Search Results (79 total)

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.

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.

We report the first evidence for a nonzero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic kinematic region. Data for the reaction ep→e^′π⁺X have been obtained using a polarized electron beam of 4.3 GeV with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The amplitude of the sin φ modulation increases with the momentum of the pion relative to the virtual photon, z. In the range z=0.5–0.8 the average amplitude is 0.038±0.005±0.003 for a missing mass M_X>1.1 GeV and 0.037±0.007±0.004 for M_X>1.4 GeV.

High-statistics cross sections and recoil polarizations for the reactions γ+p→K⁺+Λ and γ+p→K⁺+Σ⁰ have been measured at CLAS for center-of-mass energies between 1.6 and 2.3  GeV. In the K⁺Λ channel we confirm a resonance-like structure near W=1.9  GeV at backward kaon angles. Our data show more complex s- and u- channel behavior than previously seen, since structure is also present at forward angles, but not at central angles. The position and width change with angle, indicating that more than one resonance is playing a role. Large positive Λ polarization at backward angles, which is also energy dependent, is consistent with sizable s- or u-channel contributions. Presently available model calculations cannot explain these aspects of the data.

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.

We have measured the ³He(e,e^′pp)n reaction at 2.2 GeV over a wide kinematic range. The kinetic energy distribution for “fast” nucleons (p>250   MeV/c) peaks where two nucleons each have 20% or less, and the third nucleon has most of the transferred energy. These fast pp and pn pairs are back to back with little momentum along the three-momentum transfer, indicating that they are spectators. Calculations by Sargsian and by Laget also indicate that we have measured distorted two-nucleon momentum distributions by striking one nucleon and detecting the spectator correlated pair.

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.

The polarized longitudinal-transverse structure function σ_LT^′ has been measured in the Δ(1232) resonance region at Q²=0.40 and 0.65 GeV². Data for the p(e→,e^′p)π⁰ reaction were taken at Jefferson Lab with the CEBAF large acceptance spectrometer (CLAS) using longitudinally polarized electrons at an energy of 1.515 GeV. For the first time a complete angular distribution was measured, permitting the separation of different nonresonant amplitudes using a partial wave analysis. Comparison with previous beam asymmetry measurements at MAMI indicate a deviation from the predicted Q² dependence of σ_LT^′ using recent phenomenological models.

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.

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.

The differential cross section, dσ/dt, for ρ⁰ meson photoproduction on the proton above the resonance region was measured up to a momentum transfer -t  =  5 GeV² using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The ρ⁰ channel was extracted from the measured two charged-pion cross sections by fitting the π⁺π^- and pπ⁺ invariant masses. The low momentum transfer region shows the typical diffractive pattern expected from Reggeon exchange. The flatter behavior at large -t cannot be explained solely in terms of QCD-inspired two-gluon exchange models. The data indicate that other processes, like quark interchange, are important to fully describe ρ photoproduction.

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.