High-resolution hypernuclear spectroscopy at Jefferson Lab, Hall A

The experiment E94-107 in Hall A at Jefferson Lab started a systematic study of high-resolution hypernuclear spectroscopy in the $0p$-shell region of nuclei such as the hypernuclei produced in electroproduction on ${}^9\mathrm{Be},{}^{12}\mathrm{C}$, and ${}^{16}\mathrm{O}$ targets. In order to increase counting rates and provide unambiguous kaon identification, two superconducting septum magnets and a ring-imaging Cherenkov detector were added to the Hall A standard equipment. The high-quality beam, the good spectrometers, and the new experimental devices allowed us to obtain very good results. For the first time, measurable strength with sub-MeV energy resolution was observed for the core-excited states of ${}_{\mathrm{\Lambda }}{}^{12}\mathrm{B}$. A high-quality ${}_{\mathrm{\Lambda }}{}^{16}\mathrm{N}$ hypernuclear spectrum was likewise obtained. A first measurement of the $\mathrm{\Lambda }$ binding energy for ${}_{\mathrm{\Lambda }}{}^{16}\mathrm{N}$, calibrated against the elementary reaction on hydrogen, was obtained with high precision, $13.76\pm 0.16$ MeV. Similarly, the first ${}_{\mathrm{\Lambda }}{}^9\mathrm{Li}$ hypernuclear spectrum shows general agreement with theory (distorted-wave impulse approximation with the SLA and BS3 electroproduction models and shell-model wave functions). Some disagreement exists with respect to the relative strength of the states making up the first multiplet. A $\mathrm{\Lambda }$ separation energy of 8.36 MeV was obtained, in agreement with previous results. It has been shown that the electroproduction of hypernuclei can provide information complementary to that obtained with hadronic probes and the $\gamma$-ray spectroscopy technique.

Figure 1

Kinematics of hypernuclear electroproduction in the laboratory frame.

Figure 2

Schematic layout of the modifications to the HRS setup.

Figure 3

Layout of the septum insertion.

Figure 4

View of the target cell with the waterfall.

Figure 5

Target thickness vs pump speed.

Figure 6

Layout and working principle of the freon CsI proximity focusing RICH.

Figure 7

Cherenkov angle distributions for protons (0.54 rad), kaons (0.64 rad), and pions (0.68 rad). The particle identification technique is explained in the text.

Figure 8

The CsI evaporator system.

Figure 9

The quantum-efficiency measurement system. The quantum efficiency ($\mathcal{Q}$) is given by $\mathcal{Q}=(I_{\mathrm{chamber}}/I_{\mathrm{PMT}}){\mathcal{Q}}_{\mathrm{PMT}}$, where the ratio of measured currents in the wire chamber and PMT multiplies the quantum efficiency of the calibrated reference PMT (see text).

Figure 10

Hadron plus electron arm coincidence time spectra. Left panel: The unfilled histogram is obtained by selecting kaons with only the threshold aerogel Cherenkov detectors. The filled histogram (expanded in the right panel) also includes the RICH kaon selection. The remaining contamination is due to accidental $(e,e^{\prime })\otimes (e,K^{+})$ coincidences. The $\pi$ and $p$ contamination is clearly reduced to a negligible contribution.

Figure 11

Excitation energy spectra of ${}_{\mathrm{\Lambda }}{}^{12}\mathrm{B}$ using, for kaon identification, only the aerogel (upper plot) or also the RICH (lower plot). The counts are for 200-keV bins.

Figure 12

From Ref. [6]. One peak of the excitation energy spectrum of the hypernucleus ${}_{\mathrm{\Lambda }}{}^9\mathrm{L}\mathrm{i}$ obtained through the reaction ${}^9\mathrm{Be}(e,e^{\prime }{K}^{+}){}_{\mathrm{\Lambda }}{}^9\mathrm{Li}$ as predicted by the Monte Carlo simc when including all effects (dashed red curve) and turning off the radiative effects (solid blue curve). The position of the peak has been made coincident with the ground state.

Figure 13

Elastic ${}^{12}\mathrm{C}$ scattering spectrum as seen in one arm of the HRS + septum configuration after optimization. The width of all the peaks, elastic and inelastic, is ${10}^{-4}$ (FWHM).

Figure 14

Excitation energy spectrum of the $p(e,e^{\prime }{K}^{+})\mathrm{\Lambda },{\mathrm{\Sigma }}^0$ on hydrogen used for energy-scale calibration. The fitted positions (not shown on the plot) for the peaks are $-0.04\pm 0.08$ MeV and $76.33\pm 0.24$ MeV.

Figure 15

Differential cross sections for photoproduction are plotted as a function of the center-of-mass angle at $W=2.21$ GeV. Photoproduction data are from Refs. [54] (CLAS), [55] (LEPS), and [56] (SAPHIR). The electroproduction data “Brown” [57] is very near to the photoproduction point at $Q^2$ = 0.18 (GeV/${\mathrm{c})}^2$ and $W=2.17$ GeV. The larger angle data can constrain the curves but the utility of new small-angle results is evident.

Figure 16

The same as Fig. 15 but for three values of the center-of-mass energy. The result of this experiment (E94-107) is shown in part (a). The predictions of the Regge-plus-resonance model (RPR-1) [36] are comparred with SLAC data [59] above the resonance region in parts (b) and (c). The problem of normalization of the SLAC data is apparent.

Figure 17

The radiative-corrected experimental spectrum of ${}_{\mathrm{\Lambda }}{}^9\mathrm{Li}$ in comparison with the theoretical prediction (dashed and dash-dotted lines). The solid line shows the result of fitting the data with four Gaussians of a common width. The theoretical curves were calculated with the width extracted from the fit (FWHM = 730 keV). Note that for a better comparison between theory and experiment, the theoretical curves have been shifted by $-0.34$ MeV with respect to the excitation energies given in Table 2.

Figure 18

The calculated spectrum of ${}_{\mathrm{\Lambda }}{}^{12}\mathrm{B}$. The ${}^{11}\mathrm{B}$ core states are shown on the left along with the spectroscopic factors for proton removal from ${}^{12}\mathrm{C}$. All excitation energies are in keV. On the right, the factors giving the relative population of purely non-spin-flip ($\mathrm{\Delta }S=0$) and purely spin-flip ($\mathrm{\Delta }S=1$) production reactions on ${}^{12}\mathrm{C}$ with $\mathrm{\Delta }L=1$ ($\pi =-$) or $\mathrm{\Delta }L=2$ ($\pi =+$) are given.

Figure 19

The radiative-corrected experimental excitation-energy spectrum of ${}_{\mathrm{\Lambda }}{}^{12}\mathrm{B}$ (the points with statistical errors) in comparison with the theoretical prediction (dashed and dash-dotted lines). The solid line shows the result of fits to the data with six Gaussians with independent widths. The theoretical curves were calculated with the average width extracted from the fit (FWHM = 820 keV) without any background.

Figure 20

The ${}_{\mathrm{\Lambda }}{}^{16}\mathrm{N}$ binding-energy spectrum. The solid line shows the best fit using Voigt functions (see text for details). The theoretical curves (dashed and dash-dotted lines) were calculated with an average width extracted from the fit (FWHM = 1177 keV).