Low Momentum Region Research Articles

2D-ACAR Studies on Swift Heavy Ion Si-Implanted GaAs

Abstract Material properties modification by high energy heavy ion implantation is a prospective technology leading to many device fabrications. This technique induces defects and hence the physical properties of the materials are modified. The effects of swift heavy ion implantation induced defects by 120 MeV 28+ Si ion implantation and doping in SI-GaAs are presented from the electron momentum distribution (EMD) of vacancy-type defects studied by two-dimensional angular correlation of annihilation radiation (2D-ACAR). The positron trapping due to the influence of high-energy Si- implantation in GaAs ( n -type) is compared with the corresponding spectra of SI- GaAs and with Si-doped ( n -type) GaAs. The EMD of the implanted sample shows a distinct increased isotropic distribution with a characteristic transform of its structure as evident from the low momentum region compared to the pristine sample. The characteristics of defects created by Si doping and by 120 MeV 28+ Si ion implantation of undoped semi-insulating (SI) GaAS are discussed. These results indicate the nature of positron trapping in open volume defects such as vacancy clusters created by implantation.

Low- and high-speed structures in the outer region of an adverse-pressure-gradient turbulent boundary layer

Large- and very large-scale structures in the form of elongated regions of low and high streamwise momentum have been studied in the outer region of a turbulent boundary layer subjected to a strong adverse pressure gradient. Large sets of streamwise–spanwise instantaneous velocity fields are acquired by particle image velocimetry at three wall-normal positions (0.2 δ, 0.5 δ, 0.8 δ) at three different streamwise locations and at 0.1 δ at the last streamwise location which allows us to study the wall-normal and streamwise variations of the structures. Subsequently, a pattern-recognition method and a classification scheme are employed in order to detect, classify and characterize the structures in an efficient and rigorous manner. Like in the case of zero-pressure-gradient turbulent boundary layers, long meandering streaky regions of low and high momentum are observed in the outer region of the present flow but they appear less frequently; especially in the lower part (at 0.1 δ and 0.2 δ) of the large-velocity-defect zone, i.e. near detachment. The dimensions of these large structures scale on boundary-layer thickness ( δ) and are generally comparable to those previously reported for such structures in the overlap region of zero-pressure-gradient turbulent boundary layers. Interestingly, the adverse pressure gradient does not significantly affect the dimensions and arrangement of the large-scale structures in the upper part (at 0.5 δ and 0.8 δ) a segment of the outer region where the scaled Reynolds stresses also remain fairly self-similar.

Particle Image Velocimetry Measurements of Turbulent Flow Within Outdoor and Indoor Urban Scale Models and Flushing Motions in Urban Canopy Layers

We investigate the spatial characteristics of urban-like canopy flow by applying particle image velocimetry (PIV) to atmospheric turbulence. The study site was a Comprehensive Outdoor Scale MOdel (COSMO) experiment for urban climate in Japan. The PIV system captured the two-dimensional flow field within the canopy layer continuously for an hour with a sampling frequency of 30 Hz, thereby providing reliable outdoor turbulence statistics. PIV measurements in a wind-tunnel facility using similar roughness geometry, but with a lower sampling frequency of 4 Hz, were also done for comparison. The turbulent momentum flux from COSMO, and the wind tunnel showed similar values and distributions when scaled using friction velocity. Some different characteristics between outdoor and indoor flow fields were mainly caused by the larger fluctuations in wind direction for the atmospheric turbulence. The focus of the analysis is on a variety of instantaneous turbulent flow structures. One remarkable flow structure is termed 'flushing', that is, a large-scale upward motion prevailing across the whole vertical cross-section of a building gap. This is observed intermittently, whereby tracer particles are flushed vertically out from the canopy layer. Flushing phenomena are also observed in the wind tunnel where there is neither thermal stratification nor outer-layer turbulence. It is suggested that flushing phenomena are correlated with the passing of large-scale low-momentum regions above the canopy.

Decoupling of spurious deeply bound states with the similarity renormalization group

The Similarity Renormalization Group (SRG) is a continuous series of unitary transformations that can be implemented as a flow equation. When the relative kinetic energy ($\Trel$) is used in the SRG generator, nuclear structure calculations have shown greatly improved convergence with basis size because of the decoupling of high-energy and low-energy physics. However this generator can sometimes be problematic. A test case is provided by a study of initial interactions from chiral effective field theories with large cutoffs, which can lead to spurious deep bound states. We would like the SRG to decouple these from the physical shallow bound states. However, with $\Trel$ the high- and low-energy bound states are not decoupled in the usual sense. Replacing $\Trel$ by the momentum-space diagonal of the Hamiltonian ($H_d$) in the SRG generator does produce decoupling, such that the shallow states are in the low-momentum region and the deep bound states are at higher momentum. The flow toward universal low-momentum interactions is also restored.

Direct numerical simulation of the turbulent boundary layer over a cube-roughened wall

Direct numerical simulation (DNS) of a spatially developing turbulent boundary layer (TBL) over a wall roughened with regularly arrayed cubes was performed to investigate the effects of three-dimensional (3-D) surface elements on the properties of the TBL. The cubes were staggered in the downstream direction and periodically arranged in the streamwise and spanwise directions with pitches of px/k = 8 and pz/k = 2, where px and pz are the streamwise and spanwise spacings of the cubes and k is the roughness height. The Reynolds number based on the momentum thickness was varied in the range Reθ = 300−1300, and the roughness height was k = 1.5θin, where θin is the momentum thickness at the inlet, which corresponds to k/δ = 0.052–0.174 from the inlet to the outlet; δ is the boundary layer thickness. The characteristics of the TBL over the 3-D cube-roughened wall were compared with the results from a DNS of the TBL over a two-dimensional (2-D) rod-roughened wall. The introduction of cube roughness affected the turbulent Reynolds stresses not only in the roughness sublayer but also in the outer layer. The present instantaneous flow field and linear stochastic estimations of the conditional averaging showed that the streaky structures in the near-wall region and the low-momentum regions and hairpin packets in the outer layer are dominant features in the TBLs over the 2-D and 3-D rough walls and that these features are significantly affected by the surface roughness throughout the entire boundary layer. In the outer layer, however, it was shown that the large-scale structures over the 2-D and 3-D roughened walls have similar characteristics, which indicates that the dimensional difference between the surfaces with 2-D and 3-D roughness has a negligible effect on the turbulence statistics and coherent structures of the TBLs.

Probing the distorted wave effects in (e, 2e) reaction for the outer valence orbitals of ferrocene

The binding energy spectrum and electron momentum spectra of outer valence orbitals of ferrocene have been obtained by using our high efficiency electron momentum spectrometer at two different impact energies of 600 eV and 1500 eV. The theoretical momentum profiles of outer valence orbitals of ferrocene for the eclipsed and staggered conformations have been calculated by non-relativistic and scalar relativistic density functional methods. It was found that the outer valence orbitals in the eclipsed conformation are in one-to-one correspondence with the ones in the staggered conformation, and there is little difference between theoretical momentum profiles of them. In addition, it was found that relativistic effects have little influence on the momentum distributions of outer valence orbital of ferrocene. Strong distorted wave effect in the low momentum region related to the outer valence orbitals were observed, Which is due to that those orbitals are mainly composed of Fe 3d atomic orbital.

Study of low momentum track reconstruction for the BESIII main drift chamber

In order to overcome the difficulty brought by the circling charged tracks with transverse momentum less than 120 MeV in the BESIII Main Drift Chamber (MDC), a specialized method called TCurlFinder was developed. This tracking method focuses on the charged track reconstruction under 120 MeV and possesses a special mechanism to reject background noise hits. The performance of the package has been carefully checked and tuned by both Monte Carlo data and real data. The study shows that this tracking method could obviously enhance the reconstruction efficiency in the low transverse momentum region, providing physics analysis with more and reliable data.

Measurement of D + and Λ c + production in deep inelastic scattering at HERA

Charm production in deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 120 pb−1. The hadronic decay channels D + → K S 0 π +, Λ c + → pK S 0 and Λ c + → Λπ +, and their charge conjugates, were reconstructed. The presence of a neutral strange hadron in the final state reduces the combinatorial background and extends the measured sensitivity into the low transverse momentum region. The kinematic range is 0 < p T (D +, Λ c +) < 10 GeV, |η(D +, Λ c +)| < 1.6, 1.5 < Q 2 < 1000 GeV2 and 0.02 < y < 0.7. Inclusive and differential cross sections for the production of D + mesons are compared to next-to-leading-order QCD predictions. The fraction of c quarks hadronising into Λ c + baryons is extracted.

ATLAS measurements of [formula omitted] events at [formula omitted

The first observation of the J / ψ → μ + μ − resonance using the ATLAS detector, in 7 TeV proton-proton collisions at the LHC, is reported for an integrated luminosity of approximately 6.4 nb−1. The reconstructed invariant mass agrees with the PDG value within statistical uncertainties, and the peak width is compatible with Monte Carlo expectations. The background estimated from Monte Carlo, or reconstructed same sign muon pairs, matches well with observations. The muon trigger and reconstruction efficiencies in this very low transverse momentum region are studied using the J / ψ candidates.

Efficiency enhancement in transonic compressor rotor blades using synthetic jets: A numerical investigation

Several passive and active techniques were studied and developed by compressor designers with the aim of improving the aerodynamic behavior of compressor blades by reducing, or even eliminating, flow separation. Fluidic-based methods, in particular, have been investigated for a long time, including both steady and unsteady suction, blowing and oscillating jets. Recently, synthetic jets (zero mass flux) have been proposed as a promising solution to reduce low-momentum fluid regions inside turbomachines. Synthetic jets, with the characteristics of zero net mass flux and non-zero momentum flux, do not require a complex system of pumps and pipes. They could be very efficient because at the suction part of the cycle the low-momentum fluid is sucked into the device, whereas in the blowing part a high-momentum jet accelerates it. To the authors’ knowledge, the use of synthetic jets has never been experimented in transonic compressor rotors, where this technique could be helpful (i) to reduce the thickness and instability of blade suction side boundary layer after the interaction with the shock, and (ii) to delay the arising of the low-momentum region which can take place from the shock-tip clearance vortex interaction at low flow operating conditions, a flow feature which is considered harmful to rotor stability. Therefore, synthetic jets could be helpful to improve both efficiency and stall margin in transonic compressor rotors. In this paper, an accurate and validated CFD model is used to simulate the aerodynamic behavior of a transonic compressor rotor with and without synthetic jets. Four technical solutions were evaluated, different for jet position and velocity, and one was investigated in detail.

An experimental and theoretical study of the HOMO of W(CO) 6: Vibrational effects on the electron momentum density distribution

The unexpected higher intensity at low momentum region for HOMO of W(CO) 6 was investigated using electron momentum spectroscopy at various impact energies, as well as non-relativistic, scalar relativistic and spin–orbit relativistic DFT-B3LYP calculations. There was no evident difference between the experimental momentum profiles at various impact energies. It was found that the low frequency vibration of W(CO) 6 can evidently change the momentum profile. The semi-quantitative vibrational averaged calculations agreed well with the experimental results. Therefore, it was suggested that the unexpected higher intensity at low momentum regions was possibly due to the vibration effects.

Study of strange particle production in pp collisions with the ALICE detector

ALICE is well suited for strange particle production studies since it has very good reconstruction capabilities in the low transverse momentum (pt) region and it also allows the identification to be extended up to quite high pt. Charged strange mesons (K+, K−) are reconstructed via energy loss measurements whereas neutral strange mesons (K0s) and strange hyperons (Λ, Ξ, Ω) are identified via vertex reconstruction. All these particles carry important information: firstly, the measurement of production yields and the particle ratio within the statistical models can help to understand the medium created, and secondly the dynamics at intermediate pt investigated via the baryon over meson ratio (Λ/K0s) allows a better understanding of the hadronization mechanisms and of the underlying event processes. We present these two aspects of the strange particle analysis in pp collisions using simulated data.

TeV scale seesaw model and a flavorfulZ′at the LHC

Small neutrino masses and their large mixing angles can be generated at the TeV scale by augmenting the standard model with an additional generation-dependent, anomaly-free $U(1{)}_{\ensuremath{\nu}}$ symmetry, in the presence of three right-handed neutrinos. The ${Z}^{\ensuremath{'}}$ gauge boson associated with the breaking of the $U(1{)}_{\ensuremath{\nu}}$ symmetry can be produced at the LHC. The flavorful nature of the ${Z}^{\ensuremath{'}}$ can be established by measuring its nonuniversal couplings to the charged leptons as determined by the lepton's $U(1{)}_{\ensuremath{\nu}}$ charges, which also govern the neutrino flavor structure. While the LHC has the potential of discovering the ${Z}^{\ensuremath{'}}$ up to ${M}_{{Z}^{\ensuremath{'}}}=4.5\text{ }\text{ }\mathrm{TeV}$ with $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data at the center of mass energy $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, to establish the flavorful nature of the ${Z}^{\ensuremath{'}}$ requires much higher integrated luminosity. For our benchmark parameters that are consistent with neutrino oscillation data, at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, a $5\ensuremath{\sigma}$ distinction between the dielectron and dimuon channels for ${M}_{{Z}^{\ensuremath{'}}}=3\text{ }\text{ }\mathrm{TeV}$ requires $500\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data. We find that the forward backward asymmetry distributions can also be useful in distinguishing the dielectron and dimuon channels in the low invariant mass and transverse momentum regions.

Photon production in relativistic heavy ion collisions with equilibrium and non-equilibrium QGP

We consider the production sources of prompt and thermal photons which include the contribution of gluons in relativistic heavy ion collisions. Considering in our calculation the shadowing and iso-spin effects of the nucleus we can properly estimate the prompt photon production. We develop a new thermal jet-photon conversion mechanism which plays a vital role in the low transverse momentum region. The effect of the non-equilibrium quark-gluon plasma enhances the contribution of thermal photons.

Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) Modelling of Carotid Artery Haemodynamics under Steady Flow: A Validation Study

Particle image velocimetry (PIV) and computational fluid dynamics (CFD) modelling of blood flow through a carotid artery bifurcation has been carried out in order to assess the role of haemodynamics in atherosclerosis and validate a novel wall shear stress (WSS) measurement method via detailed quantitative data analysis. Velocity and WSS data, obtained by PIV was compared against CFD-predicted data obtained for the same geometry and boundary conditions. The results demonstrate that both methods are capable of capturing essential flow characteristics, and are in good agreement. The axial velocity data clearly show the important flow features such as skewed velocity profiles and a low-momentum region near the sinus outer wall, although with slight errors between the two techniques in the order of 6.1-13.7%. The secondary velocity plots and three-dimensional particle traces reveal complex flow features within the sinus, such as Dean vortices and helicoidal flow. WSS data obtained by both techniques reveal the presence of a low-momentum and reversed-flow regions, and show agreement with errors in the order of 1% in the common carotid artery and 0.9-9.8% in the sinus. The error sources in both the experimental and numerical methodology are discussed and recommendations for future applications are made.

Coherent structures in turbulent boundary layers with adverse pressure gradients

The coherent structures in turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) were investigated by analyzing a database of direct numerical simulations. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution. The population trends of the spanwise vortices show that the outer regions of the APG TBLs are densely populated with hairpin-like vortices. These vortical structures induce low-momentum regions in the middle of the boundary layers, which result in an outer peak in the Reynolds shear stress. The 3-D features of the hairpin packets were deduced from their spatial characteristics in the spanwise-wall-normal plane. The conditionally averaged velocity fields show that there are counter-rotating v−w swirling motions that represent cross-sectional evidences of the packets. Moreover, two-point correlations and linear stochastic estimations were used to provide statistical information about the hairpin packet motions in the ...

Electron Momentum Distributions for 4a1 Orbitals of CFxCl4xin Low Momentum Region: a Possible Evidence of Molecular Geometry Distortion

Electron momentum distributions for 4a1 orbitals of serial freon molecules CF3Cl, CF2Cl2, and CFCl3 (CFxCl4x, x = 13) have been reanalyzed due to the severe discrepancies between theory and experiment in low momentum region. The tentative calculations using equilibrium geometries of molecular ions have exhibited a great improvement in agreement with the experimental data, which suggests that the molecular geometry distortion may be responsible for the observed high intensities at p 0.5 a.u. Further analyses show that the severe discrepancies at low momentum region mainly arise from the influence of molecular geometry distortion on CCl bonding electron density distributions.

Structures in turbulent boundary layers subjected to adverse pressure gradients

The effects of adverse pressure gradients on turbulent structures were investigated by carrying out direct numerical simulations of turbulent boundary layers subjected to adverse and zero pressure gradients. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution U∞ ~ xm. Two-point correlations of velocity fluctuations were used to show that the spanwise spacing between near-wall streaks is affected significantly by a strong adverse pressure gradient. Low-momentum regions are dominant in the middle of the boundary layer as well as in the log layer. Linear stochastic estimation was used to provide evidence for the presence of low-momentum regions and to determine their statistical properties. The mean width of such large-scale structures is closely associated with the size of the hairpin-like vortices in the outer layer. The conditionally averaged flow fields around events producing Reynolds stress show that hairpin-like vortices are the structures associated with the production of outer turbulence. The shapes of the vortices beyond the log layer were found to be similar when their length scales were normalized according to the boundary layer thickness. Estimates of the conditionally averaged velocity fields associated with the spanwise vortical motion were obtained by using linear stochastic estimation. These results confirm that the outer region of the adverse pressure gradient boundary layer is populated with streamwise-aligned vortex organizations, which are similar to those of the vortex packet model proposed by Adrian, Meinhart &amp; Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1–54). The adverse pressure gradient augments the inclination angles of the packets and the mean streamwise spacing of the vortex heads in the packets.

Jet-Photon Production at RHIC and LHC

We calculate the production of prompt and thermal photons which includes the contribution of gluons in relativistic heavy ion collisions with the equilibrium and non-equilibrium quark-gluon plasma. We develop a new thermal jet-photon conversion mechanism which plays a vital role in the low transverse momentum region. The effect of the non-equilibrium quark-gluon plasma enhances the contribution of the thermal photons. The shadowing and iso-spin of the nucleus which can properly estimate the prompt photon production are also considered in our calculation.

Positron lifetime and Doppler broadening study of defects in hydrogen charged Al–Mg alloys

The results of positron lifetime and Doppler broadening spectrum of defects in the hydrogen charged non-heat treatable 5xxx Al alloys are presented in this work. The yield stress of the sample was reduced for about 20 MPa after hydrogen was charged. A similar trend was observed in positron lifetime measurement, as the average lifetime τav descended remarkably to almost the level of Al matrix. The change in coincidence Doppler broadening (CDB) spectroscopy was also significant, exhibited by the characteristic change in CDB radio curves of a sample before and after hydrogen was charged. After hydrogen charging, there is an obvious enhancement in the high momentum region compensating dehancement in the low momentum region. This indicates the existence of hydrogen filling effect. The vacancies around the Mg atoms should be preferential filling sites for hydrogen because Mg has a strong affinity for hydrogen. The formation of an Mg–H bond parallel to a grain boundary is an important factor in weakening the grain boundary cohesion.