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When laser radiation is skilfully applied, atoms and molecules can be cooled1–3, allowing the precise measurements and control of quantum systems. This is essential for the fundamental studies of physics as well as practical applications such as precision spectroscopy4–7, ultracold gases with quantum statistical properties8–10and quantum computing. In laser cooling, atoms are slowed to otherwise unattainable velocities through repeated cycles of laser photon absorption and spontaneous emission in random directions. Simple systems can serve as rigorous testing grounds for fundamental physics—one such case is the purely leptonic positronium11,12, an exotic atom comprising an electron and its antiparticle, the positron. Laser cooling of positronium, however, has hitherto remained unrealized. Here we demonstrate the one-dimensional laser cooling of positronium. An innovative laser system emitting a train of broadband pulses with successively increasing central frequencies was used to overcome major challenges posed by the short positronium lifetime and the effects of Doppler broadening and recoil. One-dimensional chirp cooling was used to cool a portion of the dilute positronium gas to a velocity distribution of approximately 1 K in 100 ns. A major advancement in the field of low-temperature fundamental physics of antimatter, this study on a purely leptonic system complements work on antihydrogen13, a hadron-containing exotic atom. The successful application of laser cooling to positronium affords unique opportunities to rigorously test bound-state quantum electrodynamics and to potentially realize Bose–Einstein condensation14–18in this matter–antimatter system.

The Arctic is notable as a region where the greatest rate of increase in precipitation associated with global warming is anticipated. The Arctic precipitation simulated by the Coupled Model Intercomparison Project Phase 6 models showed a strong increasing trend since the 1980s. We found that the forcing factor of the trend is a combination of the continued strengthening of greenhouse gas forcing and the leveling off of aerosol forcing dominated in earlier periods. From an energetic perspective, we found that the increased atmospheric radiative cooling and reduced sensible heat transport from lower latitudes contributed equally to the recent increase in Arctic precipitation. The combination of these two energetic factors suggests a doubling of the Arctic amplification factor for precipitation relative to that for temperature. Future Arctic precipitation will change in proportion to the temperature change, and the fractional contributions of the energetic factors will remain stable across various scenarios. The Arctic region is inherently a low‐precipitation area. However, because of global warming, precipitation is expected to increase substantially in the Arctic region compared with the global average when viewed as a percentage change from the original precipitation. This severely affects climate change in the Arctic environment. The latest climate model simulations show that there has been a rapid increase in precipitation in the Arctic region in recent decades. The driving factors behind the rapid increase are the effects of the accelerating growth of greenhouse gas concentrations, which were previously suppressed by the increasing anthropogenic aerosol emissions before the 1980s. Based on the heat budget of the atmosphere, we identified important factors contributing to these precipitation changes. These include enhanced radiative cooling (responding locally to increased air temperature) and reduced heat transport from lower latitudes due to greater temperature increases at higher latitudes. Future precipitation will change in proportion to the temperature change while maintaining consistent fractional contributions across different scenarios. Trends in Arctic precipitation in the recent and future decades are examined from multimodel simulationsThe recent rapid increase is driven by accelerating greenhouse gas concentrations and plateauing growth in anthropogenic aerosol emissionsIncreased radiative cooling and reduced poleward sensible heat transport equally contributed to the Arctic precipitation changes Trends in Arctic precipitation in the recent and future decades are examined from multimodel simulations The recent rapid increase is driven by accelerating greenhouse gas concentrations and plateauing growth in anthropogenic aerosol emissions Increased radiative cooling and reduced poleward sensible heat transport equally contributed to the Arctic precipitation changes

Deposition of black carbon (BC) aerosol in the Arctic lowers snow albedo, thus contributing to warming in the region. However, the processes and impacts associated with BC deposition are poorly understood because of the scarcity and uncertainties of measurements of BC in snow with adequate spatiotemporal resolution. We sampled snowpack at two sites (11 m and 300 m above sea level) at Ny‐Ålesund, Spitsbergen, in April 2013. We also collected falling snow near the surface with a windsock from September 2012 to April 2013. The size distribution of BC in snowpack and falling snow was measured using a single‐particle soot photometer combined with a characterized nebulizer. The BC size distributions did not show significant variations with depth in the snowpack, suggesting stable size distributions in falling snow. The BC number and mass concentrations (CNBCand CMBC) at the two sites agreed to within 19% and 10%, respectively, despite the sites' different snow water equivalent (SWE) loadings. This indicates the small influence of the amount of SWE (or precipitation) on these quantities. Average CNBCand CMBCin snowpack and falling snow at nearly the same locations agreed to within 5% and 16%, after small corrections for artifacts associated with the sampling of the falling snow. This comparison shows that the dry deposition was a small contributor to the total BC deposition. CMBCwere highest (2.4 ± 3.0 μg L−1) in December–February and lowest (1.2 ± 1.2 μg L−1) in September–November. The concentrations of black carbon measured in snowpack in Spitsbergen were somewhat insensitive to the snow water equivalentConcentrations of black carbon in falling snow and ambient air in midwinter were higher than those in fall on averageThe BC concentrations in snowpack measured by this study were lower than those previously reported by factors of 2–6

An improved understanding of the variations in the mass concentration and size distribution of black carbon (BC) in the free troposphere (FT) over East Asia, where BC emissions are very high, is needed to reliably estimate the radiative forcing of BC in climate models. We measured these parameters and the carbon monoxide (CO) concentration by conducting the Aerosol Radiative Forcing in East Asia (A‐FORCE) 2013W aircraft campaign in East Asia in winter 2013 and compared these data with measurements made in the same region in spring 2009. The median BC concentrations in the FT originating from North China (NC) and South China (SC) showed different seasonal variations, which were primarily caused by variations in meteorological conditions. CO concentrations above the background were much higher in SC than in NC in both seasons, suggesting a more active upward transport of CO. In SC, precipitation greatly increased from winter to spring, leading to an increased wet deposition of BC. As a result, the median BC concentration in the FT was highest in SC air in winter. This season and region were optimal for the effective transport of BC from the planetary boundary layer to the FT. The count median diameters of the BC size distributions generally decreased with altitude via wet removal during upward transport. The altitude dependence of the BC size distributions was similar in winter and spring, in accord with the similarity in the BC mixing state. The observed BC concentrations and microphysical properties will be useful for evaluating the performance of climate models. Seasonal variation of black carbon concentrations over East Asia is shownBlack carbon concentrations were influenced by uplift and wet depositionThe size distribution of black carbon shifted to smaller sizes with altitude

Aerosols significantly affect Earth's radiation budget, thus influencing global climate. In the Arctic, sulfate aerosols are thought to have reduced the warming during the twentieth century. However, trends in past sulfate aerosols are poorly known, especially the aerosol sizes and compositions. Here, we analyze a high‐resolution ice core from southeastern Greenland, comparing the seasonal deposition flux of large sulfate salt particles and small sulfur compounds, including non‐neutralized sulfuric acid, between the anthropogenic sulfate maximum (1973–1975) and after sulfur emissions control (2010–2012). Between these periods, we find that the large‐diameter (>0.4 μm) flux remains roughly unchanged, yet the small‐diameter (<0.4 μm) aerosol flux significantly decreases. The results indicate that small sulfates were efficiently activated as cloud condensation nuclei during the 1970s, and thus likely increased cloud albedo, offsetting the warming. Sulfate aerosol compositions and size distributions are important factors in Earth's radiation budget. However, we have little information on past sulfate aerosol sizes and compositions over the Arctic due to a lack of reliable observations, and as such, we have less confidence in modeling historical aerosol composition and transport. We present here the first observational evidence that small sulfate particles increased in the Arctic during the 1970s. With emission controls, anthropogenic SO2emissions from eastern North America and Western Europe decreased after the 1980s, thus increasing the relative importance of large sulfate particles. Thus, our findings should help to improve the understanding of aerosol‐cloud interactions, and provide new constraints for the parameterization in the model. Ice core analyses provide the first evidence of past seasonal deposition flux of large sulfate salt particles and small sulfatesCompared to the 1970s, the small sulfates (<0.4 μm) aerosol flux significantly decreased in the 2010sSmall sulfates were efficiently activated as cloud condensation nuclei during anthropogenic sulfate maximum (1970s) Ice core analyses provide the first evidence of past seasonal deposition flux of large sulfate salt particles and small sulfates Compared to the 1970s, the small sulfates (<0.4 μm) aerosol flux significantly decreased in the 2010s Small sulfates were efficiently activated as cloud condensation nuclei during anthropogenic sulfate maximum (1970s)

The aim of this study is to investigate efficiency of stainless steel powder inoculation into pure aluminum for microstructure refinement. Refiners consisting of pure aluminum powder (powder size: 106~180m) and stainless steel powder (powder size: 25~53m) have been fabricated through spark plasma sintering (SPS). The stainless steels used in the study include SUS304L, SUS316L and SUS434L. SUS 304L powder has achieved a great grain refinement in cast aluminum, for which fading phenomenon has been considerably avoided. SUS316L and SUS434L powders develop fine dendrite structures, which can lead to high hardness of cast aluminum.

Positron annihilation lifetime spectroscopy (PALS) is an experimental technique whereby the lifetime spectrum of positrons injected into a material is measured. Analysis of this spectrum can be used to characterize defects in the material. While radioisotope positron sources are often used for PALS, there are several advantages of using mono-energetic, slow positron beams. In order to measure lifetime spectra with such beams it is necessary to produce a pulsed beam with an extremely short pulse length (~ 200 ps). In this article we discuss the advantages of using pulsed, slow-positron beams, the various methods of beam production and pulsing methods. We focus in particular on intense beams generated by electron accelerators and describe the current status and future plans for the accelerator based facility at AIST.

The nanoscopic structure of three kinds of commercially available composite membranes (LF10, NTR729HF, and NTR7250) for water purification has been studied by energy-variable positron annihilation. The membranes consist of a functional polymer film and porous polysulfone substrate supported on a nonwoven polyester fabric. Positron annihilation γ-ray and positron lifetime techniques were employed to probe the layer structure of the composite membranes and subnanometer-sized free-volume holes in the functional films, respectively. It was found that the functional film on the porous substrate in LF10 contains two different layers on the substrate, while that in NTR729HF and NTR7250 consists of a single top layer. Comparison of the rejections of different uncharged solutes, determined by a pressure-driven experiment, with the hole size of the functional film, obtained from the positron lifetime data, revealed that the holes probed by positrons can well explain the hindering effect of molecular transport. Moreover, the higher rejections by LF10 were found to arise from the denser top layer than the top layers of the other membranes. A combined use of the present positron techniques was demonstrated to be useful for examining the hole structure as well as the layer structure of the composite membranes.

The structural colour of the neon tetra is distinguishable from those of, e.g., butterfly wings and bird feathers, because it can change in response to the light intensity of the surrounding environment. This fact clearly indicates the variability of the colour-producing microstructures. It has been known that an iridophore of the neon tetra contains a few stacks of periodically arranged light-reflecting platelets, which can cause multilayer optical interference phenomena. As a mechanism of the colour variability, the Venetian blind model has been proposed, in which the light-reflecting platelets are assumed to be tilted during colour change, resulting in a variation in the spacing between the platelets. In order to quantitatively evaluate the validity of this model, we have performed a detailed optical study of a single stack of platelets inside an iridophore. In particular, we have prepared a new optical system that can simultaneously measure both the spectrum and direction of the reflected light, which are expected to be closely related to each other in the Venetian blind model. The experimental results and detailed analysis are found to quantitatively verify the model.

We report on a measurement of the ratio of the differential cross sections for Wand Zboson production as a function of transverse momentum in proton–antiproton collisions at s=1.8 TeV. This measurement uses data recorded by the DØ detector at the Fermilab Tevatron in 1994–1995. It represents the first investigation of a proposal that ratios between Wand Zobservables can be calculated reliably using perturbative QCD, even when the individual observables are not. Using the ratio of differential cross sections reduces both experimental and theoretical uncertainties, and can therefore provide smaller overall uncertainties in the measured mass and width of the Wboson than current methods used at hadron colliders.

We present a search for electroweak production of single top quarks in ≈90 pb−1of data collected with the DØ detector at the Fermilab Tevatron collider. Using arrays of neural networks to separate signals from backgrounds, we set upper limits on the cross sections of 17 pb for the s-channel process pp̄→tb+X, and 22 pb for the t-channel process pp̄→tqb+X, both at the 95% confidence level.

Abstract: Pulsed slow positrons were produced using a time-varying moderator bias with an interval of 82 ns; 97% of the positrons were compressed within 2 ns width at the target position. Both the positron annihilation lifetime and Doppler broadening of the positron annihilation radiation (DBPR) of polytetrafluoroethylene (PTFE) were measured as a function of the incident energy of slow positrons. It was shown that the lifetime and intensity of the long-lived component of positron annihilation are independent of the positron incident energy above 1.2 keV. However, the width of the Doppler-broadened annihilation γ-ray increased in the energy region below 1.2 keV.

A theoretical model for estimating the effective elastic modulus of particulate hybrid composites is proposed. A concept of "equivalent virtual volume fraction" is in troduced in order to take interaction among a matrix and inclusions into account. The model is illustrated by the effective shear modulus of particulate hybrid composites based upon a modified self-consistent method for two-phase composites. The obtained results reflect the influence of the matrix phase geometry more adequately than simple "self-consistent" models.

A study of the properties of charm particles produced in 360 GeV/c p-p interactions is reported. The experiment was performed using the high resolution hydrogen bubble chamber LEBC in association with the European Hybrid Spectrometer at the CERN SPS. Details of the exposure and operation of the spectrometer are given and the methods used to extract the charm data are presented. The essential physics results on the decay properties (lifetime, branching ratios) as well as on the hadroproduction properties (cross sections forD, ,F, ?c,D, correlations between charm particles) are given.

A study of the properties of charm particles produced in 400 GeV/cpp interactions is reported. The experiment was performed using the high resolution hydrogen bubble chamber LEBC in association with the European Hybrid Spectrometer at the CERN SPS. Details of the experimental set-up and operational procedures are given and the methods to extract samples of charm decays are discussed. Results are presented on the intrinsic properties of charm particles (masses, lifetimes, decay modes and branching ratios), adding, whenever appropriate, the relevant information obtained in a similar study made with 360 GeV/cp-p interactions. The hadroproduction properties of charm states (total and differential cross sections, correlations) are presented and discussed in the context of current QCD inspired phenomenology.

A comparative analysis of the global properties of 400 GeV/c proton-proton interactions with and without charm production is presented. Multiplicities, momentum distributions, sphericity and thrust of these interactions are compared.

Charm data from 360 GeV/c p-p and 400 GeV/cpp interactions are used to give results onD-meson branching ratios. The analysis is based on 438 charm events containing 608 spatially resolved charm particle decays. We present topological branching ratios, as well as the following inclusive branching ratios ofD-mesons into kaons: $$\begin{gathered} B(D^ \pm \to K^ \mp + anything) = 0.17 \pm 0.07, \hfill \\ B(D^ \pm \to K^ \pm + anything) = 0.08_{ - 0.05}^{ + 0.06} , \hfill \\ B(D^0 \to K^ \pm + anything) = 0.42 \pm 0.08, \hfill \\ B(D^0 \to K^ + + anything) = 0.03_{ - 0.02}^{ + 0.05} , \hfill \\ \end{gathered} $$ and the following semi-electronic branching ratios ofD-mesons: $$\begin{gathered} B(D^ \pm \to e^ \pm + anything) = 0.20_{ - 0.07}^{ + 0.09} , \hfill \\ B(D^0 \to e^ \pm + anything) = 0.15 \pm 0.05. \hfill \\ \end{gathered} $$

Abstract: The polymerization process of bisphenol-A dicyanate (BADCy) has been studied using a positron-annihilation lifetime technique (PAL). The polymerization was conducted at 150°C, and the process was followed by PAL. Seven kinds of samples with different curing times were also formed at 150°C, and the relation between the period of the curing time and the degree of polymerization was studied. It has been shown that theo-Ps lifetime increases in samples with a higher polymerization than 85%, which is consistent with measurements of the specific volume of BADCy.

The measurement of 12D0 and 10D± exclusive branching ratios are presented. The analysis is based on 608 spatially resolved charm particle decays produced in 360 GeV/c p-p and 400 GeV/cpp interactions.

In a study of proton-proton interactions at , inclusive distributions of single pions and systems of pions of various charge combinations are presented, as well as the production ratio obtained in association with various trigger particles. The results are compared to current phenomenological models in an attempt to understand multiparticle productions in hadron collisions.

D-meson lifetimes have been determined from decays observed in an exposure of the high resolution hydrogen bubble chamber LEBC in association with the European Hybrid Spectrometer (EHS) to an incident 360 GeV p- beam. Various techniques for lifetime determination are presented and the results compared. A new model-independent analysis based on the distribution in transverse decay length is introduced and preferred over other techniques. Best values are found to be: $$\begin{gathered} \tau (D^0 ) = \left( {4.1_{ - 0.3}^{ + 0.7} } \right) \times 10^{ - 13} s (60 decays) \hfill \\ \tau (D^ \pm ) = \left( {10.7_{ - 1.8}^{ + 2.8} } \right) \times 10^{ - 13} s (40 decays) \hfill \\ \end{gathered} $$ . The lifetime ratio is: $$T = \frac{{\tau (D^ \pm )}}{{\tau (D^0 )}} = 2.6_{ - 0.6}^{ + 0.8} $$ .

Black carbon (BC) aerosol deposited in and onto Arctic snow increases the snow's absorption of sunlight and accelerates snowmelt. Wet removal of BC from the atmosphere plays a key role in determining its abundance in the Arctic atmosphere and in Arctic snow. However, this process is poorly understood, mainly due to the scarcity of relevant measurements. To study wet deposition of BC, we made measurements of mass concentration of BC in snow and rain (CMBC) and of BC in air (MBC) with high accuracy (16% and 10%, respectively) at the Barrow Atmospheric Baseline Observatory, Alaska, from July 2013 to August 2017 and analyzed them along with routinely measured meteorological parameters from Barrow. Monthly mean MBCnear the surface and CMBCwere poorly correlated from midwinter to early spring, when CMBCwas close to the annual median while MBCwas at its annual peak. Seasonal variations in the altitude distribution of MBCmay lead to these differences in seasonal variation of MBCnear the surface and CMBC. About 50% of the annual wet deposition of BC occurred in the 3 months of summer, associated with high values of total precipitation and BC originating from biomass burning. Size distributions of BC in snow and rain were stable throughout the year, suggesting that the size distribution of BC in the lower troposphere was similarly stable. Calculations by two global models reproduced the observed seasonal variations of CMBCand showed that BC from biomass burning dominated CMBCin summer. Black carbon (BC) aerosol, a component of soot, efficiently absorbs solar radiation and heats the atmosphere. BC deposited in and onto Arctic snow increases the absorption of sunlight, accelerates snowmelt, and reduces the snow albedo. Wet deposition of BC greatly influences the concentrations of BC in the atmosphere and in fallen snow. Because measurements of BC wet deposition are scarce, we measured mass concentrations of BC in snow and rain (CMBC) and of BC in ambient air near the surface (MBC) with high accuracy (16% and 10%, respectively) at the Barrow Atmospheric Baseline Observatory, Alaska, from 2013 to 2017. MBCand CMBCshowed different seasonal variations. Differences between BC concentrations near the surface and in the free troposphere may be one cause of this difference. About 50% of the annual wet deposition of BC occurred in the 3 months of summer, associated with high levels of BC emitted from biomass burning and high precipitation amounts. Calculations by two global models reproduced the observed seasonal variations of BC in snow and rain and showed that in summer, this BC was predominantly from biomass burning. We studied seasonal variations of black carbon (BC) concentrations in snow and rain in Arctic Alaska using measurements with 16% accuracyAbout 50% of annual wet deposition occurred in summer, indicating the importance of wet deposition of BC emitted from biomass burning (BB)Model calculations, which reproduce the observed BC in hydrometeors within a factor of 2, suggest large contributions of BB during summer

Aircraft measurements of aerosols were conducted over the Yellow Sea and the East China Sea from February 14 to March 10, 2013, during the Aerosol Radiative Forcing in East Asia 2013 Winter campaign. Ground‐based measurements of aerosols were also conducted on Fukue Island (32.8°N, 128.7°E), located near the boundary of the Yellow Sea and the East China Sea. The aircraft measurements revealed large increases in particle number concentrations in the nucleation mode (2.5–10 nm and up to ~105cm−3) at altitudes of ~0.5–3 km over the Yellow Sea. This was attributed to enhanced new particle formation (NPF) above the marine boundary layer over the Yellow Sea. The number ratios of Aitken‐mode (10–90 nm) or accumulation‐mode (>90 nm) particles to black carbon particles (tracer for primary emissions) were used to quantify the effects of NPF on particle number concentrations. We estimated that NPF increased median Aitken‐mode particle number concentrations by an order of magnitude at altitudes of ~1–3 km over the Yellow Sea. Synchronized aircraft and ground‐based data suggest that vertical transport of Aitken‐mode particles formed over the Yellow Sea led to a large variability in the particle growth events observed on Fukue Island. A shift of sea surface temperature near Fukue Island and the resultant promotion of vertical mixing are probably the key mechanisms responsible for this complex feature. Evolution of accumulation‐mode particles was observed farther downwind over the East China Sea, suggesting significant impacts of NPF on the regional distributions of cloud condensation nuclei. Airborne measurements in Feb–Mar 2013 revealed large enhancements of nanoparticles above the marine boundary layer over the Yellow SeaWe estimate that new particle formation increased particle number concentrations by a factor of ~10 at ~1–3 km over the Yellow SeaAitken‐mode particles formed over the Yellow Sea had significant impacts on cloud condensation nuclei over the East China Sea

Knowledge of black carbon (BC) concentrations and size distributions within surface snow in Antarctica is limited. However, these measurements are important to understanding global aerosol transport from combustion sources, and BC contributes to positive radiative forcing. This study analyzed the concentrations and size distributions of BC and inorganic ions in snow samples collected at the Syowa station in Antarctica from April to December 2011 and along a traverse route to an inland (Mizuho) station. The BC size distributions in snow were bimodal with mass median diameters of ~140 and ~690 nm. We also estimated the mass median diameter from unimodal distributions and found smaller diameters than were reported by other studies. The mass concentrations of BC in snow were higher in inland samples than in Syowa samples. Among Syowa samples, the BC concentrations in December (2117.3 ng L−1on average) were higher than in other periods (288.2 ng L−1on average). The December samples experienced ambient temperatures above 0 °C, and the atmospheric BC concentrations did not increase simultaneously. Inorganic ions originated from the ocean and decreased with increasing distance from the coastal area. We conclude that the BC concentrations in surface snow increased mainly by postdeposition processes through the loss of water mass due to melting, evaporation, and sublimation. Our study is the first to report detailed BC concentrations and size distributions in eastern Antarctica, and the results will help to evaluate BC global transport, the snow albedo estimations in this region, and the climate impacts of BC. BC concentrations and size distributions in snow samples were measured during a 9‐month observation period in AntarcticaBC in our snow samples showed bimodal mass size distributions and smaller diameters than those in other regionsThe snow samples that experienced temperatures above 0 degrees had the highest BC concentrations, possibly due to postdeposition processes

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Coordinatively saturated cationic Ru(II) complexes (I)‐(III) are prepared by the reaction of ((η6‐C6H6)RuCl2)2 with cyclopentadiene, pentamethylcyclopentadiene, 1,3‐cyclohexadiene, 1,3‐cycloheptadiene, 1,3,5‐cycloheptatriene, and 1,5‐cyclooctadiene, resp., in ethanol in the presence of AgBF4.