Search Results (70 total)

We used our (perviously described) system for high sensitivity measurements of β-delayed α-particle emission of light nuclei to measure upper limits of the branching ratios of the first-forbidden rank-one β decay of the 2⁺ ground state of ²⁰F to the 3^-,5.62 MeV, and 1^-,5.79 MeV excited states of ²⁰Ne to be 1.5×10^-6 and 7×10^-7, respectively. These limits are more than a factor of 300 smaller than previously measured. The obtained ft values are at least a factor of 10 smaller than that of the first-forbidden rank-zero β decay to the 2^- state at 4.97 MeV in ²⁰Ne, whose branching ratio was also measured to be 1.2(6)×10^-4, in agreement with the precise value obtained by Alburger and Warburton of 0.90(4)×10^-4.

We measured the β-delayed α-particle emission spectrum of ¹⁶N with a sensitivity for β-decay branching ratios of the order of 10^-10. The ¹⁶N nuclei were produced using the d(¹⁵N,¹⁶N)p reaction with 70 MeV ¹⁵N beams and a deuterium gas target 7.5 cm long at a pressure of 1250 torr. The ¹⁶N nuclei were collected (over 10 s) using a thin aluminum foil with an areal density of 180 μg/cm² tilted at 7^° with respect to the beam. The activity was transferred to the counting area by means of a stepping motor in less than 3  s with the counting carried out over 8  s. The β-delayed α-particles were measured using a time-of-flight method to achieve a sufficiently low background. Standard calibration sources (¹⁴⁸Gd, ²⁴¹Am, ^208,209Po, and ²²⁷Ac) as well as α particles and ⁷Li from the ¹⁰B(n,α)⁷Li reaction were used for an accurate energy calibration. The energy resolution of the catcher foil (180–220 keV) was calculated and the time-of-flight resolution (3–10 nsec) was measured using the β-delayed α-particle emission from ⁸Li that was produced using the d(⁷Li,⁸Li)p reaction with the same setup. The line shape was corrected to account for the variation in the energy and time resolution and a high statistics spectrum of the β-delayed α-particle emission of ¹⁶N is reported. However, our data (as well as earlier Mainz data and unpublished Seattle data) do not agree with an earlier measurement of the β-delayed α-particle emission of ¹⁶N taken at TRIUMF after averaging over the energy resolution of our collection system. This disagreement, among other issues, prohibits accurate inclusion of the f-wave component in the R-matrix analysis.

The Coulomb dissociation (CD) of ⁸B has emerged as a landmark testing ground of the very method of CD for measuring the cross section of the low-energy ⁷Be(p,γ)⁸B direct capture (DC) reaction. Recent claims of evidence of slope difference between CD and DC results are critically examined. We include all relevant RIKEN2 data and all previously published DC data, and we examine the extracted so-called average scale-independent slope (b). The parametrization used by the Seattle group to extract the so-called b-slope parameter is also examined at energies above 300 keV. Considering the physical slope (S^'=dS/dE) above 300 keV, we observe a (1.7σ) agreement between slopes (S^') measured in CD and DC above 300 keV. The claim that S₁₇(0) values extracted from CD data are inconsistent and lower than DC results arises from a neglect of substantial systematic uncertainty of low-energy CD data. A consideration of the published CD S₁₇(0) results yields very consistent S₁₇(0) values that agree with most recent DC measurements. The recent correction of the b-slope parameter suggested by Esbensen, Bertsch, and Snover (EBS) was applied to the wrong b slope calculated using part of the RIKEN2 data. When the correct slope of the RIKEN2 data is used, the EBS correction in fact leads to a substantial disagreement between the slopes of the RIKEN2 data and DC data. In spite of an agreement between CD and DC data neither allow for extracting the slope above 300 keV with high accuracy. Uncertainty of the slope (S^') leads to an additional uncertainty of the extrapolated S₁₇(0). The slope of the astrophysical cross-section factor S₁₇ must be measured with high precision to enable extraction of the d/s ratio and a high-precision extrapolation of S₁₇(0).

A Comment on the Letter by H. Esbensen, G. F. Bertsch, and K. A. Snover, Phys. Rev. Lett. 94, 042502 (2005). The authors of the Letter offer a Reply.

The ep→e^'π⁺n reaction was studied in the first and second nucleon resonance regions in the 0.25 GeV²<Q²<0.65 GeV² range by use of the CLAS detector at Thomas Jefferson National Accelerator Facility. For the first time, to our knowledge, the absolute cross sections were measured, covering nearly the full angular range in the hadronic center-of-mass frame. We extracted the structure functions σ_TL,σ_TT, and the linear combination σ_T+εσ_L by fitting the ϕ dependence of the measured cross sections and compared them with the MAID and Sato-Lee models.

An exclusive measurement of the Coulomb breakup of ⁸B into ⁷Be+p at 254A MeV was used to infer the low-energy ⁷Be(p,γ)⁸B cross section. The radioactive ⁸B beam was produced by projectile fragmentation of 350A MeV ¹²C and separated with the FRagment Separator (FRS) at Gesellschaft für Schwerionenforschung in Darmstadt, Germany. The Coulomb-breakup products were momentum-analyzed in the KaoS magnetic spectrometer; particular emphasis was placed on the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates within the angular cuts selected for the analysis. The deduced astrophysical S₁₇ factors exhibit good agreement with the most recent direct ⁷Be(p,γ)⁸B measurements. By using the energy dependence of S₁₇ according to the recently refined cluster model for ⁸B of P. Descouvemont [Phys. Rev. C 70, 065802 (2004)], we extract a zero-energy S factor of S₁₇(0)=20.6±0.8(stat)±1.2(syst) eV b. These errors do not include the uncertainty of the theoretical model to extrapolate to zero relative energy, estimated by Descouvemont to be about 5%.

We investigate the transition from the nucleon-meson to the quark-gluon description of the strong interaction using the photon energy dependence of the d(γ,p)n differential cross section for photon energies above 0.5 GeV and center-of-mass proton angles between 30° and 150°. A possible signature for this transition is the onset of cross-section s^-11 scaling with the total energy squared, s, at some proton transverse momentum P_T. The results show that the scaling has been reached for proton transverse momentum above about 1.1  GeV/c. This may indicate that the quark-gluon regime is reached above this momentum.

Nearly complete angular distributions of the two-body deuteron photodisintegration differential cross section have been measured using the CEBAF Large Acceptance Spectrometer detector and the tagged photon beam at the Thomas Jefferson National Accelerator Facility. The data cover photon energies between 0.5 and 3.0  GeV and center-of-mass proton scattering angles 10°–160°. The data show a persistent forward-backward angle asymmetry over the explored energy range, and are well described by the nonperturbative quark gluon string model.

As part of a measurement [E. Anciant , Phys. Rev. Lett. 85, 4682 (2000)] of the cross section of ϕ meson photoproduction to high momentum transfer, we measured the polar angular decay distribution of the outgoing K⁺ in the channel ϕ→K⁺K⁻ in the ϕ center-of-mass frame (the helicity frame). We find that s-channel helicity conservation (SCHC) holds in the kinematical range where t-channel exchange dominates (up to −t∼2.5  GeV² for E_γ=3.6  GeV). Above this momentum, u-channel production of a ϕ meson dominates and induces a violation of SCHC. The deduced value of the ϕNN coupling constant lies in the upper range of previously reported values.

The absolute branching ratio of the β-delayed γ-ray emission of ¹⁸N was measured, providing absolute normalization of the previous work by Olness [Nucl. Phys. A373, 13 (1982)] who measured the relative branching ratios for the individual γ-rays. We find the total absolute branching ratio for β-delayed γ-ray emission of ¹⁸N to be 76.7±7.2(stat)±5.8(norm)%. A combination of other results suggests a value consistent with our result, but smaller than that calculated by Millener as quoted in Olness

Measurements of the angular distributions of target and double-spin asymmetries for the Δ⁺(1232) in the exclusive channel p→(e→,e^′p)π⁰ obtained at the Jefferson Lab in the Q² range from 0.5 to 1.5 GeV²/c² are presented. Results of the asymmetries are compared with the unitary isobar model [D. Drechsel et al., Nucl. Phys. A645, 145 (1999)], dynamical models [T. Sato and T. S. Lee, Phys. Rev. C 54, 2660 (1996); S. S. Kamalov et al., Phys. Lett. B 27, 522 (2001)], and the effective Lagrangian theory [R. M. Davidson et al., Phys. Rev. D 43, 71 (1991)]. Sensitivity to the different models was observed, particularly in relation to the description of background terms on which the target asymmetry depends significantly.

The ratios of inclusive electron scattering cross sections of ⁴He, ¹²C, and ⁵⁶Fe to ³He have been measured for the first time. It is shown that these ratios are independent of x_B at Q²>1.4 GeV² for x_B>1.5, where the inclusive cross section depends primarily on the high momentum components of the nuclear wave function. The observed scaling shows that the momentum distributions at high-momenta have the same shape for all nuclei and differ only by a scale factor. The observed onset of the scaling at Q²>1.4 GeV² and x_B>1.5 is consistent with the kinematical expectation that two-nucleon short range correlations (SRC) dominate the nuclear wave function at p_m≳300 MeV/c. The values of these ratios in the scaling region can be related to the relative probabilities of SRC in nuclei with A>~3. Our data, combined with calculations and other measurements of the ³He/deuterium ratio, demonstrate that for nuclei with A>~12 these probabilities are 4.9–5.9 times larger than in deuterium, while for ⁴He it is larger by a factor of about 3.8.

The cross section for the reaction ep→e^′pπ⁺π^- was measured in the resonance region for 1.4<W<2.1  GeV and 0.5<Q²<1.5  GeV²/c² using the CLAS detector at Jefferson Laboratory. The data show resonant structures not visible in previous experiments. The comparison of our data to a phenomenological prediction using available information on N^* and Δ states shows an evident discrepancy. A better description of the data is obtained either by a sizable change of the properties of the P₁₃(1720) resonance or by introducing a new baryon state, not reported in published analyses.

An exclusive measurement of the Coulomb breakup of ⁸B into ⁷Be+p at 254A  MeV allowed the study of the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates and that E2 multipolarity can be neglected. By using a simple single-particle model for ⁸B and treating the breakup in first-order perturbation theory, we extract a zero-energy S factor of S₁₇(0)=18.6±1.2±1.0  eV b, where the first error is experimental and the second one reflects the theoretical uncertainty in the extrapolation.

We report the results of a new measurement of spin structure functions of the deuteron in the region of moderate momentum transfer [Q²=0.27–1.3 (GeV/c)²] and final hadronic state mass in the nucleon resonance region (W=1.08–2.0 GeV). We scattered a 2.5 GeV polarized continuous electron beam at Jefferson Lab off a dynamically polarized cryogenic solid state target (¹⁵ND₃) and detected the scattered electrons with the CEBAF large acceptance spectrometer. From our data, we extract the longitudinal double spin asymmetry A_|| and the spin structure function g₁^d. Our data are generally in reasonable agreement with existing data from SLAC where they overlap, and they represent a substantial improvement in statistical precision. We compare our results with expectations for resonance asymmetries and extrapolated deep inelastic scaling results. Finally, we evaluate the first moment of the structure function g₁^d and study its approach to both the deep inelastic limit at large Q² and to the Gerasimov-Drell-Hearn sum rule at the real photon limit (Q²→0). We find that the first moment varies rapidly in the Q² range of our experiment and crosses zero at Q² between 0.5 and 0.8 (GeV/c)², indicating the importance of the Δ resonance at these momentum transfers.

We measured the inclusive electron-proton cross section in the nucleon resonance region (W<2.5 GeV) at momentum transfers Q² below 4.5 (GeV/c)² with the CLAS detector. The large acceptance of CLAS allowed the measurement of the cross section in a large, contiguous two-dimensional range of Q² and x, making it possible to perform an integration of the data at fixed Q² over the significant x interval. From these data we extracted the structure function F₂ and, by including other world data, we studied the Q² evolution of its moments, M_n(Q²), in order to estimate higher twist contributions. The small statistical and systematic uncertainties of the CLAS data allow a precise extraction of the higher twists and will require significant improvements in theoretical predictions if a meaningful comparison with these new experimental results is to be made.

The first measurements of the transferred polarization for the exclusive e→p→e^′K⁺Λ→ reaction have been performed at Jefferson Laboratory using the CLAS spectrometer. A 2.567 GeV beam was used to measure the hyperon polarization over Q² from 0.3 to 1.5   (GeV/c)², W from 1.6 to 2.15 GeV, and over the full K⁺ center-of-mass angular range. Comparison with predictions of hadrodynamic models indicates strong sensitivity to the underlying resonance contributions. A nonrelativistic quark-model interpretation of our data suggests that the ss̅ quark pair is produced with spins predominantly antialigned. Implications for the validity of the most widely used quark-pair creation operator are discussed.

The differential cross section, dσ/dt, for ω meson exclusive photoproduction on the proton above the resonance region (2.6<W<2.9  GeV) was measured up to a momentum transfer -t=5  GeV² using the CLAS detector at Jefferson Laboratory. The ω channel was identified by detecting a proton and π⁺ in the final state and using the missing mass technique. While the low momentum transfer region shows the typical diffractive pattern expected from Pomeron and Reggeon exchange, at large -t the differential cross section has a flat behavior. This feature can be explained by introducing quark interchange processes in addition to the QCD-inspired two-gluon exchange.

Differential cross sections for γp→ηp have been measured with tagged real photons for incident photon energies from 0.75 to 1.95 GeV. Mesons were identified by missing mass reconstruction using kinematical information for protons scattered in the production process. The data provide the first extensive angular distribution measurements for the process above W=1.75  GeV. Comparison with preliminary results from a constituent quark model support the suggestion that a third S₁₁ resonance with mass ∼1.8  GeV couples to the ηN channel.

Models of baryon structure predict a small quadrupole deformation of the nucleon due to residual tensor forces between quarks or distortions from the pion cloud. Sensitivity to quark versus pion degrees of freedom occurs through the Q² dependence of the magnetic (M₁₊), electric (E₁₊), and scalar (S₁₊) multipoles in the γ^*p→Δ⁺→pπ⁰ transition. We report new experimental values for the ratios E₁₊/M₁₊ and S₁₊/M₁₊ over the range Q²  =  0.4–1.8 GeV², extracted from precision p(e,e^′p)π⁰ data using a truncated multipole expansion. Results are best described by recent unitary models in which the pion cloud plays a dominant role.

The double spin asymmetry in the e→p→→e^′π⁺n reaction has been measured for the first time in the resonance region for four-momentum transfer Q²  =  0.35–1.5 GeV². Data were taken at Jefferson Lab with the CLAS detector using a 2.6 GeV polarized electron beam incident on a polarized solid NH₃ target. Comparison with predictions of phenomenological models shows strong sensitivity to resonance contributions. Helicity- 1/2 transitions are found to be dominant in the second and third resonance regions. The measured asymmetry is consistent with a faster rise with Q² of the helicity asymmetry A₁ for the F₁₅(1680) resonance than expected from the analysis of the unpolarized data.

We studied the exclusive reaction ep→e^′p^′φ using the φ→K⁺K^- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Our experiment covers the range in Q² from 0.7 to 2.2 GeV², and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of φ production on the proton. Our measurement shows the expected decrease of the t slope with the vector-meson formation time cΔτ below 2 fm. At 〈cΔτ〉=0.6 fm, we measure b_φ=2.27±0.42 GeV^-2. The cross section dependence on W as W^0.2±0.1 at Q²=1.3 GeV² was determined by comparison with φ production at HERA after correcting for threshold effects. This is the same dependence as observed in photoproduction.

We report the first results of the beam-spin asymmetry measured in the reaction e→p→epγ at a beam energy of 4.25 GeV. A large asymmetry with a sinφ modulation is observed, as predicted for the interference term of deeply virtual compton scattering (DVCS) and the Bethe-Heitler process. The amplitude of this modulation is α  =  0.202±0.028. In leading-order and leading-twist perturbative QCD, the α is directly proportional to the imaginary part of the DVCS amplitude.