Search Results (29 total)

The Θ⁺ was searched for via the K⁺p→π⁺X reaction using the 1.2 GeV/c K⁺ beam at the K6 beam line of the KEK-PS 12 GeV Proton Synchrotron. In the missing mass spectrum of the K⁺p→π⁺X reaction, no clear peak structure was observed. Therefore a 90% C.L. upper limit of 3.5 μb/sr was derived for the differential cross section averaged over 2^° to 22^° in the laboratory frame of the K⁺p→π⁺Θ⁺ reaction. This upper limit is much smaller than the theoretical calculation for the t-channel process where a K^0* is exchanged. From the present result, either the t-channel process is excluded or the coupling constant of g_K*NΘ is quite small.

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q²=0.15, 0.25  (GeV/c)². The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2γ exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12≤Q²≤1.0  GeV². These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q² dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

We report on a measurement of the parity violating asymmetry in the elastic scattering of polarized electrons off unpolarized protons with the A4 apparatus at MAMI in Mainz at a four momentum transfer value of Q²=0.108   (GeV/c)² and at a forward electron scattering angle of 30°<θ_e<40°. The measured asymmetry is A_LR(e→p)=[-1.36±0.29(stat)±0.13(syst)]×10^-6. The expectation from the standard model assuming no strangeness contribution to the vector current is A₀=(-2.06±0.14)×10^-6. We have improved the statistical accuracy by a factor of 3 as compared to our previous measurements at a higher Q². We have extracted the strangeness contribution to the electromagnetic form factors from our data to be G_E^s+0.106G_M^s=0.071±0.036 at Q²=0.108   (GeV/c)². We again find the value for G_E^s+0.106G_M^s to be positive, this time at an improved significance level of two σ.

We report on a measurement of the asymmetry in the scattering of transversely polarized electrons off unpolarized protons, A_⊥, at two Q² values of 0.106 and 0.230   (GeV/c)² and a scattering angle of 30°<θ_e<40°. The measured transverse asymmetries are A_⊥(Q²=0.106   (GeV/c)²)=(-8.59±0.89_stat±0.75_syst)×10^-6 and A_⊥(Q²=0.230   (GeV/c)²)=(-8.52±2.31_stat±0.87_syst)×10^-6. The first errors denote the statistical error and the second the systematic uncertainties. From comparison with theoretical estimates of A_⊥ we conclude that πN-intermediate states give a substantial contribution to the imaginary part of the two-photon amplitude. There is no obvious reason why this should be different for the real part of the two-photon amplitude, which enters into the radiative corrections for the Rosenbluth separation measurements of the electric form factor of the proton.

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q² of 0.230   (GeV/c)² and a scattering angle of θ_e=30°–40°. Using a large acceptance fast PbF₂ calorimeter with a solid angle of ΔΩ=0.62   sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A_phys=(-5.44±0.54_stat±0.26_sys)×10^-6. The standard model expectation assuming no strangeness contributions to the vector form factors is A₀=(-6.30±0.43)×10^-6. The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G_E^s+0.225G_M^s=0.039±0.034 or F₁^s+0.130F₂^s=0.032±0.028.

Parity violating asymmetry in inclusive π⁰ or π⁺ meson production by longitudinally polarized electrons on unpolarized protons, is calculated as a function of the momentum transfer square k² and the total energy W of the πN system. We consider the Δ contribution in the s channel, the standard Born contributions and the vector mesons (ρ and ω) exchanges in the t channel. The parity-odd term is the sum of two contributions. The main term (found to be linear in k²) comes from the isovector component of the electromagnetic currents. It is model independent and can be calculated exactly in terms of fundamental constants. The second term is isoscalar in nature. Near threshold and in the Δ region, it is found to be much smaller (in absolute value) than the isovector one.

Tensor polarization observables ( t₂₀, t₂₁, and t₂₂) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)². The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q² the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.

The A(Q²) structure function in elastic electron-deuteron scattering was measured at six momentum transfers Q² between 0.66 and 1.80 (GeV/c)² in Hall C at Jefferson Laboratory. The scattered electrons and recoil deuterons were detected in coincidence, at a fixed deuteron angle of 60.5°. These new precise measurements resolve discrepancies between older sets of data. They put significant constraints on existing models of the deuteron electromagnetic structure, and on the strength of isoscalar meson exchange currents.

The analyzing power A_y0 of the reaction H(p→,d)π⁺ has been measured at a fixed value of the Mandelstam variable u_d=-0.17 GeV² for nine proton energies between 1000 and 1300 MeV. The experiment was performed at SATURNE with the SPES1 spectrometer. The data exhibit structure around sqrt[s]≃2.37 GeV. The origin of this structure could be related to a resonancelike behavior of the ¹S₀P or ¹G₄F partial amplitudes.

Polarization observables for strange particle threshold production in the processes N+N→K+Y+N (where Y is a Λ or a Σ) are studied. The spin structure of the corresponding matrix elements is derived, assuming S-wave production. Using the P invariance of the strong interaction and the Pauli principle, it is shown that these reactions can be described in terms of two (in the case of pp collisions) or three (in the case of np-collisions) independent complex amplitudes. This results in a specific behavior for polarization observables which is described. Polarization observables are calculated in the S- and P-wave approximations. In the case of collinear kinematics, relationships between spin correlation coefficients and the spin structure of the nucleon are explained.

Polarization observables in η production induced by NN collisions p+p→p+p+η, n+p→n+p+η, and n+p→d+η, are derived. The unique S wave spin structure of the threshold amplitude for p+p→p+p+η allows us to calculate all polarization observables in an universal form. Asymmetries in n→p→ collisions, characterized by noninterfering singlet and triplet amplitudes, are calculated. From the large experimental ratio of the differential cross sections of the processes n+p→n+p+η and p+p→p+p+η, near threshold, one predicts large spin correlation coefficients C_xx (C_yy). The spin structure of S- and P- wave contributions is derived.

The tensor polarization of the recoil deuteron in elastic electron-deuteron scattering has been measured at the Bates Linear Accelerator Center at three values of four-momentum transfer Q=3.78, 4.22, and 4.62 fm^-1, corresponding to incident electron energies of 653, 755, and 853 MeV. The scattered electrons and the recoil deuterons were detected in coincidence. The recoil deuterons were transported to a liquid hydrogen target to undergo a second scattering. The angular distribution of the d→-p scattering was measured using a polarimeter. The polarimeter was calibrated in an auxiliary experiment using a polarized deuteron beam at the Laboratoire National Saturne. A Monte Carlo procedure was used to generate interpolated calibration data because the energy spread in the deuteron energies in the Bates experiment spanned the range of deuteron energies in the calibration experiment. The extracted values of t₂₀ are compared to predictions of different theoretical models of the electromagnetic form factors of the deuteron: nonrelativistic and relativistic nucleon-meson dynamics, Skyrme model, quark models, and perturbative quantum chromodynamics. Along with the world data the structure functions A(Q) and B(Q) are used to separate the charge monopole and charge quadrupole form factors of the deuteron. A node in the charge monopole form factor is observed at Q=4.39±0.16 fm^-1.

The tensor polarization t₂₀ of the recoil deuteron in elastic e-d scattering has been measured for three values of four-momentum transfer, q=3.78, 4.22, and 4.62 fm^-1. The data have been used to locate the first node in the charge monopole form factor of the deuteron at q=4.39±0.16 fm^-1. The results for t₂₀ are in reasonable agreement with expectations based on the nucleon-meson description of nuclear dynamics.

Measurements of 20 vector and tensor spin observables in d-p elastic scattering at a deuteron bombarding energy of 1.6 GeV (800 MeV equivalent proton beam energy) are reported. Of these 10 are two-spin observables and the remaining ten are three-spin observables. The two-spin observables, six of which have been measured for the first time, were measured in the four-momentum-transfer range 0.12 to 0.85 (GeV/c)². The three-spin observables, of which eight were measured for the first time, were measured in the momentum-transfer range 0.2 to 0.85 (GeV/c)². The magnitude of the average vector polarization of the deuteron beam was 0.3, and of the average tensor alignment was 0.85. A frozen spin-polarized proton target was utilized for these measurements. These results are compared with previous measurements when possible, and show a general agreement. They are also compared with the predictions of the relativistic-impulse approximation, and its modified version, which takes into account a nontrivial off-mass-shell behavior of nucleon-nucleon scattering amplitudes.

Direct measurements of total reaction cross sections (σ_R) have been performed in the energy range of 10–300 MeV/nucleon for heavy ion collisions. A decrease of σ_R with increasing energy was observed for a wide range of masses of the colliding systems. The data suggest that σ_R reaches a minimum located around 300 MeV/nucleon independently of the projectile target combination. A dependence of σ_R on mass asymmetry of the system is also demonstrated. Trends of σ_R in this energy range are well reproduced by the predictions of a simple microscopic model based on individual nucleon-nucleon collisions. Our data have been employed in this framework to derive a new semi- empirical parametrization of σ_R. Most of the experimental results in the intermediate and high energy range have been reproduced by this parametrization using a single energy-dependent parameter.

Inelastic pion scattering from ¹⁶O has been studied by measuring the spectrum of pions down to 40 MeV at five angles between 30° and 134° for three incident pion energies: 114, 163, and 240 MeV. The spectra are dominated by a broad peak due to quasielastic scattering from a single nucleon in the nucleus. The systematics of the spectra are discussed with emphasis on the pion-nucleus interaction dynamics including the effect of absorption. The partial π⁺-¹⁶O cross sections for all nonradiative channels are estimated.

NUCLEAR REACTIONS ¹⁶O(π, π^′) E=114, 163, 240 MeV. θ=30-134°, full inelastic spectrum measured; σ(θ). Discussion of nuclear medium effects on quasifree processes, Δ propagation.

The tensor analyzing power T₂₀ for d-p elastic scattering at or near θ_c.m.=180° has been measured at sixteen deuteron beam energies between 0.3 and 2.3 GeV. In marked disagreement with earlier work, large negative values (|T₂₀|>0.6) are found at all energies in this range, with a dip to T₂₀=-1.25 at T_d=0.5 GeV. The present results are compared with various model calculations.

We have measured the angular and momentum distributions of the scattered pions from the reaction ¹²C(π⁺, π⁺d)¹⁰B in a coincidence experiment. We compare our results with two theoretical models based on the impulse approximation. We also present some data on the reactions ¹²C(π^±, π^±t)⁹B.

NUCLEAR REACTIONS ¹²C(π^±, π^±d)¹⁰B, ¹²C(π^±, π^±t)⁹B, E_π=130, 150 MeV; measured σ(p), σ(θ), compared with PWIA.

The vector analyzing power (iT₁₁) in elastic π-d_pol scattering has been measured for several angles at T_π=219, 256, 275, and 294 MeV. Strong oscillations are confirmed at 256 MeV. The data are compared with Faddeev calculations, alone and in combination with either of two resonances. At 219 MeV the Faddeev calculations are consistent with the data, while at the higher energies the general trend of the data favors the admixing of the ¹G₄ dibaryon resonance. The forward-angle data at 294 MeV are not well described by any existing calculation.

We have studied the reaction ¹²C(π⁺,π⁺p)¹¹B in a coincidence experiment that completely determined the kinematics of each event. The energy resolution was sufficient to identify the truly quasifree events, i.e., those that left the residual nucleus in its ground state. A comparison of our data with empirically normalized plane wave impulse approximation calculations shows reasonable agreement as long as only one parameter is varied at a time. Some effects of the nuclear shell structure can be seen. There is an indication that the (3 / 2,3 / 2) resonance may be narrowed inside a nucleus.

NUCLEAR REACTIONS ¹²C(π⁺,π⁺p), E=130-200 MeV; measured σ(E,θ,E^′), PWIA analysis. Resolution ∼ 1 MeV, θ_π=-60°-130°, θ_p=30°-45°.

The vector analyzing power (iT₁₁) in elastic π-d_pol scattering has been measured for several angles at T_π=142 and 256 MeV. The results are compared with calculations reported in the literature. At the lower energies, Faddeev calculations agree fairly well with the data. At the higher energies, the experimental results differ markedly from any conventional calculation, but agree surprisingly well with predictions in which effects of dibaryon resonances are explicitly included.

The cross section of the pion-induced proton-knockout reaction ¹²C→¹¹B has been measured with positive and negative pions. The ratio of the cross sections is compared with the ratio calculated in the plane-wave impulse approximation and agreement is found for those events that lead to particle-stable final states.

Differential elastic cross-section ratios and absolute cross sections have been measured for ^{12,  13}C at 29.2-and 49.5-MeV average π^- energy and for ^{16,  18}O at 29.2 MeV. Range telescopes detected the scattered pions. The ratio data were compared with different optical-potential calculations to extract neutron radii of 2.35 ± 0.03 fm for ¹³C and 2.81 ± 0.03 fm for ¹⁸O, relative to the neutron radii of ¹²C (2.31) and ¹⁶O (2.60), respectively. Our studies indicate little sensitivity to the optical model used.

Inelastic scattering of 163- and 241-MeV positive pions by ^natCa has been studied with special attention paid to the giant-resonance region. Pronounced giant resonance structures are observed at both incident energies. The analysis indicates a strong excitation of the giant quadrupole resonance at 18.2 MeV (Γ∼3-3.5 MeV).