arXiv subjects

J. K. Ahn

Publications and source records attributed to J. K. Ahn.

At least 19 recordsLinked to original sources

arXiv2026

Direct measurement of the in-medium $η^{\prime}$ mass spectrum through the $γγ$ decay channel

We measured the invariant mass of the $η^{\prime}(958)$ meson through the $η^{\prime}\toγγ$ decay channel to search for possible character change of $η^{\prime}$ in a nuclear medium. The measured invariant-mass spectra were analyzed by fitting realistic spectral functions. An enhancement was observed in the lower tail of the $η^{\prime}$ mass peak for low-momentum ($P_{γγ} < 1$~$\mathrm{GeV/c}$) events. Three statistical methods were applied to assess the significance of the enhancement. All methods yield a statistical significance above 3.7$σ$. The spectral fitting indicates a mass reduction of $57.5^{+~5.7}_{-27.8}$~$\mathrm{MeV/c}^{2}$. Our result provides the first evidence for an in-medium spectral modification of the $η^{\prime}$ meson obtained through a direct measurement of its invariant mass.

nucl-ex↗

arXiv2026

Evidence for a $\bar{K}NN$ quasi-bound state in the $γd \to K^0Λp$ reaction

The $γd \to K^{0}Λp$ reaction has been studied to search for a $\bar{K}NN$ quasi-bound state using the LEPS2 solenoid spectrometer at SPring-8. A localized enhancement concentrated at low $q$ is observed in the $(M_{Λp},q)$ distribution below the $K^-pp$ mass threshold, where $M_{Λp}$ is the $Λp$ invariant mass and $q \equiv |\vec{p}_γ-\vec{p}_{K^{0}}| = |\vec{p}_{Λp}|$ is the momentum transfer. A two-dimensional $(M_{Λp},q)$ fit demonstrates that the enhancement near the threshold is statistically significant, yielding a local significance of $7.3,σ$. The enhancement is characterized by effective shape parameters including the Breit--Wigner mass $M$ and width $Γ$, and a Gaussian form-factor momentum scale $Q$: $M = 2.354 \pm 0.011(\mathrm{stat.})^{+0.009}_{-0.005}(\mathrm{syst.})~\mathrm{GeV}/c^{2}$, $Γ= 0.055 \pm 0.023(\mathrm{stat.})^{+0.031}_{-0.009}(\mathrm{syst.})~\mathrm{GeV}/c^{2}$, and $Q = 0.350 \pm 0.041(\mathrm{stat.})^{+0.035}_{-0.010}(\mathrm{syst.})~\mathrm{GeV}/c$. The obtained results support the existence of a $\bar{K}NN$ quasi-bound state in photoproduction.

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arXiv2026

Observation of the decays $B^{+} \to Σ_{c}(2455)^{++} \barΞ_{c}^{\prime-}$ and $B^{0} \to Σ_{c}(2455)^{0} \barΞ_{c}^{\prime0}$

We report the first observation of the decays $B^{+} \to Σ_{c}(2455)^{++} \barΞ_{c}^{\prime-}$ and $B^{0} \to Σ_{c}(2455)^{0} \barΞ_{c}^{\prime0}$, with significances of $6.4\,σ$ and $5.3\, σ$, respectively, including systematic uncertainties. This analysis is based on data samples containing $771.6 \times 10^{6}$ $Υ(4S)$ decays collected with the Belle detector at the KEKB collider and $520.6 \times 10^{6}$ $Υ(4S)$ decays collected with the Belle~II detector at the SuperKEKB collider. The branching fractions are measured to be $\mathcal{B}(B^+ \to Σ_c(2455)^{++} \barΞ_c^{\prime -}) = (1.68 \pm 0.31 \pm 0.12^{+1.49}_{-0.54}) \times 10^{-3}$ and $\mathcal{B}(B^0 \to Σ_c(2455)^{0} \barΞ_c^{\prime 0}) = (1.28 \pm 0.32 \pm 0.10^{+0.30}_{-0.21}) \times 10^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third arises from the uncertainties in the absolute branching fractions of $\barΞ_{c}^{-}$ and $\barΞ_{c}^{0}$ decays. This result represents the first observation of $B$-meson decays into a pair of charmed baryon-antibaryon states belonging to the same $SU(3)$ flavor sextet.

hep-ex↗

arXiv2026

The Search for $K_L \rightarrow π^0π^0γγ$ and $K_L\rightarrow π^0π^0X$ where $X\rightarrow 2γ$ at the KOTO Experiment

We performed searches for $K_L\rightarrow π^0π^0X$ where $X$ may be an axion-like particle which promptly decays to two photons, and the first search for $K_L \rightarrow π^0π^0γγ$ at the KOTO experiment using data taken in 2021. The search is performed for $X$ mass in the range of 160$\unicode{x2013}$220 MeV/$c^2$. Three events were observed in the signal region, with two events near an $X$ mass of 177 MeV/$c^2$. This result led to a range of upper limits on the branching ratio, BR($K_L\rightarrow π^0π^0X$) $< (1\unicode{x2013}20) \times 10^{-7}$ at the 95% confidence level (C.L.). No events were observed for the analysis of $K_L \rightarrow π^0π^0γγ$, setting an upper limit on the branching ratio, BR($K_L \rightarrow π^0π^0γγ$) $< 1.69 \times 10^{-6}$ at the 95% C.L.

hep-ex↗

arXiv2026

Comprehensive measurement of $η^\prime$ photoproduction off the proton at $E_γ< 2.4$ $\mathrm{GeV}$

For the spectroscopy of nucleon resonances at the total energies from the $η^\prime$-meson production threshold to $2.32$ $\mathrm{GeV}$, photon beam asymmetries of the reaction $γp \to η^\prime p$ were measured together with total and differential cross sections by analyzing the two decay modes $η^\prime \to γγ$ and $π^0 π^0 η$. New constraints for amplitude decomposition were given by the first-time result of photon beam asymmetries at $E_γ> 1.84$ $\mathrm{GeV}$ and the most precise data of differential cross sections to date at extremely backward $η^\prime$ angles. The possibility of a larger coupling constant of the $η^\prime$-nucleon system to the $N(2250)$ resonance was implied in the partial wave analyses using the present data.

nucl-ex↗

arXiv2025

Measurement of $Λ$ Polarization in the $π^{-}p \to K^{0} Λ$ Reaction at $p_{π^{-}}=1.33$ GeV/$c$ toward a New $Λp$ Scattering Experiment

This paper presents high-precision experimental data of the polarization of the $Λ$ hyperon in the $π^{-}p \to K^{0} Λ$ reaction, measured in the angular range $0.6<\cos θ^{CM}_{K0}<1.0$ with a fine bin width of $d\cos θ^{CM}_{K0}=0.05$. The data were obtained from the J-PARC E40 experiment at the K1.8 beamline in the J-PARC Hadron Experimental Facility. The observed average polarization of $Λ$ in the range $0.60<\cos θ^{CM}_{K0}<0.85$ was $0.932 \pm 0.058 \,(\text{stat}) \pm 0.028 \,(\text{syst})$, demonstrating the successful extraction of precise polarization observables. This result provides essential experimental input for partial wave analysis (PWA) of dynamical coupled-channel (DCC) models, which aim to uncover the underlying mechanisms of $N^{*}$ resonances that emerge in intermediate states of $πN$ and $γN$ interactions. Besides, it indicates the feasibility of a strongly polarized $Λ$ beam suitable for future $Λp$ scattering experiments (e.g., J-PARC E86).

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arXiv2025

Measurement of the $Λ_c^+$ lifetime

An absolute measurement of the $Λ^{+}_c$ lifetime is reported using $Λ_c^+\rightarrow pK^-π^+$ decays in events reconstructed from data collected by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. The total integrated luminosity of the data sample, which was collected at center-of-mass energies at or near the $Υ(4S)$ resonance, is $207.2~\mbox{fb}^{-1}$. The result, $τ(Λ^{+}_c) = 203.20 \pm 0.89 \,\mathrm{(stat)} \pm 0.77 \,\mathrm{(syst)}$ fs, is the most precise measurement to date and is consistent with previous determinations.

hep-ex↗

arXiv2025

Search for a pentaquark state decaying into $pJ/ψ$ in $Υ(1,2S)$ inclusive decays at Belle

Using the data samples of 102 million $Υ(1S)$ and 158 million $Υ(2S)$ events collected by the Belle detector, we search for a pentaquark state in the $pJ/ψ$ final state from $Υ(1,2S)$ inclusive decays. Here, the charge-conjugate $\bar{p}J/ψ$ is included. We observe clear $pJ/ψ$ production in $Υ(1,2S)$ decays and measure the branching fractions to be $B[Υ(1S) \to pJ/ψ+ anything] = [8.1 \pm 0.6(stat.) \pm 0.5(syst.)] \times 10^{-5}$ and $B[Υ(2S) \to pJ/ψ+ anything] = [4.3 \pm 0.5(stat.) \pm 0.4(syst.)] \times 10^{-5}$. We also measure the cross section of inclusive $pJ/ψ$ production in $e^+e^-$ annihilation to be $σ(e^+e^- \to pJ/ψ+ anything) = [108 \pm 11 (stat.) \pm 6(syst.)]$~fb at $\sqrt{s} = 10.52~\hbox{GeV}$ using an 89.5~fb$^{-1}$ continuum data sample. There is no significant $P_c(4312)^+$, $P_c(4440)^+$ or $P_c(4457)^+$ signal found in the $pJ/ψ$ final states in $Υ(1,2S)$ inclusive decays. We determine the upper limits of $B[Υ(1,2S)\to P_c^{+} + anything] \cdot B(P_c^{+}\to pJ/ψ)$ to be at the $10^{-6}$ level.

hep-ex↗

arXiv2025

Measurements of the branching fractions of $Ξ_{c}^{+}\to Σ^{+}K_{S}^{0}$, $Ξ_{c}^{+}\to Ξ^{0}π^{+}$, and $Ξ_{c}^{+}\to Ξ^{0}K^{+}$ at Belle and Belle II

Using 983.0 $\rm{fb}^{-1}$ and 427.9 $\rm{fb}^{-1}$ data samples collected with the Belle and Belle II detectors at the KEKB and SuperKEKB asymmetric energy $e^+e^-$ colliders, respectively, we present studies of the Cabibbo-favored $Ξ_c^+$ decays ${Ξ_{c}^{+}\to Σ^{+}K_{S}^{0}}$ and $Ξ_{c}^{+}\to Ξ^{0}π^{+}$, and the singly Cabibbo-suppressed decay $Ξ_{c}^{+}\to Ξ^{0}K^{+}$. The ratios of branching fractions of ${Ξ_{c}^{+}\to Σ^{+}K_{S}^{0}}$ and $Ξ_{c}^{+}\to Ξ^{0}K^{+}$ relative to that of $Ξ_{c}^{+}\toΞ^{-}π^{+}π^{+}$ are measured for the first time, while the ratio ${\cal B}(Ξ_{c}^{+}\toΞ^{0}π^{+})/{\cal B}(Ξ_{c}^{+}\toΞ^{-}π^{+}π^{+}) $ is also determined and improved by an order of magnitude in precision. The measured branching fraction ratios are $\frac{\cal{B}(Ξ_{c}^{+} \to Σ^{+}K_{S}^{0})}{\cal{B}(Ξ_{c}^{+}\to Ξ^{-}π^{+}π^+)}= 0.067 \pm 0.007 \pm 0.003$, $\frac{\cal{B}(Ξ_c^{+} \to Ξ^{0}π^{+})}{\cal{B}(Ξ_{c}^{+}\to Ξ^{-}π^{+}π^+)} = 0.251 \pm 0.005 \pm 0.010$, $\frac{\cal{B}(Ξ_c^{+} \to Ξ^{0}K^{+})}{\cal{B}(Ξ_{c}^{+}\to Ξ^{-}π^{+}π^+)} = 0.017 \pm 0.003 \pm 0.001$. Additionally, the ratio ${\cal B}(Ξ_{c}^{+}\toΞ^{0}K^{+})/{\cal B}(Ξ_{c}^{+}\toΞ^{0}π^{+})$ is measured to be $ 0.068 \pm 0.010 \pm 0.004$. Here, the first and second uncertainties are statistical and systematic, respectively. Multiplying the ratios by the branching fraction of the normalization mode, ${\mathcal B}(Ξ_{c}^{+}\toΞ^{-}π^{+}π^+)= (2.9\pm 1.3)\%$, we obtain the following absolute branching fractions ${\cal B}(Ξ_{c}^{+}\toΣ^{+}K^{0}_{S}) = (0.194 \pm 0.021 \pm 0.009 \pm 0.087 )%$, ${\cal B}(Ξ_{c}^{+}\toΞ^{0}π^{+}) = (0.728 \pm 0.014 \pm 0.027 \pm 0.326 )%$, ${\cal B}(Ξ_{c}^{+}\toΞ^{0}K^{+}) = (0.049 \pm 0.007 \pm 0.003 \pm 0.022 )%$.

hep-ex↗

arXiv2025

Search for Pcs(4459) and Pcs(4338) in Upsilon(1S,2S) inclusive decays at Belle

Using data samples of 102 million Upsilon(1S) events and 158 million Upsilon(2S) events collected by the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider, we search for [udsccbar] pentaquark states decaying to Jpsi Lambda. Using the first observations of Upsilon(1S, 2S) inclusive decays to Jpsi Lambda, we find evidence of the P_ccbars(4459)0 state with a local significance of 3.3 standard deviations, including statistical and systematic uncertainties. We measure the mass and width of the Pccbars(4459)0 to be (4471.7 +- 4.8 +- 0.6) MeV/c2 and (21.9 +- 13.1 +- 2.7) MeV, respectively. The branching fractions for P_ccbars(4459)0 production are measured to be B[Upsilon(1S) -> P_ccbars(4459)0/ Pbar_ccbars(4459)0 + anything] = (3.5 +- 2.0 +- 0.2)*10-6 and B[Upsilin(2S) -> P_ccbars(4459)0/ Pbar_ccbars(4459)0 +anything] = (2.9 +- 1.7 +- 0.4)*10-6. The inclusive branching fractions of Upsilon(1S, 2S) -> Jpsi Lambda/Lambdabar are measured to be B[Upsilin(1S) -> Jpsi Lambda/Lambdabar + anything] = (36.9 +- 5.3 +- 2.4)*10-6 and B[Upsilon(2S) -> Jpsi Lambda/Lambdabar + anything] = (22.3 +- 5.7 +- 3.1)*10-6. We measure the visible cross section $σ(e^+e^- \to J/psi Λ/\barΛ$ + anything) = (90 +- 14 +- 6) fb for the continuum production at $\sqrt{s} = 10.52$ GeV. In all cases, the first uncertainties are statistical and the second are systematic.

hep-ex↗

arXiv2025

Experimental Study of Rare Kaon Decays at J-PARC with KOTO and KOTO II

The rare kaon decay $K_L\toπ^0ν\barν$ is extremely sensitive to new physics, because the contribution to this decay in the Standard Model (SM) is highly suppressed and known very accurately; the branching ratio is $3\times 10^{-11}$ in the SM with a theoretical uncertainty of just 2%. The measurement of this branching ratio could provide essential new information about the flavor structure of the quark sector from the $s\to d$ transition. The decay is being searched for in the KOTO experiment at J-PARC, which has obtained the current best upper limit on the branching ratio of $2.2\times 10^{-9}$; a sensitivity to branching ratios below $10^{-10}$ is achievable by the end of the decade. A next-generation experiment at J-PARC, KOTO II, was proposed in 2024 with 82 members worldwide, including significant contributions from European members. The goal of KOTO II is to measure the $K_L\toπ^0ν\barν$ branching ratio with sensitivity below $10^{-12}$ in the 2030s. Discovery of the decay with $5σ$ significance is achievable at the SM value of the branching ratio. An indication of new physics with a significance of 90% is possible if the observed branching ratio differs by 40% from the SM value. Another important goal of KOTO II is to measure the branching ratio of the unobserved $K_L\to π^0e^+e^-$ decay, which can give an input to flavor structures of new physics. Other rare $K_L$ decays and hidden-sector particles are also in the scope of the study. After 2026, KOTO will be the only dedicated rare kaon decay experiment in the world, and KOTO II is the only future rare kaon decay project currently proposed. We would like to lead a global initiative for the experimental study of rare kaon decays, with significant contributions and support from the European community.

hep-ex↗

arXiv2025

Measurement of the Branching Fraction of $Λ_c^+ \to p K_S^0 π^0$ at Belle

We report a precise measurement of the ratio of branching fractions $\mathcal{B}(Λ_c^+\to p K_S^0 π^0)/\mathcal{B}(Λ_c^+\to p K^- π^+)$ using 980 fb$^{-1}$ of $e^+e^-$ data from the Belle experiment. We obtain a value of $\mathcal{B}(Λ_c^+\to p K_S^0 π^0)/\mathcal{B}(Λ_c^+\to p K^- π^+)=0.339\pm 0.002\pm 0.009$, where the first and second uncertainties are statistical and systematic, respectively. This Belle result is consistent with the previous measurement from the CLEO experiment but has a fivefold improvement in precision. By combining our result with the world average $\mathcal{B}(Λ_c^+\to p K^- π^+)$, we obtain the absolute branching fraction $\mathcal{B}(Λ_c^+\to p K_S^0 π^0)=(2.12\pm 0.01\pm 0.05 \pm 0.10)\%$, where the uncertainties are statistical, systematic, and the uncertainty in the absolute branching fraction scale $\mathcal{B}(Λ_c^+\to p K^- π^+)$, respectively. This measurement can shed light on hadronic decay mechanisms in charmed baryon decays.

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arXiv2025

Prospects for Pentaquark Baryon Search with the Upgraded LEPS2 Facility

We present prospects for the $Θ^+$ pentaquark baryon search using the newly constructed LEPS2 facility at SPring-8. The LEPS2 detector system features significant improvements in acceptance for multi-particle final states compared to previous experiments. Our search employs two complementary strategies: direct production in the $γn \to K^-Θ^+$ reaction using a liquid deuterium target with a photon beam up to 2.4 GeV, and $\bar{K}^{*0}$-associated $Θ^+$ production using a liquid hydrogen target with a photon beam up to 2.9 GeV. The extended acceptance covers both forward and large angle regions, effectively spanning the kinematic regions explored by previous LEPS and CLAS experiments. The large acceptance and improved resolution of LEPS2, combined with these complementary approaches, provide unprecedented sensitivity for establishing the existence of the $Θ^+$ or placing definitive upper limits on its production.

hep-ex↗

arXiv2025

Search for the $K_{L} \to π^{0} ν\barν$ Decay at the J-PARC KOTO Experiment

We performed a search for the $K_L \to π^{0} ν\barν$ decay using the data taken in 2021 at the J-PARC KOTO experiment. With newly installed counters and new analysis method, the expected background was suppressed to $0.252\pm0.055_{\mathrm{stat}}$$^{+0.052}_{-0.067}$$_{\mathrm{syst}}$. With a single event sensitivity of $(9.33 \pm 0.06_{\rm stat} \pm 0.84_{\rm syst})\times 10^{-10}$, no events were observed in the signal region. An upper limit on the branching fraction for the decay was set to be $2.2\times10^{-9}$ at the 90% confidence level (C.L.), which improved the previous upper limit from KOTO by a factor of 1.4. With the same data, a search for $K_L \to π^{0} X^{0}$ was also performed, where $X^{0}$ is an invisible boson with a mass ranging from 1 MeV/$c^{2}$ to 260 MeV/$c^{2}$. For $X^{0}$ with a mass of 135 MeV/$c^{2}$, an upper limit on the branching fraction of $K_L \to π^{0} X^{0}$ was set to be $1.6\times10^{-9}$ at the 90% C.L.

hep-ex↗

arXiv2024

Observation of $Ω(2012)^- \to Ξ(1530)\bar{K}$ and measurement of the effective couplings of $Ω(2012)^-$ to $Ξ(1530)\bar{K}$ and $Ξ\bar{K}$

Using $Υ(1S)$, $Υ(2S)$, and $Υ(3S)$ data collected by the Belle detector, we discover a new three-body decay, $Ω(2012)^-\toΞ(1530)\bar K\toΞπ\bar K$, with a significance of 5.2~$σ$. The mass of the $Ω(2012)^-$ is $(2012.5\pm0.7\pm0.5)$ MeV and its effective couplings to $Ξ(1530)\bar{K}$ and $Ξ\bar{K}$ are $(39^{+31}_{-39}\pm9)\times10^{-2}$ and $(1.7\pm0.3\pm0.3)\times10^{-2}$, where the first uncertainties are statistical and the second are systematic. The ratio of the branching fraction for the three-body decay to that for the two-body decay to $Ξ\bar{K}$ is $0.99\pm0.26\pm0.06$, assuming isospin symmetry.

hep-ex↗

arXiv2024

Performance of the prototype beam drift chamber for LAMPS at RAON with proton and Carbon-12 beams

Beam Drift Chamber (BDC) is designed to reconstruct the trajectories of incident rare isotope beams provided by RAON (Rare isotope Accelerator complex for ON-line experiments) into the experimental target of LAMPS (Large Acceptance Multi-Purpose Spectrometer). To conduct the performance test of the BDC, the prototype BDC (pBDC) is manufactured and evaluated with the high energy ion beams from HIMAC (Heavy Ion Medical Accelerator in Chiba) facility in Japan. Two kinds of ion beams, 100 MeV proton, and 200 MeV/u $^{12}$C, have been utilized for this evaluation, and the track reconstruction efficiency and position resolution have been measured as the function of applied high voltage. This paper introduces the construction details and presents the track reconstruction efficiency and position resolution of pBDC.

physics.ins-det↗

arXiv2024

Measurement of the integrated luminosity of data samples collected during 2019-2022 by the Belle II experiment

A series of data samples was collected with the Belle~II detector at the SuperKEKB collider from March 2019 to June 2022. We determine the integrated luminosities of these data samples using three distinct methodologies involving Bhabha ($e^+e^- \to e^+e^-(nγ)$), digamma ($e^+e^- \to γγ(nγ)$), and dimuon ($e^+e^- \to μ^+ μ^- (nγ)$) events. The total integrated luminosity obtained with Bhabha, digamma, and dimuon events is ({426.88} $\pm$ 0.03 $\pm$ {2.61})~fb$^{-1}$, ({429.28} $\pm$ 0.03 $\pm$ {2.62})~fb$^{-1}$, and ({423.99} $\pm$ 0.04 $\pm$ {3.83})~fb$^{-1}$, where the first uncertainties are statistical and the second are systematic. The resulting total integrated luminosity obtained from the combination of the three methods is ({427.87 $\pm$ 2.01})~fb$^{-1}$.

hep-ex↗

arXiv2024

Search for charmed baryons in the $Λ_c^+η$ system and measurement of the branching fractions of $Λ_c(2880)^+$ and $Λ_c(2940)^+$ decaying to $Λ_c^+η$ and $pD^0$ relative to $Σ_c(2455)π$

We search for excited charmed baryons in the $Λ_c^+η$ system using a data sample corresponding to an integrated luminosity of 980 $\rm fb^{-1}$. The data were collected by the Belle detector at the KEKB $e^{+}$$e^{-}$ asymmetric-energy collider. No significant signals are found in the $Λ_c^+η$ mass spectrum, including the known $Λ_c(2880)^+$ and $Λ_c(2940)^+$. Clear $Λ_c(2880)^+$ and $Λ_c(2940)^+$ signals are observed in the $pD^0$ mass spectrum. We set upper limits at 90\% credibility level on ratios of branching fractions of $Λ_c(2880)^+$ and $Λ_c(2940)^+$ decaying to $Λ_c^+η$ relative to $Σ_c(2455)π$ of $<0.13$ for the $Λ_c(2880)^+$ and $<1.11$ for the $Λ_c(2940)^+$. We measure ratios of branching fractions of $Λ_c(2880)^+$ and $Λ_c(2940)^+$ decaying to $pD^0$ relative to $Σ_c(2455)π$ of $0.75 \pm 0.03(\text{stat.}) \pm 0.07(\text{syst.})$ for the $Λ_c(2880)^+$ and $3.59 \pm 0.21(\text{stat.}) \pm 0.56(\text{syst.})$ for the $Λ_c(2940)^+$.

hep-ex↗