Search Results (88 total)

We present a measurement of the spin-dependent cross sections for the ³He→(e→,e^′)X reaction in the quasielastic and resonance regions at a four-momentum transfer 0.1≤Q²≤0.9  GeV². The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt-Cottingham and extended Gerasimov-Drell-Hearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact three-body Faddeev calculation in the quasielastic region.

The reaction K^-p→Σ⁰π⁰→ neutrals was studied with the Crystal Ball detector at the BNL Alternating Gradient Synchrotron for beam momenta between 514 and 750 MeV/c. The photons from Σ⁰ and π⁰ decays were detected, as well as the neutron in a small fraction of the events. The differential cross section and the Σ⁰ polarization are reported at eight momenta and nearly the full angular range. Total cross sections are derived from Legendre polynomial fits to the data. The measurements presented are of considerably higher precision than previous results.

The data analysis for the reaction ¹H(e,e^'π⁺)n, which was used to determine values for the charged pion form factor F_π for values of Q²= 0.6–1.6 GeV², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract F_π from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of F_π are presented.

We measured angular distributions of differential cross section, beam analyzing power, and recoil polarization for neutral pion electroproduction at Q²=1.0 (GeV/c)² in 10 bins of 1.17≤W≤1.35 GeV across the Δ resonance. A total of 16 independent response functions were extracted, of which 12 were observed for the first time. Comparisons with recent model calculations show that response functions governed by real parts of interference products are determined relatively well near the physical mass, W=M_Δ≈1.232 GeV, but the variation among models is large for response functions governed by imaginary parts, and for both types of response functions, the variation increases rapidly with W>M_Δ. We performed a multipole analysis that adjusts suitable subsets of ℓ_π≤2 amplitudes with higher partial waves constrained by baseline models. This analysis provides both real and imaginary parts. The fitted multipole amplitudes are nearly model independent—there is very little sensitivity to the choice of baseline model or truncation scheme. By contrast, truncation errors in the traditional Legendre analysis of N→Δ quadrupole ratios are not negligible. Parabolic fits to the W dependence around M_Δ for the multiple analysis gives values for Re(S₁₊/M₁₊)=(-6.61±0.18)% and Re(E₁₊/M₁₊)=(-2.87±0.19)% for the pπ⁰ channel at W=1.232 GeV and Q²=1.0 (GeV/c)² that are distinctly larger than those from the Legendre analysis of the same data. Similarly, the multipole analysis gives Re(S₀₊/M₁₊)=(+7.1±0.8)% at W=1.232 GeV, consistent with recent models, while the traditional Legendre analysis gives the opposite sign because its truncation errors are quite severe.

The ¹H(e,e^′π⁺)n cross section was measured at four-momentum transfers of Q²=1.60 and 2.45  GeV² at an invariant mass of the photon nucleon system of W=2.22  GeV. The charged pion form factor (F_π) was extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F_π is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q² by one sigma, but is still far from its perturbative quantum chromodynamics prediction.

The recoil proton polarization was measured in the ²H(e→,e^′p→)n reaction in Hall A of the Thomas Jefferson National Accelerator Facility. The electron kinematics were centered on the quasielastic peak (x_Bj≈1) and included three values of the squared four-momentum transfer, Q²=0.43,1.00 and 1.61 (GeV/c)². For Q²=0.43 and 1.61 (GeV/c)², the missing momentum, p_m, was centered at zero, whereas for Q²=1.00  (GeV/c)² two values of p_m were chosen: 0 and 174  MeV/c. At low p_m, the Q² dependence of the longitudinal polarization, P_z^′, is not well described by a state-of-the-art calculation. Further, at higher p_m, a 3.5σ discrepancy was observed in the transverse polarization, P_x^′. Understanding the origin of these discrepancies is important to confidently extract the neutron electric form factor from the analogous ²H(e→,e^′n→)p experiment.

A new upper limit is presented for the branching ratio (BR) of η→3γ. This decay is not allowed by charge-conjugation invariance. We have determined that BR(η→3γ)<4×10^-5. We also obtained BR(η→π⁰γ→3γ)<9×10^-5; this decay mode is absolutely forbidden, as it violates conservation of angular momentum. Finally, we have established that BR(η→π⁰π⁰γγ)<1.2×10^-3 in a restricted diphoton-energy region. All three upper limits are at the 90% confidence level. They were obtained by using the Crystal Ball multiphoton spectrometer at the AGS and are based on the analysis of 28 million η mesons produced in the π^-p→ηn reaction close to threshold.

We measured angular distributions of recoil-polarization response functions for neutral pion electroproduction for W=1.23  GeV at Q²=1.0  (GeV/c)², obtaining 14 separated response functions plus 2 Rosenbluth combinations; of these, 12 have been observed for the first time. Dynamical models do not describe quantities governed by imaginary parts of interference products well, indicating the need for adjusting magnitudes and phases for nonresonant amplitudes. We performed a nearly model-independent multipole analysis and obtained values for Re (S₁₊/M₁₊)=-(6.84±0.15)% and Re (E₁₊/M₁₊)=-(2.91±0.19)% that are distinctly different from those from the traditional Legendre analysis based upon M₁₊ dominance and ℓ_π≤1 truncation.

The branching ratio (BR) for the rare decay η→π⁰γγ was measured with the Crystal Ball multiphoton spectrometer. The result, BR(η→π⁰γγ)=(3.5±0.7_stat±0.6_syst)×10^-4, is in agreement with calculations of chiral perturbation theory to third order. We have used a sample of 28 million η mesons produced at the BNL Alternating Gradient Synchrotron using the π^-p→ηn reaction close to threshold. We detail the intricate extraction of the signal, which has about 1.6 thousand η→π⁰γγ events, from the overwhelming background of η→3π⁰ decays and from the π^-p→π⁰π⁰n reaction.

We report a new measurement of the differential cross section for π^-p→π⁰n from p_π=649 to 752 MeV/c, which is around the opening of the η channel (685 MeV/c). Our data support the main features of the π^-p charge-exchange differential cross sections generated by the SAID πN partial-wave analysis. The opening of the η channel has a clear effect on the shape of the excitation function for dσ(π^-p→π⁰n), which is most noticeable in the backward direction.

The differential cross section for η production in reaction π^-p→ηn has been measured over the full angular range at seven incident π^- beam momenta from threshold to p_π^-=747 MeV/c using the Crystal Ball multiphoton spectrometer. The angular distributions are S wave dominated. At 10 MeV/c above threshold, a small D-wave contribution appears that interferes with the main S wave. The total η production cross section σ^tot is obtained by integration of dσ/dΩ. Starting at threshold, σ^tot rises rapidly, as expected for S-wave-dominated production. The features of the π^-p→ηn cross section are strikingly similar to those of the SU(3) flavor-related process K^-p→ηΛ. Comparison of the π^-p→ηn reaction is made with η photoproduction.

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

The ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

We report on the first determination of upper limits on the branching ratio (BR) of η decay to π⁰π⁰γ and to π⁰π⁰π⁰γ. Both decay modes are strictly forbidden by charge conjugation (C) invariance. Using the Crystal Ball multiphoton detector, we obtained BR(η→π⁰π⁰γ)<5×10^-4 at the 90% confidence level, in support of C invariance of isoscalar electromagnetic interactions of the light quarks. We have also measured BR(η→π⁰π⁰π⁰γ)<6×10^-5 at the 90% confidence level, in support of C invariance of isovector electromagnetic interactions.

The generalized forward spin polarizabilities γ₀ and δ_LT of the neutron have been extracted for the first time in a Q² range from 0.1 to 0.9  GeV². Since γ₀ is sensitive to nucleon resonances and δ_LT is insensitive to the Δ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on δ_LT show significant disagreement with chiral perturbation theory calculations, while the data for γ₀ at low Q² are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

Differential cross sections for the process π⁻p→γn have been measured at Brookhaven National Laboratory’s Alternating Gradient Synchrotron with the Crystal Ball multiphoton spectrometer. Measurements were made at 18 pion momenta from 238 to 748  MeV∕c, corresponding to E_γ for the inverse reaction from 285 to 769  MeV. The data have been used to evaluate the γn multipoles in the vicinity of the N(1440) resonance. We compare our data and multipoles to previous determinations.

The physics program in Hall A at Jefferson Lab commenced in the summer of 1997 with a detailed investigation of the ¹⁶O(e,e^′p) reaction in quasielastic, constant (q,ω) kinematics at Q²≈0.8  (GeV∕c)², q≈1  GeV∕c, and ω≈445  MeV. Use of a self-calibrating, self-normalizing, thin-film waterfall target enabled a systematically rigorous measurement. Five-fold differential cross-section data for the removal of protons from the 1p-shell have been obtained for 0<p_miss<350  MeV∕c. Six-fold differential cross-section data for 0<E_miss<120  MeV were obtained for 0<p_miss<340  MeV∕c. These results have been used to extract the A_LT asymmetry and the R_L, R_T, R_LT, and R_L+TT effective response functions over a large range of E_miss and p_miss. Detailed comparisons of the 1p-shell data with Relativistic Distorted-Wave Impulse Approximation (RDWIA), Relativistic Optical-Model Eikonal Approximation (ROMEA), and Relativistic Multiple-Scattering Glauber Approximation (RMSGA) calculations indicate that two-body currents stemming from meson-exchange currents (MEC) and isobar currents (IC) are not needed to explain the data at this Q². Further, dynamical relativistic effects are strongly indicated by the observed structure in A_LT at p_miss≈300  MeV∕c. For 25<E_miss<50  MeV and p_miss≈50  MeV∕c, proton knockout from the 1s_1∕2-state dominates, and ROMEA calculations do an excellent job of explaining the data. However, as p_miss increases, the single-particle behavior of the reaction is increasingly hidden by more complicated processes, and for 280<p_miss<340  MeV∕c, ROMEA calculations together with two-body currents stemming from MEC and IC account for the shape and transverse nature of the data, but only about half the magnitude of the measured cross section. For 50<E_miss<120  MeV and 145<p_miss<340  MeV∕c, (e,e^′pN) calculations which include the contributions of central and tensor correlations (two-nucleon correlations) together with MEC and IC (two-nucleon currents) account for only about half of the measured cross section. The kinematic consistency of the 1p-shell normalization factors extracted from these data with respect to all available ¹⁶O(e,e^′p) data is also examined in detail. Finally, the Q²-dependence of the normalization factors is discussed.

Reaction K⁻p→π⁰π⁰Σ⁰ was measured with the Crystal Ball multiphoton spectrometer at eight K⁻ momenta from 514 to 750  MeV∕c. Dynamics and the energy dependence of the reaction are shown by Dalitz plots, invariant mass spectra, production angular distributions, and total cross sections. The dipion invariant-mass spectra for K⁻p→π⁰π⁰Σ⁰ show an enhancement in the low mass region; the opposite is the case for the K⁻p→π⁰π⁰Λ and π⁻p→π⁰π⁰n reactions. There is no direct evidence for a f₀(600)-meson contribution to π⁰π⁰ production. Everywhere σ_t(K⁻p→π⁰π⁰Σ⁰)⪡σ_t(K⁻p→π⁰π⁰Λ).

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P_LL-P_TT/ϵ and P_LT, and the electric and magnetic generalized polarizabilities (GPs) α_E(Q²) and β_M(Q²) at momentum transfer Q²=0.92 and 1.76  GeV². The electric GP shows a strong falloff with Q², and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q², compensated at higher Q² by a paramagnetic contribution from πN intermediate states.

Data for pion-single-charge exchange reactions at 750  MeV∕c on complex nuclei are analyzed for y-scaling, or single scattering quasifree, responses. The angular dependence of the data is used to separate the spin and nonspin isovector responses, with comparisons to electron scattering results.

Measured values of the differential cross section for pion-nucleon charge exchange, π⁻p→π⁰n, are presented at π⁻ momenta of 148, 174, 188, 212, 238, 271, 298, and 323  MeV∕c, a region dominated by the Δ(1232) resonance. Complete angular distributions were obtained using the Crystal Ball detector at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL). Statistical uncertainties of the differential cross sections are typically 2–6  %, exceptions being the results at the lowest momentum and at the most forward measurements at the five lowest momenta. We estimate the systematic uncertainties to be 3–6  %.

Reaction K⁻p→π⁰π⁰Λ was measured at eight incident K⁻ momenta between 514 and 750  MeV∕c using the Crystal Ball multiphoton spectrometer. The reaction dynamics are displayed in total cross sections, Dalitz plots, invariant-mass spectra, production angular distributions, and the Λ polarization. The π⁰π⁰Λ production is dominated by the π⁰Σ⁰(1385) intermediate state; no trace of other light Σ^∗ states is observed, and the role of the f₀(600) meson appears to be insignificant. A striking similarity is seen between K⁻p→π⁰π⁰Λ and π⁻p→π⁰π⁰n; this can be understood as a consequence of dynamical flavor symmetry.

Exclusive electroproduction of π⁰ mesons on protons in the backward hemisphere has been studied at Q²=1.0  GeV² by detecting protons in the forward direction in coincidence with scattered electrons from the 4  GeV electron beam in Jefferson Lab’s Hall A. The data span the range of the total (γ*p) center-of-mass energy W from the pion production threshold to W=2.0  GeV. The differential cross sections σ_T+ϵσ_L, σ_TL, and σ_TT were separated from the azimuthal distribution and are presented together with the MAID and SAID parametrizations.

Reaction π⁻p→π⁰π⁰n has been measured with high statistics in the beam momentum range 270–750  MeV∕c. The data were obtained using the Crystal Ball multiphoton spectrometer, which has 93% of 4π solid angle coverage. The dynamics of the π⁻p→π⁰π⁰n reaction and the dependence on the beam energy are displayed in total cross sections, Dalitz plots, invariant-mass spectra, and production angular distributions. Special attention is paid to the evaluation of the acceptance that is needed for the precision determination of the total cross section σ_t(π⁻p→π⁰π⁰n). The energy dependence of σ_t(π⁻p→π⁰π⁰n) shows a shoulder at the Roper resonance [i.e., the N(1440)1 / 2⁺], and there is also a maximum near the N(1520)3 / 2⁻. It illustrates the importance of these two resonances to the π⁰π⁰ production process. The Dalitz plots are highly nonuniform; they indicate that the π⁰π⁰n final state is dominantly produced via the π⁰Δ⁰(1232) intermediate state. The invariant-mass spectra differ much from the phase-space distributions. The production angular distributions are also different from the isotropic distribution, and their structure depends on the beam energy. For beam momenta above 550  MeV∕c, the density distribution in the Dalitz plots strongly depends on the angle of the outgoing dipion system (or equivalently on the neutron angle). The role of the f₀(600) meson (also known as the σ) in π⁰π⁰n production remains controversial.

We have measured the spin structure functions g₁ and g₂ of ³He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ³He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q² evolution of Γ₁(Q²)=∫₀¹g₁(x,Q²)dx, Γ₂(Q²)=∫₀¹g₂(x,Q²)dx, and d₂(Q²)=∫₀¹x²[2g₁(x,Q²)+3g₂(x,Q²)]dx for the neutron in the range 0.1≤Q²≤0.9  GeV² with good precision. Γ₁(Q²) displays a smooth variation from high to low Q². The Burkhardt-Cottingham sum rule holds within uncertainties and d₂ is nonzero over the measured range.