Polarization Transfer Coefficients Research Articles

Spin effects in backward deuteron scattering and baryonic degrees of freedom in the deuteron

The effect of the baryon-resonance admixture to the deuteron wave function on the momentum dependences of the differential cross section, the tensor analyzing power T20, and the polarization-transfer coefficient κ0 for backward elastic deuteron-proton scattering at high energies is investigated. The reaction in question is assumed to proceed predominantly through nucleon and nucleon-resonance exchanges. The formalism used in this study is based on light-front dynamics. The effect of various parameters of the problem on the results of the calculations is analyzed, and it is shown that, even at a 1% level of the total admixture of nucleon resonances to the deuteron wave function, a description of experimental data that is superior to that within the one-nucleon-exchange approximation can be achieved by appropriately choosing the contributions of various resonances. For qualitative agreement between the results of the calculations and experimental data to be attained, it is sufficient to take into account the contributions of the lightest negative-parity baryon resonances. Upon taking into account baryon exchanges, results computed for observables with different deuteron wave functions show a smaller scatter than analogous results obtained within the one-nucleon-exchange approximation.

Precise measurement of dp elastic scattering at 270 MeV and three-nucleon force effects

The cross section, the deuteron vector A(d)(y) and tensor analyzing powers A(ij), the polarization transfer coefficients K(y('))(ij), and the induced polarization P(y(')) were measured for the dp elastic scattering at 270 MeV. The cross section and A(d)(y) are well reproduced by Faddeev calculations with modern data-equivalent nucleon-nucleon forces plus the Tucson-Melbourne three-nucleon force. In contrast, A(ij), K(y('))(ij), or P(y(')) are not described by such calculations. These facts indicate the deficiencies in the spin dependence of the Tucson-Melbourne force and call for extended three-nucleon force models.

Proton recoil polarization in exclusive(e,e′pp)reactions

The general formalism of nucleon recoil polarization in the (${\vec e},e'{\vec N}N$) reaction is given. Numerical predictions are presented for the components of the outgoing proton polarization and of the polarization transfer coefficient in the specific case of the exclusive $^{16}$O(${\vec e},e'{\vec p}p$)$^{14}$C knockout reaction leading to discrete states in the residual nucleus. Reaction calculations are performed in a direct knockout framework where final-state interactions and one-body and two-body currents are included. The two-nucleon overlap integrals are obtained from a calculation of the two-proton spectral function of $^{16}$O where long-range and short-range correlations are consistently included. The comparison of results obtained in different kinematics confirms that resolution of different final states in the $^{16}$O(${\vec e},e'{\vec p}p$)$^{14}$C reaction may act as a filter to disentangle and separately investigate the reaction processes due to short-range correlations and two-body currents and indicates that measurements of the components of the outgoing proton polarization may offer good opportunities to study short-range correlations.

Polarization Transfer Invariants at Forward Angle and Spin-Dependent Excitations in Nuclei

Polarization study has long been a fundamental tool for understanding of reaction mechanisms and the spin structure of the target. 1) In particular, polarization transfer measurements for inelastic scattering of intermediate energy nucleon have provided rich information on spin-dependent excitations in nuclei. 2) 5) For instance, the spin transfer in (p, n) reactions at forward angles allowed a detailed study of the Gamow-Teller strengths. Recently, a similar analysis has been performed for the isoscalar 1+ state in 12C, where the polarization transfer at forward angles was measured for the (p, p′) reaction. 6) Here, the spin-spin central interaction is less dominant, and other components of the interaction may give rise to a large effect on some of the polarization transfer coefficients. As the standard analysis of these reactions relies upon a model of the reaction process, it would be helpful if one could postulate a constraint on the relevant observables in a model-independent manner. The purpose of the present paper is to provide such a constraint at forward angles by defining rotation invariants (scalars) from polarization transfer coefficients (PTCs). 7) For nucleon inelastic scattering, the invariant is related to what is called the total spin transfer in Refs. 8) and 6). This invariant was shown to provide a signature of spin-dependent excitations. We first summarize the notation and define invariants of PTCs. Let us consider a reaction A(a, b)B, where the spins of the projectile a and the ejectile b are given respectively by sa and sb, and those of the target/residue by SA and SB . The scattering amplitude in the c.m. frame is given by

Reaction mechanism and characteristics ofT20ind+3Hebackward elastic scattering at intermediate energies

For backward elastic scattering of deuterons by ^3He, cross sections \sigma and tensor analyzing power T_{20} are measured at E_d=140-270 MeV. The data are analyzed by the PWIA and by the general formula which includes virtual excitations of other channels, with the assumption of the proton transfer from ^3He to the deuteron. Using ^3He wave functions calculated by the Faddeev equation, the PWIA describes global features of the experimental data, while the virtual excitation effects are important for quantitative fits to the T_{20} data. Theoretical predictions on T_{20}, K_y^y (polarization transfer coefficient) and C_{yy} (spin correlation coefficient) are provided up to GeV energies.

Testing microscopic medium effects on nucleons and mesons using polarization observables in high-spin, unnatural-parity(p→,p′)reactions at 200 MeV

We compare measurements of the cross section, analyzing power, induced polarization, and polarization transfer coefficients for $(\stackrel{\ensuremath{\rightarrow}}{p},{p}^{\ensuremath{'}})$ reactions leading to unnatural-parity, high-spin states in ${}^{16}\mathrm{O}$ and ${}^{28}\mathrm{Si}$ with distorted wave impulse approximation calculations. We use an effective nucleon-nucleon (NN) interaction obtained from a microscopic treatment of nuclear medium effects. The NN potential is generated from a one-boson-exchange model of the nuclear force that reproduces NN scattering data well. Medium effects are incorporated through a G matrix obtained within a Dirac-Brueckner approach to nuclear matter. While agreement for some observables is good, differences for the polarization transfer coefficients indicate that systematic problems with the relative sizes of the spin-orbit and tensor interaction components exist. The differences are larger for ${}^{28}\mathrm{Si}$ than for ${}^{16}\mathrm{O}.$ A new model that incorporates density-dependent meson mass reductions produces only small effects that do not change the quality of the agreement. Connections to previous calculations are investigated.

Production of a nuclearly polarized3He+beam by multiple electron capture and stripping collisions

A ${}^{3}{\mathrm{He}}^{+}$ beam with nuclear polarization was produced by passing a fast ${}^{3}{\mathrm{He}}^{+}$-ion beam through a thick polarized Rb vapor target in a strong magnetic field. Observed polarization transfer coefficients as a function of the Rb vapor thickness are qualitatively explained by an electron pumping model based on multiple processes of electron capture and stripping. The experimentally obtained polarization and beam current for the ${}^{3}{\mathrm{He}}^{+}$ beam were, respectively, 0.07 and 2 $\ensuremath{\mu}\mathrm{A}$ for a Rb polarization of 0.2.

Strange particle production and polarization of Λ hyperons in the HERMES experiment

Semi-inclusive and inclusive reactions with Λ, Λ , and K s in the final states are studied in the HERMES experiment using unpolarized gas targets and the polarized 27.5 GeV positron beam of the HERA collider. Semi-inclusive DIS kinematics of Λ production in current fragmentation region show a good agreement with LEPTO based Monte Carlo calculations for the accepted by the HERMES spectrometer events. Longitudinal Λ polarization for the semi-inclusive Λ production is measured. The false asymmetry resulting from the spectrometer acceptance is excluded by analyzing two data sets with opposite beam polarizations. Polarization transfer coefficient from the knocked out quark to the Λ is found to be 0.04 ± 0.17( stat) ± 0.09( syst). In inclusive case transverse polarizations of Λ and Λ are measured. Averaged over the range of the transverse Λ, Λ momentum 0 < p t < 1.7 GeV/c it is found P n = 0.066 ± 0.011( stat) ± 0.025( syst) for Λ and P n = −0.066 ± 0.037( stat) ± 0.025( syst) for Λ .

Polarization phenomena for [formula omitted] in collinear kinematics as a test of polarized strangeness

Polarization effects for the process p ̄ +p→ Λ ̄ +Λ are studied for the collinear Λ-production. Using the triplet channel dominance hypothesis for the p ̄ p -annihilation, we prove that the longitudinal polarization transfer coefficient from the initial proton to final Λ hyperon must be positive in this kinematical regime, independently of any model. The exact value of this coefficient, however, depends on the sign of the strange quark contribution to the proton spin, for relatively small momentum transfer.

Polarization-Transfer Coefficients in $D(\vec {p},\vec {p})d$ and $D(\vec {p},\vec {d})p$ Reactions at $E_{p} = 19$ MeV: Experiment Compared to Calculations with Recent Nuclear-Interaction Models

The polarization-transfer coefficients K\(\), K\(\) and K\(\), K\(\) have been measured in the elastic scattering reactions D(\(\), \(\))p and D(\(\), \(\))d at \(\) MeV, respectively. They are compared to solutions of the three-nucleon Faddeev equations obtained with the recent nucleon-nucleon interactions AV18, CD Bonn, NijmI and II. Effects of the Tucson-Melbourne three-nucleon force, adjusted separately to reproduce the triton binding energy for each of these potentials, are studied. Both \(\) and \(\) exhibit a scaling behaviour with the triton binding energy. For \(\) and \(\) the various predictions with two-nucleon forces only agree practically with each other but spread after inclusion of the three-nucleon force. The agreement of theory and data is fair but the neglect of the proton-proton Coulomb force precludes a final conclusion.

Polarization transfer in the3H(p→,n→)3Hereaction and the0−level in4He

Longitudinal polarization-transfer coefficients for the ${}^{3}\mathrm{H}(\stackrel{\ensuremath{\rightarrow}}{p},\stackrel{\ensuremath{\rightarrow}}{n}{)}^{3}\mathrm{He}$ reaction have been measured at zero degrees for proton energies of 1.3\char21{}2.8 MeV. The results show a striking resonance behavior for energies corresponding to excitation of the ${0}^{\ensuremath{-}}$ level in ${}^{4}\mathrm{He}$ at 21.0 MeV. In agreement with $R$-matrix calculations, the value approaches unity at 1.52 MeV, the peak of the resonance. Near this same energy, at 1.62 MeV, the transverse polarization-transfer coefficient was measured to be consistent with zero.

Polarization transfer coefficientKyy′forn−pscattering at 15.8 MeV

The polarization transfer coefficient ${K}_{y}^{{y}^{\ensuremath{'}}}$ has been measured for neutron-proton scattering at ${E}_{n}=15.8\mathrm{MeV}$ and ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}=132.4\ifmmode^\circ\else\textdegree\fi{}.$ Using a quasimonoenergetic polarized neutron beam produced via the ${}^{2}\mathrm{H}{(d,n)}^{3}\mathrm{He}$ reaction, and a liquid-helium proton polarimeter, a value of ${K}_{y}^{{y}^{\ensuremath{'}}}=0.160\ifmmode\pm\else\textpm\fi{}0.020$ has been obtained which is appreciably smaller than the predictions of all realistic, meson-based $N\ensuremath{-}N$ potentials but agrees well with the result of an earlier measurement at 17.4 MeV.

New approach to polarization phenomena indpbackward elastic scattering at intermediate and high energies

The tensor analyzing power $T_{20}$ and the polarization transfer coefficients $\kappa_0 (= 3/2 K_y^y)$ and $K_{xz}^y$ are investigated for dp backward elastic scattering by the invariant-amplitude method. Discrepancies between the conventional calculations and the experimental data on $T_{20}$ and $\kappa_0$ at high and intermediate energies are mostly dissolved by including imaginary parts in the amplitudes. The quantity $K_{xz}^y$ is shown to be useful in criticizing nuclear force assumptions.

Transverse polarization transfer in the2H(d→,n→)3Hereaction atθ=0°

The transverse polarization transfer coefficient ${K}_{y}^{{y}^{\ensuremath{'}}}$ has been measured for the ${}^{2}\mathrm{H}(\stackrel{\ensuremath{\rightarrow}}{d},\stackrel{\ensuremath{\rightarrow}}{n}{)}^{3}\mathrm{He}$ reaction at 0\ifmmode^\circ\else\textdegree\fi{} for deuteron energies around 13 MeV, using a purely vector-polarized deuteron beam. At $0.666\ifmmode\pm\else\textpm\fi{}0.013,$ ${K}_{y}^{{y}^{\ensuremath{'}}}(0\ifmmode^\circ\else\textdegree\fi{})$ was found to be significantly larger than the values used frequently in the past to deduce the neutron polarization from the polarization of the deuteron beam.

Strength observed in Gamow-Teller resonance plus continuum in the (,) reaction at 295 MeV

Abstract The double differential cross sections for θlab between 0.0° and 14.7° and the polarization transfer coefficient DNN(0°) for the 27 Al ( p → , n → ) reaction have been measured at a bombarding energy of 295 MeV. A multipole decomposition technique is applied for the cross section data to extract L = 0, 1, 2, and 3 contributions. The Gamow-Teller (GT) strength B(GT) deduced from the L = 0 contribution is compared with the B(GT) values calculated in a full sd shell-model space. The sum of B(GT) values up to 20 MeV excitation is Sβ− = 4.0 ± 0.1 ± 0.1. A fairly large L = 0 contribution is observed in the continuum region up to 50 MeV, which could be in part ascribed to the quenched GT strength. A limit on the effect that the Δ(1232)-isobar nucleon-hole admixture has upon the GT strength is estimated.

Cross sections and complete polarization transfer measurements for the [formula omitted] reaction at 346 MeV

Cross sections and complete polarization transfer coefficients D ij for the charge-exchange 2H ( p, n)pp reaction have been measured at E p = 346 MeV and θ lab = 0° – 22°. In the forward angles the prominent narrow peak due to the strong 1S 0 final state interaction (FSI) between the two residual protons is observed while at the large angles the broad bump due to the quasi-free scattering (QFS) becomes prominent. Characteristic features of FSI and QFS are well described by the simple plane-wave impulse approximation (PWIA) calculations.

Measurements at Low Energies of the Polarization-Transfer Coefficient

Measurements of the transverse polarization-transfer coefficient for the reaction H He are reported for outgoing neutron energies of 1.94, 5.21, and 5.81 MeV. This reaction is important both as a source of polarized neutrons for nuclear-physics experiments, and as a test of theoretical descriptions of the nuclear four-body system. Comparison is made to previous measurements, confirming that the H He reaction can be used as a polarized neutron source with the polarization known to an accuracy of approximately 5%. Comparison to R-matrix theory suggests that the sign of the phase-shift parameter is incorrect. Changing the sign of this parameter dramatically improves the agreement between theory and experiment.

Forward polarization observables in 3He(d,p) 4He reactions

In 3He(d,p) 4He reactions, tensor analyzing powers T 20 and deuteron-proton polarization transfer coefficients κ 0(= 3 2 K y y) for forward proton emissions are theoretically predicted. Measurements of these observables will be valuable in criticizing nuclear-force assumptions as well as in investigating the reaction mechanism.

Facility for the (p, n) polarization transfer measurement

We have designed and constructed a facility to measure the complete polarization transfer coefficient D ij for the (p, n) reaction at intermediate energies. The facility consists of a beam swinger dipole magnet used for angular distribution measurements and a 100 m tunnel for the neutron energy measurement by the time-of-flight method. Typical neutron energy resolution is about 1.85 MeV for 290 MeV neutrons including a contribution of 1 MeV due to the target thickness. The background associated with this tunnel system was carefully investigated and found to be negligibly small. A dipole magnet for the neutron spin rotation is installed in the neutron flight path to measure the longitudinal polarization as well as the induced polarization. The neutron polarimeter which consists of 6 planes of the two-dimensional position-sensitive neutron detectors with a size of 1 m × 1 m × 0.1 m has been constructed. Its effective left-right analyzing power 〈 A y eff〉 has been calibrated at 194, 290 and 384 MeV. The figure-of-merit value defined by 〈 A y eff〉 2 × ε DS at 290 MeV is 4.05 × 10 −4, where ε DS is the double scattering efficiency.

Forward polarization observables in ( d, p) reactions at intermediate energies

In A( d, p) B reactions, the tensor analyzing power T 20 and the polarization transfer coefficient κ 0(= 3 2 K y y) for forward proton emissions are investigated by the PWBA at intermediate energies. For neutron captures into S 1 2 or P 1 2 states, model-independent formulae for these quantities as well as K xz y are derived by the invariant-amplitude method within the non-relativistic framework. Data of dp backward elastic scattering and inclusive breakup of deuterons are analyzed.