Azimuthal Angle Dependence Research Articles

Physical mechanisms of terahertz pulse emission from photoexcited surfaces of tellurium crystals

We report on terahertz (THz) emission from tellurium crystal surfaces excited by femtosecond optical pulses. Measurements were performed on three differently cut Te samples and with different wavelength optical excitation pulses. THz pulse amplitude dependences on the azimuthal angle measured at various excitation wavelengths have evidenced that three different mechanisms are responsible for THz generation in tellurium: second order nonlinear optical rectification effect, dominating at lower excitation photon energies, as well as transverse and ordinary photo-Dember effects, which emerge at energies larger than 0.9 eV. The shapes of the azimuthal angle dependences were also explained by theoretical model.

Emission angle and particle mass dependence of HBT radii in Au+Au collisions at sNN=200 GeV

Abstract Hydrodynamic features of quark–gluon plasma (QGP) are being studied via various observables in high-energy heavy-ion collisions. Measurement of space–time structure via two-particle interferometry is one of those and is known as Hanbury Brown and Twiss (HBT) method. This gives geometrical and temporal information as well as dynamical information such as collective expansion during the freeze-out stage. The azimuthal-angle dependence of the HBT radii for various particle species ( ππ and KK ) and for various event plane orders (2nd and 3rd) are measured in s NN = 200 GeV Au+Au collisions at RHIC-PHENIX experiment. The extracted parameters for KK pairs are slightly different from those for ππ pairs for a given m T , which may indicate earlier freeze-out and longer emission duration for K than for π . The azimuthal dependences relative to the 2nd and 3rd order event planes are observed to be consistent with a strong and rapid expansion, which has also been known as sudden freeze-out with a short duration time. Large v 2 events are selected by the 2nd order flow vector magnitude Q 2 in order to test the Event Shape Engineering (ESE) via final eccentricity ϵ final measurement. The results confirm a significant correlation between elliptic flow v 2 and final eccentricity ϵ final , which is supposed to originate from the initial geometrical eccentricity ϵ initial .

Macroscopic and microscopic strain of SiO2 glass under uniaxial compression

Although SiO2 glass is brittle due to its covalency and the lack of dislocation movement seen in crystals, it can deform without fracturing when compressed to high pressures. The phenomenon may be attributable to the well-known permanent densification by the reconstruction of the network structure consisting of SiO4 tetrahedra. To explore so-called plastic deformation without permanent densification, we measured the change in size (macroscopic strain) of uniaxially-compressed disk-shaped SiO2 glass by an optical microscope [1]. Also, to understand the anisotropy in structure (microscopic strain), we measured the azimuth-angle dependence of the position of the first sharp diffraction peak (FSDP) of uniaxially-compressed SiO2 glass with a radial X-ray diffraction technique [2]. In the microscope observation, the glass was found to deform largely without fracturing up to at least 20 GPa from 6-8 GPa, where uniaxial conditions were achieved. In the X-ray diffraction observation, a large anisotropy was found in the FSDP which corresponds to the intermediate-range network structure of the glass. The recovered glass was examined by the radial X-ray diffraction up to a high-Q range and was found to remain largely anisotropic (equivalent to about 2 GPa in differential stress) in the intermediate-range network structure and not to remain anisotropic in the short-range SiO4 tetrahedral structure. It seems intuitive that the residual anisotropy is due to the anisotropic reconstruction of the network structure during permanent densification. However, the macroscopic strain measured in the microscope observation was an order of magnitude larger than the microscopic strain in the X-ray diffraction observation, and therefore it cannot be explained solely by the anisotropic permanent densification. The permanent densification may also enhance the reconstruction of the network structure and therefore plastic deformation.

Hydrodynamics of charge fluctuations and balance functions

We apply stochastic hydrodynamics to the study of charge density fluctuations in QCD matter undergoing Bjorken expansion. We find that the charge density correlations are given by a time integral over the history of the system, with the dominant contribution coming from the QCD crossover region where the change of susceptibility per entropy, chi T/s, is most significant. We study the rapidity and azimuthal angle dependence of the resulting charge balance function using a simple analytic model of heavy-ion collision evolution. Our results are in agreement with experimental measurements, indicating that hydrodynamic fluctuations contribute significantly to the measured charge correlations in high energy heavy-ion collisions. The sensitivity of the balance function to the value of the charge diffusion coefficient D allows us to estimate the typical value of this coefficient in the crossover region to be rather small, of the order of 1/(2pi T), characteristic of a strongly coupled plasma.

Azimuthal-angle dependence of charged-pion-interferometry measurements with respect to second- and third-order event planes in Au+Au collisions at √[S(NN)]=200 GeV.

Charged-pion-interferometry measurements were made with respect to the second- and third-order event plane for Au+Au collisions at sqrt[s_{NN}]=200 GeV. A strong azimuthal-angle dependence of the extracted Gaussian-source radii was observed with respect to both the second- and third-order event planes. The results for the second-order dependence indicate that the initial eccentricity is reduced during the medium evolution, which is consistent with previous results. In contrast, the results for the third-order dependence indicate that the initial triangular shape is significantly reduced and potentially reversed by the end of the medium evolution, and that the third-order oscillations are largely dominated by the dynamical effects from triangular flow.

Effects of dispersion and absorption in resonant Bragg diffraction of x-rays

Resonant diffraction of x-rays by crystals with anisotropic optical properties is investigated theoretically, to assess how the intensity of a Bragg spot is influenced by effects related to dispersion (birefringence) and absorption (dichroism). Starting from an exact but opaque expression, simple analytic results are found to expose how intensity depends on dispersion and absorption in the primary and secondary beams and, also, the azimuthal angle (rotation of the crystal about the Bragg wavevector). If not the full story for a given application, our results are more than adequate to explore consequences of dispersion and absorption in the intensity of a Bragg spot. Results are evaluated for antiferromagnetic copper oxide, and low quartz. For CuO, one of our results reproduces all salient features of a previously published simulation of the azimuthal-angle dependence of a magnetic Bragg peak. It is transparent in our analytic result that dispersion and absorption effects alone cannot reproduce published experimental data. Available data for the azimuthal-angle dependence of space-group forbidden reflections (0,0, l), with l≠3n, of low quartz depart from symmetry imposed by the triad axis of rotation symmetry. The observed asymmetry can be induced by dispersion and absorption even though absorption coefficients are constant, independent of the azimuthal angle, in this class of reflections.

Free streaming and sudden equilibration approximation for the early stages of relativistic heavy-ion collisions

Using a multi-phase transport (AMPT) model, we study the equilibration process of the early collision stages with free streaming and sudden equilibration approximation. In addition, the results of the transverse momentum spectra, the elliptic flow, the Hanbury-Brown twiss (HBT) parameters and the freeze-out eccentricity from the azimuthal angle dependence of the HBT radii are described within our approach. The results of the transverse momentum spectra and the elliptic flow are close to the experimental data. The HBT radii result is similar to that from the AMPT model. The eccentricity results indicate that the approximation leads to a slow drop of the eccentricity with increasing time. The slow decrease can be reflected in the freeze-out eccentricity obtained by HBT radii.

LONG-RANGE RAPIDITY CORRELATIONS IN HEAVY-LIGHT ION COLLISIONS

We study two-particle long-range rapidity correlations arising in the early stages of heavy ion collisions in the saturation/Color Glass Condensate framework, assuming for simplicity that one colliding nucleus is much larger than the other. We calculate the two-gluon production cross section while including all-order saturation effects in the heavy nucleus with the lowest-order rescattering in the lighter nucleus. We find four types of correlations in the two-gluon production cross section: (i) geometric correlations, (ii) HBT correlations accompanied by a back-to-back maximum, (iii) away-side correlations, and (iv) near-side azimuthal correlations which are long-range in rapidity. The geometric correlations (i) are due to the fact that nucleons are correlated by simply being confined within the same nucleus and may lead to long-range rapidity correlations for the produced particles without strong azimuthal angle dependence. Somewhat surprisingly, long-range rapidity correlations (iii) and (iv) have exactly the same amplitudes along with azimuthal and rapidity shapes: one centered around Δϕ = π with the other one centered around Δϕ = 0 (here Δϕ is the azimuthal angle between the two produced gluons). We thus observe that the early-time CGC dynamics in nucleus-nucleus collisions generates azimuthal non-flow correlations which are qualitatively different from jet correlations by being long-range in rapidity. If strong enough, they have the potential of mimicking the elliptic (and higher-order even-harmonic) flow in the di-hadron correlators: one may need to take them into account in the experimental determination of the flow observables.

Simulation of energy scan of pion interferometry in central Au+Au collisions at relativistic energies

We present a systematic analysis of two-pion interferometry for the central Au+Au collisions at √sNN = 3, 5, 7, 11, 17, 27, 39, 62, 130 and 200 GeV/c with the help of a multiphase transport (AMPT) model. Emission source-size radius parameters Rlong, Rout, Rside and the chaotic parameter λ are extracted and compared with the experimental data. Transverse momentum and azimuthal angle dependencies of the HBT radii are also discussed for central Au+Au collisions at 200 GeV/c. The results show that the HBT radii in central collisions do not change much above 7 GeV/c. For central collisions at 200 GeV/c, the radii decrease with the increasing of transverse momentum pT but are not sensitive to the azimuthal angle. These results provide a theoretical reference for the energy scan program of the RHIC-STAR experiment.

The creation, destruction, and transfer of multipole moments in electron- and proton-impact ionization of atoms and ions

Expanding on previous works that involved elastic and inelastic scattering of electrons by atoms and ions, we use the wave-packet propagation scheme of Dollard to define multipole moment creation, destruction and transfer cross sections for electron- and proton-impact ionization of atoms and ions. The electron-impact cross sections can then be used by defining appropriate rate coefficients for use in Fujimoto’s population-alignment collisional-radiative model for cylindrically symmetric plasmas. Our result for the alignment creation cross section is in agreement with those formulae that were obtained earlier intuitively or by semi-classical collisional methods. The multipole cross sections obtained here can be used also for modelling the relaxation behaviour of laser-excited plasmas under cylindrical symmetry conditions. We have also derived the electron- and proton-impact ionization multipole cross sections in terms of Liouville-space quantities, which then enabled us by using group theoretical methods to obtain the azimuthal-angle dependence of the multipole cross sections and symmetry properties that are results of reflection across a plane over the collisional axis.

Evaluation of the dosimetric properties of a synthetic single crystal diamond detector in high energy clinical proton beams

To investigate the dosimetric properties of a synthetic single crystal diamond Schottky diode for accurate relative dose measurements in large and small field high-energy clinical proton beams. The dosimetric properties of a synthetic single crystal diamond detector were assessed by comparison with a reference Markus parallel plate ionization chamber, an Exradin A16 microionization chamber, and Exradin T1a ion chamber. The diamond detector was operated at zero bias voltage at all times. Comparative dose distribution measurements were performed by means of Fractional depth dose curves and lateral beam profiles in clinical proton beams of energies 155 and 250 MeV for a 14 cm square cerrobend aperture and 126 MeV for 3, 2, and 1 cm diameter circular brass collimators. ICRU Report No. 78 recommended beam parameters were used to compare fractional depth dose curves and beam profiles obtained using the diamond detector and the reference ionization chamber. Warm-up∕stability of the detector response and linearity with dose were evaluated in a 250 MeV proton beam and dose rate dependence was evaluated in a 126 MeV proton beam. Stem effect and the azimuthal angle dependence of the diode response were also evaluated. A maximum deviation in diamond detector signal from the average reading of less than 0.5% was found during the warm-up irradiation procedure. The detector response showed a good linear behavior as a function of dose with observed deviations below 0.5% over a dose range from 50 to 500 cGy. The detector response was dose rate independent, with deviations below 0.5% in the investigated dose rates ranging from 85 to 300 cGy∕min. Stem effect and azimuthal angle dependence of the diode signal were within 0.5%. Fractional depth dose curves and lateral beam profiles obtained with the diamond detector were in good agreement with those measured using reference dosimeters. The observed dosimetric properties of the synthetic single crystal diamond detector indicate that its behavior is proton energy independent and dose rate independent in the investigated energy and dose rate range and it is suitable for accurate relative dosimetric measurements in large as well as in small field high energy clinical proton beams.

Differential strain and residual anisotropy in silica glass

To understand the behavior of SiO2 glass under high pressure and differential stress, we conducted radial x-ray diffraction measurements on SiO2 glass up to 60 GPa, in which x-rays irradiate the sample from a direction perpendicular to the compression axis of a uniaxial apparatus. The differential strain of SiO2 glass, determined from the azimuth angle dependence of the position of the first sharp diffraction peak, was very large especially at pressures below 20 GPa and decreased with increasing pressure. After decompression, a large differential strain, equivalent to about 2 GPa in differential stress, remained in the glass at ambient conditions. We attribute this residual anisotropy to the anisotropic permanent densification, which is caused by the anisotropic change in intermediate-range structure, i.e., the anisotropic reconstruction of the network structure consisting of SiO4 tetrahedra.

Direct photon-hadron correlations with the PHENIX detector

Direct photon tagged jets, in the form of photon-hadron correlations, are well suited to provide unique insight into how jets interact with the quark gluon plasma. Since photons do not interact strongly with medium produced at RHIC, at leading order the measured photon momentum approximately balances that of the away-side parton. The modification to the effective fragmentation function can be measured by comparing integrated away-side yields in direct photon-hadron correlations in Au+Au collisions to those in p+p. By varying the away-side integration range, the angular dependence of the observed modification can also be studied. Direct photon-hadron correlations have been measured with PHENIX in p+p and Au+Au using a statistical subtraction technique to remove the decay photon contribution from the inclusive photon-hadron correlations, with an additional isolation cut applied in p+p to reduce uncertainties. Recently published results showing a significant modification to the fragmentation function with an azimuthal angle dependence indicative of broadening are discussed in the context of related theoretical models and complementary results from other experiments.

Surface-grating deflection of fast atom beams

For energetic atomic beams grazingly incident at the surface along the low index directions, fast motion parallel to the surface and slow motion perpendicular to the surface lead to the quantum diffraction pattern in the scattered beam. In this experimental and theoretical joint study we show that when the incident beam is misaligned with respect to an axial channel, the characteristic deformation of the diffraction pattern reflects an overall deflection of the scattered beam from the specular direction. The deflection is maximum for the azimuthal misalignment angles close to the rainbow angle and we show how this effect can be explained with the detailed balance principle relating diffraction of misaligned and perfectly aligned beams. We also demonstrate that using the detailed balance principle the diffraction charts for the incident beams aligned along the axial channel can be reconstructed from the azimuthal incidence angle dependence of the data obtained with misaligned beams.

ATLAS results from lead-lead collisions at sNN=2.76TeV

Abstract An overview of the recent results on particle production in lead–lead collisions at s NN = 2.76 TeV obtained with the ATLAS detector is presented. Two samples of collision events recorded in 2010 and 2011 LHC heavy-ion runs, with integrated luminosities of approximately 9 μ b − 1 and 0.15 nb − 1 respectively, were used. The measurements of collective flow phenomena in lead–lead collisions, related to the initial geometry and its fluctuations, are shown providing new constraints on the initial geometry models and on hydrodynamic evolution of the system. They are complemented with studies of high-transverse momentum probes. The latter include measurements of electro-weakly interacting probes such as Z bosons and prompt photons as well as strongly interacting probes like charged hadrons, heavy quarks and jets. Yields of electro-weak probes are found to be consistent with binary collision scaling for all collision centralities. On the contrary, charged hadrons, heavy quarks and jets are suppressed in central collisions relative to peripheral events. The azimuthal angle dependence of jet yields is consistent with the expected dependence on the path length traversed by partons through the medium, while the measured jet fragmentation function shows some centrality-dependent modifications. These measurements, supplemented with the results on correlations between electro-weak probes and jets, provide new insights into the mechanism of in-medium parton energy loss.

Long-range rapidity correlations in heavy–light ion collisions

We study two-particle long-range rapidity correlations arising in the early stages of heavy ion collisions in the saturation/Color Glass Condensate framework, assuming for simplicity that one colliding nucleus is much larger than the other. We calculate the two-gluon production cross section while including all-order saturation effects in the heavy nucleus with the lowest-order rescattering in the lighter nucleus. We find four types of correlations in the two-gluon production cross section: (i) geometric correlations, (ii) HBT correlations accompanied by a back-to-back maximum, (iii) away-side correlations, and (iv) near-side azimuthal correlations which are long-range in rapidity. The geometric correlations (i) are due to the fact that nucleons are correlated by simply being confined within the same nucleus and may lead to long-range rapidity correlations for the produced particles without strong azimuthal angle dependence. Somewhat surprisingly, long-range rapidity correlations (iii) and (iv) have exactly the same amplitudes along with azimuthal and rapidity shapes: one centered around Δ ϕ = π with the other one centered around Δ ϕ = 0 (here Δ ϕ is the azimuthal angle between the two produced gluons). We thus observe that the early-time CGC dynamics in nucleus–nucleus collisions generates azimuthal non-flow correlations which are qualitatively different from jet correlations by being long-range in rapidity. If strong enough, they have the potential of mimicking the elliptic (and higher-order even-harmonic) flow in the di-hadron correlators: one may need to take them into account in the experimental determination of the flow observables.

REFLECTANCE CALIBRATION SCHEME FOR AIRBORNE FRAME CAMERA IMAGES

Abstract. The image quality of photogrammetric images is influenced by various effects from outside the camera. One effect is the scattered light from the atmosphere that lowers contrast in the images and creates a colour shift towards the blue. Another is the changing illumination during the day which results in changing image brightness within an image block. In addition, there is the so-called bidirectional reflectance of the ground (BRDF effects) that is giving rise to a view and sun angle dependent brightness gradient in the image itself. To correct for the first two effects an atmospheric correction with reflectance calibration is chosen. The effects have been corrected successfully for ADS linescan sensor data by using a parametrization of the atmospheric quantities. Following Kaufman et al. the actual atmospheric condition is estimated by the brightness of a dark pixel taken from the image. The BRDF effects are corrected using a semi-empirical modelling of the brightness gradient. Both methods are now extended to frame cameras. Linescan sensors have a viewing geometry that is only dependent from the cross track view zenith angle. The difference for frame cameras now is to include the extra dimension of the view azimuth into the modelling. Since both the atmospheric correction and the BRDF correction require a model inversion with the help of image data, a different image sampling strategy is necessary which includes the azimuth angle dependence. For the atmospheric correction a sixth variable is added to the existing five variables visibility, view zenith angle, sun zenith angle, ground altitude, and flight altitude – thus multiplying the number of modelling input combinations for the offline-inversion. The parametrization has to reflect the view azimuth angle dependence. The BRDF model already contains the view azimuth dependence and is combined with a new sampling strategy.

Terahertz Pulse Emission from Nanostructured (311) Surfaces of GaAs

In the present work, azimuthal angle dependences of the terahertz (THz) pulse emission from lower symmetry, (311) planes of porous GaAs samples have been studied. GaAs porous layers were prepared by pulsed anodic electrochemical etching of n-type (311) GaAs wafers in mixed acidic fluoride-iodide electrolyte. It has been discovered that the anodic electrochemical etching of the GaAs sample significantly enhances its terahertz radiation emissivity. It was shown theoretically that for this crystallographic plane the contributions of both optical rectification (OR) and electric-field-induced optical rectification (EFIOR) effects are characterized by different azimuthal angle dependences. Experimental measurements were compared with the theoretical calculations of the azimuthal angle dependencies; it has been shown that both nonlinear optical effects are necessary to take into account when explaining the experimental observations.

Resonant X-Ray Diffraction Study of Multipole Ordering in the Ferromagnetic Compound CePd3S4

Resonant X-ray diffraction experiments on CePd 3 S 4 were performed at the Ce- L 3 absorption edge to study the states of quadrupolar ordering below the ferromagnetic transition temperature T C = 6.3 K. The magnetic structure of CePd 3 S 4 was reported as a canted ferromagnetic structure with a canting angle of 51°. It has therefore been pointed out that the complex magnetic structure arises from the simultaneous ordering of quadrupole moments. We observed the azimuthal angle dependence of the diffraction intensity of two forbidden reflections, specifically, 003 and 104 reflections, for both π–π' and π–σ' scattering processes. The results revealed that O 2 0 -type antiferroquadrupolar ordering coexists with magnetic ordering. We infer that the magnetic structure of CePd 3 S 4 is not a canted structure but a ferrimagnetic structure and that the quadrupole moment is also a primary-order parameter. These findings well explain the anomalously high transition temperature of CePd 3 S 4 , which is very much high...

Three-dimensional spin orientation in antiferromagnetic domain walls of NiO studied by x-ray magnetic linear dichroism photoemission electron microscopy

A determination of the three-dimensional spin directions in all types of domain walls (DWs) of antiferromagnetic NiO has been successfully performed by photoemission electron microscopy combined with x-ray magnetic linear dichroism (XMLD), both for $s$- and $p$-polarized light. By comparing the azimuthal angle dependence of the XMLD contrast in the DWs with cluster model calculations which include the crystal symmetry and full-multiplet splitting, we determine the spin structures in the ${001}$ T walls, ${011}$ T walls, 120\ifmmode^\circ\else\textdegree\fi{} S walls, and 180\ifmmode^\circ\else\textdegree\fi{} S walls. In some cases, distinct S walls are not formed between two adjacent S domains, and the spin direction changes gradually over a wide range of the S domain structures. In the S walls, the spin direction is parallel to the magnetic easy ${111}$ plane. These spin configurations arise from the large difference in anisotropy energy between the in-plane and out-of-plane directions. Unexpectedly large widths in the several hundred nanometer range were observed for all the DWs. This also shows that NiO has a small magnetocrystalline anisotropy energy. Together with Monte Carlo simulation results, the qualitative phenomena concerning the wall energies are discussed. We further investigated the difference in wall energy between the ${001}$ T wall and the ${011}$ T wall. From the Monte Carlo simulation and an experimental study of heating effects, it is revealed that the ${001}$ T wall energy is smaller than the ${011}$ T wall energy.