Search Results (117 total)

High-precision measurements of the proton elastic form-factor ratio, μ_pG_E^p/G_M^p, have been made at four-momentum transfer, Q², values between 0.2 and 0.5  GeV². The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q² range the deviation from unity is primarily due to G_E^p being smaller than expected.

Precision measurements of the relative analyzing powers of five electron beam polarimeters, based on Compton, Møller, and Mott scattering, have been performed using the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory). A Wien filter in the 100 keV beam line of the injector was used to vary the electron spin orientation exiting the injector. High statistical precision measurements of the scattering asymmetry as a function of the spin orientation were made with each polarimeter. Since each polarimeter receives beam with the same magnitude of polarization, these asymmetry measurements permit a high statistical precision comparison of the relative analyzing powers of the five polarimeters. This is the first time a precise comparison of the analyzing powers of Compton, Møller, and Mott scattering polarimeters has been made. Statistically significant disagreements among the values of the beam polarization calculated from the asymmetry measurements made with each polarimeter reveal either errors in the values of the analyzing power or failure to correctly include all systematic effects. The measurements reported here represent a first step toward understanding the systematic effects of these electron polarimeters. Such studies are necessary to realize high absolute accuracy (ca. 1%) electron polarization measurements, as required for some parity violation measurements planned at Jefferson Laboratory. Finally, a comparison of the value of the spin orientation exiting the injector that provides maximum longitudinal polarization in each experimental hall leads to an independent and very precise (better than 10^-4) absolute measurement of the final electron beam energy.

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.

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.

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.

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.

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.

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.

Using a data sample with an integrated luminosity of 3.9 fb^-1 collected in e⁺e^- annihilation with the CLEO-II detector at the Cornell Electron Storage Ring, we have measured the branching ratios for the decay modes D_s⁺→(η,η^′)π⁺ and D_s⁺→(η,η^′)ρ⁺ relative to D_s⁺→φπ⁺. These decay modes are among the most common hadronic decays of the D_s⁺, and can be related by factorization to the semileptonic decays D_s⁺→(η,η^′)l⁺ν_l. The results obtained are compared with previous CLEO results and with the branching ratios measured for the related semileptonic decays. We also report results on the Cabibbo-suppressed decays of the D⁺ to the same final states.

The decays ϒ(4S)→BB̅ , followed by B→D^*π and D^*→Dπ, permit reconstruction of all kinematic quantities that describe the sequence without reconstruction of the D, with reasonably low backgrounds. Using an integrated e⁺e^- luminosity of 3.1 fb^-1 accumulated at the ϒ(4S) by the CLEO-II detector, we report measurements of B(B̅ ⁰→D^*+π^-)  =  (2.81±0.11±0.21±0.05)×10^-3 and B(B^-→D^*0π^-)  =  (4.34±0.33±0.34±0.18)×10^-3.

Using data taken with the CLEO II detector at the Cornell Electron Storage Ring, we present the first full angular analysis in the color-suppressed modes B⁰→J/ψ K^*0 and B⁺→J/ψ K^*+. This leads to a complete determination of the decay amplitudes of these modes including the longitudinal polarization γ_L/γ  =  0.52±0.07±0.04 and the P wave component |P|²  =  0.16±0.08±0.04. In addition, we update the branching fractions for B→J/ψ K and B→J/ψ K^*.

Using data collected in the region of the ϒ(4S) resonance with the CLEO-II detector, we report on the first observation of exclusive decays of the B meson to final states with a charmed baryon. We have measured the branching fractions B(B^-→Λ_c⁺p̅ π^-)  =  (0.62_-0.20^+0.23±0.11±0.10)×10^-3 and B(B̅ ⁰→Λ_c⁺p̅ π⁺π^-)  =  (1.33_-0.42^+0.46±0.31±0.21)×10^-3, where the first error is statistical, the second is systematic, and the third is due to uncertainty in the Λ_c⁺ branching fractions. In addition, we report upper limits for final states of the form B̅ →Λ_c⁺p̅ (nπ) and Λ_c⁺p̅ (nπ)π⁰, where (nπ) denotes up to four charged pions.

We report new measurements of the differential and total branching ratios for inclusive B decay to D⁰, D⁺, and D^*+ and the first measurement of the same quantities for inclusive B decay to D^*0. Here B is the mixture of B_d and B_u from Υ(4S) decay. Furthermore, since more than one charm particle (or antiparticle) of the same kind can be produced in B decay, here “inclusive B branching ratio” is used to mean the average number of charm particles and their antiparticles of a certain species produced in B decay. We obtain the following results (the first error is statistical, the second systematic of this analysis, the third propagated from other measurements): B(B→D⁰X)=(0.636±0.014±0.019±0.018), B(B→D⁺X)=(0.235±0.009±0.009±0.024), B(B→D^*0X)=(0.247±0.012±0.018±0.018), B(B→D^*+X)=(0.239±0.011±0.014±0.009). The following ratio of branching ratios is not affected by most of the systematic errors: B(B→D^*0X)/B(B→D^*+X)=(1.03±0.07±0.09±0.08). We also report the first measurement of the momentum-dependent D^*0 polarization and a new measurement of the D^*+ polarization in inclusive B decay. Using these measurements and other CLEO results and making some additional assumptions, we calculate the average number of c and c̅ quarks produced in B decay to be 〈n_c〉=1.10±0.05.

Using the CLEO detector at the Cornell e⁺e^- storage ring CESR we study the two-photon production of ΛΛ̅ , making the first observation of γγ→ΛΛ̅ . We present the cross section for γγ→ΛΛ̅ as a function of the γγ center of mass energy and compare it to that predicted by the quark-diquark model.

Using the CLEO-II data set we have searched for the decays B⁰→D^(*)+D^(*)-. We observe one candidate signal event for the decay B⁰→D^*+D^*- with an expected background of 0.022±0.011 events. This yield corresponds to a branching fraction of B(B⁰→D^*+D^*-)  =  [5.3_-3.7^+7.1(stat)±1.0(syst)]×10^-4 and an upper limit of B(B⁰→D^*+D^*-)<2.2×10^-3 at the 90% C.L. We see no significant excess of signal above the expected background level in the other modes, and we calculate the 90% C.L. upper limits on the branching fractions to be B(B⁰→D^*±D^∓)<1.8×10^-3 and B(B⁰→D⁺D^-)<1.2×10^-3.

Using data collected with the CLEO II detector at the Cornell Electron Storage Ring, we determine the ratio R_chrg for the mean charged multiplicity observed in Υ(1S)→ggγ events, 〈n_gluon^±〉, to the mean charged multiplicity observed in e⁺e^-→qq̅ γ events, 〈n_quark^±〉. We find R_chrg≡〈n_gluon^±〉/〈n_quark^±〉=1.04 ±0.02(stat)±0.05(syst) for jet-jet masses less than 7 GeV.

We have searched for the decays B→μν̅ _μγ and B→eν̅ _eγ in a sample of 2.7×10⁶ charged B decays collected with the CLEO II detector. In the muon channel, we observe no candidates in the signal region and set an upper limit on the branching fraction of B(B→μν̅ _μγ)<5.2×10^-5 at the 90% confidence level. In the electron channel, we observe five candidates in the signal region and set an upper limit on the branching fraction of B(B→eν̅ _eγ)<2.0×10^-4 at the 90% confidence level.

We have measured the spectral shape Michel parameters ρ and η using leptonic decays of the τ, recorded by the CLEO II detector. Assuming e-μ universality in the vectorlike couplings, we find ρ_eμ  =  0.735±0.013±0.008 and η_eμ  =  -0.015±0.061±0.062, where the first error is statistical and the second systematic. We also present measurements for the parameters for e and μ final states separately.

We report a measurement of the magnitude of the ν_τ helicity from τ-pair events taken with the CLEO detector at the CESR electron-positron storage ring. Events in which each τ undergoes the decay τ→hν, with h a charged pion or kaon, are analyzed for energy correlations between the daughter hadrons, yielding |ξ_h| = 1.03±0.06±0.04, with the first error statistical and the second systematic.

Using data taken with the CLEO II detector at the Cornell Electron Storage Ring, we have determined the ratio of branching fractions: R_γ≡Γ(Υ(1S)→γgg)/Γ(Υ(1S)→ggg)=[2.75±0.04(stat)±0.15(syst)]%. From this ratio, we have determined the QCD scale parameter Λ_MS̅ (defined in the modified minimal subtraction scheme) to be Λ_MS̅ =233±11±59 MeV, from which we determine a value for the strong coupling constant α_s(M_Υ(1S))=0.163±0.002±0.014, or α_s(M_Z)=0.110±0.001±0.007.