[Paper Review] The (K-,p) reaction on C12 at KEK

V. K. Magas, J. Yamagata-Sekihara+3 more|arXiv (Cornell University)|Nov 11, 2009

Quantum Chromodynamics and Particle Interactions3 citations

TL;DR

This study reanalyzes the (K⁻,p) reaction on ¹²C at KEK using Monte Carlo simulations to assess whether fast protons observed in coincidence with decay products imply a deeply attractive kaon-nucleus optical potential. The authors demonstrate that the coincidence requirement significantly distorts the proton spectrum, and conventional processes—such as multi-scattering and two-nucleon absorption—can reproduce the data with a standard shallow potential of -60 MeV, challenging the claim of a 200 MeV depth from Kishimoto et al. (2007zz).

ABSTRACT

We study the (K-,p) reaction on C12 with a kaon beam of 1 GeV momentum, paying a special attention to the region of emitted protons having kinetic energy above 600 MeV, which was used to claim a deep kaon nucleus optical potential [1]. The experiment looks for fast protons emitted from the absorption of in flight kaons by nuclei, but in coincidence with at least one charged particle in the decay counters sandwiching the target. The analysis of the data is done in [1] assuming that the coincidence requirement does not change the shape of the final spectra. However our detailed calculations show that this assumption doesn't hold, and, thus, the final conclusion of this experiment is doubtful. We perform Monte Carlo simulation of this reaction. The advantage of our method with respect to Green's function method used in [1] is that it allows to account not only for quasi-elastic K- p scattering, but also for the other processes which contribute to the proton spectra. We investigated the effect of the multi-scatterings and of the K- absorptions by one and two nucleons (K- N --> pi Y and K- N N --> Y N) followed by the decay of the hyperon in pi N. We show that all these mechanisms allow us to explain reasonably well the observed spectrum with standard shallow kaon nucleus optical potential, obtained in chiral models. [1] T. Kishimoto et al., Prog. Theor. Phys. 118, 181 (2007).

Motivation & Objective

• To reevaluate the experimental claim of a deeply bound K⁻-nuclear state in ¹²C based on (K⁻,p) reaction data with fast protons in coincidence with decay counters.
• To challenge the assumption in Kishimoto et al. (2007zz) that the coincidence requirement does not alter the final-state proton spectrum shape.
• To investigate whether conventional reaction mechanisms—beyond quasi-elastic K⁻p scattering—can reproduce the observed proton spectrum with a standard shallow optical potential.
• To quantify the impact of the experimental coincidence detection on the reconstructed proton spectrum using a realistic simulation framework.
• To determine whether the data truly support a strongly attractive kaon-nucleus potential or can be explained by standard dynamics and detection biases.

Proposed method

• Employed a Monte Carlo simulation to model the full cascade of K⁻-nucleus interactions, including quasi-elastic scattering, single- and two-nucleon absorption, and hyperon decays.
• Tracked all final-state particles, including protons, pions, and kaons, through multiple scattering and absorption processes in the nuclear medium.
• Implemented a minimal coincidence requirement to simulate the experimental detection condition: at least one charged particle detected in counters flanking the target.
• Compared simulated spectra with and without the coincidence cut to assess its impact on the shape of the proton energy spectrum.
• Applied suppression factors to simulate incomplete detection efficiency, particularly for backward-going kaons and low-angle particles.
• Used a standard chiral-motivated optical potential of V_opt = (-60, -60) ρ/ρ₀ MeV to test whether conventional dynamics can reproduce the data.

Experimental results

Research questions

• RQ1 Does the coincidence detection condition in the KEK experiment significantly alter the shape of the observed proton energy spectrum?
• RQ2 Can the observed peak in the proton spectrum at high kinetic energy (above 600 MeV) be explained by conventional reaction mechanisms rather than a deeply bound K⁻-nuclear state?
• RQ3 To what extent do multi-scattering, single-nucleon, and two-nucleon absorption processes contribute to the production of fast protons in the (K⁻,p) reaction on ¹²C?
• RQ4 How does the experimental coincidence requirement suppress certain final-state configurations, and what is the resulting distortion of the reconstructed spectrum?
• RQ5 Is the claimed evidence for a 200 MeV attractive K⁻-nucleus potential robust, or can it be explained by standard dynamics and detection biases?

Key findings

• The coincidence requirement in the KEK experiment significantly distorts the proton spectrum, invalidating the assumption in Kishimoto et al. (2007zz) that the spectrum shape remains unchanged.
• Multi-scattering and single-nucleon absorption processes contribute substantially to the proton spectrum, particularly in the region above 600 MeV kinetic energy.
• Two-nucleon absorption processes contribute to the entire spectrum, including the low-energy region down to -300 MeV binding energy, and cannot be simulated by a simple parametric background.
• The minimal coincidence requirement suppresses a large fraction of quasi-elastic events where the kaon is scattered backward, altering the spectrum shape and reducing the yield in the peak region.
• The suppression factor in the bound region (-E_B < 0 MeV) is approximately 0.7, while it becomes negligible for continuum states above 50 MeV, indicating a strong angular and momentum dependence in detection efficiency.
• The observed spectrum can be well reproduced using a standard shallow optical potential of -60 MeV, suggesting that the data do not require a deeply bound K⁻-nuclear state.