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2 Science Questions INTRODUCTION This chapter discusses in more detail the recent accomplishments and direc- tions that are expected to be taken in nuclear physics in upcoming years. Where the discussion in Chapter 1 focused on four overarching questions being addressed by the field, this chapter is separated into more traditional subfields of nuclear physics—(1) nuclear structure, whose goal is to build a coherent framework for explaining all properties of nuclei and nuclear matter and how they interact; (2) nuclear astrophysics, which explores those events and objects in the universe shaped by nuclear reactions; (3) quark-gluon plasma, which examines the state of “melted” nuclei and with that knowledge seeks to shed light on the beginnings of the uni- verse and the nature of those quarks and gluons that are the constituent particles of nuclei; (4) hadron structure, which explores the remarkable characteristics of the strong force and the various mechanisms by which the quarks and gluons interact and result in the properties of the protons and neutrons that make up nuclei; and (5) fundamental symmetries, those areas on the edge of nuclear physics where the understandings and tools of nuclear physicists are being used to unravel limitations of the Standard Model and to provide some of the understandings upon which a new, more comprehensive Standard Model will be built. 30
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Science Questions 31 PERSPECTIVES ON THE STRUCTURE OF ATOMIC NUCLEI The goal of nuclear structure research is to build a coherent framework that explains all the properties of nuclei, nuclear matter, and nuclear reactions. While extremely ambitious, this goal is no longer a dream. With the advent of new generations of exotic beam facilities, which will greatly expand the variety and intensity of rare isotopes available, new theoretical concepts, and the extreme-scale computing platforms that enable cutting-edge calculations of nuclear properties, nuclear structure physics is poised at the threshold of its most dramatic expansion of opportunities in decades. The overarching questions guiding nuclear structure research have been expressed as two general and complementary perspectives: a microscopic view focusing on the motion of individual nucleons and their mutual interactions, and a mesoscopic one that focuses on a highly organized complex system exhibiting special symmetries, regularities, and collective behavior. Through those two per- spectives, research in nuclear structure in the next decade will seek answers to a number of open questions: • What are the limits of nuclear existence and how do nuclei at those limits live and die? • What do regular patterns in the behavior of nuclei divulge about the nature of nuclear forces and the mechanism of nuclear binding? • What is the nature of extended nucleonic matter? • How can nuclear structure and reactions be described in a unified way? New facilities and tools will help to explore the vast nuclear landscape and iden- tify the missing ingredients in our understanding of the nucleus. A huge number of new nuclei are now available—proton rich, neutron rich, the heaviest elements, and the long chains of isotopes for many elements. Together, they comprise a vast pool from which key isotopes—designer nuclei—can be chosen because they isolate or amplify specific physics or are important for applications. At the same time, research with intense beams of stable nuclei continues to produce innovative science, and, in the long term, discoveries at exotic beam facili- ties will raise new questions whose answers are accessible with stable nuclei. Examples of the current program that offer a glimpse into future areas of inquiry are the investigation of new forms of nuclear matter such as neutron skins occurring on the surfaces of nuclei having large excesses of neutrons over protons, the ability to fabricate the superheavy elements that are predicted to exhibit unusual stability in spite of huge electrostatic repulsion, and structural studies in exotic isotopes whose properties defy current textbook paradigms. Hand in hand with experimental developments, a qualitative change is taking
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32 Nuclear Physics place in theoretical nuclear structure physics. With the development of new con- cepts, the exploitation of symbiotic collaborations with scientists in diverse fields, and advances in computing technology and numerical algorithms, theorists are progressing toward understanding the nucleus in a comprehensive and unified way. Revising the Paradigms of Nuclear Structure Shell Structure: A Moving Target The concept of nucleons moving in orbits within the nucleus under the influ- ence of a common force gives rise to the ideas of shell structure and resulting magic numbers. Like an electron’s motion in an atom, nucleonic orbits bunch together in energy, forming shells, and nuclei having filled nucleonic shells (nuclear “noble gases”) are exceptionally well bound. The numbers of nucleons needed to fill each successive shell are called the magic numbers: The traditional ones are 2, 8, 20, 28, 50, 82, and 126 (some of these are exemplified in Figure 2.1). Thus a nucleus such as lead-208, with 82 protons and 126 neutrons, is doubly “magic.” The concept of magic numbers in turn introduces the idea of valence nucleons—those beyond a magic number. Thus, in considering the structure of nuclei like lead-210, one can, to some approximation, consider only the last two valence neutrons rather than all 210. When proposed in the late 1940s, this was a revolutionary concept: How could individual nucleons, which fill most of the nuclear volume, orbit so freely without generating an absolute chaos of collisions? Of course, the Pauli exclusion principle is now understood to play a key role here, and the resulting model of nucleonic orbits has become the template used for over half a century to view nuclear structure. One experimental hallmark of nuclear structure is the behavior of the first excited state with angular momentum 2 and positive parity in even-even nuclei. This state, usually the lowest energy excitation in such nuclei, is a bellwether of structure. Its excitation energy takes on high values at magic numbers and low values as the number of valence nucleons increases and collective behavior emerges. The picture of nuclear shells leads to the beautiful regularities and simple repeated patterns, illustrated in Figure 1.2 and seen here in the energies of the 2+ states shown at the top of Figure 2.2. The concept of magic numbers was forged from data based on stable or near-stable nuclei. Recently, however, the traditional magic numbers underwent major revisions as previously unavailable species became accessible. The shell structure known from stable nuclei is no longer viewed as an immutable construct but instead is seen as an evolving moving target. Indeed the elucidation of changing shell structure is one of the triumphs of recent experiments in nuclear structure at exotic beam facilities worldwide. For example, experiments
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Science Questions 33 electronic nucleonic shells of shells of the atom the nucleus 3p1/2 5p 4d 2f5/2 5s 1i13/2 3p3/2 4p 1h9/2 3d 2f7/2 noble gases 4s magic nuclei (closed shells) (closed shells) 2d3/2 3p 1h11/2 3s 3s1/2 1g7/2 2d5/2 2p 2s 1g9/2 FIGURE 2.1 Shell structure in atoms and nuclei. Left: Electron energy levels forming the atomic shell structure. In the noble gases, shells of valence electrons are completely filled. Right: Representative nuclear shell structure characteristic of stable or long-lived nuclei close to the valley of stability. In the 2-01.eps “magic” nuclei with proton or neutron numbers 2, 8, 20, 28, 50, 82, and 126, which are analogous to noble gases, proton and/or neutron shells are completely filled. The shell structure in very neutron- rich nuclei is not known. New data on light nuclei with N >> Z tell us that significant modifications are expected. SOURCE: Adapted and reprinted with permission from K. Jones and W. Nazarewicz, 2010, The Physics Teacher 48 (381). Copyright 2010, American Association of Physics Teachers. at Michigan State University (MSU) in the United States and at the Gesellschaft für Schwerionenforschung (GSI) have shown that in the very neutron-rich isotope oxygen-24, with 8 protons and twice as many neutrons, N = 16 is, in fact, a new magic number. One of the most interesting regions exhibiting the fragility of magic numbers is nuclei with 12 to 20 protons and 18 to 30 neutrons. The experimental evidence is exemplified in the lower portion of Figure 2.2 by the energies of the first excited 2+ states in this region. The figure shows the disappearance of neutron number N = 20 as a magic number in magnesium while it persists for neighboring elements.
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34 Nuclear Physics 2-2 top or left NEW FIGURE 2.2 Measured energies of the lowest 2+ states in even-even nuclei. Top: Color-coded Z-N plot spanning the entire nuclear chart clearly show the filaments of magic behavior at particular neutron 2-02bottom.eps and proton numbers (denoted by dashed lines) and the lowering of these states as nucleons are added type outlined and collective behavior emerges. The legend bar relates the colors to an energy scale in MeV. Bottom: Close-up view of the data for the neutron-rich magnesium (Mg), silicon (Si), sulfur (S), argon (Ar), and calcium (Ca) isotopes. The fingerprint of magic numbers is missing in the neutron-rich isotopes of Mg, Si, S, and Ar, in which the “standard” magic numbers at either N = 20 or 28 have dissipated. As of 2011, no data exist on Si, S, and Ar nuclei with N = 32. SOURCES: (Top) Courtesy of R. Burcu Cakirli, Max Planck Institute for Nuclear Physics, private communication, 2011; (bottom) Courtesy of Alexandra Gade, MSU.
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Science Questions 35 Similarly, N = 28 loses its magic character for silicon, sulfur, and argon, while cal- cium, which is also magic in protons, retains its doubly magic character at N = 28. There are at least three factors leading to such changes in shell structure: changes in how nucleons interact with each other as the proton-neutron asym- metry varies, the influence of scattering and decay states near the isotopic limits of nuclear existence (the “drip lines”), and the increasing role of many-body effects in weakly bound nuclei where correlations determine the mere existence of the nucleus. This new perspective on shell structure affects many facets of nuclear structure, from the existence of short-lived light nuclei, to the emergence of col- lectivity, to the stability of the superheavy elements. Recent studies of calcium, nickel, and tin isotopes using techniques such as Coulomb excitation and light-ion single nucleon transfer reactions, both near traditional magic numbers and along extended isotopic chains, are beginning to answer questions about effective internucleon forces in the presence of large neu- tron excess, the relevance of the detailed shell-model template in the presence of weak binding, and the nature of nuclear collective motion. Excellent tests of the nuclear shell model were offered by recent studies of the tin (Sn) isotopes. The nucleus tin has a magic number (50) of protons, and its short-lived isotopes tin-100 and tin-132, with 50 and 82 neutrons, respectively, are expected to be rare examples of new doubly magic heavy nuclei. Unique data in the tin-132 region (see Figure 2.3) shows that tin-132 indeed behaves as a good doubly magic nucleus. Other experiments providing data around tin-100, in particular the first structural infor- mation on tin-101, have led to theoretical surprises. Further tests of shell structure and interactions in the heaviest elements will be discussed below. It is expected that the shell model will undergo sensitive tests in the region of superheavy nuclei, whose very existence hinges on a dynamical competition between short-range nuclear attraction and huge long-range Coulomb repulsion. Interestingly, a similar interplay takes place in low-density, neutron-rich matter found in crusts of neutron stars, where “Coulomb frustration” produces rich and complex collective structures, discussed later in this chapter in “Nuclear Astrophys- ics.” Figure 2.4 shows the calculated shell energy—that is, the quantum enhance- ment in nuclear binding due to the presence of nucleonic shells. The nuclei from the tin region are excellent examples of the shell-model paradigm: the magic nuclei with Z = 50, N = 50, and N = 82 have the largest shell energies, and the associ- ated closed shells provide exceptional stability. In superheavy nuclei, the density of single-particle energy levels is fairly large, so small energy shifts, such as the regions of enhanced shell stabilization in the super-heavy region near N = 184, are generally expected to be fairly broad; that is, the notion of magic numbers and the energy gaps associated with them becomes fluid there. Another dimension in studies of shells in nuclei has been opened by precision studies, at the Thomas Jefferson National Accelerator Facility (JLAB) and at the
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36 Nuclear Physics Proton number (Z ) Z = 82 Z = 50 132Sn N = 126 Z = 28 N = 82 Z = 20 N = 50 Z=8 N = 28 Z=2 N = 20 N=8 Neutron number (N) N=2 FIGURE 2.3 Top: All known nuclides are Proton shown as black (if stable) or yellow (unsta- 60 CD target 2-03_top_high res Cottle.eps lines indicate the traditional 2 (5/2–) ble). Dashed (9/2–) 2,005 keV 1,561 keV (1/2–) 1,363 keV 50 132 bitmap with vector type magic numbers of protons and neutrons. Two 3/2 854 keV & some ruling – Sn beam Sn 133 doubly-magic nuclei, tin-132 and nickel-78, 40 beam 7/2 0 keV are adjacent to the r-process region (blue) – Counts of as-yet-unseen nuclides that are thought 30 to be involved in the creation of the heavi- est elements in supernovae. By adding neu- 20 trons or protons to a stable nucleus, one 10 enters the territory of radioactive nuclei, first long-lived, then short-lived, until finally the 0 nuclear drip line is reached, where there is –2 –1 0 1 no longer enough binding force to prevent Q (MeV) the last nucleons from dripping off the nuclei. The proton and neutron drip lines form the borders of nuclear existence. Bottom: Experimental spec- trum for a transfer reaction in which an incident deuteron grazes a tin-132 target, depositing a neutron 2-03_bottom_high res nature the exiting proton (that is, d + tin-132 → p + tin-133). The solid to make tin-133 with detection of 09048-f2.eps bitmaps with vector masks, rules, in green, red, blue, and lavender in the level scheme (inset). line shows a fit to the four peaks shown and type The top left inset displays a cartoon of the reaction employed. The investigations revealed that low energy states in tin-133 have even purer single-particle character than their counterparts in lead-209, outside the doubly-magic nucleus lead-208, the previous benchmark. SOURCE: (Top) Reprinted by permission from Macmillan Publishers Ltd., B. Schwarzschild. August 2010. Physics Today 63:16, copyright 2010; (Bottom) Reprinted by permission from Macmillan Publishers Ltd., K.L. Jones, A.S. Adekola, D.W. Bardayan, et al. 2010. Nature 465: 454, copyright 2010. Portions of the figure caption are extracted from K.L. Jones, W. Nazarewicz, 2010. Designer nuclei – making atoms that barely exist, The Physics Teacher 48: 381.
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Science Questions 37 60 Tin region 0 -3 50 -6 -9 -12 Proton number 40 40 60 80 100 140 Superheavy region 0 130 -2 -4 -6 -8 120 -10 110 160 180 200 Neutron number FIGURE 2.4 Contribution to nuclear binding due to shell effects (in MeV) for nuclei from the tin region (top) and super-heavy elements (bottom) calculated in the nuclear density functional theory. The nuclei colored in darker red are those2-04_she1.eps enhanced by quantum effects. The nuclei whose binding is most predicted to be stable to beta decay are marked by dots. SOURCE: Reprinted and adapted from M. Bender, W. Nazarewicz, and P.G. Reinhard, Shell stabilization of super- and hyperheavy nuclei without magic gaps, Physics Letters B 515: 42, Copyright 2001, with permission from Elsevier. Japanese National Laboratory for High Energy Physics (KEK), of hypernuclei— nuclei that contain at least one hyperon, a strange baryon, in addition to nucleons. By adding a hyperon, nuclear physicists can explore inner regions of nuclei that are impossible to study with protons and neutrons, which must obey the con- straints imposed by the Pauli principle. The experimental work goes hand in hand with advanced theoretical calculations of hyperon-nucleon and hyperon-hyperon interactions, with the ultimate goal being the comprehensive understanding of all baryon-baryon interactions. Exploring and Understanding the Limits of Nuclear Existence An important challenge is to delineate the proton and neutron drip lines—the limits of proton and neutron numbers at which nuclei are no longer bound by
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38 Nuclear Physics the strong force and nuclear existence ends—as far into the nuclear chart as pos- sible (see Figure 2.3 [top]). For example, experiments at MSU have produced the heaviest magnesium and aluminum isotopes accessible to date and have shown that magnesium-40, aluminum-42, and possibly aluminum-43 exist. Nuclei near the drip lines are very weakly bound quantum systems, often with extremely large spatial sizes. In recent years, experiments at Argonne National Laboratory (ANL), TRIUMF, Grand Accélérateur National d’Ions Lourds (GANIL), GSI, the European Organization for Nuclear Research (CERN), and Rikagaku Kenkyūjo (RIKEN) using high-precision laser spectroscopy have determined the charge radii of halo nuclei helium-6, helium-8, beryllium-11, and lithium-11 with an accuracy of 1 percent through the determination of isotope shifts of atomic electronic levels. With the advanced-generation Facility for Rare Isotope Beams (FRIB) it should be possible to extend such studies and to delineate most of the drip line up to mass 100 using the high-power beams available and the highly efficient and selective FRIB frag- ment separators. Drip line nuclei often exhibit exotic decay modes. An example is the extremely proton-rich nucleus iron-45 that decays by beta decay or by ejecting two protons from its ground state. Another example of exotic decay modes, proton-rich nuclei exhibiting “superallowed” beta decays, is discussed in “Fundamental Symmetries,” later in this chapter. Moving toward the drip lines, the coupling between different nuclear states, via a continuum of unbound states, becomes systematically more important, eventually playing a dominant role in determining structure. Such sys- tems where both bound and unbound states exist and interact are called “open” quantum systems. Many aspects of nuclei at the limits of the nuclear landscape are generic and are currently explored in other open systems: molecules in strong external fields, quan- tum dots and wires and other solid-state microdevices, crystals in laser fields, and microwave cavities. Radioactive nuclear beam experimentation will answer ques- tions pertaining to all open quantum systems: What are their properties around the lowest energies, where the reactions become energetically allowed (reaction thresholds)? What is the origin of states in nuclei, which resemble groupings of nucleons into well-defined clusters, especially those of astrophysical importance? What should be the most important steps in developing the theory that will treat nuclear structure and reactions consistently? The Heaviest Elements What are the heaviest nuclei that can exist? Is there an island of very long-lived nuclei in the N-Z plane? What are the chemical properties of superheavy atoms? These questions present challenges to both experiment and theory. As discussed earlier, the repulsive electrostatic Coulomb force between protons grows so much
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Science Questions 39 in those nuclei with large proton number that they would not be bound except for subtle quantum effects. Theory predicts that stability will increase with the addition of neutrons in these systems as one approaches N = 184 (see Figure 2.5), but there is no consensus about the precise location of the projected island of long-lived superheavy elements and their lifetimes (some are predicted to have lifetimes as long as 105-107 years. By using actinide targets and rare stable beams, such as calcium-48, elements up to Z = 118 have been produced and observed. The discovery of a nucleus with Z = 117, with a target of berkelium-249, is a case in point as well as an excellent example of international cooperation in nuclear physics (Box 2.1). Not only did this work discover a new element but new information obtained on the half lives of several nuclei in its decay path provided experimental support for the existence of the long-predicted island of stability in superheavy nuclei. Further incremen- tal progress approaching Z = 118 and beyond is possible, but it requires new actinide targets beyond berkelium, and intense beams of rare stable isotopes such as titanium-50. However, there is a range of options for synthesizing heavy elements with exotic beams. By using neutron-rich radioactive targets and beams a highly excited system can be formed, which would decay into the superheavy ground state via evaporation of the excess neutrons. An area of related importance is the further study of the spectroscopy of the heaviest nuclei possible using reaccelerated beams and large acceptance spectrometers, looking at alpha-decay and gamma-ray spectroscopy up to at least Z = 106. Neutron-Rich Matter in the Laboratory and the Cosmos Neutron-rich matter is at the heart of many fascinating questions in nuclear physics and astrophysics: What are the phases and equations of state of nuclear and neutron matter? What are the properties of short-lived neutron-rich nuclei through which the chemical elements around us were created? What is the structure of neu- tron stars, and what determines their electromagnetic, neutrino, and gravitational- wave radiations? To explain the nature of neutron-rich matter across a range of densities, an interdisciplinary approach is essential in order to integrate laboratory experiments with astrophysical theory, nuclear theory, condensed matter theory, atomic physics, computational science, and electromagnetic and gravitational-wave astronomy. Figure 2.6 summarizes such linkages in this interdisciplinary endeavor. In heavy neutron-rich nuclei, the excess of neutrons predominantly collects at the nuclear surface creating a skin, a region of weakly bound neutron matter. The presence of a skin can lead to curious collective excitations, for example, “pygmy resonances,” characterized by the motion of the partially decoupled neutron skin against the remainder of the nucleus. Such modes could alter neutron capture cross sections important to r-process nucleosynthesis (discussed further in “Nuclear
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40 Nuclear Physics Physics of Superheavy Elements Chemistry of Superheavy Elements region of expected long-lived nuclei Cn Z=112: Copernicium very volatile noble metal FIGURE 2.5 Left: Calculated properties of even-even superheavy nuclei. The upper-left diagram shows the deformation energy (in MeV) 2-05_she2.eps between the ground state energy and the defined as a difference energy at the spherical shape. Several Z = 110-113 alpha decay chains found at GSI and RIKEN with bitmaps with some vector type & ruling fusion reactions using lead or bismuth targets are marked by pink squares and those obtained in hot fusion reactions at the Joint Institute for Nuclear Reactions (JINR) in Dubna are marked by yellow squares. The region of anticipated long-lived superheavy nuclei is schematically marked. Lower left: contour map of predicted ground-state quadrupole deformations and nuclear shapes for selected nuclei. Prolate shapes are red-orange; oblate shapes, blue-green; and spherical shapes, light yellow. The symbols 274112, 290118, 296124, and 310126 refer to unnamed nuclei having the given number of nucleons (superscript) and protons (base). Right: Periodic table of elements as of 2010 including the element Z = 112 discovered at GSI and accorded the name copernicium (chemical symbol Cn) in honor of astronomer Nicolaus Copernicus. Its chemistry suggests it is a member of the metallic group 12 (containing zinc, cadmium, and mercury). SOURCES: (Left) Adapted by permission from Macmillan Publishers Ltd., S. Ćwiok, P.H. Heenen, and W. Nazarewicz. 2005. Nature 433: 705; (right) K.L. Jones and W. Nazarewicz, 2010, Designer nuclei—Making atoms that barely exist, The Physics Teacher 48: 381. Reprinted with permission from The Physics Teacher, Copyright 2010, American Association of Physics Teachers.
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Science Questions 139 (evidence that neutrinos and antineutrinos are the same particle) are a particular focus in nuclear physics. Beta Decays of Nuclei and the Free Neutron The beta-decays of nuclei in which both the parent and daughter nuclear states have zero angular momentum and positive parity (“superallowed” nuclear decays) provide a value for the largest and most precise element Vud in the Stan- dard Model Cabibbo-Kobayashi-Maskawa (CKM) matrix that relates quark flavor states to quark mass states. The CKM matrix transforms the quark description with well-defined masses (for which there are no special names) into states with well-defined flavors (down, strange, and bottom). Because there must be a one-to- one relationship between the two descriptions (i.e., no additional quarks beyond the three known to exist), the matrix is unitary, which defines some relationships between the elements and reduces the number of independent parameters to only three, plus a phase that has no effect on the size of the parameters. When combined with the results of kaon and B meson decay studies, which yield the small terms Vus and Vub, the superallowed nuclear decays provide a stringent test of the unitarity property of the CKM matrix (see Figure 2.38). If this unitarity requirement were found to be violated, it might imply the existence of new interactions such as right- handed weak interactions; an additional generation of quarks and leptons; or the effects of virtual supersymmetric particles that modify the dynamics of the decay. The correlation between the spin axis of a radioactive nucleus and the emission direction of a beta particle or a neutrino can also yield information about possible non-Standard-Model structure of the weak interaction. Neutrons are neutral particles heavy enough that they are unstable: A free neutron can decay with a half-life of about 10 minutes into a proton, electron, and antineutrino. This decay process makes the neutron a microlaboratory for the study of the weak interaction. When combined with the results of neutron decay correlations, the lifetime of the free neutron provides an independent test of CKM unitarity. The neutron lifetime itself is also one of the key inputs in big bang nucleosynthesis that provides a framework for explaining the abundance of the light elements hydrogen, deuterium, helium-3, helium-4, and lithium-7 in the universe. Notwithstanding its importance, to make a precise measurement of the lifetime at better than the desired one part in a thousand level is very challenging. Improved measurements are needed. Much more can be learned from a careful study of neutron beta decay. The correlations between the measurable quantities—namely, the neutron spin direction, the emission directions of the electron and the neutrino, the electron spin direction, and the electron energy spectrum—each illuminate a different facet of Standard Model predictions that may disclose the influence of NSM
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140 Nuclear Physics nuclear meson decay with new symmetry-breaking corrections: 0.9996(7) FIGURE 2.38 Experimental evidence supporting the requirement that the Cabibbo-Kobayashi-Mas- kawa matrix is unitary, as predicted by the Standard Model. The three terms should add up to exactly 2-38.eps one, which they do within experimental uncertainty. The colored bars indicate the contributions to the uncertainty from each term, and the yellow dot is type, mask, ruling bitmap with some vector the central value. The steady reduction of the uncertainty over the years comes from a worldwide decay-spectroscopy effort, involving rare isotope research at laboratories in the United States, Canada, and Europe, coupled with theoretical advances in calculating the radiative and isospin-breaking corrections. SOURCE: Courtesy of G. Savard, ANL, and J.C. Hardy, Texas A&M University. physics. A vigorous worldwide program of precise weak decay studies aims to achieve significant improvements in sensitivity. It involves ongoing studies of the superallowed nuclear decays at ANL, Texas A&M University, TRIUMF, Jyvaskyla, ISOLDE, and Munich, and in the future FRIB, where rare, unstable isotopes will provide enhanced sensitivity for testing the theory of correction terms. Improved measurements of the neutron lifetime and neutron decay correlations are planned at the Institut Laue-Langevin, the Los Alamos Neutron Science Cen- ter (LANSCE), NIST, TRIUMF, Munich, and the Fundamental Neutron Physics Beamline (FNPB) at the SNS. Sterile Neutrinos The unitarity of the CKM matrix supports the conclusion that the known quarks are the only ones that exist in nature. In the neutrino world, however, there are intriguing indications that the three known flavors may be accompanied by
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Science Questions 141 other, so-called sterile neutrinos that mix slightly with the known neutrinos. Data from the Liquid Scintillator Neutrino Detector at Los Alamos, from a number of short-baseline reactor oscillation experiments, from the MiniBooNE neutrino oscillation search at Fermilab, and from radioactive-source tests of the Soviet- American Gallium Experiment (SAGE) and Gallex/Gallium Neutrino Observa- tory (GNO) solar neutrino detectors all appear to exhibit small deviations from the three-neutrino expectation. A consistent interpretation has been elusive. The results are limited by statistical and systematic uncertainties, pointing to a need for new tests and theoretical work. The low-energy solar neutrino spectrum remains imprecisely known, and the ongoing measurements by the Borexino experiment in Italy as well as new experiments being designed would permit a comparison between the sun’s energy production and its neutrino production. Carried out with sufficient precision, the comparison would be a test of both neutrino unitarity and of our understanding of solar energy generation. Despite the general success of solar models, there are small but significant discrepancies related particularly to the abundance of elements heavier than helium, and new experiments could also provide insight into this problem. Weak Interactions Between Nucleons The same weak interaction that gives rise to beta decay also contributes to the force between quarks and therefore between nucleons. Its magnitude is tiny (10–14) by comparison with the strong force, but it discloses its presence through parity violation because the strong force respects parity. Highly sensitive experiments reveal its presence unequivocally, but one particular part of the weak interaction between nucleons, in which a pion is exchanged, has defied experimental and theoretical quantification. New experiments are under way to try to observe the parity-violating rotation of neutron spin as neutrons pass through matter and a possible preference in spin direction as neutrons are captured by protons. At the same time, advanced lattice-gauge theory is being applied in the hope of achieving a theoretical understanding of the apparent suppression of this part of the force. Theory is currently limited by existing computational resources. Weak Interactions of Electrons A somewhat complementary avenue involves the measurement of parity-vio- lating (PV) asymmetries in the scattering of longitudinally polarized electrons from nuclei or from other electrons. Historically, the measurement of such an asymmetry in deep inelastic scattering from deuterium at SLAC played a key role in confirming the fundamental prediction of the Standard Model that there were neutral weak interactions. Indeed, two more accurate versions of this classic experiment, the
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142 Nuclear Physics Parity Violation in Deep Inelastic Scattering (PVDIS) and the PVDIS-SOLID, are planned at JLAB. After the initial work at SLAC, PV electron scattering was used with great success to probe the contributions of strange quarks to the nucleon’s electromagnetic properties through a program of measurements at MIT-Bates, the Mainz MAMI facility, and the CEBAF beam at JLAB. A measurement of the PV asymmetry in Moller scattering (in which polarized electrons are scattered from unpolarized ones) performed at SLAC yielded the most precise determination of the dependence of the weak mixing angle on energy scale, one of the more dra- matic and novel predictions of the Standard Model. The weak mixing angle, or Weinberg angle, is a parameter of the Standard Model that defines (among other things) the extent to which interactions mediated by the Z boson violate parity. A still more precise version of this experiment, Moller, is planned at JLAB after the completion of the energy upgrade. It would complement another PV experiment, Q-weak, currently under way at JLAB involving elastic scattering from a proton target. Together, the comparison of results of purely leptonic (Moller scattering) and semileptonic (electron-proton scattering) experiments can provide a powerful test of the Standard Model. Muon Decay The properties and decays of muons—structureless particles like electrons but with a mass about 200 times greater—are among the most sensitive probes of the Standard Model. The example of the muon anomalous moment was already described above. In addition, nuclear physicists have recently reported new results on the correlation parameters in muon decay, the muon lifetime, and muon capture in hydrogen, which have improved previous experimental values by factors of 10 or more. The muon lifetime, now determined to part-per-million accuracy, defines the strength of the weak interaction. These new results give the tightest limits now available on interactions beyond those in the Standard Model. Over the next decade, new measurements with muons will continue to push the precision frontier. An experiment to search for the decay of a muon into an electron and photon with a 100-fold better sensitivity than previous measure- ments is under way now at the Paul Scherrer Institute (PSI). This conversion is essentially forbidden in the Standard Model but is predicted to occur in certain proposed theoretical extensions. Two experiments—a new, even more precise measurement of the anomalous moment of the muon and a sensitive search for the conversion of a muon to an electron in the field of a nucleus—are being planned by collaborations of high-energy and nuclear physicists for Fermilab following its intensity upgrade.
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Science Questions 143 Pion Decay Like the neutron, the pion is a composite particle that undergoes beta decay, but because of its large mass, it can decay to either an electron or a muon with an associated neutrino. The relative decay probabilities of charged pions into a muon or an electron have provided stringent (better than 0.1 percent) tests of the lepton universality property of the Standard Model weak interaction, which simply states that the weak interaction acts with the same strength in every family of elemen- tary particles. With the operation of the LHC and its prospective sensitivity to the existence of new particles with multi-TeV masses, the forefront sensitivity for many of the weak decay studies will rise to 1 part in 10,000 within the next decade. Two new pion beta decay measurements are planned at TRIUMF and PSI. Two Challenges Certain experimental efforts in nuclear physics are motivated by specific expec- tations for the physics that the NSM is likely to display. Two specific research thrusts having great discovery potential are searches for the permanent electric dipole moments (EDMs) of the nucleon, neutral atoms, and charged leptons and searches for the neutrinoless double beta decay of heavy nuclei. Hand in hand with these experimental initiatives is a focused program of theoretical nuclear physics studies that aim to interpret the results of these and other experiments in terms of the NSM. Search for a Permanent Electric Dipole Moment The goal of the EDM searches is to discover a mechanism for the violation of CP symmetry (or time-reversal symmetry) beyond the CP violation that can be accounted for by the Standard Model weak interactions. The reason is that an explanation of the excess of matter over antimatter in the present universe requires the existence of a not-yet-understood source of CP violation in the early universe. Perhaps it may be found in the neutrinos, as we consider below. Alternatively, if the matter-antimatter asymmetry was produced when the universe was roughly 10 picoseconds old—during the era of so-called electroweak symmetry breaking— then the next generation of EDM experiments would have a good chance of observ- ing it. EDM searches look for a small shift in the precision frequency of a quantum system with spin (such as the neutron) in the presence of electric and magnetic fields. An EDM violates both parity (P) and time-reversal (T) symmetry, but not the matter-antimatter symmetry C. In addition to uncovering the CP violation needed to explain the matter-antimatter asymmetry, the EDM searches could also
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144 Nuclear Physics reveal the presence of CP violation in the strong interaction. The present limits on the latter are so stringent as to imply the possible existence of another symmetry, known as Peccei-Quinn symmetry, a symmetry invoked specifically to explain why the CP symmetry is not violated in the strong interactions at more than about 10–10. The violation of this symmetry in such a way as to lead to a nonvanishing EDM would imply the existence of a new particle called the axion. If it exists, the axion itself could also make up the cosmic dark matter. The next generation of EDM searches is expected to improve the level of sensitivity by up to two orders of magnitude over present limits. Intensive efforts to reach this level of sensitivity are under way in the United States, Canada, and Europe. They include searches for (1) the neutron EDM at the Fundamental Neutron Physics Beamline at the Oak Ridge SNS, the Institut Laue-Langevin in Grenoble, and the Paul Scherrer Institute in Switzerland; (2) the atomic EDMs of mercury, radium, radon, and xenon at various laboratories and universities; and (3) the EDM of the electron, using molecular or solid-state systems, in the United States and Europe. In addition, nuclear scientists at BNL are developing a possible measurement of the proton EDM using a storage-ring technique. The “physics reach” of these searches expressed in terms of the mass of new, pres- ently unknown particles is in many cases at a scale beyond that accessible at the LHC. The LHC gets its sensitivity by directly trying to produce and detect new particles involved in CP-violating interactions, while the experiments that are the main subject of this paragraph look for the effects of the same interactions at much lower energies by seeking rare effects induced by quantum fluctuations. The present EDM limits generically imply that the mass scale of any new CP- violating interaction (in other words, the mass of some new particle that could mediate the interaction) is greater than several TeV, and improvements by two orders of magnitude would extend this scale by a factor of 10, well beyond the scale accessible at the energy frontier. Search for Neutrinoless Double-Beta Decay Because neutrinos lack electric charge they can in principle be their own anti- particles. Whether some symmetry preserves a distinction between matter and antimatter for neutrinos is presently unknown. The answer to this question may be at the heart of why the universe contains matter and essentially no antimatter, because the violation of total lepton number could be associated with the genera- tion of the matter-antimatter asymmetry at times much earlier than 10 picoseconds after the big bang. It is also a question that needs an answer for the construction of the NSM, because it leads to a novel mechanism for the generation of particle mass, one that does not exist in the Standard Model. The only practical experimental approach to this problem is the search for
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Science Questions 145 neutrinoless double beta decay. The pairing property of the nuclear force leads to a number of nuclei that are stable against all decay modes except the simultane- ous emission of two electrons and two antineutrinos. The process, while allowed, rarely occurs. Of approximately 10 examples known, the shortest half-life is still a billion times longer than the age of the universe. If neutrinos and antineutrinos are the same particle, then the decay can proceed with the emission of just the two electrons and no neutrinos—that is, neutrinoless double beta decay. That process has not yet been seen, with lifetime limits some 104 times longer still than the two- neutrino mode. A major experimental attack on this problem, calling ultimately for detectors containing a ton or more of an enriched isotope, is a priority in nuclear physics. In addition to answering the question of whether a neutrino is the same as an antineutrino, a Majorana particle, or is different, a Dirac particle, a positive observation would help to define the mass of neutrinos. As with the EDM experiments, there exists a worldwide program of searches for neutrinoless double beta decay. U.S. nuclear scientists are involved in several of these efforts, including the CUORE experiment at Gran Sasso, the EXO experi- ment at the Waste Isolation Pilot Plant (WIPP) in New Mexico, the Majorana Demonstrator Project at the Sanford Underground Laboratory, SNO+ at SNOLAB, and KamLAND-Xen at Kamioka. Majorana neutrinos with masses in the presently allowed range may produce a signal in these experiments. If necessary, larger and more ambitious experiments using enriched isotopes could improve the sensitivity substantially. A next-generation ton-scale neutrinoless double beta decay experi- ment could be carried out 7,400 feet down in the Sanford Underground Research Facility (SURF) in the Homestake mine in Lead, South Dakota. Nuclear Theory at the Precision Frontier For these experimental efforts at the precision frontier, input and guidance from nuclear theory is vital. For example, interpreting the results of EDM searches in terms of a new mechanism for CP violation and relating the latter to the cosmic matter-antimatter asymmetry requires a web of nuclear theory computations along with calculations from cosmology and astrophysics. Starting from the computation of low-energy matrix elements in strongly interacting systems such as the neutron or mercury nucleus, one must then derive values for the parameters of an under- lying model at the elementary particle level, taking into account the constraints from studies at the high-energy and astrophysical frontiers. Computations of the matter-antimatter asymmetry require calculations analogous to those performed when interpreting the results of relativistic heavy ion collisions. A similar chain of theoretical analyses is needed to interpret the neutrinoless double-beta decay results, as well as those from weak decays and PV electron scattering, in terms of the structure of the NSM. The increasing scope of the experimental effort in this
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146 Nuclear Physics area of nuclear science calls for concomitant increases in the related theoretical effort as well as advances in computational tools. Connections with Cosmology Much remains to be understood about neutrino mass and mixing, and new experiments are under construction or in operation. Oscillations set a lower limit on the mass, but other techniques are required to determine the actual magnitude of neutrino mass. The mass of the lightest neutrino cannot be less than zero, and it follows from oscillation data that the sum of the three masses must be at least 0.06 eV. An upper limit that is independent of assumptions about the properties of neutrinos comes from laboratory measurements of the shape of beta spectra near the end point. There the electron energy approaches the maximum value for the decay, which is limited by the rest mass of the accompanying neutrino. Experi- mental measurements of the shape of the tritium beta spectrum yield an upper limit on the sum of the three masses of 6 eV, setting a range of 0.06 to 6 eV in which the mass sum must lie. A new, large-scale tritium experiment, the KArlsruhe TRItium Neutrino (KATRIN) experiment, is under construction that will have 0.6 eV sensitivity. New ideas for extending the sensitivity of beta experiments are being explored should the mass sum turn out to be smaller than 0.6 eV. There is a strong prediction, but not yet direct experimental proof, of a cosmo- logical relic neutrino background. The energies of these neutrinos are so low that detecting them appears all but impossible. However, cosmological arguments also relate the large-scale structure in the universe to neutrino mass. These arguments are model-dependent, being sensitive to the equation of state of dark energy and to the power spectral index that describes how quantum fluctuations in the big bang were distributed in scale. For reasonable assumptions, they limit the mass sum to about 0.6 eV or less. The ESA Planck satellite, launched in 2009, together with new galaxy surveys, may be able to extend the sensitivity to about 0.1 eV. A laboratory measurement at this level would be the most direct laboratory confirmation of the existence of the relic neutrino background that can presently be envisaged and would subject cosmological models to an important test. Overwhelming evidence from observational astronomy for the existence of dark matter demands an understanding of its particle nature. Neutrinos are now known to be insufficiently massive, and no other known Standard Model par- ticle can explain the data. Many candidates have been advanced, of which two are strongly motivated by theoretical considerations outside of astronomy. The lightest neutral particle in theories such as supersymmetry would be long-lived or stable and could have the mass (still many times the proton mass) and the inter- action cross section to be the dark matter. Alternatively, a new symmetry would explain why CP is so well conserved in the strong interactions and would imply
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Science Questions 147 the existence of a very light, long-lived particle, the axion, which could also be the dark matter. Detection of the former type of particle, the weakly interacting massive particle (WIMP), might be achieved by observing the recoil energy imparted to a nucleus struck by a WIMP present in the galactic dark-matter cloud. The energies are small, the interactions are rare, and the backgrounds present significant challenges, but there has been steady progress toward achieving the necessary sensitivity and redundant criteria for identification. Nuclear physics techniques are widely used in this field, and nuclear physicists have much to contribute; indeed, there is great enthusiasm in the nuclear physics community for addressing this challenge. Underground Science Key parts of the experimental program in fundamental symmetries and neu- trino astrophysics demand an underground location shielded from the steady rain of cosmic rays that arrive at Earth’s surface. The signals from solar neutrinos, supernova neutrinos, and geoneutrinos, from neutrinoless double-beta decay, and from dark-matter particles are so rare that the cosmic ray background at Earth’s surface overwhelms them. Deep underground, the flux of energetic muons, the most penetrating cosmic ray particle other than neutrinos, decreases by a factor of about 10 for each 300 m. The deepest underground research laboratory today is SNOLAB in Canada, where the SNO experiment was carried out at a depth of 2,000 m. A smaller but even deeper laboratory is being commissioned at Jinping in China. Many countries have deep underground research laboratories: the Gran Sasso National Laboratory in Italy at an effective depth of 1,300 m is the largest in the world. In the United States, Ray Davis’s experiment on solar neutrinos, for which he shared the 2002 Nobel prize, was carried out 1,600-m down in the Homestake gold mine in South Dakota. Other research sites in the United States include WIPP and the Soudan mine in Minnesota, where neutrinos from Fermilab are detected. Both are about 700 m deep and are confirming the atmospheric neutrino signal and providing increasingly precise data on the “atmospheric” mass-squared splitting. The priority of the research goals that need underground space, including neutrinoless double beta decay, dark matter searches, and solar neutrino phys- ics, prompted the National Science Foundation to solicit proposals for a science program and a laboratory. Eight sites were proposed, and the Homestake mine was selected for the final design and facility proposal. The owners of the mine had decided in 2000 to terminate commercial operations there. Once closed, the mine flooded and required rehabilitation. The importance of the science and its location in South Dakota attracted private funding in excess of $70 million, unprecedented in the field of nuclear and particle physics. That funding, with additional support
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148 Nuclear Physics from the state of South Dakota, was used to prepare surface facilities for research and to rehabilitate the mine to a depth of 1,300 m. In the interim, the field of high- energy physics became increasingly interested in this area of research and developed plans for a neutrino beam that would originate at Fermilab, 1,300 km to the East. At Homestake, a very large detector would make possible studies of the neutrino mass hierarchy (that is, the ordering of the mass eigenstates by increasing mass), the possible violation of the CP symmetry in neutrinos, and searches for proton decay. If CP violation is observable among neutrinos, it is a tantalizing possibility for explaining why the universe contains mostly matter and not much antimat- ter. There are both logistical and intellectual advantages for nuclear and particle physicists to collocate in the Sanford Underground Research Facility at Homestake.5 However, at the time this report was being prepared, the future of that facility and the experiments planned for it were uncertain. Fundamental Symmetries Studies in the United States and Around the World The field of fundamental symmetries is a microcosm for some of the difficulties encountered in managing science in the United States and elsewhere because it does not always fit comfortably within the mainstream of a field. The questions often call for exploration of physics that lies at the interface between two or more disciplines. The physics outcomes are often highly uncertain when a project is starting up. Many nations have elected to organize a separate research field that, broadly speaking, encompasses particle and nuclear physics, high-energy astrophysics, and cosmology. In the United States, the core disciplines of nuclear physics, particle physics, astronomy, and space sciences have been preserved at the federal agency level and are the homes for investigations in the interface areas often explored in the area of fundamental symmetries. Agency decisions on which discipline area will consider funding an investigation may appear to be arbitrary, but there has been a commendable effort to be flexible and to prevent research from falling into the cracks. Nevertheless, in the competition for scarce resources, core studies in a particular discipline area are likely to enjoy the home-arena advantage when competing against studies that might arguably belong to another discipline. The European approach, forming a separate discipline area, is one solution, but there is also merit in the continuous competition between research at the core of a discipline and research at its boundaries. From such competition, the center of a discipline can begin to shift. 5 NRC, 2012, An Assessment of the Science Proposed for the Deep Underground Science and Engineering Laboratory (DUSEL), Washington, D.C.: The National Academies Press.
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Science Questions 149 The Workforce The field of fundamental symmetries and neutrinos serves as a magnet for attracting new talent into physics and its related disciplines. Scientists young and old find the questions at once grand and simple. Motivation does not need to be accompanied by specialized knowledge at the beginning. University physics faculties as well are enthusiastic about the potential of the field for discovery and about the fact that the basic concepts are easily communicated to students and to colleagues. As a result, departments have hired new faculty working in this field. The experimental tools and expertise lie at the field’s boundary with particle, atomic, and molecular physics. For students, the field provides exposure to a vari- ety of experimental and theoretical techniques and the opportunity to work at the interface of several disciplines. That breadth of experience is attractive to future employers whether in academe, at the national laboratories, or in industry.