We respond to the recent suggestion by A. Gal [arXiv:2604.18259] that the sharp pion-momentum peak at p_π^-≈ 113.8 MeV/c observed in our ^7Li(e,e^' K^+) electroproduction experiment at MAMI [Phys. Rev. Lett. 136, 152301 (2026)] originates from ^7_ΛHe weak decay rather than from ^3_ΛH→ π^- + ^3He as we reported. We present quantitative arguments against this interpretation and conclude that the ^3_ΛH assignment remains the most well-supported interpretation of the data.

Decay pion spectroscopy provides a direct and model-independent approach to the precise mass measurement of light hypernuclei, such as the hypertriton (3ΛH), through the monochromatic momentum of pions emitted in two-body weak decays. In this experiment at the Mainz Microtron (MAMI), decay pions from stopped hypernuclei produced in the p(e,e’K+)Λ reaction on a natural lithium target were measured with Spectrometer A (SpekA). Since the achievable precision of the hypertriton mass is directly determined by the accuracy of the pion momentum, this proceedings focuses on the momentum calibration essential for decay pion spectroscopy. A comprehensive calibration based on elastic electron scattering was performed using 181Ta and 12C targets. The reconstructed scattering angle, momentum linearity over the full spectrometer acceptance, and the dependence on the reaction point along the beam axis were systematically evaluated and corrected. These procedures constrained the systematic uncertainty of the elastic-scattering-based momentum calibration to below 10 keV, excluding the beam-energy contribution. The established calibration provides the foundation for a high-precision determination of the hypertriton mass using decay pion spectroscopy.

We performed high-precision decay-pion spectroscopy of light Λ hypernuclei at the Mainz Microtron (MAMI) using the A1 spectrometer facility. By measuring the monochromatic π^{-} momentum from the two-body weak decay _{Λ}^{3}H→^{3}He+π^{-} and referencing it to the _{Λ}^{4}H→^{4}He+π^{-} decay, we determined the Λ binding energy of _{Λ}^{3}H with unprecedented accuracy. The obtained value, B_{Λ}(_{Λ}^{3}H)=0.523±0.013_{stat}±0.075_{syst}.  MeV, is consistent with the STAR result, but indicates a significantly deeper binding than inferred from earlier measurements. This result implies a stronger Λ-deuteron interaction and provides stringent constraints on hyperon-nucleon interactions.

Experimental measurements of the radial spreading of photoelectrons emitted from a multi-alkali photocathode in a dedicated MCP-based photomultiplier tube have shown that, for photon wavelengths of 460 nm, 515 nm and 625 nm, the maximum initial energies of the emitted photoelectrons are approximately 0.3 eV, 0.2 eV and 0.1 eV respectively. Combining these experimental results with simulations performed using the SIMION simulation package, a compact radio-frequency photoelectron multiplier tube with a temporal resolution better than 10 ps is proposed. The device is suitable for applications in several fields, particularly in medical optical instruments employing time-correlated single-photon counting.

Decay pion spectroscopy provides a direct and model-independent approach to the precise mass measurement of light hypernuclei, such as the hypertriton ( ^3_Λ H), through the monochromatic momentum of pions emitted in two-body weak decays. In this experiment at the Mainz Microtron (MAMI), decay pions from stopped hypernuclei produced in the p(e,e'K^+)Λ reaction on a natural lithium target were measured with a high-resolution magnetic spectrometer, Spectrometer A (SpecA). Since the achievable precision of the hypertriton mass is directly determined by the accuracy of the pion momentum, this contribution focuses on the momentum calibration essential for decay pion spectroscopy. A comprehensive calibration based on elastic electron scattering was performed using ^181 Ta and ^12 C targets. The reconstructed scattering angle, momentum linearity over the full spectrometer acceptance, and the dependence on the reaction point along the beam axis were systematically evaluated and corrected. These procedures constrained the systematic uncertainty of the elastic-scattering-based momentum calibration to below 10 keV, excluding the beam-energy contribution. The established calibration procedure provides the foundation for a high-precision determination of the hypertriton mass relying on decay pion spectroscopy.

We present the design, modeling, and experimental validation of a radio-frequency based time-to-position conversion system for keV electrons incorporating a helical deflector operating in the 400-1000 MHz range. The device performs circular deflection of the electrons when driven by a single RF frequency and enables spiral scanning when two phase-locked RF voltages with slightly different frequencies are applied. The superposition of the two phase-locked RF voltages produces an amplitude-beating field whose slowly varying envelope modulates the deflection radius, transforming the circular scan into a controlled spiral on the detector plane. A detailed theoretical model describing the electron dynamics under two phase-locked RF voltages with different frequencies was derived, yielding analytical expressions for the transverse velocity and radius-vector components at the deflector exit. The experimental studies demonstrated good agreement with the model predictions. Spiral scanning will allow measurements with picosecond resolution in a temporal dynamic range 1-2 orders of magnitude larger than the period of the circular scanning.

A high-precision measurement of the Λ binding energies of light hypernuclei was performed at the Mainz Microtron (MAMI) using the decay-pion spectroscopy technique. To improve both statistical and systematic precision, two major upgrades were introduced: a newly developed natural lithium target to enhance the 3 H yield and a novel beam energy measurement based on undulator radiation interference to reduce the beam energy uncertainty. The experiment was carried out in 2022 with a total beam time of 287 hours. The momentum spectra obtained from the SpekA spectrometer clearly show two distinct peaks corresponding to the monoenergetic decays 4^H → 4He + π− and 3^ H → 3He + π−. The statistical significances exceed 5ε and 3ε, respectively, marking the first successful observation of 3^H using decay-pion spectroscopy. The corresponding statistical uncertainties in the Λ binding energies are less than 7 keV for 4^H and 15 keV for 3^H, with a systematic Λ Λ uncertainty estimated below 30 keV. These results demonstrate the capability of the decay-pion spectroscopy method for high-precision hypernuclear mass measurements and open the way toward detailed studies of ΛN interactions in few-body systems.

We performed high-precision decay-pion spectroscopy of light A hypernuclei at the Mainz Microtron (MAMI) using the A1 spectrometer facility. By measuring the monochromatic pi- momentum from the two-body weak decay 3 AH -* 3He + pi- and referencing it to the 4AH -* 4He + pi- decay, we determined the A binding energy of 3 AH with unprecedented accuracy. The obtained value, BA(3AH) = 0.523 + 0.013stat + 0.075syst: MeV, is consistent with the STAR result, but indicates a significantly deeper binding than inferred from earlier measurements. This result implies a stronger A-deuteron interaction and provides stringent constraints on hyperon-nucleon interactions.

In this paper, we present the design and preliminary performance evaluation of a new heavy-ion detector for direct measurements of heavy Lambda hypernuclei lifetime. The detector employs the previously developed 10 picosecond resolution Radio Frequency (RF) Timer, which converts the temporal information of incident particles into spatial coordinates of secondary or photoelectrons on a position-sensitive detector by means of circular RF scanning in the 500-1000 MHz range. Here, we report the detector design to achieve efficient suppression of accidental background and effective separation of prompt reaction products and delayed events from Lambda hypernuclei decays, results of test studies carried out with RF synchronized laser as well as preliminary results obtained by using alpha particles. Dedicated Monte-Carlo simulations have been performed to estimate the detector's performance under realistic experimental conditions at RF-driven electron, photon, or proton beams. The results confirm the feasibility of the proposed design and provide a basis for upcoming experimental measurements, based on the delayed fission detection.

We performed high-precision decay-pion spectroscopy of light Λ hypernuclei at the Mainz Microtron (MAMI) using the A1 spectrometer facility. By measuring the monochromatic π^- momentum from the two-body weak decay ^3_ΛH→^3He + π^- and referencing it to the ^4_ΛH→^4He + π^- decay, we determined the Λ binding energy of ^3_ΛH with unprecedented accuracy. The obtained value, B_Λ(^3_ΛH) = 0.523 ± 0.013 (stat.) ± 0.075 (syst.) MeV, is consistent with the STAR result, but indicates a significantly deeper binding than inferred from earlier measurements. This result implies a stronger Λ-deuteron interaction and provides stringent constraints on hyperon-nucleon interactions.

The potential of the intense secondary muon, neutrino, and (hypothetical) light dark matter beams at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) is explored. These are produced in the high-power dumps with high-current electron beams. Light dark matter searches with the approved Beam Dump eXperiment (BDX) are driving the realization of a new underground vault behind Hall A that could be extended to a Beamdump Facility with little additional installations. High-energy muons created via the Bethe–Heitler process uniquely do not proceed through the more common pion production and decay channels. Several possible muon physics applications are highlighted. Neutrino detector technologies and experiments suitable for a beamdump facility are outlined.

The neutron-rich hydrogen isotope (6)His produced for the first time in an electron-scattering experiment at MAMI-A1. Making use of the three-spectrometer setup in the A1-hall, triple coincidences between the scattered electron, produced proton, and pi(+), were measured with the reaction 7Li(e,e ' p pi(+))H-6 which allowed to determine the missing-mass spectrum of H-6. The preliminary analysis signals a ground state energy of H-6 near 3 MeV. Our work presents a new possibility to produce neutron-rich nuclei with electron-scattering experiments.

This White Paper is exploring the potential of intense secondary muon, neutrino, and (hypothetical) light dark matter beams produced in interactions of high-intensity electron beams with beam dumps. Light dark matter searches with the approved Beam Dump eXperiment (BDX) are driving the realization of a new underground vault at Jefferson Lab that could be extended to a Beamdump Facility with minimal additional installations. The paper summarizes contributions and discussions from the International Workshop on Secondary Beams at Jefferson Lab (BDX Beyond). Several possible muon physics applications and neutrino detector technologies for Jefferson Lab are highlighted. The potential of a secondary neutron beam will be addressed in a future edition.

The role of the electron-helicity-dependent cross-section term and the structure function f01′ in the quasi-elastic A(e→,e′p) process was studied. The f01′ was measured for proton knockout from the 1d3/2 shell in 40Ca via the 40Ca(e→,e′p)39Kg.s. reaction, leaving the residual nucleus in a well-defined state. It requires a longitudinally polarized electron beam and out-of-plane proton detection. This structure function vanishes in the absence of final-state interactions (FSI) involving the ejected proton. Presented are the dependencies of f01′ on the missing momentum (closely related to the initial proton’s Fermi momentum) and the angle between the knocked-out proton and the virtual photon momenta. The role of the spin-orbit interaction in FSI through the L→·S→ term in a nuclear optical potential is discussed.

The spin structure functions of the proton and the deuteron were measured during the EG4 experiment at Jefferson Lab in 2006. Data were collected for longitudinally polarized electron scattering off longitudinally polarized NH_3 and ND_3 targets, for Q^2 values as small as 0.012 and 0.02 GeV^2, respectively, using the CEBAF Large Acceptance Spectrometer (CLAS). This is the archival paper of the EG4 experiment that summaries the previously reported results of the polarized structure functions g_1, A_1F_1, and their moments Γ_1, γ_0, and I_TT, for both the proton and the deuteron. In addition, we report on new results on the neutron g_1 extracted by combining proton and deuteron data and correcting for Fermi smearing, and on the neutron moments Γ_1, γ_0, and I_TT formed directly from those of the proton and the deuteron. Our data are in good agreement with the Gerasimov-Drell-Hearn sum rule for the proton, deuteron, and neutron. Furthermore, the isovector combination was formed for g_1 and the Bjorken integral Γ_1^p-n, and compared to available theoretical predictions. All of our results provide for the first time extensive tests of spin observable predictions from chiral effective field theory (χEFT) in a Q^2 range commensurate with the pion mass. They motivate further improvement in χEFT calculations from other approaches such as the lattice gauge method.

Developing an understanding of phenomena driven by the emergence of hadron mass (EHM) is one of the most challenging problems in the Standard Model. This discussion focuses on the impact of results on nucleon resonance (N*) electroexcitation amplitudes (or γvpN* electrocouplings) obtained from experiments during the 6 GeV era in Hall B at Jefferson Lab on understanding EHM. Analyzed using continuum Schwinger function methods (CSMs), these results have revealed new pathways for the elucidation of EHM. A good description of the Δ(1232)3/2+, N(1440)1/2+, and Δ(1600)3/2+ electrocouplings, achieved by CSM analyses that express a realistic dressed quark mass function, sheds light on the strong interaction dynamics underlying EHM. Extensions to N* studies for higher-mass states are outlined, as well as experimental results anticipated in the 12 GeV era at Jefferson Lab and those that would be enabled by a further increase in the beam energy to 22 GeV.

For the first time the neutron-rich hydrogen isotope ^{6}H was produced in an electron scattering experiment in the reaction ^{7}Li(e,e^{'}pπ^{+})^{6}H using the spectrometer facility of the A1 Collaboration at the Mainz Microtron accelerator. By measuring the triple coincidence between the scattered electron, the produced proton, and π^{+}, the missing mass spectrum of ^{6}H was obtained. A clear peak above ^{3}H+n+n+n energy threshold was seen resulting in a ground state energy of ^{6}H at 2.3±0.5(stat.)±0.4(syst.)  MeV with a width of 1.9±1.0(stat.)±0.4(syst.)  MeV. This Letter challenges the understandings of multinucleon interactions and presents a new method to study light neutron-rich nuclei with electron scattering experiments.

The ratio of the transverse and longitudinal components of polarization transfer to protons in the quasielastic ((e) over right arrow, e'(p) over right arrow) reaction, P'(x)/P'(z), is sensitive to the proton's electromagnetic form factor ratio, G(E)/G(M). To explore density-dependent in-medium modifications, a comparison of polarization transfer ratios involving protons from distinct nuclear shells, each with different local nuclear densities, has been proposed. In this study, we present such comparisons between four shells, 1s(1/2), 1p(3/2) in C-12 and 1d(3/2), 2s(1/2) in Ca-40. In an effort to account for other many-body effects that may differ between shells, we use a state-of-the-art relativistic distorted-wave impulse-approximation (RDWIA) calculation and present the double ratios (P'(x)/P'(z))(Data)/(P'(x)/P'(z))(RDWIA) as well as the superratios [(P'(x)/P'(z))(A)/(P'(x)/P'(z))(B)](Data)/[(P'(x)/P'(z))(A)/(P'(x)/P'(z))(B)](RDWIA), for chosen shells A and B, as a function of effective local nuclear densities. We find that double ratios for individual shells show a dependence on the probed effective nuclear densities. Studying the super-ratios, we observed a systematic variation between pairs of higher- and lower-density shells.

Measuring deeply virtual Compton scattering (DVCS) on the neutron is one of the necessary steps to understand the structure of the nucleon in terms of generalized parton distributions (GPDs). Neutron targets play a complementary role to transversely polarized proton targets in the determination of the GPD E. This poorly known and poorly constrained GPD is essential to obtain the contribution of the quarks' angular momentum to the spin of the nucleon. DVCS on the neutron was measured for the first time selecting the exclusive final state by detecting the neutron, using the Jefferson Lab longitudinally polarized electron beam, with energies up to 10.6 GeV, and the CLAS12 detector. The extracted beam-spin asymmetries, combined with DVCS observables measured on the proton, allow a clean quark-flavor separation of the imaginary parts of the Compton form factors H and E.