Annual Review of Nuclear and Particle Science - Volume 70, 2020
Volume 70, 2020
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“Why Do We Do Physics? Because Physics Is Fun!”
Vol. 70 (2020), pp. 1–20More LessIn this informal memoir, the author describes his passage through a golden age of elementary particle physics. It includes not only his career trajectory as a theoretical physicist but also his excursions into experimental physics and particle accelerator theory. While his successes are highlighted, some unsuccessful efforts are included in the narrative as well. Those “losers” were arguably as pleasurable as the less-frequent “winners.” Since retirement, the author has become interested in gravitation theory and cosmology—a new golden age. This activity is also briefly described.
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Covariant Density Functional Theory in Nuclear Physics and Astrophysics
Vol. 70 (2020), pp. 21–41More LessHow does subatomic matter organize itself? Neutron stars are cosmic laboratories uniquely poised to answer this fundamental question that lies at the heart of nuclear science. Newly commissioned rare isotope facilities, telescopes operating across the entire electromagnetic spectrum, and ever more sensitive gravitational wave detectors will probe the properties of neutron-rich matter with unprecedented precision over an enormous range of densities. A coordinated effort between observation, experiment, and theoretical research is of paramount importance for realizing the full potential of these investments. Theoretical nuclear physics provides valuable insights into the properties of neutron-rich matter in regimes that are not presently accessible to experiment or observation. In particular, nuclear density functional theory is likely the only tractable framework that can bridge the entire nuclear landscape by connecting finite nuclei to neutron stars. This compelling connection is the main scope of the present review.
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Parton Distributions in Nucleons and Nuclei
Vol. 70 (2020), pp. 43–76More LessWe review the current status of parton distribution function (PDF) determinations for unpolarized and longitudinally polarized protons and for unpolarized nuclei, which are probed by high-energy hadronic scattering in perturbative quantum chromodynamics (QCD). We present the established theoretical framework, the experimental information, and the methodological aspects inherent to any modern PDF extraction. Furthermore, we summarize the present knowledge of PDFs and discuss their limitations in both accuracy and precision relevant to advancing our understanding of QCD proton substructure and pursuing our quest for precision in the Standard Model and beyond. In this respect, we highlight various achievements, discuss contemporary issues in PDF analyses, and outline future directions of progress.
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The Shortage of Technetium-99m and Possible Solutions
Vol. 70 (2020), pp. 77–94More LessFollowing a major shortage of 99Mo in the 2009–2010 period, concern grew that the aging reactor production facilities needed to be replaced. Most producers were using highly enriched 235U (HEU) as the target material. The Organisation for Economic Co-operation and Development and the International Atomic Energy Agency sought to remedy these issues by removing HEU from medical isotope production and implementing full cost recovery to enable new production entities to compete with the existing multipurpose reactor facilities, which were heavily subsidized by their respective governments. This review examines the various approaches to producing 99Mo and/or 99mTc with a critical eye toward their potential success in (a) producing the medical isotopes and (b) being able to successfully enter and compete in the market. Because many of the new approaches are adapting existing technologies for commercial businesses, some of the details are of a proprietary nature and not available for in-depth technical review.
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The Dynamics of Binary Neutron Star Mergers and GW170817
Vol. 70 (2020), pp. 95–119More LessWith the first observation of a binary neutron star merger through gravitational waves and light, GW170817, compact binary mergers have now taken the center stage in nuclear astrophysics. They are thought to be one of the main astrophysical sites of production of r-process elements, and merger observations have become a fundamental tool to constrain the properties of matter. Here, we review our current understanding of the dynamics of neutron star mergers in general and of GW170817 in particular. We discuss the physical processes governing the inspiral, merger, and postmerger evolution, and we highlight the connections between these processes, the dynamics, and the multimessenger observables. Finally, we discuss open questions and issues in the field and the need to address them through a combination of better theoretical models and new observations.
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Theoretical Prediction of Presupernova Neutrinos and Their Detection
C. Kato, K. Ishidoshiro, and T. YoshidaVol. 70 (2020), pp. 121–145More LessMore than 30 years have passed since the successful detection of supernova (SN) neutrinos from SN 1987A. In the last few decades, remarkable progress has been made in neutrino detection techniques, through which it may be possible to detect neutrinos from a new source, presupernova (pre-SN) neutrinos. They are emitted from a massive star prior to core bounce. Because neutrinos escape from the core freely, they carry information about the stellar physics directly. Pre-SN neutrinos may play an important role in verifying our understanding of stellar evolution for massive stars. Observation of pre-SN neutrinos, moreover, may serve as an alarm regarding an SN explosion a few days in advance if the progenitor is located in our vicinity, enabling us to observe the next galactic SN. In this review, we summarize the current status of pre-SN neutrino studies from both the theoretical and observational points of view.
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Nuclear Reactions in Astrophysics: A Review of Useful Probes for Extracting Reaction Rates
Vol. 70 (2020), pp. 147–170More LessAstrophysical simulations require knowledge of a wide array of reaction rates. For a number of reasons, many of these reaction rates cannot be measured directly and instead are probed with indirect nuclear reactions. We review the current state of the art regarding the techniques used to extract reaction information that is relevant to describe stars, including their explosions and collisions. We focus on the theoretical developments over the last decade that have had an impact on the connection between the laboratory indirect measurement and the astrophysical desired reaction. This review includes three major probes that have been, and will continue to be, widely used in our community: transfer reactions, breakup reactions, and charge-exchange reactions.
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Tracking Triggers for the HL-LHC
Vol. 70 (2020), pp. 171–195More LessHardware-based track reconstruction in the CMS and ATLAS trigger systems for the high-luminosity upgrade of the LHC (HL-LHC) will provide unique capabilities. In this review, we present an overview of earlier track trigger systems at hadron colliders, in particular those at the Tevatron CDF and DØ experiments. We discuss the plans of the CMS and ATLAS experiments to implement hardware-based track reconstruction for the HL-LHC. Particular focus is placed on the track trigger capability of the upgraded CMS experiment. We discuss the challenges and opportunities of this novel capability, review the alternatives that were considered for its implementation, and discuss its expected performance. The planned track trigger systems for CMS and ATLAS have different goals, and we compare and contrast the two approaches.
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Extended Scalar Sectors
Vol. 70 (2020), pp. 197–223More LessExtended scalar sectors appear in various extensions of the Standard Model of particle physics, such as supersymmetric models. They are also generic extensions of the Standard Model and can address a number of its shortcomings. Direct searches for additional Higgs bosons and measurements of the 125-GeV Higgs boson, both of which provide insights into the different possible sectors, are carried out at the LHC. This review gives an overview of searches for the additional Higgs bosons and their implications for different models. The discussed analyses comprise searches for neutral and charged Higgs bosons that decay in various final states. In addition, the review summarizes the constraints from precision measurements, including in particular the observed couplings of the 125-GeV Higgs boson. While several models naturally incorporate a Higgs boson with couplings that are similar to the ones in the Standard Model, the measurements of the 125-GeV Higgs boson provide constraints on all considered extensions.
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What Is the Top Quark Mass?
Vol. 70 (2020), pp. 225–255More LessThis review provides an overview of the conceptual issues regarding the interpretation of so-called direct top quark mass measurements, which are based on the kinematic reconstruction of top quark decay products at the Large Hadron Collider (LHC). These measurements quote the top mass parameter
of Monte Carlo event generators with current uncertainties of around 0.5 GeV. The problem of finding a rigorous relation between
and top mass renormalization schemes defined in field theory is unresolved to date and touches perturbative as well as nonperturbative aspects and the limitations of state-of-the-art Monte Carlo event generators. I review the status of LHC top mass measurements, illustrate how conceptual limitations enter the picture, and explain a controversy that has permeated the community in the context of the interpretation problem related to
. I then summarize recent advances in acquiring first principles insights and outline what else has to be understood to fully resolve the issue. I conclude with recommendations on how to deal with the interpretation problem for the time being when making top mass–dependent theoretical predictions.
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The Nuclear Legacy Today of Fukushima
Vol. 70 (2020), pp. 257–292More LessThe accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) following the Great East Japan Earthquake and the subsequent tsunami in March 2011 changed people's perceptions regarding nuclear power generation in Japan and worldwide. The failure to prevent the accident and the response to it had an enormous impact specifically on the communities close to the site but also across Japan and globally. In this review, I discuss radiation detection technologies, their use and limits in the immediate assessment and response, and improvements since then. In particular, I examine recent developments in radiation detection and imaging systems that, in combination with the enormous advances in computer vision, provide new means to detect, map, and visualize radiation using manned and unmanned deployment platforms. In addition to smarter and more adaptable technologies to prevent and minimize the impact of such events, an important outcome of this accident is the need for informed and resilient citizens who are empowered by knowledge and technologies to make rational decisions. The accident at FDNPS leaves a legacy concerning the importance of historical information, technologies, and resilience as well as challenges regarding powerful technologies that can provide substantial benefits to human society but that are also associated with risks of which we must be aware.
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Chiral Magnetic Effects in Nuclear Collisions
Vol. 70 (2020), pp. 293–321More LessThe interplay of quantum anomalies with strong magnetic fields and vorticity in chiral systems could lead to novel transport phenomena, such as the chiral magnetic effect (CME), the chiral magnetic wave (CMW), and the chiral vortical effect (CVE). In high-energy nuclear collisions, these chiral effects may survive the expansion of a quark–gluon plasma fireball and be detected in experiments. The experimental searches for the CME, the CMW, and the CVE have aroused extensive interest over the past couple of decades. The main goal of this article is to review the latest experimental progress in the search for these novel chiral transport phenomena at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory and the Large Hadron Collider at CERN. Future programs to help reduce uncertainties and facilitate the interpretation of the data are also discussed.
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Photonuclear and Two-Photon Interactions at High-Energy Nuclear Colliders
Vol. 70 (2020), pp. 323–354More LessUltraperipheral collisions (UPCs) of heavy ions and protons are the energy frontier for electromagnetic interactions. Both photonuclear and two-photon collisions are studied at collision energies that are far higher than those available elsewhere. In this review, we discuss physics topics that can be addressed with UPCs, including nuclear shadowing, nuclear structure, and searches for physics beyond the Standard Model.
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Primordial Black Holes as Dark Matter: Recent Developments
Vol. 70 (2020), pp. 355–394More LessAlthough the dark matter is usually assumed to be made up of some form of elementary particle, primordial black holes (PBHs) could also provide some of it. However, various constraints restrict the possible mass windows to 1016–1017 g, 1020–1024 g, and 10–103M⊙. The last possibility is contentious but of special interest in view of the recent detection of black hole mergers by LIGO/Virgo. PBHs might have important consequences and resolve various cosmological conundra even if they account for only a small fraction of the dark matter density. In particular, those larger than 103M⊙ could generate cosmological structures through the seed or Poisson effect, thereby alleviating some problems associated with the standard cold dark matter scenario, and sufficiently large PBHs might provide seeds for the supermassive black holes in galactic nuclei. More exotically, the Planck-mass relics of PBH evaporations or stupendously large black holes bigger than 1012M⊙ could provide an interesting dark component.
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Polarization and Vorticity in the Quark–Gluon Plasma
Vol. 70 (2020), pp. 395–423More LessThe quark–gluon plasma (QGP) produced by collisions between ultrarelativistic heavy nuclei is well described in the language of hydrodynamics. Noncentral collisions are characterized by very large angular momentum, which in a fluid system manifests as flow vorticity. This rotational structure can lead to a spin polarization of the hadrons that eventually emerge from the plasma, and thus these collisions provide experimental access to flow substructure at unprecedented detail. Recently, the first observations of Λ hyperon polarization along the direction of collisional angular momentum were reported. These measurements are in broad agreement with hydrodynamic and transport-based calculations and reveal that the QGP is the most vortical fluid ever observed. However, there remain important tensions between theory and observation that might be fundamental in nature. In the relatively mature field of heavy-ion physics, the discovery of global hyperon polarization and 3D simulations of the collision have opened an entirely new direction of research. We discuss the current status of this rapidly developing area and directions for future research.
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The Search for Electroweakinos
Anadi Canepa, Tao Han, and Xing WangVol. 70 (2020), pp. 425–454More LessIn this review, we consider a general theoretical framework for fermionic color-singlet states—including a singlet, a doublet, and a triplet under the Standard Model SU(2)L gauge symmetry, corresponding to the bino, higgsino, and wino in supersymmetric theories—generically dubbed electroweakinos for their mass eigenstates. Depending on the relations among these states’ three mass parameters and their mixing after the electroweak symmetry breaking, this sector leads to a rich phenomenology that may be accessible in current and near-future experiments. We discuss the decay patterns of electroweakinos and their observable signatures at colliders, review the existing bounds on the model parameters, and summarize the current statuses of the comprehensive searches by the ATLAS and CMS Collaborations at the Large Hadron Collider. We also comment on the prospects for future colliders. An important feature of the theory is that the lightest neutral electroweakino can be identified as a weakly interacting massive particle cold dark matter candidate. We take into account the existing bounds on the parameters from the dark matter direct detection experiments and discuss the complementarity of the electroweakino searches at colliders.
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The Fermi–LAT Galactic Center Excess: Evidence of Annihilating Dark Matter?
Vol. 70 (2020), pp. 455–483More LessThe center of the Galaxy is one of the prime targets in the search for a signal of annihilating (or decaying) dark matter. If such a signal were to be detected, it would shed light on one of the biggest mysteries in physics today: What is dark matter? Fundamental properties of the particle nature of dark matter, such as its mass, annihilation cross section, and annihilation final states, could be measured for the first time. Several experiments have searched for such a signal, and some have measured excesses that are compatible with it. A long-standing and compelling excess is observed in γ-rays by the Fermi Large Area Telescope (Fermi–LAT). This excess is consistent with a dark matter particle with a mass of approximately 50 (up to ∼200) GeV annihilating with a velocity-averaged cross section of ∼10−26 cm3 s−1. Although a dark matter origin of the excess remains viable, other interpretations are possible. In particular, there is some evidence that the excess is produced by a population of unresolved point sources of γ-rays—for example, millisecond pulsars. In this article, I review the current status of the observation of the Fermi–LAT Galactic center excess, the possible interpretations of the excess, the evidence and counterevidence for each, and the prospects for resolving its origin with future measurements.
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Previous Volumes
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Volume 74 (2024)
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Volume 73 (2023)
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Volume 72 (2022)
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Volume 71 (2021)
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Volume 70 (2020)
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Volume 69 (2019)
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Volume 68 (2018)
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Volume 67 (2017)
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Volume 66 (2016)
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Volume 65 (2015)
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Volume 64 (2014)
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Volume 63 (2013)
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Volume 62 (2012)
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Volume 61 (2011)
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Volume 60 (2010)
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Volume 59 (2009)
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Volume 58 (2008)
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Volume 57 (2007)
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Volume 56 (2006)
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Volume 55 (2005)
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Volume 54 (2004)
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Volume 53 (2003)
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Volume 52 (2002)
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Volume 51 (2001)
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Volume 50 (2000)
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Volume 49 (1999)
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Volume 48 (1998)
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Volume 47 (1997)
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Volume 46 (1996)
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Volume 45 (1995)
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Volume 44 (1994)
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Volume 43 (1993)
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Volume 42 (1992)
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Volume 41 (1991)
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Volume 40 (1990)
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Volume 39 (1989)
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Volume 38 (1988)
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Volume 37 (1987)
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Volume 36 (1986)
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Volume 35 (1985)
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Volume 34 (1984)
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Volume 33 (1983)
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Volume 32 (1982)
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Volume 31 (1981)
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Volume 30 (1980)
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Volume 29 (1979)
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Volume 28 (1978)
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Volume 27 (1977)
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Volume 26 (1976)
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Volume 25 (1975)
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Volume 24 (1974)
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Volume 23 (1973)
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Volume 22 (1972)
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Volume 21 (1971)
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Volume 20 (1970)
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Volume 19 (1969)
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Volume 18 (1968)
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Volume 17 (1967)
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Volume 16 (1966)
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Volume 15 (1965)
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Volume 14 (1964)
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Volume 13 (1963)
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Volume 12 (1962)
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Volume 11 (1961)
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Volume 10 (1960)
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Volume 9 (1959)
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Volume 8 (1958)
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Volume 7 (1957)
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Volume 6 (1956)
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Volume 5 (1955)
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Volume 4 (1954)
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Volume 3 (1953)
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Volume 2 (1953)
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Volume 1 (1952)
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Volume 0 (1932)