Search Results (55 total)

We measured the lifetime and the mesonic and nonmesonic decay rates of the _Λ⁴He hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K^- beam on a liquid ⁴He target by the reaction ⁴He (K^-,π^-)_Λ⁴He. The _Λ⁴He lifetime was directly measured using protons from Λp→np nonmesonic decay (also referred to as proton-stimulated decay) and was found to have a value of τ=245±24 ps. The mesonic decay rates were determined from the observed numbers of π^-'s and π^○'s as Γ_π^-/Γ_tot=0.270±0.024 and Γ_π^○/Γ_tot=0.564±0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Γ_p/Γ_tot=0.169±0.019 and Γ_n/Γ_tot≤0.032 (95% CL), respectively. The effects of final-state interactions and possible three-body ΛNN decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the nonmesonic branching fractions. The implications of the results of this analysis were considered for the empirical ΔI=1 / 2 rule and the decay rates of the _Λ⁴H hypernucleus.

A pioneering experiment in Λ hypernuclear spectroscopy, undertaken at the Thomas Jefferson National Accelerator Facility (JLab), was recently reported. The experiment used the high precision, continuous electron beam at JLab, and a special arrangement of spectrometer magnets to measure the hypernuclear spectrum from C and ⁷Li targets using the (e,e'K⁺) reaction. The _Λ¹²B spectrum found in this investigation was previously published, but is reported here in more detail, with improved resolution. In addition, the results of a _Λ⁷He spectrum also obtained in the experiment, are shown. This latter spectrum indicates the need for a more detailed few-body calculation of the hypernucleus and the reaction process. The success of the experiment demonstrates the potential of the (e,e'K⁺) reaction for high resolution spectroscopy of hypernuclear spectra.

In order to study the Σ-nucleus optical potential, we measured inclusive (π⁻,K⁺) spectra on medium-to-heavy nuclear targets CH₂, Si, Ni, In, and Bi. The CH₂ target was used to calibrate the excitation energy scale by using the elementary process p+π⁻→K⁺+Σ⁻, where the C spectrum was also extracted. The calibration was done with ±0.1  MeV precision. The angular distribution of the elementary cross section was measured and agreed well with the previous bubble chamber data, but with better statistics, and the magnitudes of the cross sections of the measured inclusive (π⁻,K⁺) spectra were also well calibrated. All of the inclusive spectra were found to be similar in shape at a region near to the Σ⁻ binding energy threshold, showing a weak mass-number dependence on the magnitude of the cross section. The measured spectra were compared with a theoretical calculation performed within the framework of the distorted-wave impulse approximation. It has been demonstrated that a strongly repulsive Σ-nucleus potential with a nonzero size of the imaginary part is required to reproduce the shape of the measured spectra.

Electroproduction of the ω meson was investigated in the ¹H(e,e^′p)ω reaction. The measurement was performed at a four-momentum transfer Q²≈0.5  GeV². Angular distributions of the virtual photon-proton center-of-momentum cross sections have been extracted over the full angular range. These distributions exhibit a strong enhancement over t-channel parity exchange processes in the backward direction. According to a newly developed electroproduction model, this enhancement provides significant evidence of resonance formation in the γ*p→ωp reaction channel.

We report on precision measurements of the elastic cross section for electron-proton scattering performed in Hall C at Jefferson Lab. The measurements were made at 28 distinct kinematic settings covering a range in momentum transfer of 0.4<Q²<5.5  (GeV∕c)². These measurements represent a significant contribution to the world’s cross section data set in the Q² range, where a large discrepancy currently exists between the ratio of electric to magnetic proton form factors extracted from previous cross section measurements and that recently measured via polarization transfer in Hall A at Jefferson Lab. This data set shows good agreement with previous cross section measurements, indicating that if a heretofore unknown systematic error does exist in the cross section measurements, then it is intrinsic to all such measurements.

We have measured the neutron spectra emitted in the weak decay of _Λ¹²C and _Λ⁸⁹Y over the energy region above 10  MeV in the (π⁺,K⁺) reaction with better statistics and an improved signal-to-background ratio. The neutron yields in the nonmesonic weak decays were obtained from the spectra. By using the proton yield of _Λ¹²C, the Γ_n∕Γ_p ratio was estimated to be 0.51±0.15  (stat) from the neutron-to-proton yield ratio for the first time, which suggests the Γ_n∕Γ_p ratio is significantly less than unity.

High-energy, cw electron beams at new accelerator facilities allow electromagnetic production and precision study of hypernuclear structure, and we report here on the first experiment demonstrating the potential of the (e,e^′K⁺) reaction for hypernuclear spectroscopy. This experiment is also the first to take advantage of the enhanced virtual photon flux available when electrons are scattered at approximately zero degrees. The observed energy resolution was found to be ≈900  keV for the _Λ¹²B spectrum, and is substantially better than any previous hypernuclear experiment using magnetic spectrometers. The positions of the major excitations are found to be in agreement with a theoretical prediction and with a previous binding energy measurement, but additional structure is also observed in the core excited region, underlining the future promise of this technique.

We report measurements of cross sections for the reaction ¹H(e,e^′K⁺)Y, for both the Λ and Σ⁰ hyperon states, at an invariant mass of W=1.84 GeV and four-momentum transfers 0.5<Q²<2 (GeV/c)². Data were taken for three values of virtual photon polarization ε, allowing the decomposition of the cross sections into longitudinal and transverse components. The Λ data are a revised analysis of prior work, whereas the Σ⁰ results have not been previously reported.

The quasielastic (e,e^′p) reaction was studied on targets of deuterium, carbon, and iron up to a value of momentum transfer Q² of 8.1 (GeV/c)². A nuclear transparency was determined by comparing the data to calculations in the plane-wave impulse approximation. The dependence of the nuclear transparency on Q² and the mass number A was investigated in a search for the onset of the color transparency phenomenon. We find no evidence for the onset of color transparency within our range of Q². A fit to the world’s nuclear transparency data reflects the energy dependence of the free-proton–nucleon cross section.

We have studied the (π^-,K⁺) reaction on a silicon target to investigate the sigma-nucleus potential. The inclusive spectrum was measured at a beam momentum of 1.2  GeV/c with an energy resolution of 3.3 MeV (FWHM) by employing the superconducting kaon spectrometer system. The spectrum was compared with theoretical calculations within the framework of the distorted-wave impulse approximation, which demonstrates that a strongly repulsive sigma-nucleus potential with a nonzero size of the imaginary part reproduces the observed spectrum.

The _Λ¹³C hypernucleus was studied by measuring γ rays in coincidence with the ¹³C(K^-,π^-) reaction. γ rays from the 1/2^- and 3/2^- states, which are the partners of the spin-orbit doublet states with a predominant configuration of [¹²C_g.s.(0⁺)⊗p_Λ], to the ground state were measured. The splitting of the states was found to be ΔE(1/2^--3/2^-)=+152±54(stat)±36(syst) keV. This value is 20–30 times smaller than that of single particle states in nuclei around this mass region. The j_Λ=l_Λ-1/2[(p_1/2)_Λ] state appeared higher in energy, as in normal nuclei. The value gives new insight into the YN interaction. The excitation energies of the 1/2^- and 3/2^- states were obtained as 10.982±0.031(stat)±0.056(syst) and 10.830±0.031(stat)±0.056(syst) MeV, respectively. We also observed γ rays from the 3/2⁺ state, which has a [¹²C(2⁺)⊗s_Λ] configuration, to the ground state in _Λ¹³C. The excitation energy of the 3/2⁺ state was obtained as 4.880±0.010(stat)±0.017(syst) MeV. Nuclear γ rays with energies of 4.438 and 15.100 MeV had similar yields, which suggests that a quasifree knockout of a Λ particle is dominant in highly excited regions.

With a germanium detector array (Hyperball), we observed two γ-ray peaks corresponding to the two transitions ( 5/2⁺→1/2⁺ and 3/2⁺→1/2⁺) in the _Λ⁹Be hypernucleus which was produced by the ⁹Be(K^-,π^-) reaction. The energies of the γ rays are 3029±2±1 keV and 3060±2±1 keV. The energy difference was measured to be 31.4_-3.6^+2.5 keV, which indicates a very small Λ-spin-dependent spin-orbit force between a Λ and a nucleon. This is the smallest level splitting by far ever measured in a hypernucleus.

The coherent ³He(e,e^′π⁺)³H reaction was measured at Q²=0.4 (GeV/c)² and W=1.6 GeV for two values of the virtual photon polarization, ε, allowing the separation of longitudinal and transverse cross sections. The results from the coherent process on ³He were compared to H(e,e^′π⁺)n data taken at the same kinematics. This marks the first direct comparison of these processes. At these kinematics (p_π=1.1 GeV/c), pion rescattering from the spectator nucleons in the ³He(e,e^′π⁺)³H process is expected to be small, simplifying the comparison to π⁺ production from the free proton.

Separated longitudinal and transverse cross sections for charged pion electroproduction from ¹H, ²H, and ³He were measured at Q²  =  0.4 (GeV/c)² for two values of the invariant mass, W  =  1.15 GeV and W  =  1.60 GeV, in a search for a mass dependence which would signal the effect of nuclear pions. This is the first such study that includes recoil momenta significantly above the Fermi surface. The longitudinal cross section, if dominated by the pion-pole process, should be sensitive to nuclear pion currents. Comparisons of the longitudinal cross section target ratios to a quasifree calculation reveal a significant suppression in ³He at W  =  1.60 GeV. The W  =  1.15 GeV results are consistent with simple estimates of the effect of nuclear pion currents, but are also consistent with pure quasifree production.

Excitation energy spectra of _Λ⁸⁹Y, _Λ⁵¹V, and _Λ¹²C have been measured via the (π⁺,K⁺) reaction by using the SKS spectrometer at the K6 beam line in the KEK 12-GeV Proton Synchrotron. In the _Λ⁸⁹Y spectrum, obtained with an energy resolution of 1.65 MeV (FWHM) and in the highest statistics so far, we have succeeded in clearly observing a characteristic fine structure in heavy Λ hypernuclear systems and precisely obtaining a series of Λ single-particle energies in the very wide excitation energy range of more than 20 MeV, for the first time. Also in the _Λ⁵¹V spectrum, a similar structure to that of _Λ⁸⁹Y was observed. In the _Λ¹²C spectrum, new core-excited states were clearly resolved thanks to the best energy resolution of 1.45 MeV so far achieved by using the SKS spectrometer.

Quasielastic scattering cross sections have been measured with a 950 MeV/c π^- beam on targets of ²H, ⁶Li, C, Ca, Zr, and ²⁰⁸Pb, over a range of three-momentum transfers from 350 through 650 MeV/c. Results for carbon are compared to a finite-nucleus continuum random-phase approximation calculation including distortions. The pion spectra at our lowest range of momentum transfers show less scalar/isoscalar correlation than predicted.

The first measurements of the d(γ,p)n differential cross section at forward angles and photon energies above 4 GeV were performed at the Thomas Jefferson National Accelerator Facility (JLab). The results indicate evidence of an angular dependent scaling threshold. Results at θ_cm  =  37° are consistent with the constituent counting rules for E_γ≳4 GeV, while those at 70° are consistent with the constituent counting rules for E_γ≳1.5 GeV.

We report the first measurement using a solid polarized target of the neutron electric form factor G_Eⁿ via d→(e→,e^′n)p. G_Eⁿ was determined from the beam-target asymmetry in the scattering of longitudinally polarized electrons from polarized deuterated ammonia ( ¹⁵ND₃). The measurement was performed in Hall C at Thomas Jefferson National Accelerator Facility in quasifree kinematics with the target polarization perpendicular to the momentum transfer. The electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle segmented detector. We find G_Eⁿ  =  0.04632±0.00616(stat)±0.00341(syst) at Q²  =  0.495 (GeV/c)².

The spin-orbit splitting of Λ single-particle states in _Λ¹³C was measured. The ¹³C(K^-,π^-)_Λ¹³C reaction was used to excite both the 1/2^- and 3/2^- states simultaneously, which have predominantly ¹²C(0⁺)×p_Λ configuration. γ rays from the states to the ground state were measured in coincidence with the π^-'s, by which ℓs splitting was found to be 152±54(stat)±36(syst) keV. The value is 20–30 times smaller than exhibited by the ℓs splitting in the nuclear shell model. This value gives us new insight into the YN interaction.

We report on the first measurement of a hypernuclear γ-transition probability. γ rays emitted in the E2(5/2⁺→1/2⁺) transition of _Λ⁷Li were detected by a large-acceptance germanium detector array (Hyperball), and the lifetime of the parent state ( 5/2⁺) was determined by the Doppler shift attenuation method. The obtained result, 5.8_-0.7^+0.9±0.7 ps, was then converted into the reduced transition probability [ B(E2)] to be B(E2;5/2⁺→1/2⁺)  =  3.6±0.5_-0.4^+0.5 e² fm⁴. Compared with the B(E2) of the corresponding E2(3⁺→1⁺) transition in the ⁶Li nucleus, our result gives evidence that the size of the ⁶Li core in _Λ⁷Li is smaller than the ⁶Li nucleus in the free space.

A search for the H dibaryon has been conducted at the Brookhaven National Laboratory AGS, using a 1.8 GeV/c K^- beam. Ξ^- hyperons were produced in a liquid-hydrogen target via the reaction K^-+p→K⁺+Ξ^-. The hyperons were slowed in degraders and those most likely to stop in an adjacent liquid-deuterium target were tagged by silicon detectors. Monoenergetic neutrons were sought as the signature for H formation in (Ξ^-,d)_atom→H+n. The experiment was designed for optimal sensitivity to a loosely-bound H, complementing recent (K^-,K⁺) measurements on nuclear targets. In addition, the experiment’s sensitivity was independent of lifetime and of decay modes of the H. No statistically significant evidence for H formation was seen. Upper limits on the branching ratio for H formation in the above reaction have been set in a mass range extending from slightly above ΛΛ threshold to ∼100 MeV of binding and are compared with a corresponding theoretical prediction.

Precise values of the normal-component spin observables D_NN^′, P, and A_y have been determined at an incident proton energy of 198 MeV for the T=0 (12.71 MeV) and T=1 (15.11 MeV) 1⁺ states in ¹²C for momentum transfers between 80 and 250 MeV/c, and for the T=0 (17.78 and 19.81 MeV) and T=1 (18.98 MeV) 4^- states in ¹⁶O for momentum transfers of 225–400 MeV/c. The data are compared with distorted wave impulse approximation calculations in which a microscopic treatment of the nuclear medium has been used to obtain an effective nucleon-nucleon interaction. Nuclear binding and Pauli blocking effects, as well as those due to the strong scalar and vector mean field potentials that arise in covariant treatments of nuclear matter, are incorporated using a G-matrix approach. Our results suggest that while the isovector channel of the effective nucleon-nucleon interaction is reasonably well understood, pronounced discrepancies between theory and experiment for the isoscalar transitions point towards problems with the relative strengths of the spin-orbit and tensor components. For the ¹²C states, these discrepancies are more difficult to interpret due to ambiguities in the nuclear structure.

We report a measurement of charge symmetry breaking in the NNη system. We have measured the ratio of the differential cross sections of the charge-symmetric reactions π⁺d→ppη and π^-d→nnη in the energy region of the η threshold. Our result is R≡dσ(π⁺d→ppη)/dσ(π^-d→nnη)=0.938±0.009 after a phase-space correction is made for the difference in the threshold energies of the two reactions. The deviation of R from unity is an indication of charge symmetry breaking, which is mostly due to π⁰-η mixing. A theoretical model for η production which includes π⁰-η mixing was used to fit the data and yields a mixing angle of (1.5±0.4)°. Our result is consistent with the mixing angle determined in particle decay and isospin-forbidden processes as well as predictions by several theoretical analyses which yield ≈1° and another which yields ≈2°.