Search Results (86 total)

Azimuthal single-spin asymmetries of leptoproduced pions and charged kaons were measured on a transversely polarized hydrogen target. Evidence for a naive-T-odd, transverse-momentum-dependent parton distribution function is deduced from nonvanishing Sivers effects for π⁺, π⁰, and K^±, as well as in the difference of the π⁺ and π^- cross sections.

Gd₂Sn₂O₇ is believed to be a good approximation to a Heisenberg antiferromagnet on a pyrochlore lattice with exchange and dipole-dipole interactions. The system is known to enter a long-range ordered ground state (the “Palmer Chalker” state) below T_c=1 K with k_ord=(000). However, persistent electronic spin fluctuations have been observed as T→0. Using inelastic neutron scattering, we have studied the buildup of short-range spin-spin correlations as the temperature is lowered, and the eventual formation of a gapped long-range ordered state that is able to sustain spin waves below T_c. As a magnetic field is applied, new magnetic phases develop and the gap widens. These measurements show that Gd₂Sn₂O₇ completely relieves itself of frustration, but the self-selected ground state is very delicate.

We have performed a neutron polarization analysis study of the short-range nuclear and magnetic correlations present in the dilute alloy, β-Mn_1−xAl_x with 0.03≤x≤0.16, in order to study the evolution of the magnetic ground state of this system as it achieves static spin-glass order at concentrations x>0.09. To this end we have developed a reverse–Monte Carlo algorithm which has enabled us to extract Warren-Cowley nuclear short-range order parameters and magnetic spin correlations. Using conventional neutron powder diffraction, we show that the nonmagnetic Al substituents preferentially occupy the magnetic site II Wyckoff positions in the β-Mn structure—resulting in a reduction of the magnetic topological frustration of the Mn atoms. These Al impurities are found to display strong anticlustering behavior. The magnetic spin correlations are predominantly antiferromagnetic, persisting over a short range which is similar for all the samples studied—above and below the spin-liquid-spin-glass boundary—while the observed static (disordered) moment is shown to increase with increasing Al concentration.

We present inelastic neutron scattering results on the geometrically frustrated pyrochlore Tb₂Sn₂O₇. At high temperature T>50  K, this system resembles the cooperative paramagnet Tb₂Ti₂O₇, while at low temperature T∼60  mK, it displays remarkably different behavior. Powder neutron scattering, susceptibility, and specific heat techniques have shown that below 0.87  K Tb₂Sn₂O₇ enters a partially ordered state that is characterized by two-sublattice ferrimagnetic long-range order which coexists with paramagnetic spin components. We show that (i) the low-temperature state produces a large internal field and collective excitations and (ii) the coexisting paramagnetic state persists down to 0.1  K, with spins fluctuating at a rate greater than 0.04  THz, resulting in a diffuse magnetic background to the diffraction patterns. A low-lying excitation at 1.2  meV partially softens as short-range correlations build up while cooling in the paramagnetic state.

The HERMES experiment has measured the transverse polarization of Λ and Λ̅ hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization P_n^Λ of the Λ hyperon is found to be positive while the observed Λ̅ polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are P_n^Λ=0.078±0.006(stat)±0.012(syst) and P_n^Λ̅ =-0.025±0.015(stat)±0.018(syst) for Λ and Λ̅ , respectively. The dependences of P_n^Λ and P_n^Λ̅ on the fraction ζ of the beam’s light-cone momentum carried by the hyperon and on the hyperon’s transverse momentum p_T were investigated. The measured Λ polarization rises linearly with p_T and exhibits a different behavior for low and high values of ζ, which approximately correspond to the backward and forward regions in the center-of-mass frame of the γ^*N reaction.

The many-body expansion of the interaction potential between atoms and molecules is analyzed in detail for different types of interactions involving up to seven atoms. Elementary clusters of Ar, Na, Si, and, in particular, Au are studied, using first-principles wave-function- and density-functional-based methods to obtain the individual n-body contributions to the interaction energies. With increasing atom number the many-body expansion converges rapidly only for long-range weak interactions. Large oscillatory behavior is observed for other types of interactions. This is consistent with the fact that Au clusters up to a certain size prefer planar structures over the more compact three-dimensional Lennard-Jones-type structures. Several Au model potentials and semiempirical PM6 theory are investigated for their ability to reproduce the quantum results. We further investigate small water clusters as prototypes of hydrogen-bonded systems. Here, the many-body expansion converges rapidly, reflecting the localized nature of the hydrogen bond and justifying the use of two-body potentials to describe water-water interactions. The question of whether electron correlation contributions can be successfully modeled by a many-body interaction potential is also addressed.

We have produced and detected molecules using a p-wave Feshbach resonance between ⁴⁰K atoms. We have measured the binding energy and lifetime for these molecules and we find that the binding energy scales approximately linearly with the magnetic field near the resonance. The lifetime of bound p-wave molecules is measured to be 1.0±0.1  ms and 2.3±0.2  ms for the m_l=±1 and m_l=0 angular momentum projections, respectively. At magnetic fields above the resonance, we detect quasibound molecules whose lifetime is set by the tunneling rate through the centrifugal barrier.

The force concept inventory and a 10-question context-modified test were given to 647 students enrolled in introductory physics classes at the University of Arkansas. Context changes had an effect ranging from −3% to 10% on the individual questions. The average student score on the ten transformed questions was 3% higher than the average student score on the corresponding 10 force concept inventory questions. Therefore, the effect of contextual changes on the total of the 10 questions is not sufficient to affect normal use of the force concept inventory as a diagnostic instrument.

The first observation of an azimuthal cross section asymmetry with respect to the charge of the incoming lepton beam is reported from a study of hard exclusive electroproduction of real photons. The data have been accumulated by the HERMES experiment at DESY, in which the HERA 27.6 GeV electron or positron beam scattered off an unpolarized hydrogen gas target. The observed asymmetry is attributed to the interference between the Bethe-Heitler process and the deeply virtual Compton scattering (DVCS) process. The interference term is sensitive to DVCS amplitudes, which provide the most direct access to generalized parton distributions.

The phase transitions of the geometrically frustrated antiferromagnet TbNiAl in a magnetic field are studied by means of neutron powder diffraction, ac susceptibility, and muon spin relaxation (μSR) measurements. Neutron powder diffraction reveals that, in addition to antiferromagnetic order, ferromagnetic order is induced in a field as low as B∼0.02 T. At higher fields, ferromagnetic and antiferromagnetic order coexist in different domains in the sample, and the domain balance depends on both magnetic field and temperature. Antiferromagnetic Bragg reflections are observed below a Néel temperature of T_N=47 K which is independent of the field. Ferromagnetic Bragg peaks are observed below a field-dependent Curie temperature which increases from T_C=52 K at B=0.2 T to T_C=70 K at B=5 T. Both phase transitions are concurrently observed in ac susceptibility and μSR measurements.

Precise measurements of the spin structure functions of the proton g₁^p(x,Q²) and deuteron g₁^d(x,Q²) are presented over the kinematic range 0.0041≤x≤0.9 and 0.18  GeV²≤Q²≤20  GeV². The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function g₁ⁿ is extracted by combining proton and deuteron data. The integrals of g₁^p,d at Q²=5  GeV² are evaluated over the measured x range. Neglecting any possible contribution to the g₁^d integral from the region x≤0.021, a value of 0.330±0.011(theo)±0.025(exp)±0.028(evol) is obtained for the flavor-singlet axial charge a₀ in a leading-twist next-to-next-to-leading-order analysis.

We present a measurement of the potential energy of an ultracold trapped gas of ⁴⁰K atoms in the BCS-BEC crossover and investigate the temperature dependence of this energy at a wide Feshbach resonance, where the gas is in the unitarity limit. In particular, we study the ratio of the potential energy in the region of the unitarity limit to that of a noninteracting gas, and in the T=0 limit we extract the universal many-body parameter β. We find β=-0.54_-0.12^+0.05; this value is consistent with previous measurements using ⁶Li atoms and also with recent theory and Monte Carlo calculations. This result demonstrates the universality of ultracold Fermi gases in the strongly interacting regime.

Using wide-line deuterium NMR, the effects of pressure on saturated-chain orientational order and gel–to–liquid-crystal phase transition temperature were observed in bilayers of 16:0-18:1 PC-d₃₁ (POPC-d₃₁) and 16:0-18:2 PC-d₃₁ (PLPC-d₃₁). Spectra were recorded for a range of pressures at selected temperatures and for a range of temperatures at selected pressures up to 193  MPa. For 16:0-18:1 PC-d₃₁, the main transition temperature increased by ∼0.18  K∕MPa, a rate that is similar to what is found for bilayers of disaturated PC’s. For 16:0-18:2 PC-d₃₁, the increase in transition temperature with pressure was slightly smaller at ∼0.13  K∕MPa. To investigate the isothermal response of chain orientational order to pressure, spectra for each lipid were obtained for three pressures (ambient, 55  MPa, and 110  MPa) near room temperature (∼25  °C) and for three pressures (ambient, 110  MPa, and 193  MPa) at higher temperature (∼40  °C). These temperatures were chosen such that the difference between the higher observation temperature and the main transition of 16:0-18:1 PC-d₃₁ would be similar to the difference between the lower observation temperature and the main transition of 16:0-18:2 PC-d₃₁. Application of a given pressure was found to raise the orientational order for all methylene groups on the saturated chain of a particular lipid by roughly similar amounts. For comparable pressure differences, the pressure-induced ordering of the 16:0-18:1 PC-d₃₁ saturated chain at ∼40  °C was greater than that of the corresponding chain in 16:0-18:2 PC-d₃₁ at ∼25  °C. These observations suggest that increasing levels of chain unsaturation may reduce the sensitivity of bilayer order to variations in pressure at corresponding temperatures relative to their ambient pressure transitions.

The transfer of polarization from a high-energy positron to a Λ0 hyperon produced in semiinclusive deep-inelastic scattering has been measured. The data have been obtained by the HERMES experiment at DESY using the 27.6 GeV longitudinally polarized positron beam of the HERA collider and unpolarized gas targets internal to the positron (electron) storage ring. The longitudinal spin-transfer coefficient is found to be DLL′Λ=0.11±0.10(stat)±0.03(syst) at an average fractional energy carried by the Λ0 hyperon ⟨z⟩=0.45. The dependence of DLL′Λ on both the fractional energy z and the fractional longitudinal momentum xF is presented.

A Reply to the Comment by L. Raymond, G. Albinet, and A.-M. S. Tremblay.

A neutron spin-echo investigation of the low temperature spin dynamics in two well-characterized kagomé bilayer compounds SrCr_9xGa_12-9xO₁₉ (x=0.95, SCGO) and Ba₂Sn₂ZnCr_7xGa_10-7xO₂₂ (x=0.97, BSZCGO) reveals two novel features. One is the slowing down of the relaxation rate without critical behavior at T_g, where a macroscopic spin-glass-like freezing occurs. The second is, in SCGO at 4 K (≈T_g)<T<7  K, the relaxation rate activation energy E_a=7±0.4  meV, equal to the energy of a phonon mode, pointing out the role of spin-lattice coupling.

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.

The effect of Fe doping on the magnetic properties of La_0.7Pb_0.3Mn_1−xFe_xO₃(0⩽x⩽0.2) colossal magnetoresistance perovskites is studied by muon spin rotation (μSR) spectroscopy and macroscopic static magnetization measurements. The latter quantities show a cusp at temperatures below that signaling magnetic ordering (Curie temperature T_C) as well as a kink at lower temperatures which are taken as signatures of magnetic irreversibility. Random substitution of Mn by Fe ions gives rise to antiferromagnetic couplings mediated by conventional superexchange interaction. This manifests itself by the decrease of Curie temperatures and a concomitant decrease of the spontaneous magnetization with increased doping. The muon relaxation data under zero and applied longitudinal fields is interpreted in terms of progressive spin freezing concomitant with a reduction of the ferromagnetic component as Fe substitution is increased. The spin dynamics as T_C is crossed from above shows clear departures from those expected for exchange-coupled Heisenberg or Ising systems but rather it displays features common to magnetically interacting-cluster systems. This effect leads to the appearance of a peak in the muon relaxation rate versus temperature curves that is located at temperatures below that signaling magnetic ordering (T_C). Finally, direct evidence of the existence of antiferromagnetic and ferromagnetically short-range-ordered regions within the x=0.2 sample is provided by polarized neutron diffraction. The picture that emerges from our study identifies competing positive (double exchange) and negative (superexchange) interactions at random sites as the entities responsible for the low-temperature freezing monitored by using μSR spectroscopy.

The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6  GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A_zz^d and the tensor structure function b₁^d for average values of the Bjorken variable 0.01<⟨x⟩<0.45 and of the negative of the squared four-momentum transfer 0.5  GeV²<⟨Q²⟩<5  GeV². The quantities A_zz^d and b₁^d are found to be nonzero. The rise of b₁^d for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.

Spin correlations in the paramagnetic phase of La₂CuO₄ have been studied using polarized neutron scattering, with two important results. First, the temperature dependence of the characteristic energy scale of the fluctuations and the amplitude of the neutron structure factor are shown to be in quantitative agreement with the predictions of the quantum nonlinear sigma model. Second, a comparison of a high-temperature series expansion of the equal-time spin correlations with the diffuse neutron intensity provides definitive experimental evidence for ring exchange.

Pair-correlated fermionic atoms are created through dissociation of weakly bound molecules near a magnetic-field Feshbach resonance. We show that correlations between atoms in different spin states can be detected using the atom shot noise in absorption images. Furthermore, using time-of-flight imaging we have observed atom pair correlations in momentum space.

A search for an exotic baryon resonance with S=-2, Q=-2 has been performed in quasireal photoproduction on a deuterium target through the decay channel Ξ^-π^-→Λπ^-π^-→pπ^-π^-π^-. No evidence for a previously reported Ξ^--(1860) resonance is found in the Ξ^-π^- invariant mass spectrum. An upper limit for the photoproduction cross section of 2.1 nb is found at the 90% confidence level. The photoproduction cross section for the Ξ⁰(1530) is found to be between 9 and 24 nb.

Polarized deep-inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double-spin asymmetries of cross sections. Inclusive and semi-inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a reanalysis of previously published data. Inclusive and semi-inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti-up, anti-down, and strange sea quarks were computed from these asymmetries in a “leading order” QCD analysis. The polarization of the up-quark is positive and that of the down-quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data.

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion (ϕ) and the target spin axis (ϕ_S) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component ⟨sin⁡(ϕ+ϕ_S)⟩_UT^π is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component ⟨sin⁡(ϕ-ϕ_S⟩_UT^π arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.

Double-spin asymmetries of semiinclusive cross sections for the production of identified pions and kaons have been measured in deep inelastic scattering of polarized positrons on a polarized deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with reanalyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.