The scattering of slow neutrons by cobalt

Different techniques have been used to determine with slow neutrons the coherent scattering amplitude b_c and the total scattering cross-section σ_sc of Co. We found values for the bound atom b_c=2.78+0.04 fm and for the free nucleus σ_sc=5.95+0.05 barns.From these quantities, the spin state scattering amplitudes b₊ and b_? corresponding to the compound nucleus states of spin I+1/2 or I?1/2 have been determined as b₊=?2.78±0.04 fm and b_?=+9.91±0.06 fm. These results are not in agreement with previous data, but they are found to be consistent with data obtained from resonance parameters.     

B* production in Z0 decays

The process B*→Bγ has been observed in a sample of approximately 4.1 million hadronic Z⁰ decays collected by the OPAL detector at LEP in the years 1991–1995. Inclusively reconstructed B mesons have been combined with converted photons to reconstruct approximately 1900 B* mesons. The B*–B mass difference is found to be ΔM(B* ?B) = 46.2 ± 0.3 ± 0.8 MeV/c ², and the rate of B* meson production relative to that of B mesons is found to be σ _B*/σ _B = 0.760 ± 0.036 ± 0.083, where the first errors are statistical and the second systematic. The angular distribution of the transition photon in the B* rest frame has been measured and the relative contribution of longitudinal B* polarization states found to be σ _l/(σ _l + σ _t ) = 0.36 ± 0.06 ± 0.07, consistent with a simple spin counting picture. These results average over B⁰, B± and B _s ⁰ mesons.     

Eigenvalues, eigenfunctions, and surface states in finite periodic systems

Using a simple approach that requires neither the Bloch functions nor the reciprocal lattice, new, compact, and rigorous analytical formulas are derived for an accurate evaluation of resonant energies, resonant states, energy eigenvalues and eigenfunctions of open and bounded n-cell periodic systems with arbitrary 1D potential shapes, provided the single cell transfer matrix is given. These formulas are applied to obtain the energy spectra and wave functions of a number of simple but representative open and bounded superlattices. We solve the fine structure in bands and exhibit unambiguously that the true eigenfunctions do no not fulfill the periodicity property |Ψ_μ,ν (z + l_c)|² = |Ψ_μ,ν (z)|², with l_c the single cell length. We show that the well known surface states and surface energy levels come out naturally. We analyze the surface repulsion effect and calculate exactly the surface energy levels for different potential discontinuities an the ends.     

Spin structure of the pion in a light-cone representation

The spin structure of the pion is discussed by transforming the wave function for the pion in the naive quark model into a light-cone representation. It is shown that there are higher helicity (λ ₁+λ ₂=±1) states in the full light-cone wave function for the pion besides the ordinary helicity (λ ₁+λ ₂=0) component wave functions as a consequence from the Melosh rotation relating spin states in light-front dynamics and those in instantform dynamics. Some low energy properties of the pion, such as the electromagnetic form factor, the charged mean square radius, and the weak decay constant, could be interrelated in this representation with reasonable parameters.     

Phase shift analysis of the reaction K+N → KN below 1.6 GeV/c

An energy-dependent partial wave analysis was made for the K⁺ nucleon scattering in the energy range between 0.2 and 1.6 GeV/c using the data including our new polarization data of K⁺n elastic and charge-exchange scattering. Counterclockwise Argand plots were found for the partial waves P₁₃, P₀₁ and D₀₃.     

Untersuchung der Polarisationsebene von Photonen nach Compton-Streuung an polarisierten Elektronen

Two polarization phenomena in Compton scattering by polarized electrons were investigated. In the first experiment, the rotation of the polarization plane of photons passing through magnetized iron and gadolinium was measured. This effect arises from a spin dependence of the Compton forward scattering amplitude. For 228 and 333 keV photons and iron absorbers, the observed rotation angles areφ ₀=(3.90±0.57) ×10^?3 rad · cm^?2 and (4.75±0.58)×10^?3 rad · cm^?2, respectively. Secondly, the orientation of the photon polarization plane after scattering of unpolarized photons by polarized electrons was measured. This experiment tests time reversal invariance in quantum electrodynamics. No dependence of the polarization plane on the direction of the electron spin was found within 2×10^?3.     

Dynamic spin susceptibility of hole-doped high-temperature superconductors in a singlet-correlated conduction band model

We have derived an expression for the dynamical spin susceptibility of a hole-doped high-temperature superconductor taking into account a strong correlation between the magnetization of spins of the localized and itinerant electrons. This formula has been used to calculate the imaginary part of the susceptibility as a function of the frequency and wave vector. The results are compared to experimental data on the inelastic neutron scattering in compounds of the YBa₂Cu₃O_6+y type. A peak in the scattering intensity observed at an energy of about 40 meV in the region of wave vectors Q = (π, π) and an arc-shaped dispersion relief are interpreted as manifestations of the collective spin excitations in the system, the energy of which falls within a superconducting gap (spin exciton). The U-shaped divergent relief observed in the neutron scattering intensity is assigned to collective short-rage-order spin oscillations.     

Neutron diffraction study on the hyperfine-induced nuclear spin order in the antiferromagnet PrSn3

At several $\text{(h+}\text{1}\text{2}\text{0 0)}$ reciprocal lattice positions of PrSn₃, we observed Bragg scattering of neutrons which is purely due to the nuclear spin polarization of ¹⁴¹Pr, and could measure the temperature dependence of the polarization between 10 mK and 4.2 K. From the perfectly polarized state of the nuclear spins, the difference between the spin-dependent nuclear scattering lengths b⁺ and b^- has been determined to be b⁺ - b^- = -0.110 ± 0.006 × 10^-12 cm, which is significantly larger in magnitude than the previously reported value. An additional electronic polarization which is linearly proportional to the nuclear spin polarization has also been observed.     

Über die Bestimmung des Spin-Abschneidekoeffizienten σ

A value ofσ=3, 5±1 is obtained for the spin cut off coefficient in the Fermi gas level density formula from a comparison of calculated and experimental (n, γ)-intensities feeding the observed levels of deformed nuclei in the rare earths region. The model used for the computations is tested at two nuclei with compound states of low spin (I _c =1/2) and high spin (I _c =13/2 or 15/2). The calculations can help to determine spins of nuclear levels in some cases.     

Hard photon emission in spectatorB meson decays

We study the emission of a hard photon in the spectator, free quark approximation ofB meson decays. We thus obtain an estimate for the high end of the photon spectrum in the processb→X _u,c+γ and in the processesb→ \(\bar c\) +X _u,c+γ andb→l \(\bar v\) +X _u,c+γ, whereX _q=q+X. We show that the hard photon spectrum will allow to distinguish between decays via spectator and “annihilation” amplitudes, and could be used as well for an independent estimate of |V _ub/V _cb|. Further implications of our calculation are discussed.     

Meson-photon transition form factors and resonance cross-sections in e+e− collisions

Meson-photon-photon transition form factors for S-, P-, and D-wave states are calculated, the meson being treated as a non-relativistic heavy-quark-antiquark pair. The full dependence on both photon virtualities is included. Cross-section formulas for charge-conjugation even mesons with J^p = 0⁻, 0⁺, 1⁺, 2⁺, and 2⁻ in electron-positron collisions are presented and numerical results for LEP energies are given. In particular, we find two-photon event rates for χ_c1, η_c(2S), and η_b(1S) within reach of LEP.With minor modifications to incorporate SU(3)-flavour breaking we estimate rates for 18 light mesons as well, based on the observation that their two-photon decay widths agree remarkably well with measured data. Finally we point out that e⁺e⁻ cross sections for 1⁺ states do not vanish at low Q², the Landau-Yang suppression factors of the two-photon cross sections being compensated by the photon propagators.     

The scattering of the Manning-Rosen potential with centrifugal term

In this Letter the approximately analytical scattering state solutions of the l-wave Schrödinger equation for the Manning-Rosen potential are carried out by a proper approximation to the centrifugal term. The normalized radial wave functions of l-wave scattering states are presented and the calculation formula of phase shifts is derived. It is well shown that the poles of the S-matrix in the complex energy plane correspond to bound states for real poles and scattering states for complex poles in the lower half of the energy plane. We consider and verify two special cases: the l=0 and the s-wave Hulthén potential.     

T invariance and polarization effects in the reactions p+3He → π++4He and π++4He → p+3He

On the basis of T invariance, it is established that the dependence of the effective cross section for a binary reaction of the a+b → c+d type on the polarization vectors of primary particles a and b determines completely the polarization vectors and spin correlations for the same particles in the inverse reaction c+d → a+b induced by collisions between unpolarized primary particles c and d. By using the formalism of helicity amplitudes, polarization effects are studied in the process p+³He → π⁺+⁴He and in the inverse process π⁺+⁴He → p+³He. It is shown that, in the reaction π⁺+⁴He → p+³He, the spins of the final-state particles (proton and ³He nucleus) are strongly correlated. An expression for the correlation tensor is obtained for arbitrary values of the (p, ³He) emission angle.     

Transverse spin correlations in a random anisotropy system: Amorphous ErCo2

We provide a coherent interpretation of early small angle scattering experiments performed by some of us on amorphous ErCo₂ [9]. At low temperature the zero field transverse spin-spin correlation function is found to fit a simple exponential for large length scales (l >l_c), supporting the lower critical dimensionality d_c=4. For shorter length scales (l<l_c) the correlation function is of the Ornstein-Zernike type. These results are physically understood in terms of the breaking of ferromagnetism into Imry and Ma domains.A further physical interpretation leads us to consider the localization of ferromagnetic spin waves within Imry and Ma domains in zero field, and their delocalization by application of an external field.     

Raman scattering from spin fluctuations and phonons in the heavy-fermion superconductor UPt3

Quasielastic scattering from spin fluctuations has been observed in UPt₃ by Raman spectroscopy. The experiments for wave vectors q≈0 show a nearly temperature independent linewidth for 5 K ⩽ T ⩽ 300 K Complementary to neutron scattering results this establishes the q independence of the spin relaxation rate, indicating the localized nature of the spin fluctuations. A Raman-active phonon near 79 cm^-1 (10 meV) shows a drastic increase in line-width with decreasing temperature, demonstrating strong electron-phonon coupling.     

Polarization transfer in relativistic electron-nucleus bremsstrahlung

The emission of circularly polarized photons during the scattering of fast spin-polarized electrons from heavy nuclei is studied within the Dirac-Sommerfeld-Maue approach. Predictions are made for the dependence of the polarization correlations C₃₂ and C₁₂ on collision energy, photon energy and nuclear charge. A comparison with pilot measurements of the transmission asymmetry, sensitive to C₃₂, for 3.5 MeV e + Pb verifies that the polarization transfer increases with photon energy for small emission angles.     

Magnetic excitations in triangular lattice NaCrO2

We report the first inelastic neutron scattering measurements on the rhombohedrally stacked triangular antiferromagnet NaCrO₂ which has recently been shown via μSR and NMR measurements to exhibit an unusually broad fluctuating cross-over regime extending far below the onset of spin freezing at T_c. Our results show that strong spin fluctuations persist far below T_c and that dispersive spin wave excitations only appear near the cross-over temperature 0.75T_c.     

Polarization of Σ0 hyperons in inclusive production from 28.5 GeV/c protons on beryllium

The polarization of Σ⁰ hyperons produced in an inclusive reactons hasbeen measured for the first time. From a sample of 11 000 events produced by 28.5 GeV/c ptorons in the reaction p+Be→Σ⁰+X, the Σ⁰ polarization has a value of +0.28±0.13 at p_t=1.01 GeV/c and x_f=0.60. The polarization of 53 000 Λ hyperons produced from 28.5 Gev/c protons inthe reaction p+Be→Λ+X has been measured in the kinematic range 0.64<p_t<1.14 GeV/c and 0.42<x_f<0.62. The average Λ polarization is found to be −0.188±0.024, consistent with previous results.     

Pair production of pions with symmetric momenta in the range 0.5⩽pT⩽2.0 GeV/c In 70 GeV p-p collisions

The invariant cross-section slope of the pp→π⁺π^?+X process as a function of p_T is found to have a break near 1 GeV/c. Fitting the cross section by a sum of two exponents gives the values of powers (12.3±0.9)(GeV/c)^?1 and (8.7±0.6)(GeV/c)^?1. The experimental points at p_T?1 GeV/c are significantly higher than predictions based on hard scattering models such as QCD and CIM.     

Coherent Exchange Cluster Approach for two-dimensional disordered Heisenberg spin systems

The spin wave properties of disordered two-dimensional and quasi two-dimensional Heisenberg spin systems, with an antiferromagnetic ground state (Neèl state) induced by single-ion anisotropy, are discussed within a Coherent Exchange Cluster Approach (Cluster CEA).The configuration averaged Green's functions are described by an effective spin wave Hamiltonian, with two sets of complex and energy dependent coherent exchange integralsJ _lm ¹ (E) andJ _lm ² (E) appropriate to the consideration of two different spins of the binary alloy constituents.J _lm ¹ andJ _lm ² , depending only on the distance of the sitesl andm, are taken to be non zero only for nearest neighbours. The remaining two quantitiesJ ¹(E) andJ ²(E) are determined self-consistently from the requirement that the most important matrix elements of the scatteringT matrix vanish when the configuration averaging has been performed.Numerical results are presented for the antiferromagnetic quasi two-dimensional systems K₂Ni _c Mn_1-c F₄ and Rb₂Fe _c Mn_1-c F₄.Both the density of states and the transverse susceptibilities, determining essentially the neutron scattering cross-sections, are calculated.The density of spin wave states for K₂Ni _c Mn_1-c F₄ is compared for different concentrations with exact computer calculations for finite 30 × 30 arrays. The agreement is excellent.Based on the thesis of H.J. Schlichting, Fachbereich Physik der Universität Hamburg, 1974.