Recent Upgrades of the Gas Handling System for the Cryogenic Stopping Cell of the FRS Ion Catcher

In this paper, the major upgrades and technical improvements of the buffer gas handling system for the cryogenic stopping cell of the FRS Ion Catcher at GSI/FAIR (in Darmstadt, Germany) are described. The upgrades include implementation of new gas lines and gas purifiers to achieve a higher buffer gas cleanliness for a more efficient extraction of reactive ions as well as suppression of the molecular background ionized in the stopping cell. Furthermore, additional techniques have been implemented for improved monitoring and quantification of the purity of the helium buffer gas

Mass measurements of As, Se and Br nuclei and their implication on the proton-neutron interaction strength towards the N=Z line

Mass measurements of the nuclides 69As, 70,71Se, and 71Br, produced via fragmentation of a 124Xe primary beam at the Fragment Separator (FRS) at GSI, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher with an unprecedented mass resolving power of almost 1000000. Such high resolving power is the only way to achieve accurate results and resolve overlapping peaks of short-lived exotic nuclei, whose total number of accumulated events is always limited. For the nuclide 69As, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only ten events. For the nuclide 70Se, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of δm/m=4.0×10−8, with less than 500 events. The masses of the nuclides 71Se and 71Br have been measured with an uncertainty of 23 and 16 keV, respectively. Our results for the nuclides 70,71Se and 71Br are in good agreement with the 2016 Atomic Mass Evaluation, and our result for the nuclide 69As resolves the discrepancy between the previous indirect measurements. We measured also the mass of the molecule 14N15N40Ar (A=69) with a relative accuracy of δm/m=1.7×10−8, the highest yet achieved with an MR-TOF-MS. Our results show that the measured restrengthening of the proton-neutron interaction (δVpn) for odd-odd nuclei along the N=Z line above Z=29 (recently extended to Z=37) is hardly evident at the N−Z=2 line, and not evident at the N−Z=4 line. Nevertheless, detailed structure of δVpn along the N−Z=2 and N−Z=4 lines, confirmed by our mass measurements, may provide a hint regarding the ongoing ≈500 keV discrepancy in the mass value of the nuclide 70Br, which prevents including it in the world average of Ft value for superallowed 0+→0+β decays. The reported work sets the stage for mass measurements with the FRS Ion Catcher of nuclei at and beyond the N=Z line in the same region of the nuclear chart, including the nuclide 70Br.peerReviewe

Observation of azimuth-dependent suppression of hadron pairs in electron scattering off nuclei

We present the first measurement of di-hadron angular correlations in electron-nucleus scattering. The data were taken with the CLAS detector and a 5.0 GeV electron beam incident on deuterium, carbon, iron, and lead targets. Relative to deuterium, the nuclear yields of charged-pion pairs show a strong suppression for azimuthally opposite pairs, no suppression for azimuthally nearby pairs, and an enhancement of pairs with large invariant mass. These effects grow with increased nuclear size. The data are qualitatively described by the GiBUU model, which suggests that hadrons form near the nuclear surface and undergo multiple-scattering in nuclei. These results show that angular correlation studies can open a new way to elucidate how hadrons form and interact inside nucleiComment: 6 pages, 4 figure

Observation of Azimuth-Dependent Suppression of Hadron Pairs in Electron Scattering Off Nuclei

We present the first measurement of dihadron angular correlations in electron-nucleus scattering. The data were taken with the CLAS detector and a 5.0 GeV electron beam incident on deuterium, carbon, iron, and lead targets. Relative to deuterium, the nuclear yields of charged-pion pairs show a strong suppression for azimuthally opposite pairs, no suppression for azimuthally nearby pairs, and an enhancement of pairs with large invariant mass. These effects grow with increased nuclear size. The data are qualitatively described by the gibuu model, which suggests that hadrons form near the nuclear surface and undergo multiple scattering in nuclei. These results show that angular correlation studies can open a new way to elucidate how hadrons form and interact inside nuclei

First Measurement of Hard Exclusive π- Δ++ Electroproduction Beam-Spin Asymmetries off the Proton

The polarized cross-section ratio σLT′/σ0 from hard exclusive π-Δ++ electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a 10.2 GeV/10.6 GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab. The study, which provides the first observation of this channel in the deep-inelastic regime, focuses on very forward-pion kinematics in the valence regime, and photon virtualities ranging from 1.5 GeV2 up to 7 GeV2. The reaction provides a novel access to the d-quark content of the nucleon and to p→Δ++ transition generalized parton distributions. A comparison to existing results for hard exclusive π+n and π0p electroproduction is provided, which shows a clear impact of the excitation mechanism, encoded in transition generalized parton distributions, on the asymmetry

First Measurement of Λ\Lambda Electroproduction off Nuclei in the Current and Target Fragmentation Regions

We report results of $\Lambda$ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014~GeV electron beam. These results represent the first measurements of the $\Lambda$ multiplicity ratio and transverse momentum broadening as a function of the energy fraction~($z$) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high~$z$~and~an enhancement at~low~$z$. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high~$z$. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.Comment: 14 pages, 6 figure

Measurement of the helicity asymmetry E for the γ→ p→ → pπ reaction in the resonance region: The CLAS Collaboration

The double-spin-polarization observable E for γ→ p→ → pπ has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies Eγ from 0.367 to 2.173GeV (corresponding to center-of-mass energies from 1.240 to 2.200GeV) for pion center-of-mass angles, cosθπ0c.m. , between - 0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will allow us to refine our understanding of the single-pion production contribution to the Gerasimov-Drell-Hearn sum rule and improve the determination of resonance properties, which will be presented in a future publication