Two-dimensional Angular Correlation Research Articles

Momentum density spectroscopy of Pd: Comparison of 2D-ACAR and Compton scattering using a 1D-to-2D reconstruction method

Two-dimensional angular correlation of annihilation radiation (2D-ACAR) and Compton scattering are both powerful techniques to investigate the bulk electronic structure of crystalline solids through the momentum density of the electrons. Here we apply both methods to a single crystal of Pd to study the electron momentum density and the occupancy in the first Brillouin zone, and to point out the complementary nature of the two techniques. To retrieve the 2D spectra from 1D Compton profiles, a new direct inversion method (DIM) is implemented and benchmarked against the well-established Cormack's method. The comparison of experimental spectra with first principles density functional theory calculations of the electron momentum density and the two photon momentum density clearly reveals the importance of positron probing effects on the determination of the electronic structure. While the calculations are in good agreement with the experimental data, our results highlight some significant discrepancies.

Measurement of D0 -meson + hadron two-dimensional angular correlations in Au+Au collisions at sNN=200 GeV

Open heavy flavor hadrons provide unique probes of the medium produced in ultra-relativistic heavy-ion collisions. Due to their increased mass relative to light-flavor hadrons, long lifetime, and early production in hard-scattering interactions, they provide access to the full evolution of the partonic medium formed in heavy-ion collisions. This paper reports two-dimensional (2D) angular correlations between neutral $D$-mesons and unidentified charged particles produced in minimum-bias Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. $D^0$ and $\bar{D}^0$ mesons are reconstructed via their weak decay to $K^{\mp} \pi^{\pm}$ using the Heavy Flavor Tracker (HFT) in the Solenoidal Tracker at RHIC (STAR) experiment. Correlations on relative pseudorapidity and azimuth $(\Delta\eta,\Delta\phi)$ are presented for peripheral, mid-central and central collisions with $D^0$ transverse momentum from 2 to 10 GeV/$c$. Attention is focused on the 2D peaked correlation structure near the triggered $D^0$-meson, the {\em near-side} (NS) peak, which serves as a proxy for a charm-quark containing jet. The correlated NS yield of charged particles per $D^0$-meson and the 2D widths of the NS peak increase significantly from peripheral to central collisions. These results are compared with similar correlations using unidentified charged particles, consisting primarily of light-flavor hadrons, at similar trigger particle momenta. Similar per-trigger yields and widths of the NS correlation peak are observed. The present results provide additional evidence that $D^0$-mesons undergo significant interactions with the medium formed in heavy-ion collision and show, for the first time, significant centrality evolution of the NS 2D peak in the correlations of particles associated with a heavy-flavor hadron produced in these collisions.

Study on anisotropies and momentum densities in AlN, GaN and InN by positron annihilation

The independent particle model (IPM) coupled with empirical pseudopotential method (EPM) was used to compute the thermalized positron charge densities in specific family of binary tetrahedrally coordinated crystals of formula ANB8-N. Initial results show a clear asymmetrical positron charge distribution relative to the bond center. It is observed that the positron density is maximum in the open interstices and is excluded not only, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001,110) planes. The results are used to analyze the positron effects in AlN, GaN and InN compounds. Our computational technique provides the theoretical means of interpreting the k-space densities obtained experimentally using the twodimensional angular correlation of annihilation radiation (2D-ACAR).

Decomposing energy balance contributions for quenched jets with CMS

Modification of energy balance in dijet events from heavy ion collisions, measured by CMS, was among the first jet quenching observations in the LHC energy domain. Here we further study the spatial extent of medium-induced modifications for such dijets, as well as potential medium response to propagating partons, using two-dimensional angular correlations of charged hadrons measured with respect to jets. New differential measurements of charged particle energy flow about the jet direction as a function of relative azimuth and relative pseudorapidity from 2.76 TeV PbPb collisions are compared with the reference pp data recorded by the CMS at the same energy. Modifications of correlated charged hadron distributions for both the leading and the subleading sides of the dijet are reported, together with comparisons of the long-range asymmetry of the underlying event in PbPb vs pp collisions.

Jet-track correlations in PbPb and pp collisions at 2.76 TeV

Jet quenching is one of the established signatures of the Quark Gluon Plasma, and this phenomenon has been studied extensively in high energy heavy ion experiments over the last decade. Despite significant progress made in those studies, the specifics of the energy loss mechanisms and the details of jet-medium interactions require further quantitative understanding. Studies of two-dimensional angular correlations of charged hadrons with respect to reconstructed jets make it possible to experimentally assess the energy radiation patterns of an energetic parton traversing the medium as well as its fragmentation function. In these proceedings, we present new differential measurements of charged particle densities and energy flow about the jet direction as a function of relative azimuth and relative pseudorapidity from 2.76 TeV PbPb and pp collisions recorded by the CMS Collaboration. With a sample of back-to-back dijets, previously used to reconstruct the event-wise momentum imbalance (missing pT), we explore modifications to correlated charged hadron distributions for both the leading and the subleading sides of the dijet. With this technique, we can individually assess the contribution of the medium-induced modifications to each side of the dijet and explore the extent of potential medium response to the jet propagation, while also extending these measurements to large angular and radial distances.

The gamma-ray spectra of 5-carbon alkane isomers in the positron annihilation process

The gamma-ray spectra of pentane (C5H12) and its two isomers, i.e., 2-Methylbutane (CH3C(CH3)HC2H5) and 2,2-Dimethylpropane (C(CH3)4) have been studied theoretically in the present work. The recent experimental gamma-ray spectra of these three molecules show that they have the same Doppler shifts, although their molecular structures are dramatically different. In order to reveal why the gamma-ray spectra of these molecules are less sensitive to the molecular structures, the one-dimensional gamma-ray spectra and spherically averaged momentum (SAM) distributions, the two-dimensional angular correlation of annihilation radiation (ACAR), and the three-dimensional momentum distributions of the positron–electron pair are studied. The one-centered momentum distributions of the electrons are found to play more important role than the multi-centered coordinate distributions. The present theoretical predictions have confirmed the experimental findings for the first time. The dominance of the inner valence electrons in the positron–electron annihilation process has also been suggested in the present work.

Local electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation.

We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV.

Charge-multiplicity dependence of single-particle transverse-rapidityytand pseudorapidityηdensities and 2D angular correlations from 200 GeVp−pcollisions

An established phenomenology and theoretical interpretation of $p\text{\ensuremath{-}}p$ collision data at lower collision energies should provide a reference for $p\text{\ensuremath{-}}p$ and other collision systems at higher energies, against which claims of novel physics may be tested. The description of $p\text{\ensuremath{-}}p$ collisions at the relativistic heavy ion collider has remained incomplete even as claims for collectivity and other novelties in data from smaller systems at the large hadron collider have emerged recently. In this study we report the charge-multiplicity dependence of two-dimensional angular correlations and of single-particle (SP) densities on transverse rapidity ${y}_{t}$ and pseudorapidity $\ensuremath{\eta}$ from 200 GeV $p\text{\ensuremath{-}}p$ collisions. We define a comprehensive and self-consistent two-component ($\text{soft}+\text{hard}$) model for hadron production and report a significant $p\text{\ensuremath{-}}p$ nonjet quadrupole component as a third (angular-correlation) component. Our results have implications for $p\text{\ensuremath{-}}p$ centrality, the underlying event, collectivity in small systems and the existence of flows in high-energy nuclear collisions.

Optimal modeling of 1D azimuth correlations in the context of Bayesian inference

Analysis and interpretation of spectrum and correlation data from high-energy nuclear collisions is currently controversial because two opposing physics narratives derive contradictory implications from the same data, one narrative claiming collision dynamics is dominated by dijet production and projectile-nucleon fragmentation, the other claiming collision dynamics is dominated by a dense, flowing QCD medium. Opposing interpretations seem to be supported by alternative data models, and current model-comparison schemes are unable to distinguish between them. There is clearly need for a convincing new methodology to break the deadlock. In this study we introduce Bayesian inference (BI) methods applied to angular correlation data as a basis to evaluate competing data models. For simplicity the data considered are projections of two-dimensional (2D) angular correlations onto a 1D azimuth from three centrality classes of 200-GeV Au-Au collisions. We consider several data models typical of current model choices, including Fourier series (FS) and a Gaussian plus various combinations of individual cosine components. We evaluate model performance with BI methods and with power-spectrum analysis. We find that FS-only models are rejected in all cases by Bayesian analysis, which always prefers a Gaussian. A cylindrical quadrupole $cos(2\ensuremath{\phi})$ is required in some cases but rejected for 0%--5%-central Au-Au collisions. Given a Gaussian centered at the azimuth origin, ``higher harmonics'' $cos(m\ensuremath{\phi})$ for $m>2$ are rejected. A model consisting of $\mathrm{Gaussian}+\mathrm{dipole}cos(\ensuremath{\phi})+\mathrm{quadrupole}cos(2\ensuremath{\phi})$ provides good 1D data descriptions in all cases.

Electron momentum distribution and singlet-singlet annihilation in the organic anthracene molecular crystals using positron 2D-ACAR and fluorescence spectroscopy.

We present the mapping of electron momentum distribution (EMD) in a single crystal of anthracene by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR). The projected EMD is explained on the basis of the crystallographic features of the material. The EMD spectra provide information about the positron states and their behavior and also about the hindrance of the positronium (Ps) formation in this material. The EMD has exhibited evidence for the absence of free volume defects. The characteristic EMD features regarding the delocalized electronic states are explained. Further, scintillation characteristics such as fluorescence and time-correlated single photon counting have also been studied. The emission peaks are attributed to vibrational bands of fluorescence emission from the singlet excitons and lifetime components are observed to be due to singlet fission and the singlet-singlet excitons annihilation.

Recovering the Fermi surface with 2D-ACAR spectroscopy in samples with defects

When two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) experiments are performed in metals containing defects, conventional analysis in which the measured momentum distribution is folded back into the first Brillouin zone is rendered ineffective due to the contribution from positrons annihilating from the defect. However, by working with the radial anisotropy of the spectrum, it is shown that an image of the Fermi surface can be recovered since the defect contribution is essentially isotropic.

Challenging the utility of third-order azimuth harmonics in the description of ultrarelativistic heavy-ion collisions

In recent years it has become conventional practice to include higher-order cylindrical harmonics in the phenomenological description of two-particle angular correlations from ultra-relativistic heavy-ion collisions. These model elements, whose dependence on relative azimuth angle has the form $\cos[m(\phi_1-\phi_2)]$ where $m > 2$, were introduced to support a hydrodynamic flow interpretation of the same-side ($|\phi_1-\phi_2| < \pi/2$) 2D peak in the correlations. Previous studies have shown that the $m > 2$ harmonics are not required by the data, that they destabilize the fitting models, and that their net effect is to decompose the same-side peak into two components, one being dependent on and the other being independent of relative pseudorapidity. Thus we are lead to question whether descriptions of angular correlation data including higher-order harmonics inform our understanding of the same-side peak or heavy-ion collisions in general. Results from analysis of two-dimensional angular correlation data from the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) show that the RHIC data do not exclude a single-Gaussian hypothesis for the same-side peak. We find that the net effect of including the $m = 3$ harmonic or azimuth sextupole in the fitting model is the inclusion of small non-Gaussian dependence in the mathematical description of the same-side peak. Those non-Gaussian effects are systematically insignificant and can be accommodated by minor perturbations to the same-side 2D Gaussian peak model, which act locally at small relative azimuth. We also demonstrate that the 0-1% 2D angular correlation data for 2.76 TeV Pb+Pb collisions from ATLAS, which display an away-side double peak on azimuth, do not require a sextupole and exclude a positive same-side sextupole.

Challenging claims of nonjet “higher harmonic” components in 2D angular correlations from high-energy heavy-ion collisions

It is conjectured that several higher harmonic flows $v_m$ may result from initial-state geometry fluctuations in \aa collisions coupled to a radially-expanding medium. But as with "elliptic flow" $v_2$ measurements, non-hydrodynamic mechanisms such as jet production may contribute to other higher azimuth multipoles $v_m$ as biases. Careful distinctions should be maintained between jet-related and nonjet (possibly hydrodynamic) contributions to $v_m$ (e.g., "nonflow" and "flow"). In this study we consider several questions: (a) To what extent do jet-like structures in two-dimensional (2D) angular correlations contribute to azimuth multipoles inferred from various $v_m$ methods? (b) If a multipole element is added to a 2D fit model is a nonzero amplitude indicative of a corresponding flow component? and (c) Can 2D correlations establish the necessity of nonjet contributions to some or all higher multipoles? Model fits to 2D angular correlations are used to establish the origins of azimuth multipoles inferred from 1D projections onto azimuth or from nongraphical numerical methods. We find that jet-like angular correlations, and specifically a 2D peak at the angular origin consistent with jet production, constitute the dominant contribution to inferred higher multipoles, and the data do not {\em require} higher multipoles in isolation from the jet-like 2D peak. Inference of "higher harmonic flows" results from identifying certain nominally jet-like structure as flow manifestations through unjustified application of 1D Fourier series analysis. Although the peak structure at the angular origin is strongly modified in more-central collisions some properties remain compatible with relevant pQCD theory expectations for jet production.

Anomalous centrality evolution of two-particle angular correlations from Au-Au collisions atsNN=62and 200 GeV

We present two-dimensional (2D) two-particle angular correlations measured with the STAR detector on relative pseudorapidity η and azimuth ϕ for charged particles from Au-Au collisions at sNN=62 and 200 GeV with transverse momentum pt≥0.15 GeV/c, |η|≤1, and 2π in azimuth. Observed correlations include a same-side (relative azimuth <π/2) 2D peak, a closely related away-side azimuth dipole, and an azimuth quadrupole conventionally associated with elliptic flow. The same-side 2D peak and away-side dipole are explained by semihard parton scattering and fragmentation (minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those structures follow N-N binary-collision scaling in Au-Au collisions until midcentrality, where a transition to a qualitatively different centrality trend occurs within one 10% centrality bin. Above the transition point the number of same-side and away-side correlated pairs increases rapidly relative to binary-collision scaling, the η width of the same-side 2D peak also increases rapidly (η elongation), and the ϕ width actually decreases significantly. Those centrality trends are in marked contrast with conventional expectations for jet quenching in a dense medium. The observed centrality trends are compared to perturbative QCD predictions computed in hijing, which serve as a theoretical baseline, and to the expected trends for semihard parton scattering and fragmentation in a thermalized opaque medium predicted by theoretical calculations and phenomenological models. We are unable to reconcile a semihard parton scattering and fragmentation origin for the observed correlation structure and centrality trends with heavy-ion collision scenarios that invoke rapid parton thermalization. If the collision system turns out to be effectively opaque to few-GeV partons the present observations would be inconsistent with the minijet picture discussed here.4 MoreReceived 17 September 2011DOI:https://doi.org/10.1103/PhysRevC.86.064902©2012 American Physical Society

Size estimation of embedded Cu nanoprecipitates in Fe by using affinitively trapped positrons

By utilizing a unique characteristic of the affinitively trapped positrons, here we demonstrate a powerful method to estimate the sizes of the embedded Cu nanoprecipitates (and subnanoprecipitates) (ECNPs) in Fe. The positron-electron momentum density distributions in the ECNPs with various sizes in a thermally aged Fe-Cu dilute alloy were measured by using the two-dimensional angular correlation of positron annihilation radiation technique and the momentum smearing around the Fermi cutoff was clearly observed. It is clarified that this smearing effect is proportional to the inverse of the average radii of the ECNPs and thus it can be employed to estimate the ECNPs' sizes. Moreover, the ECNPs' sizes obtained by the present method are compared with those obtained by the three-dimensional atom probe technique and good agreement is observed, justifying the size estimation using the smearing effect. It is noteworthy that the present technique can detect the smearing due to the ECNPs with their radii even below about 0.5 nm and thus gives an important trend in the smearing reflecting the average size of the ECNPs in this range.

Positron annihilation study of the Fermi surface of Ni2MnGa

The Fermi surface of the ferromagnetic shape-memory alloy Ni2MnGa has been determined experimentally with two-dimensional angular correlation of electron–positron annihilation radiation. Our results are supported by first principles electronic structure calculations. The measured electron occupancy within the Brillouin zone is consistent with the existence of two nesting features present in the Fermi surfaces calculated in previous studies. The nesting vectors of the calculated Fermi surface match the modulation of the pre-martensitic intermediate structure and that of the martensitic structure.

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.

In-situ Hydrogen Sorption 2D-ACAR Facility for the Study of Metal Hydrides for Hydrogen Storage

We developed a dedicated hydrogen sorption setup coupled to a positron 2D-ACAR (two-dimensional Angular Correlation of Annihilation Radiation) setup employing a 22Na-source, which will enable to collect 2D-ACAR momentum distributions in-situ as a function of temperature, hydrogen pressure and hydrogen content. In parallel, a dedicated glovebox was constructed for handling air-sensitive metal and metal hydride samples, with a special entrance for the 2D-ACAR sample insert. The 2D-ACAR setup was tested in first measurements on a Pd0.75Ag0.25 foil and on a ball-milled MgH2 powder in both the hydrogen loaded and desorbed states. The hydrogen loaded Pd0.75Ag0.25Hx sample was kept under a 1 bar hydrogen pressure to prevent partial desorption during measurements at room temperature. The collected 2D-ACAR distributions of Pd0.75Ag0.25 and Pd0.75Ag0.25Hx showed similar features as observed in previous studies. The broadening of the ACAR distributions observed for the Mg to MgH2 metal-insulator transition was compared in a quantitative manner to ab-initio calculations reported in the literature.

What Else for Improvement of Reconstructed Densities?

Reconstruction techniques, belonging to two different categories (series expansion and transform methods), are compared with regard to their applicability to line projections of a model density. It is demonstrated that in the case of this model the algebraic technique gives much better results than the filtered back projection (FBP) algorithm. However, it could not be a case for real data where a region of densities to be considered is very large comparing to the modelled one. In this connection we propose a new way of dealing with two-dimensional (2D) angular correlation of annihilation radiation (ACAR) data. Such a treatment, proposed for all reconstruction techniques, allows to improve reconstructed densities and filters the experimental noise.

Optimization of three-dimensional positron annihilation spectroscopy system (3DPASS) for three-dimensional momentum measurements

A three-dimensional positron annihilation spectroscopy system (3DPASS) was characterized. 3DPASS permits determination of three-dimensional electron–positron ( e −– e + ) momentum distributions by simultaneously measuring angles and energies for coincident two-gamma annihilation photons. 3DPASS collects a single dataset of correlated energies and positions for two coincident annihilation photons from a pair of solid-state double-sided strip detectors (DSSDs). Subpixel-interpolated positions are determined by transient charge analysis. 3DPASS performs simultaneous two-dimensional angular correlation of annihilation radiation (2D ACAR) and two-detector coincidence Doppler-broadening of annihilation radiation (CDBAR) measurements, which are typically collected independently. The 2D ACAR response of 3DPASS was measured for single-crystal Cu and 6H-SiC, with and without compensation for subpixel detection efficiency. Variation of efficiency across the width of DSSD charge collection electrodes was dominated by the event selection criteria required by the subpixel interpolation method. The DBAR resolution was optimized by adjusting the energy range of CDBAR events included in the Doppler-broadening (DB) lineshape. 2D ACAR and DBAR spectra from 3DPASS were compared to previously published results for single-crystal Cu and 6H-SiC. Detailed analysis of the ACAR spectra and the DB lineshapes highlighted momentum features not previously reported.