Search Results (21 total)

The (W,Q²) dependence of the ratio of inclusive electron scattering cross sections for ¹⁵N/¹²C was determined in the kinematic ranges 0.8<W<2 GeV and 0.2<Q²<1 GeV² using 2.285 GeV electrons and the CLAS detector at Jefferson Lab. The ratios exhibit only slight resonance structure, in agreement with a simple phenomenological model and an extrapolation of deep-inelastic scattering ratios to low Q². Ratios of ⁴He/¹²C using 1.6 to 2.5 GeV electrons were measured with very high statistical precision and were used to correct for He in the N and C targets. The (W,Q²) dependence of the ⁴He/¹²C ratios is in good agreement with that of the phenomenological model and exhibit significant resonance structure centered at W=0.94,1.23, and 1.5 GeV.

The light vector mesons (ρ,ω, and ϕ) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the ρ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to e⁺e^-. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The ρ-meson mass spectrum was extracted after the ω and ϕ signals were removed in a nearly model-independent way. Comparisons were made between the ρ mass spectra from the heavy targets (A>2) with the mass spectrum extracted from the deuterium target. With respect to the ρ-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.

The beam-spin asymmetries in the hard exclusive electroproduction of photons on the proton (e→p→epγ) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (x_B from 0.11 to 0.58, Q² from 1 to 4.8  GeV², -t from 0.09 to 1.8  GeV²), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A≃asin⁡ϕ/(1+ccos⁡ϕ). This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector.

We examine the results of two measurements by the CLAS collaboration, one of which claimed evidence for a Θ⁺ pentaquark, while the other found no such evidence. The unique feature of these two experiments was that they were performed with the same experimental setup. Using a Bayesian analysis, we find that the results of the two experiments are in fact compatible with each other, but that the first measurement did not contain sufficient information to determine unambiguously the existence of a Θ⁺. Further, we suggest a means by which the existence of a new candidate particle can be tested in a rigorous manner.

The photoproduction of vector mesons on various nuclei has been studied using the CLAS detector at Jefferson Laboratory. The vector mesons, ρ, ω, and ϕ, are observed via their decay to e⁺e^-, in order to reduce the effects of final-state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the ρ meson. The ρ mass spectrum is extracted from the data on various nuclei, ²H, C, Fe, and Ti. We observe no significant mass shift and some broadening consistent with expected collisional broadening for the ρ meson.

The reaction ²H(e,e^′p)n has been studied with full kinematic coverage for photon virtuality 1.75<Q²<5.5  GeV². Comparisons of experimental data with theory indicate that for very low values of neutron recoil momentum (p_n<100  MeV/c) the neutron is primarily a spectator and the reaction can be described by the plane-wave impulse approximation. For 100<p_n<750  MeV/c, proton-neutron rescattering dominates the cross section, while Δ production followed by the NΔ→NN transition is the primary contribution at higher momenta.

The exclusive reactions γp→K̅ ⁰K⁺n and γp→K̅ ⁰K⁰p have been studied in the photon energy range 1.6–3.8 GeV, searching for evidence of the exotic baryon Θ⁺(1540) in the decays Θ⁺→nK⁺ and Θ⁺→pK⁰. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. The integrated luminosity was about 70  pb^-1. The reactions have been isolated by detecting the K⁺ and proton directly, the neutral kaon via its decay to K_S→π⁺π^- and the neutron or neutral kaon via the missing mass technique. The mass and width of known hyperons such as Σ⁺, Σ^- and Λ(1116) were used as a check of the mass determination accuracy and experimental resolution. Approximately 100 000 Λ^*(1520)’s and 150 000 ϕ’s were observed in the K̅ ⁰K⁺n and K̅ ⁰K⁰p final state, respectively. No evidence for the Θ⁺ pentaquark was found in the nK⁺ or pK_S invariant mass spectra. Upper limits were set on the production cross section of the reaction γp→K̅ ⁰Θ⁺ as functions of center-of-mass angle, nK⁺ and pK_S masses. Combining the results of the two reactions, the 95% C.L. upper limit on the total cross section for a resonance peaked at 1540 MeV was found to be 0.7 nb. Within most of the available theoretical models, this corresponds to an upper limit on the Θ⁺ width, Γ_Θ⁺, ranging between 0.01 and 7 MeV.

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.

In an exclusive measurement of the reaction γd→K⁺K^-pn, a narrow peak that can be attributed to an exotic baryon with strangeness S=+1 is seen in the K⁺n invariant mass spectrum. The peak is at 1.542±0.005  GeV/c² with a measured width of 0.021  GeV/c² FWHM, which is largely determined by experimental mass resolution. The statistical significance of the peak is (5.2±0.6)σ. The mass and width of the observed peak are consistent with recent reports of a narrow S=+1 baryon by other experimental groups.

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 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 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.

We apply the single quark transition model to resonance transition amplitudes extracted from photo-production and electroproduction data. We use experimental data on the S₁₁(1535) and D₁₃(1520) nucleon resonances to extract the amplitudes for the electromagnetic transition from the nucleon ground state [56, 0⁺] to the [70, 1^-] supermultiplet, and make predictions for the transition amplitudes of all other states associated with the [70, 1^-]. We compare the predictions with data and find surprisingly good agreement. The comparison is hampered by the poor data quality for many of the states especially in the electroproduction sector.

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.

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.

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.

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.