Session F05: Minisymposium: Baryon-Baryon Interactions Involving Strangeness II

Constraining the repulsion of the Λ potential at high densities through Λ binding energy of hypernuclei

The hyperon puzzle in neutron stars, extensively discussed in recent decades, refers to the problem that most of the equations of state with hyperons are not sufficiently stiff to support the observed massive neutron stars. Although various solutions to the puzzle have been proposed, the presence or absence of hyperons in neutron stars remains uncertain. The Λ potential at high densities is a key ingredient in discussing the presence of Λ hyperons, but it is not well constrained by the available experimental data.

High-resolution data of the Λ binding energy in hypernuclei will become available in future experiments at J-PARC. In this presentation, we explore the possibility of constraining the repulsion of Λ potential at high densities using the Λ binding energy data. We parametrize the Λ potential by the Taylor coefficients and the effective mass at the saturation density. We then vary the parameters, calculate the Λ binding energy using the Skyrme-Hartree-Fock method, and compare the results with the available data. We found that the repulsive (attractive) Λ potentials at high densities are favored when the depth of the Λ potential at saturation density, J_Λ, is J_Λ ≥ -29 MeV (J_Λ ≤ -31 MeV). Since the value of J_Λ is sensitive to the Λ binding energy, this result suggests that future high-resolution data could exclude the scenario in which Λ hyperons appear in neutron stars.   

Exclusive K+Λ and K+Σ0 electroproduction at Q2≅0.5 (GeV/c)2

We measured the differential cross section for the p(e,e'K⁺)Λ/Σ⁰ reaction at Jefferson Lab in 2018 [1,2]. This dataset is at four-momentum transfer Q²≅0.5 (GeV/c)² ,W≅2.14 GeV, θ_γK^c.m.≅8 deg. This kinematic condition has never been covered by other experiments. In this talk, we will report the differential cross section for the extracted virtual photoproduction, p(γ*,K⁺)Λ/Σ⁰ reaction, to compare with the photoproduction.

The new results were compared with predictions of isobaric models which describe the hyperon production reaction with hadron degrees of freedom. In particular, BS models [3] developed by Bydžovský and Skoupil were optimized with extensive photoproduction experimental data reported by CLAS and LEPS in 2010 and 2006, respectively.The predictions of the hyperon elecetroproduction with the models were in good agreement with our new results. According to the BS models, the longitudinal couplings of the virtual photon had a significant contribution. This cannot be investigated by the photoproduction, but can be approached only by the electroproduction. Our new results can help to determine the magnitude of the longitudinal couplings at relatively high-Q² region.

[1] B. Panday et al., PRC 105 (2022) L051001.

[2] K.N. Suzuki et al., PTEP 013D01 (2022).

[3] D. Skoupil and P. Bydžovský, PRC 93 (2016) 025204; PRC 97 (2018) 025202.   

Experimental study of Λn interaction via FSI effect in the γ+d reaction at ELPH, Spring-8, and JLab.

The Λn interaction is the last  key of the study of Baryon-Baryon interaction. Since the Σp/Λp interaction study is developed with the modern technique in J-PARC, we need to make the Λn interaction clear to solve the problem of the charge symmetry braking in the Lambda-Nucleon (ΛN) interaction. It is difficult to make Lambda and neutron be a target and a beam so that the Final-State-Interaction (FSI).

We proposed and are preparing an experiment of Λn interaction measurement via FSI in the γ+d→K⁺+Λ+n reaction by using NKS2 spectrometer at ELPH, Tohoku University. As further experiments, we have plans to investigate ΣN-ΛN coupling effect by polarization observables as a function of K⁺ angle/momentum [1] and a cusp in the differential cross section of K⁺ as a function of momentum [2] in γ+d→K⁺+X reaction. The first one will be proposed as an experiment at LEPS2, Spring-8 and we are planning the second one using HES and HKS spectrometers at Hall C, JLab.  I will introduce those experimental plans and show the feasibility of the measurement in this talk.

 

[1] K. Miyagawa, T. Mart, C. Bennhold, and W. Glöckle, Phys. Rev. C74 (034002 (2006)

[2] H. Yamamura, K. Miyagawa, T. Mart, C. Bennhold, H. Habrezetil, and W. Glöckle, Phys. Rev. C61. 014001 (1999)   

The upcoming experiments on electroproduction of Lambda hypernuclei at JLab

The JLab hypernuclear collaboration is currently preparing for the upcoming hypernuclear spectroscopy using electromagnetic reaction at JLab in the US. We, the JLab hypernuclear collaboration, have developed the missing mass spectroscopy of Lambda hypernuclei at JLab for decades[1]. We have achieved high energy resolution of around 500 keV(FWHM) for light hypernuclei and high accuracy of absolute binding energy better than 100 keV[2]. In the next experiment, we plan to investigate the binding energy of ⁴⁰_ΛK and ⁴⁸_ΛK to extract the isospin dependency of ΛNN three-body force[3]. Before conducting the experiment, it is important to estimate the expected hypernuclear spectra quantitatively and thus, I performed a simulation under the latest experimental conditions at JLab Hall-C.In this study, I modeled a realistic experimental setup, and evaluated the resolution and signal-to-noise ratio of the expected spectra. I will present the simulation results and the prospects for the experiment will be discussed based on the simulation results.

[1] T. Miyoshi et al., Phys. Rev. Lett. 90, 232502 (2003).

[2] L. Tang et al., Phys. Rev. C 90, 034320 (2014).

[3] S.N. Nakamura et al., JLab proposal for E12-15-008(2015).   

Design of a new experiment for measuring the β-decay rate of Λ in hypernucleus

The baryon properties may be modified in nuclear matter. We consider that the baryon modification can be clarified by measuring the β-decay rate of Λ hypernucleus(Γ_β). According to the QMC model, Γ_β decreases by 20% at max in nuclear matter.

 This experiment is planned at the J-PARC K1.1 beam line. We will produce ⁵ΛHe and measure Γ_β within 4.5% accuracy . The branching ratio of the Λ’s β-decay (~8.0×10^-4) is much smaller than that of the Λ’s main decay (Λ→pπ^- and Λ→nπ⁰ and nonmesonic decay), and therefore huge background will be a severe problem. To reduce this, the target is surrounded by a 4π BGO calorimeter, and the β-decay electron is identified by the number of hit clusters in the BGO. The β-decay electron produces one-cluster hit in the BGO. The background π^- stops in BGO and emits several neutrons after nuclear absorption. Therefore, we expect that it produces two or more hit clusters. The Background reduction is estimated by GEANT4 simulation. But there is no accurate data for stopped π^- absorption in BGO nuclei. Therefore, we need to measure neutron data from stopped π^- absorption in BGO.

To measure the data, we will use π^- from K^- beam decay at the J-PARC. In this talk, how to conduct this measurement and simulation results of this measurement will be reported.   

Development of a High-Precision Electron Beam Measurement Method in the Several Hundred MeV Range Using Undulator Synchrotron Radiation Interferometry

We have pioneered the decay pion spectroscopy of electroproduced light Λ hypernuclei at the Mainz Microtron (MAMI), with a particular focus on ³_ΛH (Hypertriton) in latest experiment. In this method, high-precision magnetic spectrometers achieved Δp/p ∼ 10^-4  momentum resolution for 130 MeV/c pion [1]. Despite high precision, large systematic uncertainty has to be addressed. In the spectrometer calibration through elastic electron scattering at several hundred MeV, the absolute calibration error of the electron beam energy is a crucial factor limiting the overall experimental uncertainty.

To resolve this, we developed a novel technique using interference of synchrotron radiations from two undulators along the electron beam [2]. The phase shift of two pulses is caused by electron beam delay with respect to radiation so we can extract energy information from interference. By data-taking with a CMOS camera, we demonstrate the measurement precision Δγ/γ ∼ 10^-5.

In this talk I will report the progress of development and practical aspects of this method.

[1] A. Esser, S.Nagao, F.Schulz et al., (A1 Collaboration) PRL. 114, 232501(2015)

[2] P.Klag et.al., NIM A 910, 147 (2018)   

Development of a new TOT-ASIC circuit for SiPM with a good time resolution

Detectors with SiPMs and plastic scintillators have been widely used as timing counters. Discrete circuits with a fast amplifier are well applied and are achieved <100 ps time-resolution [1]. Since these circuits require large currents, development of miniaturized circuitry is essentially important for power-saving.

We are developing a new ASIC-based circuit to read out signals from Time-Of-Flight detectors combining a plastic scintillator and SiPM (Hamamatsu S13360-3050PE). This ASIC development is based on established TOF-PET ASIC, which obtains timing and charge information with Time Over Threshold (TOT) method [2]. We tested the circuit's performance with electrons from the accelerator at ELPH Tohoku. This new ASIC circuit is expected to be widely applicable not only to particle and nuclear experiments but also to space and medical applications.

I will present the results of the performance test of the TOF-PET ASIC with a plastic-scintillator paddle and the progress of the new ASIC development.

[1] 10.1109/TNS.2014.2321657, http://dx.doi.org/10.1109/TNS.2014. 2347576, arXiv:1402.1404.

[2] T. Orita et al., Nucl. Inst. and Meth. A 912 (2018) 303.