidence for multi-nucleon processes.

Significance. If the extraction holds, the direct escape-channel measurement supplies a new experimental handle on the imaginary part of the K^- optical potential, which is otherwise difficult to isolate. The reported W0 value, if robust against model assumptions, would indicate that multi-nucleon absorption contributes substantially beyond current one-nucleon calculations, with potential consequences for kaonic-atom analyses and K^- propagation in nuclei.

major comments (1)
  1. [results section on the likelihood fit] The simultaneous likelihood fit (described in the results section) extracts W0 = -100 MeV by assuming the chosen optical-potential form (parameterized by V0, W0 at r=0) plus the reaction model fully accounts for the observed K^- escape yield. Because the paper concludes that multi-nucleon processes are likely present to explain the discrepancy with theory, any unmodeled leakage of those channels into the escape signature would systematically bias W0 upward; no explicit test (model-space variation or microscopic comparison) is reported that bounds this contamination below the quoted ±16 MeV systematic uncertainty.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful review and the insightful comment concerning the likelihood fit and possible systematic effects from multi-nucleon processes. We respond to the major comment below.

read point-by-point responses

  1. Referee: [results section on the likelihood fit] The simultaneous likelihood fit (described in the results section) extracts W0 = -100 MeV by assuming the chosen optical-potential form (parameterized by V0, W0 at r=0) plus the reaction model fully accounts for the observed K^- escape yield. Because the paper concludes that multi-nucleon processes are likely present to explain the discrepancy with theory, any unmodeled leakage of those channels into the escape signature would systematically bias W0 upward; no explicit test (model-space variation or microscopic comparison) is reported that bounds this contamination below the quoted ±16 MeV systematic uncertainty.

    Authors: The optical potential in our analysis is an effective phenomenological potential whose imaginary part W0 is intended to encompass the net effect of all absorption mechanisms, including multi-nucleon processes. The fact that the fitted |W0| exceeds one-nucleon predictions is itself the quantitative signature of those additional contributions. The systematic uncertainty of 16 MeV was obtained by varying the potential shape, nuclear density, and distortion parameters within the reaction model. Although we did not carry out an explicit microscopic multi-nucleon calculation or additional model-space scan beyond those variations, the simultaneous likelihood fit to both the inclusive and exclusive data sets supplies internal consistency checks that constrain possible unaccounted leakage. In the revised manuscript we will add a short paragraph in the results section that explicitly discusses this point and states that the quoted systematic uncertainty is intended to cover residual model dependence of this type. revision: partial

Circularity Check

0 steps flagged

No significant circularity; direct measurement and data-driven fit

full rationale

The paper reports a new experimental measurement of the K^- escape differential cross section (436 ±6 stat ±44 syst μb/sr) from J-PARC data on the exclusive ^{12}C(K^-, p K^-_{esc}) reaction. The optical potential parameters V0 and W0 are extracted via a simultaneous likelihood fit to the measured inclusive and exclusive spectra. This is standard parameter estimation from fresh data against an external reaction model, not a derivation that reduces to its own inputs by construction. No self-citations, ansatze smuggled via prior work, or uniqueness theorems are invoked in the provided text to justify the central result. The comparison of fitted W0 to one-nucleon theoretical models is an external benchmark, not internal redefinition.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The central claim rests on the validity of the optical potential parameterization used in the likelihood fit and the assumption that the reaction can be modeled with a single complex potential at the given beam energy.

free parameters (2)
  • V0 = -72 MeV
    Real part of the optical potential strength at nuclear center, obtained from the simultaneous fit to data.
  • W0 = -100 MeV
    Imaginary part of the optical potential strength at nuclear center, obtained from the simultaneous fit to data.
axioms (1)
  • domain assumption The K- nucleus interaction can be described by a local optical potential whose imaginary part directly governs the escape probability in the (K-,p) reaction.
    Invoked to interpret the measured escape cross section as a direct probe of W0.

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discussion (0)

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