Missing mass spectroscopy with the (e, e', K+) reaction was performed at Jefferson Laboratory's Hall C for the neutron-rich Lambda hypernucleus Li-9(Lambda). The ground-state (g.s.) energy was obtained to be B-Lambda(g.s.) = 8.84 +/- 0.17(stat.) +/- 0.15(sys.) MeV by using shell-model calculations of a cross-section ratio and an energy separation of the spin doublet states (3/2(1)(+) and 5/2(1)(+)). In addition, peaks that are considered to be states of [Li-8(3(+)) circle plus s(Lambda) = 3/2(2)(+), 1/2(+)] and [Li-8(3(+)) circle times s (Lambda) = 5/2(2)(+), 7/2(+)] were observed at E-Lambda (no. 2) = 1.74 +/- 0.27(stat.) +/- 0.11(sys). and E Lambda (no. 3) = 3.30 +/- 0.24(stat.) +/- 0.11(sys.) MeV, respectively. The E Lambda (no. 3) is larger than shell-model predictions by a few hundred keV, and the difference would indicate that a He-5 + t structure is more developed for the 3(+) state than those for the 2(+) and 1(+) states in a core nucleus Li-8 as a cluster model calculation suggests.

The High Threshold Cherenkov Counter (HTCC) is one of the detector systems of the CLAS12 spectrometer, and is used to generate a fast trigger signal in electron scattering experiments in the polar angle range from 5°to 35°. The HTCC is installed in front of the drift chambers and introduces a minimal amount of additional material within the acceptance. The HTCC is one unit whose core component is a multifocal mirror that consists of 60 lightweight ellipsoidal mirrors. It is important that the HTCC provides efficient coverage of the CLAS12 forward acceptance with no gaps. In order to achieve this, each sector of the CLAS12 Forward Detector is covered by 2 identical half-sector mirrors that focus Cherenkov light on 8 phototubes. The HTCC has a total of 48 channels with Electron Tubes 9823QKB photomultipliers that have a 5-in quartz face plate to detect Cherenkov light. The system provides rejection of charged π-mesons with momenta below 4.8 GeV for the reliable identification of scattered electrons. In this paper the details of the design, construction, calibration, and performance results of the HTCC are presented.

Recommended Citation Bosted, P. E.; Biselli, A. S.; Careccia, S.; Dodge, G.; Fersch, R.; Guler, N.; Kuhn, S. E.; Pierce, J.; Prok, Y.; Zheng, X.; Adhikari, K. P.; Adikaram, D.; Akbar, Z.; Amaryan, M. J.; Pereira, S. Anefalos; Asryan, G.; Avakian, H.; Badui, R. A.; Ball, J.; Baltzell, N. A.; Battaglieri, M.; Batourine, V.; Bedlinskiy, I.; Boiarinov, S.; and Griffioen, Keith A., Target and beam-target spin asymmetries in exclusive pi(+) and pi(-) electroproduction with 1.6-to 5.7-GeV electrons (2016). PHYSICAL REVIEW C, 94(5). 10.1103/PhysRevC.94.055201

Background: Measurements of polarization observables for the reactions (cid:2) γp → K + (cid:3) and (cid:2) γp → K + (cid:4) 0 have been performed. This is part of a program of measurements designed to study the spectrum of baryon resonances in particular, and nonperturbative QCD in general. Purpose: The accurate measurement of several polarization observables provides tight constraints for phenomenological fits, which allow the study of strangeness in nucleon and nuclear systems. Beam-recoil observables for the (cid:2) γp → K + (cid:4) 0 reaction have not been reported before now. Method: The measurements were carried out using linearly polarized photon beams incident on a liquid hydrogen target, and the CLAS detector at the Thomas Jefferson National Accelerator Facility. The energy range of the results is 1 . 71 < W < 2 . 19 GeV, with an angular range − 0 . 75 < cos θ ⋆K < + 0 . 85. Results: The observables extracted for both reactions are beam asymmetry (cid:4) , target asymmetry T , and the beam-recoil double polarization observables O x and O z . Conclusions: Comparison with theoretical fits indicates that, in the regions where no previous data existed, the new data contain significant new information, and strengthen the evidence for the set of resonances used in the latest Bonn-Gatchina fit. DOI:

The lifetime of a Lambda particle embedded in a nucleus (hypernucleus) decreases from that of free Lambda decay due to the opening of the Lambda N to NN weak decay channel. However, it is generally believed that the lifetime of a hypernucleus attains a constant value (saturation) for medium to heavy hypernuclear masses, yet this hypothesis has been difficult to verify. The present paper reports a direct measurement of the lifetime of medium-heavy hypernuclei produced with a photon-beam from Fe, Cu, Ag, and Bi targets. The recoiling hypernuclei were detected by a fission fragment detector using low-pressure multi-wire proportional chambers. The experiment agrees remarkably well with the only previously-measured single-species heavy-hypernucleus lifetime, that of Fe56_Lambda at KEK, and has significantly higher precision. The experiment disagrees with the measured lifetime of an unknown combination of heavy hypernuclei with 180<A<225 and, with a small statistical and systematic uncertainty, strongly favors the expected saturation of the lifetime decrease.

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π 0 electroproduction reaction γ ∗ p → p π 0 , expanding an analysis of the γ ∗ p → n π + reaction from the same experiment. The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic ranges covered are 1.1 < W < 3 GeV and 1 < Q 2 < 6 GeV 2 . Results were obtained for about 5700 bins in W , Q 2 , cos ( θ ∗ ) , and ϕ ∗ . The beam-target asymmetries were found to generally be greater than zero, with relatively modest ϕ ∗ dependence. The target asymmetries exhibit very strong ϕ ∗ dependence, with a change in sign occurring between results at low W and high W , in contrast to π + electroproduction. Reasonable agreement is found with phenomenological fits to previous data for W < 1.6 GeV, but significant differences are seen at higher W . When combined with cross-sectional measurements, as well as π + observables, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q 2 , for resonances with masses as high as 2.4 GeV.

The missing mass spectroscopy of the 7(cid:3) He hypernucleus was performed, using the 7 Li( e; e ′ K + ) 7(cid:3) He reaction at the Thomas Jefferson National Accelerator Facility Hall C. The (cid:3)-binding energy of the ground state (1/2 + ) was determined with a smaller error than that of the previous measurement, being B (cid:3) = 5 : 55 (cid:6) 0 : 10 stat. (cid:6) 0 : 11 sys. MeV. The experiment also provided new insight into charge symmetry breaking in p -shell hypernuclear systems. Finally, a peak at B (cid:3) = 3 : 65 (cid:6) 0 : 20 stat. (cid:6) 0 : 11 sys. MeV was observed and assigned as a mixture of 3 = 2 + and 5 = 2 + states, con(cid:12)rming the \gluelike" behavior of (cid:3), which makes an unstable state in 6 He stable against neutron emission.

The missing mass spectroscopy of the He-7(Lambda) hypernucleus was performed using the Li-7(e, e' K+)(Lambda)He-7 reaction at the Thomas Jefferson National Accelerator Facility Hall C. The Lambda-binding energy of the ground-state (1/2(+)) was determined with a smaller error than that of the previous measurement, being B-Lambda = 5.55 +/- 0.10(stat.) +/- 0.11(sys.) MeV. The experiment also provided new insight into charge symmetry breaking in p-shell hypernuclear systems. Finally, a peak at B Lambda = 3.65 +/- 0.20(stat.) +/- 0.11(sys.) MeV was observed and assigned as a mixture of 3/2(+) and 5/2(+) states, confirming the "gluelike" behavior of Lambda, which makes an unstable state in He-6 stable against neutron emission.

The missing mass spectroscopy of the $^{7}_{\Lambda}$He hypernucleus was performed, using the $^{7}$Li$(e,e^{\prime}K^{+})^{7}_{\Lambda}$He reaction at the Thomas Jefferson National Accelerator Facility Hall C. The $\Lambda$ binding energy of the ground state (1/2$^{+}$) was determined with a smaller error than that of the previous measurement, being $B_{\Lambda}$ = 5.55 $\pm$ 0.10(stat.) $\pm$ 0.11(sys.) MeV. The experiment also provided new insight into charge symmetry breaking in p-shell hypernuclear systems. Finally, a peak at $B_{\Lambda}$ = 3.65 $\pm$ 0.20(stat.) $\pm$ 0.11(sys.) MeV was observed and assigned as a mixture of 3/2$^{+}$ and 5/2$^{+}$ states, confirming the gluelike behavior of $\Lambda$, which makes an unstable state in $^{6}$He stable against neutron emission.

Spectroscopy of a $^{10}_{\Lambda}$Be hypernucleus was carried out at JLab Hall C using the $(e,e^{\prime}K^{+})$ reaction. A new magnetic spectrometer system (SPL+HES+HKS), specifically designed for high resolution hypernuclear spectroscopy, was used to obtain an energy spectrum with a resolution of 0.78 MeV (FWHM). The well-calibrated spectrometer system of the present experiment using the $p(e,e^{\prime}K^{+})\Lambda,\Sigma^{0}$ reactions allowed us to determine the energy levels, and the binding energy of the ground state peak (mixture of 1$^{-}$ and 2$^{-}$ states) was obtained to be B$_{\Lambda}$=8.55$\pm$0.07(stat.)$\pm$0.11(sys.) MeV. The result indicates that the ground state energy is shallower than that of an emulsion study by about 0.5 MeV which provides valuable experimental information on charge symmetry breaking effect in the $\Lambda N$ interaction.

Differential cross sections of the exclusive process ep→e′π+n were measured with good precision in the range of the photon virtuality Q2=1.8–4.5 GeV2 and the invariant mass range of the π+n final state W=1.6–2.0 GeV using the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the nπ+ center-of-mass system. More than 37 000 cross-section points were measured. The contributions of the isospin I=12 resonances N(1675)52−,N(1680)52+, and N(1710)12+ were extracted at different values of Q2 using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-t dispersion relations, were employed in the analysis. We observe significant strength of the N(1675)52− in the A1/2 amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the N(1680)52+ we observe a slow changeover from the dominance of the A3/2 amplitude at the real photon point (Q2=0) to a Q2 where A1/2 begins to dominate. The scalar amplitude S1/2 drops rapidly with Q2 consistent with quark model prediction. For the N(1710)12+ resonance our analysis shows significant strength for the A1/2 amplitude at Q2<2.5 GeV2.15 MoreReceived 8 December 2014DOI:https://doi.org/10.1103/PhysRevC.91.045203©2015 American Physical Society

T. Gogami1 ∗, P. Achenbach2, A. Ahmidouch3, I. Albayrak4, D. Androic5, A. Asaturyan6, R. Asaturyan6, O. Ates4, P. Baturin7, R. Badui7, W. Boeglin7, J. Bono7, E. Brash8, P. Carter8, C. Chen4, A. Chiba1, E. Christy4, S. Danagoulian3, R. De Leo10, D. Doi1, M. Elaasar11, R. Ent9, Y. Fujii1, M. Fujita1, M. Furic5, M. Gabrielyan7, L. Gan12, F. Garibaldi13, D. Gaskell9, A. Gasparian3, O. Hashimoto1, T. Horn9, B. Hu14, Ed. V. Hungerford21, M. Jones9, H. Kanda1, M. Kaneta1, S. Kato19, M. Kawai1, D. Kawama1, H. Khanal7, M. Kohl4, A. Liyanage4, W. Luo14, K. Maeda1, A. Margaryan6, P. Markowitz7, T. Maruta1, A. Matsumura1, V. Maxwell7, A. Mkrtchyan6, H. Mkrtchyan6, S. Nagao1, S. N. Nakamura1, A. Narayan15, C. Neville7, G. Niculescu16, M. I. Niculescu16, A. Nunez7, Nuruzzaman15, Y. Okayasu1, T. Petkovic5, J. Pochodzalla2, X. Qiu14, J. Reinhold7, V. M. Rodriguez17, C. Samanta18, B. Sawatzky9, T. Seva5, A. Shichijo1, V. Tadevosyan6, L. Tang4, N. Taniya1, K. Tsukada1, M. Veilleux8, W. Vulcan9, F. R. Wesselmann20, S. A. Wood9, T. Yamamoto1, L. Ya4, Z. Ye4, K. Yokota1, L. Yuan4, S. Zhamkochyan6 and L. Zhu4

Differential cross sections of the exclusive process ep -> e 'pi(+)n were measured with good precision in the range of the photon virtuality Q(2) = 1.8-4.5 GeV2 and the invariant mass range of the pi(+)n final state W = 1.6-2.0 GeV using the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the n pi(+) center-of-mass system. More than 37 000 cross-section points were measured. The contributions of the isospin I = 1/2 resonances N(1675) 5/2(-), N(1680) 5/2(+), and N(1710) 1/2(+) were extracted at different values of Q(2) using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-t dispersion relations, were employed in the analysis. We observe significant strength of the N(1675)5/2(-) in the A(1/2) amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the N(1680)5/2(+) we observe a slow changeover from the dominance of the A(3/2) amplitude at the real photon point (Q(2) = 0) to a Q(2) where A(1/2) begins to dominate. The scalar amplitude S-1/2 drops rapidly with Q(2) consistent with quark model prediction. For the N(1710)1/2(+) resonance our analysis shows significant strength for the A1/2 amplitude at Q(2) < 2.5 GeV2.

The first-time measurement of the angular dependence of the beam-helicity asymmetry for γp → pK+ K− is shown and compared to γp → pπ+ π−. The data obtained were from the CLAS g12 experiment at Jefferson Lab. The experiment utilized a beam of circularly polarized photons with energies between 1.1 and 5.4 GeV incident on an unpolarized liquid hydrogen target. An unprecedented number of strange hadrons in photoproduction were observed in the g12 experiment. The production mechanism for strange hadrons is not well understood. The beam-helicity asymmetry is a polarization observable that provides information on competing production mechanisms in the reaction. It is shown that the asymmetry is sensitive to several kinematic variables that are key in modeling the reaction dynamics. Furthermore, the comparison of the beam-helicity asymmetry between the kaon and pion channels serves as a platform for the investigation of flavor dependence.

L. Tang, 2, ∗ C. Chen, T. Gogami, D. Kawama, Y. Han, L. Yuan, A. Matsumura, Y. Okayasu, T. Seva, V. M. Rodriguez, 6 P. Baturin, A. Acha, P. Achenbach, A. Ahmidouch, I. Albayrak, D. Androic, A. Asaturyan, R. Asaturyan, † O. Ates, R. Badui, O. K. Baker, F. Benmokhtar, W. Boeglin, J. Bono, P. Bosted, E. Brash, P. Carter, R. Carlini, A. Chiba, M. E. Christy, L. Cole, M. M. Dalton, 2 S. Danagoulian, A. Daniel, R. De Leo, V. Dharmawardane, D. Doi, K. Egiyan, M. Elaasar, R. Ent, H. Fenker, Y. Fujii, M. Furic, M. Gabrielyan, L. Gan, F. Garibaldi, D. Gaskell, A. Gasparian, E. F. Gibson, P. Gueye, O. Hashimoto, † D. Honda, T. Horn, 11 B. Hu, Ed V. Hungerford, C. Jayalath, M. Jones, K. Johnston, N. Kalantarians, H. Kanda, M. Kaneta, F. Kato, S. Kato, M. Kawai, C. Keppel, H. Khanal, M. Kohl, L. Kramer, K. J. Lan, Y. Li, A. Liyanage, W. Luo, D. Mack, K. Maeda, S. Malace, A. Margaryan, G. Marikyan, P. Markowitz, T. Maruta, N. Maruyama, V. Maxwell, D. J. Millener, T. Miyoshi, A. Mkrtchyan, H. Mkrtchyan, T. Motoba, 24 S. Nagao, S. N. Nakamura, A. Narayan, C. Neville, G. Niculescu, M. I. Niculescu, A. Nunez, Nuruzzaman, H. Nomura, K. Nonaka, A. Ohtani, M. Oyamada, N. Perez, T. Petkovic, J. Pochodzalla, X. Qiu, S. Randeniya, B. Raue, J. Reinhold, R. Rivera, J. Roche, C. Samanta, Y. Sato, B. Sawatzky, E. K. Segbefia, D. Schott, A. Shichijo, N. Simicevic, G. Smith, Y. Song, M. Sumihama, V. Tadevosyan, T. Takahashi, N. Taniya, K. Tsukada, V. Tvaskis, M. Veilleux, W. Vulcan, S. Wells, F. R. Wesselmann, S. A. Wood, T. Yamamoto, C. Yan, Z. Ye, K. Yokota, S. Zhamkochyan, and L. Zhu

Since the pioneering experiment, E89-009 studying hypernuclear spectroscopy using the $(e,e^{\prime}K^+)$ reaction was completed, two additional experiments, E01-011 and E05-115, were performed at Jefferson Lab. These later experiments used a modified experimental design, the Tilt Method, to dramatically suppress the large electromagnetic background, and allowed for a substantial increase in luminosity. Additionally, a new kaon spectrometer, HKS (E01-011), a new electron spectrometer, HES, and a new splitting magnet were added to produce precision, high-resolution hypernuclear spectroscopy. These two experiments, E01-011 and E05-115, resulted in two new data sets, producing sub-MeV energy resolution in the spectra of ${}^{7}_{\Lambda}\text{He}$, ${}^{12}_{\Lambda}\text{B}$ and ${}^{28}_{\Lambda}\text{Al}$ and ${}^{7}_{\Lambda}\text{He}$, ${}^{10}_{\Lambda}\text{Be}$, ${}^{12}_{\Lambda}\text{B}$ and ${}^{52}_{\Lambda}\text{V}$. All three experiments obtained a ${}^{12}_{\Lambda}\text{B}$, spectrum, which is the most characteristic $p$-shell hypernucleus and is commonly used for calibration. Independent analyses of these different experiments demonstrate excellent consistency and provide the clearest level structure to date of this hypernucleus as produced by the $(e,e^{\prime}K^+)$ reaction. This paper presents details of these experiments, and the extraction and analysis of the observed ${}^{12}_{\Lambda}\text{B}$ spectrum.