Annual Review of Nuclear and Particle Science - Volume 71, 2021
Volume 71, 2021
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Adventures with Particles
Vol. 71 (2021), pp. 1–21More LessDespite some gender-related bumps in the road, the author had the good fortune that her career spanned the evolution of the Standard Model from its inception in the late 1960s and early 1970s to its final confirmation with the discovery of the Higgs boson in 2012. Her major contributions to these developments and other facets of her career are described.
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J. David Jackson (January 19, 1925–May 20, 2016): A Biographical Memoir
Vol. 71 (2021), pp. 23–36More LessJohn David (“Dave”) Jackson, a Canadian-born theoretical physicist, contributed significantly to particle, nuclear, and atomic physics. He is best known, however, for his text Classical Electrodynamics, which has been a fixture in physics graduate education around the world for more than 50 years. It is generally referred to simply as “Jackson.” This textbook, which has inspired fear and wonder alike in generations of students, clearly reflects the author's fascination with physical phenomena, his renowned mathematical dexterity, and his appreciation of the elegance of physical laws.
Jackson's major contributions to research included the theory of muon-catalyzed fusion; the analysis, with Kurt Gottfried, of angular distributions in quasi-two-body elementary particle collisions; and the elucidation of charmonium-state decays. Jackson influenced the development of physics research throughout the United States as well as internationally—particularly through his work on the nascent Superconducting Super Collider. An active promoter of civil liberties and human rights, he was one of the leaders of the efforts to free Andrei Sakharov, Yuri Orlov, and Anatoly Shcharansky from Soviet imprisonment.
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Searches for Dark Photons at Accelerators
Vol. 71 (2021), pp. 37–58More LessDark matter particles may interact with other dark matter particles via a new force mediated by a dark photon, A′, which would be the dark-sector analog to the ordinary photon of electromagnetism. The dark photon can obtain a highly suppressed mixing-induced coupling to the electromagnetic current, providing a portal through which dark photons can interact with ordinary matter. This review focuses on A′ scenarios that are potentially accessible to accelerator-based experiments. We summarize the existing constraints placed by such experiments on dark photons, highlight what could be observed in the near future, and discuss the major experimental challenges that must be overcome to improve sensitivities.
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Mixing and CP Violation in the Charm System
Vol. 71 (2021), pp. 59–85More LessIn recent years charm physics has undergone a renaissance, which has been catalyzed by an unexpected and impressive set of experimental results from the B factories, the Tevatron, and LHCb. The existence of
oscillations is now well established, and the recent discovery of CP violation in D0 decays has further renewed interest in the charm sector. In this article, we review the current status of charm-mixing and CP-violation measurements and assess their agreement with theoretical predictions within the Standard Model and beyond. We look forward to the great improvements in experimental precision that can be expected over the coming two decades and to the prospects for corresponding advances in theoretical understanding.
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What Can We Learn About QCD and Collider Physics from N=4 Super Yang–Mills?
Vol. 71 (2021), pp. 87–112More LessTremendous ongoing theory efforts are dedicated to developing new methods for quantum chromodynamics (QCD) calculations. Qualitative rather than incremental advances are needed to fully exploit data that are still to be collected at the LHC. The maximally supersymmetric Yang–Mills theory,
super Yang–Mills (sYM), shares with QCD the gluon sector, which contains the most complicated Feynman graphs but also has many special properties and is believed to be solvable exactly. It is natural to ask what we can learn from advances in
sYM for addressing difficult problems in QCD. With this in mind, I review several remarkable developments and highlights of recent results in
sYM. This includes all-order results for certain scattering amplitudes, novel symmetries, surprising geometrical structures of loop integrands, novel tools for the calculation of Feynman integrals, and bootstrap methods. While several insights and tools have already been carried over to QCD and have contributed to state-of-the-art calculations for LHC physics, I argue that there is a host of further fascinating ideas waiting to be explored.
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Rare Kaon Decays
Vol. 71 (2021), pp. 113–137More LessHistorically important in the development of the Standard Model (SM) of particle physics, rare kaon decays are still a privileged tool for looking beyond it. The main reasons to continue the study of rare kaon decays are to test the CKM quark-mixing and CP-violation paradigm, to make quantitative comparisons with the B sector, and to search for explicit violations of the SM. Current research on rare kaon decays focuses mostly on
decays, which are predicted with good accuracy within the SM and beyond. Experimentally, these decays, especially that of the charged kaon, have a long history. Their theoretical importance is matched only by their experimental difficulty. This article reviews the progress of the past 10 years, describes the state of the art, and looks toward future perspectives.
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Precise Measurements of the Decay of Free Neutrons
Vol. 71 (2021), pp. 139–163More LessThe impact of new and highly precise neutron β decay data is reviewed. We focus on recent results from neutron lifetime, β asymmetry, and electron–neutrino correlation experiments. From these results, weak interaction parameters are extracted with unprecedented precision, which is possible also because of progress in effective field theory and lattice QCD. Limits on New Physics beyond the Standard Model derived from neutron decay data are sharper than those derived from high-energy experiments, except for processes involving right-handed neutrinos.
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New Developments in Flavor Evolution of a Dense Neutrino Gas
Vol. 71 (2021), pp. 165–188More LessNeutrino–neutrino refraction dominates the flavor evolution in core-collapse supernovae, neutron star mergers, and the early Universe. Ordinary neutrino flavor conversions develop on timescales determined by the vacuum oscillation frequency. However, when the neutrino density is large enough, collective flavor conversions may arise because of pairwise neutrino scattering. Pairwise conversions are deemed fast because they are expected to occur on timescales that depend on the neutrino–neutrino interaction energy (i.e., on the neutrino number density) and are regulated by the angular distributions of electron neutrinos and antineutrinos. The enigmatic phenomenon of fast pairwise conversions has been overlooked for a long time. However, because of the fast conversion rate, pairwise conversions could occur in the proximity of the neutrino decoupling region with yet-to-be-understood implications for the hydrodynamics of astrophysical sources and the synthesis of the heavy elements. We review the physics of this fascinating phenomenon and its implications for neutrino-dense sources.
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Directional Recoil Detection
Vol. 71 (2021), pp. 189–224More LessSearches for dark matter–induced recoils have made impressive advances in the last few years. Yet the field is confronted by several outstanding problems. First, the inevitable background of solar neutrinos will soon inhibit the conclusive identification of many dark matter models. Second, and more fundamentally, current experiments have no practical way of confirming a detected signal's Galactic origin. The concept of directional detection addresses both of these issues while offering opportunities to study novel dark matter– and neutrino-related physics. The concept remains experimentally challenging, but gas time projection chambers are an increasingly attractive option and, when properly configured, would allow directional measurements of both nuclear and electron recoils. In this review, we reassess the required detector performance and survey relevant technologies. Fortuitously, the highly segmented detectors required to achieve good directionality also enable several fundamental and applied physics measurements. We comment on near-term challenges and how the field could be advanced.
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Recent Progress in the Physics of Axions and Axion-Like Particles
Vol. 71 (2021), pp. 225–252More LessThe axion is a light pseudoscalar particle postulated to solve issues with the Standard Model, including the strong CP problem and the origin of dark matter. In recent years, there has been remarkable progress in the physics of axions in several directions. An unusual type of axion-like particle termed the relaxion was proposed as a new solution to the weak scale hierarchy problem. There are also new ideas for laboratory, astrophysical, or cosmological searches for axions; such searches can probe a wide range of model parameters that were previously inaccessible. On the formal theory side, the weak gravity conjecture indicates a tension between quantum gravity and a trans-Planckian axion field excursion. Many of these developments involve axions with hierarchical couplings. In this article, we review recent progress in axion physics, with particular attention paid to hierarchies between axion couplings. We emphasize that the parameter regions of hierarchical axion couplings are the most accessible experimentally. Moreover, such regions are often where important theoretical questions in the field are addressed, and they can result from simple model-building mechanisms.
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Nuclear Dynamics and Reactions in the Ab Initio Symmetry-Adapted Framework
Vol. 71 (2021), pp. 253–277More LessWe review the ab initio symmetry-adapted (SA) framework for determining the structure of stable and unstable nuclei, along with related electroweak, decay, and reaction processes. This framework utilizes the dominant symmetry of nuclear dynamics, the shape-related symplectic
symmetry, which has been shown to emerge from first principles and to expose dominant degrees of freedom that are collective in nature, even in the lightest species or seemingly spherical states. This feature is illustrated for a broad scope of nuclei ranging from helium to titanium isotopes, enabled by recent developments of the ab initio SA no-core shell model expanded to the continuum through the use of the SA basis and that of the resonating group method. The review focuses on energies, electromagnetic transitions, quadrupole and magnetic moments, radii, form factors, and response function moments for ground-state rotational bands and giant resonances. The method also determines the structure of reaction fragments that is used to calculate decay widths and α-capture reactions for simulated X-ray burst abundance patterns, as well as nucleon–nucleus interactions for cross sections and other reaction observables.
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The Search for Feebly Interacting Particles
Vol. 71 (2021), pp. 279–313More LessAt the dawn of a new decade, particle physics faces the challenge of explaining the mystery of dark matter, the origin of matter over antimatter in the Universe, the apparent fine-tuning of the electroweak scale, and many other aspects of fundamental physics. Perhaps the most striking frontier to emerge in the search for answers involves New Physics at mass scales comparable to that of familiar matter—below the GeV scale but with very feeble interaction strength. New theoretical ideas to address dark matter and other fundamental questions predict such feebly interacting particles (FIPs) at these scales, and existing data may even provide hints of this possibility. Emboldened by the lessons of the LHC, a vibrant experimental program to discover such physics is underway, guided by a systematic theoretical approach that is firmly grounded in the underlying principles of the Standard Model. We give an overview of these efforts, their motivations, and the decadal goals that animate the community involved in the search for FIPs, and we focus in particular on accelerator-based experiments.
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Progress in the Glauber Model at Collider Energies
Vol. 71 (2021), pp. 315–344More LessWe review the theoretical and experimental progress in the Glauber model of multiple nucleon and/or parton scatterings after the last 10–15 years of operation with proton and nuclear beams at the BNL Relativistic Heavy Ion Collider and the CERN Large Hadron Collider. The main developments and the state of the art of the field are summarized. These encompass measurements of the inclusive inelastic proton and nuclear cross sections, advances in the description of the proton and nuclear density profiles and their fluctuations, inclusion of subnucleonic degrees of freedom, experimental procedures and issues related to the determination of the collision centrality, validation of the binary scaling prescription for hard scattering cross sections, and constraints on transport properties of quark–gluon matter from varying initial-state conditions in relativistic hydrodynamics calculations. These advances confirm the validity and usefulness of the Glauber formalism for quantitative studies of quantum chromodynamics matter produced in high-energy collisions of systems, from protons to uranium nuclei, of vastly different size.
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The Trojan Horse Method: A Nuclear Physics Tool for Astrophysics
Vol. 71 (2021), pp. 345–376More LessThe Trojan Horse Method (THM) represents an indirect path to determine the bare nucleus astrophysical S-factor for reactions among charged particles at astrophysical energies. This is achieved by measuring the quasi-free cross section of a suitable three-body process. The method is also suited to study neutron-induced reactions, especially in the case of radioactive ion beams. A comprehensive review of the theoretical as well as experimental features behind the THM is presented here. An overview is given of some recent applications to demonstrate the method's practical use for reactions that have a great impact on selected astrophysical scenarios.
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Study of the Strong Interaction Among Hadrons with Correlations at the LHC
Vol. 71 (2021), pp. 377–402More LessThe strong interaction among hadrons has been measured in the past by scattering experiments. Although this technique has been extremely successful in providing information about the nucleon–nucleon and pion–nucleon interactions, when unstable hadrons are considered the experiments become more challenging. In the last few years, the analysis of correlations in the momentum space for pairs of stable and unstable hadrons measured in pp and p+Pb collisions by the ALICE Collaboration at the LHC has provided a new method to investigate the strong interaction among hadrons. In this article, we review the numerous results recently achieved for hyperon–nucleon, hyperon–hyperon, and kaon–nucleon pairs, which show that this new method opens the possibility of measuring the residual strong interaction of any hadron pair.
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Chiral Effective Field Theory and the High-Density Nuclear Equation of State
Vol. 71 (2021), pp. 403–432More LessBorn in the aftermath of core-collapse supernovae, neutron stars contain matter under extraordinary conditions of density and temperature that are difficult to reproduce in the laboratory. In recent years, neutron star observations have begun to yield novel insights into the nature of strongly interacting matter in the high-density regime where current theoretical models are challenged. At the same time, chiral effective field theory has developed into a powerful framework to study nuclear matter properties with quantified uncertainties in the moderate-density regime for modeling neutron stars. In this article, we review recent developments in chiral effective field theory and focus on many-body perturbation theory as a computationally efficient tool for calculating the properties of hot and dense nuclear matter. We also demonstrate how effective field theory enables statistically meaningful comparisons among nuclear theory predictions, nuclear experiments, and observational constraints on the nuclear equation of state.
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Neutron Stars and the Nuclear Matter Equation of State
Vol. 71 (2021), pp. 433–464More LessNeutron stars provide a window into the properties of dense nuclear matter. Several recent observational and theoretical developments provide powerful constraints on their structure and internal composition. Among these are the first observed binary neutron star merger, GW170817, whose gravitational radiation was accompanied by electromagnetic radiation from a short γ-ray burst and an optical afterglow believed to be due to the radioactive decay of newly minted heavy r-process nuclei. These observations give important constraints on the radii of typical neutron stars and on the upper limit to the neutron star maximum mass and complement recent pulsar observations that established a lower limit. Pulse-profile observations by the Neutron Star Interior Composition Explorer (NICER) X-ray telescope provide an independent, consistent measure of the neutron star radius. Theoretical many-body studies of neutron matter reinforce these estimates of neutron star radii. Studies using parameterized dense matter equations of state (EOSs) reveal several EOS-independent relations connecting global neutron star properties.
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Efimov Physics and Connections to Nuclear Physics
Vol. 71 (2021), pp. 465–490More LessPhysical systems characterized by a shallow two-body bound or virtual state are governed at large distances by continuous scale invariance, which is broken into discrete scale invariance when three or more particles come into play. This symmetry induces a universal behavior for different systems that is independent of the details of the underlying interaction and rooted in the smallness of the ratio ℓ/aB ≪ 1, where the length aB is associated with the binding energy of the two-body system
, and ℓ is the natural length given by the interaction range. Efimov physics refers to this universal behavior, which is often hidden by the onset of system-specific nonuniversal effects. In this review, we identify universal properties by providing an explicit link of physical systems to their unitary limit, in which aB → ∞, and we show that nuclear systems belong to this class of universality.
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The Future of Solar Neutrinos
Vol. 71 (2021), pp. 491–528More LessIn this article we review the current state of the field of solar neutrinos, including flavor oscillations, nonstandard effects, solar models, cross section measurements, and the broad experimental program thus motivated and enabled. We describe the historical discoveries that contributed to current knowledge, and define critical open questions to be addressed in the next decade. We discuss standard solar models, including uncertainties and problems related to the solar composition, and review experimental and model solar neutrino fluxes, including future prospects. We review the state of the art of the nuclear reaction data relevant for solar fusion in the proton–proton chain and carbon–nitrogen–oxygen cycle. Finally, we review the current and future experimental programs that can address outstanding questions in this field.
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Implications of New Physics Models for the Couplings of the Higgs Boson
Vol. 71 (2021), pp. 529–551More LessIt has been almost a decade since the first hints of the Higgs boson discovery began to emerge from CERN, making a review of our updated expectations for the Higgs boson properties, in light of New Physics models, timely. In this review I attempt to draw connections between modified Higgs boson couplings and the big questions that broad classes of New Physics models aim to answer. Questions considered include whether the Higgs boson is composite and whether a new space-time supersymmetry exists. The goal is to present these topics, framed in reference to the Higgs boson, in a conceptually driven manner and, to make them accessible to a relatively broad audience, without a great deal of technicality.
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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)