Search Results (32 total)

We report on the first measurement of exclusive Ξ^-(1321) hyperon photoproduction in γp→K⁺K⁺Ξ^- for 3.2<E_γ<3.9 GeV. The final state is identified by the missing mass in p(γ,K⁺K⁺)X measured with the CLAS detector at Jefferson Laboratory. We have detected a significant number of the ground state Ξ^-(1321)1 / 2⁺ and have estimated the total cross section for its production. We also have strong evidence for the first excited state Ξ^-(1530)3 / 2⁺. Photoproduction provides a copious source of Ξ's. We discuss the possibilities of a search for the recently proposed Ξ₅^- and Ξ₅⁺ pentaquarks.

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 reaction γp→π⁺K^-K⁺n was studied at Jefferson Laboratory using a tagged photon beam with an energy range of 3–5.47 GeV. A narrow baryon state with strangeness S=+1 and mass M=1555±10  MeV/c² was observed in the nK⁺ invariant mass spectrum. The peak’s width is consistent with the CLAS resolution (FWHM=26  MeV/c²), and its statistical significance is (7.8±1.0)σ. A baryon with positive strangeness has exotic structure and cannot be described in the framework of the naive constituent quark model. The mass of the observed state is consistent with the mass predicted by the chiral soliton model for the Θ⁺ baryon. In addition, the pK⁺ invariant mass distribution was analyzed in the reaction γp→K^-K⁺p with high statistics in search of doubly charged exotic baryon states. No resonance structures were found in this spectrum.

Double-polarization asymmetries for inclusive ep scattering were measured at Jefferson Lab using 2.6 and 4.3 GeV longitudinally polarized electrons incident on a longitudinally polarized NH₃ target in the CLAS detector. The polarized structure function g₁(x,Q²) was extracted throughout the nucleon resonance region and into the deep inelastic regime, for Q²=0.15–1.64   GeV². The contributions to the first moment Γ₁(Q²)=∫g₁(x,Q²) dx were determined up to Q²=1.2   GeV². Using a parametrization for g₁ in the unmeasured low x regions, the complete first moment was estimated over this Q² region. A rapid change in Γ₁ is observed for Q²<1   GeV², with a sign change near Q²=0.3   GeV², indicating dominant contributions from the resonance region. At Q²=1.2   GeV² our data are below the perturbative QCD evolved scaling value.

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.

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.

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.

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.

The reaction ep→e^′K⁺Λ(1520) with Λ(1520)→p^′K^- was studied at electron beam energies of 4.05, 4.25, and 4.46 GeV, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cos θ_K⁺, φ_K⁺, Q², and W dependencies of Λ(1520) electroproduction are presented for the kinematic region 0.9<Q²<2.4 GeV² and 1.95<W<2.65 GeV. Also, the Q² dependence of the Λ(1520) decay angular distribution is presented for the first time. The cos θ_K⁺ angular distributions suggest t-channel diagrams dominate the production process. Fits to the Λ(1520) t-channel helicity frame decay angular distributions indicate the m_z=±1 / 2 parentage accounts for about 60% of the total yield, which suggests this reaction has a significant contribution from t-channel processes with either K⁺ exchange or longitudinal coupling to an exchanged K^*. The Q² dependence of the Λ(1520) production cross section is the same as that observed for Λ(1116) photo- and electroproduction.

New cross sections for the reaction ep→epη are reported for total center of mass energy W  =  1.5–1.86 GeV and invariant momentum transfer Q²  =  0.25–1.5 (GeV/c)². This large kinematic range allows extraction of important new information about response functions, photocouplings, and ηN coupling strengths of baryon resonances. Newly observed structure at W∼1.65 GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S₁₁(1535) resonance.

The cross section for φ meson photoproduction on the proton has been measured for the first time up to a four-momentum transfer -t  =  4  GeV², using the CLAS detector at the Thomas Jefferson National Accelerator Facility. At low four-momentum transfer, the differential cross section is well described by Pomeron exchange. At large four-momentum transfer, above -t  =  1.8  GeV², the data support a model where the Pomeron is resolved into its simplest component, two gluons, which may couple to any quark in the proton and in the φ.

The momentum-transfer dependence of the ³H and ³He knockout reactions from ⁶Li via exclusive electron scattering has been measured, and the two reactions are compared. In the absence of two-step processes, the ratio of the fivefold cross sections for these mirror reactions should simply scale by the ratio of the ³H and ³He electron-scattering cross sections. A significant deviation from this simple expectation is seen at low momentum transfer. Possible explanations for this dramatic difference in cross sections for these mirror reactions are discussed.

Low-multipolarity magnetic transitions in ⁴⁰Ca were excited by 180° inelastic electron scattering. Measurements were made in the excitation energy range of 9 to 14 MeV, at low momentum transfers corresponding to q values of 0.3 to 0.5 fm^-1. Multipolarities and transition strengths were determined in a model-independent analysis for all the observed transitions and compared with previous published and unpublished electron scattering data. The experimental M1 strength distributions are compared with configuration-mixing shell-model calculations.

Low-multipolarity magnetic transitions of ³⁰Si, ³²S, and ³⁴S were studied by 180° inelastic electron scattering at low momentum transfers (q∼0.3–0.5 fm^-1). These measurements, made in the excitation energy region from 9 to 14 MeV, revealed several previous unreported levels. These are the first low-q results obtained by electron scattering for ³⁰Si and ³⁴S. Multipolarities and transition strengths for all the observed transitions were determined in a model-independent analysis. A large fragmentation of M1 strength is observed in the 4N+2 nuclei ³⁰Si and ³⁴S, while more strength is concentrated into fewer transitions in the self-conjugate nucleus ³²S. The experimental M1 strength distributions are compared with configuration-mixing shell-model calculations. The sums of the transition strengths are in good agreement with recent shell-model calculations using an effective M1 operator.

Elastic electron scattering cross sections have been measured for ⁸⁹Y at 180° for 0.72<q<2.67 fm^-1. The M1 form factor has been extracted and a Fourier-Bessel analysis performed to obtain the ground-state magnetization density. Calculations including effects of core polarization have been performed within the framework of the finite Fermi system theory. These show a weakening of the strong repulsion in the spin-isospin channel and are in overall agreement with the data.

New cross section and analyzing power data for ⁸⁸Sr(p,p’) at E_p=200 MeV are reported and analyzed together with earlier cross section data. Neutron transition densities for low-lying normal-parity excitations were extracted using an empirical density-dependent effective interaction and an expansion of the radial density which permits analysis of uncertainties due to penetrability, distortion, incompleteness, statistics, and normalization. The densities were compared with shell-model calculations using effective operators based upon a density-dependent Hartree-Fock approximation. Good agreement between theory and experiment is obtained for the 2₁⁺, 3₁^-, and 5₁^- states, but the experimental neutron densities for the 2₂⁺ and 7₁^- states are considerably stronger than expected. Both proton and neutron transition densities for the 2₂⁺ state have strong interior lobes, but the theory fails to reproduce the accompanying surface lobes.

Differential cross sections for electron scattering from ⁸⁶Sr have been measured for excited states below 3.5 MeV over a range of effective momentum transfer of 0.4 to 2.8 fm^-1. The elastic scattering data have been analyzed to reconstruct the ground-state charge distribution. The inelastic scattering data have been analyzed to extract transition charge densities. Interpretation of the underlying nuclear structure is given in the framework of a self-consistent finite Fermi system theory.

Inelastic electron scattering cross sections have been measured from high spin transitions in ²⁰⁸Pb. Measurements were made over a range of momentum transfer of 1.1 to 2.9 fm^-1 with energy resolution between 14 and 22 keV. In addition to the previously measured 14^-, 12^-, and 12⁺ levels, electron scattering cross sections have been measured on M11, M10, M9, E9, and E7 transitions. High spin states are quenched from the predicted Woods-Saxon single particle amplitudes by 22% to 67%. Evidence is presented for the fragmentation of the 7.06 MeV 12^- transition strength. Results are interpreted in terms of the independent particle model.

Electron scattering cross sections have been measured for the elastic and the first 2⁺,3^- and low-lying 4⁺ excitations in ¹¹⁸Sn. Forward angle data were taken for momentum transfers in the range 1.24<q<2.74 fm^-1 allowing the extraction of transition charge densities in a Fourier-Bessel analysis. Comparisons are made with microscopic calculations performed in the framework of the self-consistent finite Fermi system theory and quasiparticle phonon approach.

High resolution electron scattering measurements at 180° have been performed on ¹³C at low momentum transfer. Transverse form factors have been extracted for transitions to the 3.68 MeV 3/2^- and 3.85 MeV 5/2⁺ states, with emphasis on mapping the low-q behavior of the magnetic contributions to these form factors. The magnetic transition strengths to the 1/2^- ground state have been determined. The results are compared to one-body density matrix element calculations using both Woods-Saxon and harmonic oscillator wave functions. The data are in good agreement with the parameters derived for these wave functions from previous measurements taken at higher momentum transfer.

Differential cross sections for electron scattering from ¹⁴⁰Ce have been measured for excitation energies less than 3.3 MeV over a range of effective momentum transfer of 0.4 to 3.1 fm^-1. The data have been analyzed to extract transition charge densities. These densities are interpreted in terms of a quasiparticle-phonon model and self-consistent finite Fermi system theory to disentangle the collective and noncollective modes of excitation.