Positronium spectroscopy is of continuing interest as a high-precision test of our understanding of binding in QFT. Spectroscopic studies of low-lying states (n = 1 hyperfine splitting, n = 2 fine ...structure, and the 1S - 2S interval) have reached a precision of order 1MHz, and ongoing experimental efforts give the promise of improved results. Theoretical calculations of positronium energies at order mα6 ∼ 18.7MHz are complete, but only partial results are known at order mα7 ∼ 0.14MHz. We report on the status of the positronium energy calculations and give some details of the methods employed.
Positronium (Ps) is an exotic hydrogenic atom composed of an electron bound to a positron via the Coulomb force. Being composed of two low-mass leptons, positronium is, for all practical purposes, ...fully described by quantum electrodynamics (QED). The absence of hadronic components suggests that positronium energy levels and decay rates can be calculated to very high precision, limited only by the order of the corresponding perturbative expansion and the tiny effects of heavy or weakly interacting virtual particles and exotic decay modes. Moreover, as it is a low-mass particle–antiparticle system, the QED description of positronium is strongly affected by annihilation and recoil effects that are either weaker or not present in other atoms. As a result, sufficiently precise measurements of Ps energy levels and decay properties can serve as stringent tests of bound-state QED theory, and may be sensitive to processes not present in the theory, such as axion-like particles (beyond the QCD axion), or a fifth fundamental force. In addition, since positronium is an eigenstate of the fundamental symmetries C and P, various symmetry violating mechanisms can be probed through searches for anomalous decay modes. In the last three decades, there have been significant experimental advances in positron and positronium physics which open up the possibility to test QED bound-state theory with unprecedented precision. Here we present the current state-of-the-art in experimental positronium spectroscopy, and discuss explicitly how such measurements can be used to test bound-state QED theory, and how such tests may contribute to the search for physics beyond the Standard Model.
We have dated Huon Peninsula, Papua New Guinea and Barbados corals that formed at times since the Last Interglacial Period, applying both
230Th and
231Pa dating techniques as a test of age accuracy. ...We show that Marine Isotope Stage (MIS) 5e ended prior to 113.1±0.7 kyr, when sea level was −19 m. During MIS 5b sea level was −57 m at 92.6±0.5 kyr, having dropped about 40 m in approximately 10 kyr during the MIS 5c–5b transition. Sea level then rose more than 40 m during the MIS 5b–5a transition, also in about 10 kyr. MIS 5a lasted until at least 76.2±0.4 kyr, at a level of −24 m at that time. Combined with earlier data that places MIS 4 sea level at −81 m at 70.8 kyr, our late MIS 5a data indicate that sea level fell almost 60 m in less than 6 kyr (10.6 m/kyr) during the MIS 5–4 transition. The magnitude of the drop is half that of the glacial–interglacial amplitude and approximately equivalent to the volume of the present-day Antarctic Ice Sheet. During this interval the minimum average rate of net continental ice accumulation was 18 cm/yr, likely facilitated by efficient moisture transport from lower latitudes. At three specific times (60.6±0.3, 50.8±0.3, and 36.8+0.2 kyr) during MIS 3, sea level was between −85 and −74 m. Sea level then dropped to −107 m at 23.7±0.1 kyr early in MIS 2, before dropping further to Last Glacial Maximum (LGM) values and then rising to present values during the last deglaciation. Times of rapid sea-level drop correspond to times of high winter insolation at low northern latitudes and high winter latitudinal gradients in northern hemisphere insolation, supporting the idea that these factors may have resulted in high water-vapor pressure in moisture sources and efficient moisture transport to high-latitude glaciers, thereby contributing to glacial buildup. We combined our sea-level results with deep-sea δ
18O records as a means of estimating the temperature and ice-volume components in the marine δ
18O record. This analysis confirms large deep-ocean temperature shifts following MIS 5e and during Termination I. Deep-ocean temperatures changed by much smaller amounts between MIS 5c and 2. Maximum temperature shift in the deep Pacific is about 2°, whereas the shift at a site in the Atlantic is 4°. Under glacial conditions temperatures at both sites are near the freezing point. The shift in the Atlantic is likely caused by a combination of changing proportions of northern and southern source waters as well as changing temperature at the sites where these deep waters form.
The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-π0s produced at forward pseudorapidities (2.6<η<4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass ...energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back π0 pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. Furthermore, the larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale $Q^{2}_{s}$ on the mass number A . A linear scaling of the suppression with A1/3 is observed with a slope of -0.09±0.01.
Monte Carlo modelling is used to simulate the 3D nano-porous structure of leached Raney-nickel catalyst material. Calculated surface areas from the model compare favourably to measured surface areas ...from BET analysis and also predict surface segregation of residual Al matching X-ray photoelectron spectroscopy results. In the figure, the magnified view at the top left shows a region (A), which is an exposed (1
1
1) plane and a region below (B), which is an exposed (1
0
0) plane. Dark atoms are Ni and light atoms are Al.
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► Nature of Raney-Ni catalyst quantified via experimentation and modelling. ► kMC model correctly predicts surface areas of leached Raney-Ni material. ► MMC model predicts surface segregation of Al in accord with XPS data.
This paper describes combined experimental techniques and numerical modelling of the surface condition of activated nanoscopic Raney-nickel catalysts produced via leaching of spray-atomised NiAl precursor powders. Results of off-lattice Metropolis and lattice kinetic Monte Carlo models compare favourably to experimentation quantifying the surface of Raney-nickel catalysts in terms of both surface area and the surface distribution of residual aluminium left over from the leaching process. Predicted surface areas from the kinetic Monte Carlo simulations are shown to be in good agreement with BET measurements from nitrogen physisorption for catalyst materials prepared from two different NiAl precursor alloys. Surface aluminium concentrations predicted by Metropolis Monte Carlo simulations are also in agreement with X-ray photoelectron spectroscopy results and in addition predict an absence of Al–Al bonding at the surface.
MS/MS and associated database search algorithms are essential proteomic tools for identifying peptides. Due to their widespread use, it is now time to perform a systematic analysis of the various ...algorithms currently in use. Using blood specimens used in the HUPO Plasma Proteome Project, we have evaluated five search algorithms with respect to their sensitivity and specificity, and have also accurately benchmarked them based on specified false‐positive (FP) rates. Spectrum Mill and SEQUEST performed well in terms of sensitivity, but were inferior to MASCOT, X!Tandem, and Sonar in terms of specificity. Overall, MASCOT, a probabilistic search algorithm, correctly identified most peptides based on a specified FP rate. The rescoring algorithm, PeptideProphet, enhanced the overall performance of the SEQUEST algorithm, as well as provided predictable FP error rates. Ideally, score thresholds should be calculated for each peptide spectrum or minimally, derived from a reversed‐sequence search as demonstrated in this study based on a validated data set. The availability of open‐source search algorithms, such as X!Tandem, makes it feasible to further improve the validation process (manual or automatic) on the basis of “consensus scoring”, i.e., the use of multiple (at least two) search algorithms to reduce the number of FPs.∁
HUPO initiated the Plasma Proteome Project (PPP) in 2002. Its pilot phase has (1) evaluated advantages and limitations of many depletion, fractionation, and MS technology platforms; (2) compared PPP ...reference specimens of human serum and EDTA, heparin, and citrate‐anti‐coagulated plasma; and (3) created a publicly‐available knowledge base (www.bioinformatics.med.umich.edu/hupo/ppp; www.ebi.ac.uk/pride). Thirty‐five participating laboratories in 13 countries submitted datasets. Working groups addressed (a) specimen stability and protein concentrations; (b) protein identifications from 18 MS/MS datasets; (c) independent analyses from raw MS‐MS spectra; (d) search engine performance, subproteome analyses, and biological insights; (e) antibody arrays; and (f) direct MS/SELDI analyses. MS‐MS datasets had 15 710 different International Protein Index (IPI) protein IDs; our integration algorithm applied to multiple matches of peptide sequences yielded 9504 IPI proteins identified with one or more peptides and 3020 proteins identified with two or more peptides (the Core Dataset). These proteins have been characterized with Gene Ontology, InterPro, Novartis Atlas, OMIM, and immunoassay‐based concentration determinations. The database permits examination of many other subsets, such as 1274 proteins identified with three or more peptides. Reverse protein to DNA matching identified proteins for 118 previously unidentified ORFs.
We recommend use of plasma instead of serum, with EDTA (or citrate) for anticoagulation. To improve resolution, sensitivity and reproducibility of peptide identifications and protein matches, we recommend combinations of depletion, fractionation, and MS/MS technologies, with explicit criteria for evaluation of spectra, use of search algorithms, and integration of homologous protein matches.
This Special Issue of PROTEOMICS presents papers integral to the collaborative analysis plus many reports of supplementary work on various aspects of the PPP workplan. These PPP results on complexity, dynamic range, incomplete sampling, false‐positive matches, and integration of diverse datasets for plasma and serum proteins lay a foundation for development and validation of circulating protein biomarkers in health and disease.
We report precision measurements of hypernuclei $^3_ΛH$ and $^4_ΛH$ lifetimes obtained from Au + Au collisions at $\sqrt{sNN}$ = 3.0 GeV and 7.2 GeV collected by the STAR experiment at the ...Relativistic Heavy Ion Collider, and the first measurement of $^3_ΛH$ and $^4_ΛH$ midrapidity yields in Au + Au collisions at $\sqrt{sNN}$ = 3.0 GeV. $^3_ΛH$ and $^4_ΛH$, being the two simplest bound states composed of hyperons and nucleons, are cornerstones in the field of hypernuclear physics. Their lifetimes are measured to be 221 ± 15 (stat) ± 19 (syst) ps for $^3_ΛH$ and 218 ± 6 (stat) ± 13 (syst) ps for $^4_ΛH$. The pT-integrated yields of $^3_ΛH$ and $^4_ΛH$ are presented in different centrality and rapidity intervals. It is observed that the shape of the rapidity distribution of $4_ΛH$ is different for 0%–10% and 10%–50% centrality collisions. Thermal model calculations, using the canonical ensemble for strangeness, describes the $^3_ΛH$ yield well, while underestimating the $^4_ΛH$ yield. Transport models, combining baryonic mean-field and coalescence (jam) or utilizing dynamical cluster formation via baryonic interactions (phqmd) for light nuclei and hypernuclei production, approximately describe the measured $^3_ΛH$ and $^4_ΛH$ yields. Our measurements provide means to precisely assess our understanding of the fundamental baryonic interactions with strange quarks, which can impact our understanding of more complicated systems involving hyperons, such as the interior of neutron stars or exotic hypernuclei.
The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum ...chromodynamics. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy. This background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. We report such measurements in Au+Au collisions at a nucleon-nucleon center-of-mass energy of 200 GeV at the Relativistic Heavy-Ion Collider. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral (large impact parameter) collisions. Some indication of finite CME signals is seen with a significance of 1–3 standard deviations in mid-central (intermediate impact parameter) collisions. Furthermore, significant residual background effects may, however, still be present.
Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle ...cumulants, v_{2}{2} and v_{2}{4}, for charged hadrons from U+U collisions at sqrts_{NN}=193 GeV and Au+Au collisions at sqrts_{NN}=200 GeV. Nearly fully overlapping collisions are selected based on the energy deposited by spectators in zero degree calorimeters (ZDCs). Within this sample, the observed dependence of v_{2}{2} on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. We also show that v_{2} vs multiplicity can be better described by models, such as gluon saturation or quark participant models, that eliminate the dependence of the multiplicity on the number of binary nucleon-nucleon collisions.
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