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Jung Keun Ahn

Publications and source records attributed to Jung Keun Ahn.

13 recordsLinked to original sources
arXiv2026

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

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 $436 \pm 6\:(\text{stat.}) \pm 44\:(\text{syst.})~μ\text{b/sr}$. 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.

nucl-ex↗
arXiv2026

Zero-locus diagnostic of the direct contact term in $ϕ\toπ^+π^-π^0$

We formulate, within a fixed FSI-improved amplitude convention, a zero-locus diagnostic for the direct contact term in $ϕ\toπ^+π^-π^0$. The resonant $ρπ$ contribution is dressed channel by channel with the complex Omnès function, the direct term is multiplied by the averaged Omnès factor, and a real constant background is retained as a smooth residual amplitude. In this convention, $F^{\rm FSI}=R+c e^{iδ_{\rm dir}}D$ with $c=g_{\phi3π}^{\rm eff}$, and the difference of two constrained invariant-mass gradients of the reduced Dalitz density gives a local quadratic identity for $c$ on the zero-contour. The construction provides a closure and stability diagnostic complementary to a global Dalitz-plot fit. For the benchmark input $g_{\phi3π}^{\rm eff}=-24.0~{\rm GeV}^{-3}$, we identify a numerically stable zero-contour branch around $M_{+0}=0.460\pm0.025~{\rm GeV}$ and $M_{+-}\le0.560~{\rm GeV}$, where the tight-contour median estimator reproduces $g_{\phi3π}^{\rm est}\simeq -24.0~{\rm GeV}^{-3}$, a closure recovery of the injected coupling within the chosen FSI-improved convention rather than an independent extraction. We further vary the mass-window selection of the branch. The median remains close to the injected value under shifts of the $M_{+0}$ window and moderate changes of its width, while the mean and standard deviation are more sensitive to outlier points near unstable quadratic branches. The method therefore identifies Dalitz-plane regions where the direct-term interference is locally well-conditioned and provides useful guidance for control-region choices and systematic variations in future global fits.

hep-ph↗
arXiv2026

Three-body decay $ϕ\toπ^+π^-π^0$ with Omnès-type final-state interactions

We investigate the decay $ϕ\toπ^+π^-π^0$ in an effective-Lagrangian framework that keeps the dominant $ρπ$ mechanism, the direct three-pion term, and a real constant background contribution explicitly separated at the amplitude level. The resonant contribution is described with the Gounaris-Sakurai propagator, the neutral channel includes $ρ^0$-$ω$ mixing, and the leading elastic $I=1$, $P$-wave $ππ$ final-state interaction is incorporated through the complex, $s$-dependent Omnès function $Ω_1(s+i0)$. The Omnès factor is applied channel by channel to the three two-pion subsystems, while the direct amplitude is dressed by the averaged factor $Ω_{\rm dir}=(Ω_0+Ω_++Ω_-)/3$. To avoid double-counting the elastic rescattering already contained in the physical $ρ\toππ$ coupling, we introduce $g_{ρππ}^{\rm bare}=g_{ρππ}^{\rm phys}/|Ω_1(m_ρ^2)|$ in the Omnès-dressed $ρ$-exchange amplitude. With the final parameter set used in this work, we obtain $Γ_{\rm th}=0.6915~{\rm MeV}$, slightly above the empirical estimate $Γ_{\rm exp}\approx0.660\pm0.020~{\rm MeV}$, while the direct integrated weight is reproduced at the KLOE scale, $I_{\rm dir}=8.393\times10^{-3}$. The corresponding resonant weight is $I_{ρπ}=0.9619$. The resulting Dalitz distribution retains the expected $ρπ$-dominated three-band structure and shows the localized neutral-channel deformation induced by $ρ^0$-$ω$ mixing. Remaining discrepancies in the $x$ and especially the $y$ projections indicate the limitations of the present minimal single-channel Omnès implementation and motivate a next-stage analysis based on a full dispersive crossed-channel treatment and a direct fit to the efficiency-corrected KLOE Dalitz-bin data.

hep-ph↗
arXiv2026

JLab and J-PARC for the J/{\ensuremathψ} Production at the Threshold

New threshold measurements for $γ~p\to p~J/ψ\to p~(μ^+μ^-)$ by 007 and $γ^\ast~p\to p~J/ψ\to p~(e^+e^-)$ by CLAS12 allow us to extend the previous phenomenological determination of the J/\ensuremathψ-proton scattering length, $α_{J/ψp}$, using GlueX threshold data for $γ~p\to p~J/ψ\to p~(e^+e^-)$. The agreement between all three J/\ensuremathψ data sets shows no indication of systematic differences between methodologies. Furthermore, perturbative QCD predictions support the phenomenological determination of heavy vector meson-nucleon scattering lengths. The role of the nucleon form factor, $F_N$, is discussed, and a possible correction to the phenomenological scattering length for heavy vector meson photoproduction is calculated using the pole form of $F_N$. Upcoming J-PARC threshold measurements of the reaction $π^-~p\to n~J/ψ\to n~(l^+l^-)$ will help to evaluate the possible role of heavy pentaquark, $P_c$, states in low-energy $J/ψ$ production and the effects caused by the nucleon form factors.

hep-ph↗
arXiv2025

Double $ϕ$-meson production via $\bar{p}p$ scattering

We investigate double $ϕ$ production in $\bar{p}p$ reactions near threshold using an effective Lagrangian approach, highlighting a significant OZI rule violation through hadronic processes. Our model includes $t$- and $u$-channel contributions from the nucleon and $s$-wave $N^*$ resonances ($N^*(1535,1650,1895)$), as well as the pentaquark-like state $P_s(2071)$. In the $s$-channel, scalar ($f_0$), tensor ($f_2$), and pseudoscalar ($η(2225)$) mesons are incorporated. The $N$, $N^*$, $f_0$, and $f_2$ contributions enhance the cross-section and generate peak structures near $W=2.2$ GeV, consistent with JETSET data, while cusp structures arise from $\barΛΛ$ and $\barΣΣ$ threshold openings. Polarization observables, such as the spin density matrix element (SDME), further elucidate the underlying hadronic mechanisms.

hep-ph↗
arXiv2025

Cross section Measurements for $^{12}$C$(K^-, K^+Ξ^-)$ and $^{12}$C$(K^-, K^+ΛΛ)$ Reactions at 1.8 GeV$/c$

We present a measurement of the production of $Ξ^-$ and $ΛΛ$ in the $^{12}$C$(K^-, K^+)$ reaction at an incident beam momentum of 1.8 GeV/$\mathit{c}$, based on high-statistics data from J-PARC E42. The cross section for the $^{12}$C$(K^-, K^+Ξ^-)$ reaction, compared to the inclusive $^{12}$C$(K^-, K^+)$ reaction cross section, indicates that the $Ξ^-$ escaping probability peaks at 70\% in the energy region of $E_Ξ=$100 to 150 MeV above the $Ξ^-$ emission threshold. A classical approach using eikonal approximation shows that the total cross sections for $Ξ^-$ inelastic scattering ranges between 42 mb and 23 mb in the $Ξ^-$ momentum range from 0.4 to 0.6 GeV/c. Furthermore, based on the relative cross section for the $^{12}$C$(K^-, K^+ΛΛ)$ reaction, the total cross section for $Ξ^-p\toΛΛ$ is estimated in the same approach to vary between 2.2 mb and 1.0 mb in the momentum range of 0.40 to 0.65 GeV/c. Specifically, a cross section of 1.0 mb in the momentum range of 0.5 to 0.6 GeV/c imposes a constraint on the upper bound of the decay width of the $Ξ^-$ particle in infinite nuclear matter, revealing $Γ_Ξ< \sim 0.6$ MeV.

nucl-ex↗
arXiv2025

Proposal of the KOTO II experiment

The KOTO II experiment is proposed to measure the branching ratio of the decay $K_L\toπ^0ν\barν$ at J-PARC. With a beamline to extract long-lived neutral kaons at 5 degrees from a production target, the single event sensitivity of the decay is $8.5\times 10^{-13}$, which is much smaller than the Standard Model prediction $3\times 10^{-11}$. This allows searches for new physics beyond the Standard Model and the first discovery of the decay with a significance exceeding $5σ$. As the only experiment proposed in the world dedicated to rare kaon decays, KOTO II will be indispensable in the quest for a complete understanding of flavor dynamics in the quark sector. Moreover, by combining efforts from the kaon community worldwide, we plan to develop the KOTO II detector further and expand the physics reach of the experiment to include measurements of the branching ratio of the $K_L\toπ^0\ell^+\ell^-$ decays, studies of other $K_L$ decays, and searches for dark photons, axions, and axion-like particles. KOTO II will therefore obtain a comprehensive understanding of $K_L$ decays, providing further constraints on new physics scenarios with existing $K^+$ results.

hep-ex↗
arXiv2023

Double-strangeness exchange reactions in a hybrid Regge-plus-resonance approach

We investigate double-strangeness exchange reactions, $K^-p\to K^+Ξ^-$ and $K^-p\to K^0Ξ^0$, using an effective Lagrangian approach based on a hybrid Regge-plus-resonance model involving rescattering diagrams. We consider the background processes that include $Λ$, $Σ$, and $Σ(1385)$ Regge trajectories in the $u$ channel and the $s$-channel Born-term diagrams. The $s$-channel hyperon resonances account for the bump structures in the total cross sections. The $s$-channel $Λ(2100,7/2^-)$ and $Σ(2030,7/2^+)$ resonances contribute significantly, and the $Σ(2250)$ resonance favors $J^P = 7/2^-$. The interference pattern between the $Λ(2100,7/2^-)$ and $Σ(2030,7/2^+)$ amplitudes is essential for describing forward differential cross sections in the resonance region $2.07 < \sqrt{s} < 2.15$ GeV. The absence of the $S=-2$ meson leads to the inclusion of the meson-baryon rescattering diagrams to describe the $K^- p \to KΞ$ reaction data, proving the significant contribution of the $(ϕ,ρ,ω)$--$(Λ,Σ)$ rescattering diagrams.

nucl-th↗
arXiv2021

Double-strangeness production in $Λp\to K^+X$ reaction

We investigate $S=-2$ production from the $Λp\to K^+X$ reactions within the effective Lagrangian approach. The $Λp\to K^+ΛΛ$ and $Λp\to K^+Ξ^-p$ reactions are considered to find the lightest $S=-2$ system, which is $H$-dibaryon. We assume that the $H(2250)\toΛΛ$, and $H(2270)\toΞ^-p$ decays with the intrinsic decay width of 1 MeV. According to our calculations, the total cross-sections for $Λp\to K^+ΛΛ$ and $Λp\to K^+Ξ^-p$ reactions were found to be of the order of a few $μ$b in the $Λ$ beam momentum range of up to 5 GeV$/c$. Furthermore, the direct access of information regarding the interference patterns between the $H$-dibaryon and non-resonant contributions was demonstrated.

hep-ph↗
arXiv2019

Hyperon production in $Λ_c^+\to K^-pπ^+$ and $Λ_c^+\to K^0_Spπ^0 $

We investigate $S=-1$ hyperon production from the $Λ_c^+\to K^-pπ^+$ and $Λ_c^+\to K^0_Spπ^0$ decays within the effective Lagrangian approach. We consider the $Σ/Λ$ ground states, $Λ(1520)$, $Λ(1670)(J^p=1/2^-)$, $Λ(1890)(J^p=3/2^+)$; $Λ/Σ$-pole contributions from the combined resonances between 1800 MeV and 2100 MeV; and $N/Δ$-pole and $K^\ast$-pole contributions, which include the proton, $Δ(1232)$, and $K(892)$. We calculate the Dalitz plot density $(d^2Γ/dM_{K^-p}dM_{K^-π^+}$) for the $Λ_c^+\to K^-pπ^+$ decay. The calculated result is in good agreement with experimental data from the Belle Collaboration. Using the parameters from the fit, we present the Dalitz plot density for the $Λ_c^+\to K^0_Spπ^0$ decay. In our calculation, a sharp peak-like structure near 1665 MeV is predicted in the $Λ_c^+\to K^-pπ^+$ decay because of the interference effects between the $Λ(1670)$ resonance and $η$-$Λ$ loop channels. We also demonstrate that we can access direct information regarding the weak couplings of $Λ(1670)$ and $Σ(1670)$ from the $Λ_c^+\to K^0_Spπ^0$ decay. Finally, a possible interpretation for the 1665 MeV structure beyond our prediction is briefly discussed.

hep-ph↗
arXiv2019

$Ξ(1690)^-$ resonance production via $K^-p \to K^+K^-Λ$

In this talk, we investigate $Ξ(1690)^-$ production from the $K^-p\to K^+K^-Λ$ reaction wit the effective Lagrangian method and consider the $s$- and $u$-channel $Σ/Λ$ ground states and resonances for the $Ξ$-pole contributions, in addition to the $s$-channel $Λ$, $u$-channel nucleon pole, and $t$-channel $K^-$-exchange for the $ϕ$-pole contributions. The $Ξ$-pole includes $Ξ(1320)$, $Ξ(1535)$, $Ξ(1690)(J^p=1/2^-)$, and $Ξ(1820)(J^p=3/2^-)$. We compute the Dalitz plot density of $(d^2σ/dM_{K^+K^-}dM_{K^-Λ}$ at 4.2 GeV$/c$) and the total cross sections for the $K^-p\to K^+K^-Λ$. Employing the parameters from the fit, we present the cross sections for the two-body $K^-p\to K^+Ξ(1690)^-$ reaction near the threshold. We also demonstrate that the Dalitz plot analysis for $p_{K^-}=1.915 \sim2.065$ GeV/c makes us to explore direct information for $Ξ(1690)^-$ production, which can be done by future $K^-$ beam experiments.

hep-ph↗
arXiv2018

$Ξ(1690)^-$ production in the $K^-p\to K^+K^-Λ$ reaction process near threshold

We investigate $Ξ(1690)^-$ production from the $K^-p\to K^+K^-Λ$ reaction within the effective Lagrangian approach at the tree-level Born approximation. We consider the $s$- and $u$-channel $Σ/Λ$ ground states and resonances for the $Ξ$-pole contributions, in addition to the $s$-channel $Λ$, $u$-channel nucleon pole, and $t$-channel $K^-$-exchange for the $ϕ$-pole contributions. The $Ξ$-pole includes $Ξ(1320)$, $Ξ(1535)$, $Ξ(1690)(J^p=1/2^-)$,and $Ξ(1820)(J^p=3/2^-)$. We calculate the Dalitz plot density of $(d^2σ/dM_{K^+K^-}dM_{K^-Λ}$ at 4.2 GeV$/c$) and the total cross sections for the $K^-p\to K^+K^-Λ$ reaction near the threshold. The calculation results are in good agreement with previously acquired experimental data. Using the parameters from the fit, we present the total and differential cross sections for the two-body $K^-p\to K^+Ξ(1690)^-$ reaction near the threshold. In our calculation, a strong enhancement at backward $K^+$ angles is predicted because of the dominant $u$-channel contribution. We also demonstrate that the Dalitz plot analysis for $p_{K^-}=1.915 -- 2.065$ GeV/c enables us to access direct information regarding $Ξ(1690)^-$ production, which can be tested by future $K^-$ beam experiments. The possible spin-parity states of $Ξ(1690)^-$ are briefly discussed as well.

hep-ph↗