Nuclear Physics Exploring the Heart of Matter (2013) / Chapter Skim
Currently Skimming:
2 Science Questions
2 Science Questions
Pages 30-149
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 30... ...
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)
|
From page 31... ...
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.
|
From page 32... ...
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
|
From page 33... ...
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.
|
From page 34... ...
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.
|
From page 35... ...
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 Astrophysics." Figure 2.4 shows the calculated shell energy -- that is, the quantum enhancement 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 associated closed shells provide exceptional stability.
|
From page 36... ...
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)
|
From page 37... ...
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 constraints 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.
|
From page 38... ...
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.
|
From page 39... ...
What is the structure of neutron stars, and what determines their electromagnetic, neutrino, and gravitationalwave 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.
|
From page 40... ...
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)
|
From page 41... ...
Precise data from PREX would provide constraints on the neutron pressure in neutron stars at subnuclear densities. Important insights come from experiments with cold Fermi atoms that can be tuned to probe strongly interacting fluids that are very similar to the lowdensity neutron matter found in the crusts of neutron stars (see Box 2.2)
|
From page 42... ...
. The measured half-lives of new superheavy nuclei were observed to increase with larger neutron number.
|
From page 43... ...
Science Questions 43
|
From page 44... ...
bitmaps with some vector type and the allied European detector Virgo will help understanding large-scale motions of dense neutron rich matter. Finally, advances in computing hardware and computational techniques will allow theorists to perform calculations of the neutron star crust.
|
From page 45... ...
Probing Nuclear Shapes by Rapid Rotation Gamma-ray spectroscopy is a basic tool for studying nuclear structure, shapes, and their changes -- both from the energies and decay paths of excited nuclear states and by measuring nuclear level lifetimes from Doppler effects. Recently, a great diversity of phenomena has been discovered as increasingly sensitive instrumentation reveals unexpected behavior in our quest to observe higher excitation energies and angular momentum states in nuclei.
|
From page 46... ...
The transition observed in strongly interacting cold fermionic atom clouds from paired superfluid states, analogous to superconducting electrons in a metal, to BEC states of molecules consisting of two fermion atoms, captures certain aspects of the transition from a quark-gluon plasma to ordinary hadronic matter made of neutrons, protons, and mesons. Superfluid pairing in low-density strongly interacting fermionic atomic systems is very similar to that pairing in low-density neutron matter in neutron stars.
|
From page 47... ...
And, strongly interacting clouds of atoms with differing densities of up and down spins, as can be engineered in optical traps, share some common features with strongly interacting quark matter with differing densities of up, down, and strange quarks. In both contexts, superfluid pairing gaps that are modulated in space in a peri odic pattern may develop, yielding a superfluid and crystalline phase of matter, hints of which may have been seen in very recent cold atom experiments.
|
From page 48... ...
Lower panel: This timeline, and the rapid development of more sophisticated instrumentation, are further echoed here, where the intensity of a particular gamma-ray transition (normalized to unity for transitions between low angular momentum states) between specific energy levels, as the nucleus de-excites, is plotted as a function of spin.
|
From page 49... ...
Science Questions 49
|
From page 50... ...
Because of finite-size effects and different polarization effects in nuclei and nuclear matter, a theoretical challenge will be to relate experiments on nucleonic superfluidity in finite nuclei to pairing fields in neutron stars (see Box 2.2)
|
From page 51... ...
Such a framework would allow for more accurate predictions of the nuclear processes that cannot be measured in the laboratory, from the creation of new elements in exploding stars to the reactions occurring in cores of nuclear reactors. Developing such a theory requires theoretical and experimental investigations of rare isotopes, new theoretical concepts, and extreme-scale computing, all carried out in partnership with applied mathematicians and computer scientists (see Box 2.3)
|
From page 52... ...
program, where distributed resources and expertise are combined to address complex ques tions and solve key problems.1 In each partnership, mathematicians and computer scientists are collaborating with nuclear physicists to remove barriers to progress in nuclear structure and reactions, QCD, stellar explosions, accelerator science, and computational infrastructure. Computational resources required for these calculations are currently obtained from a combina tion of dedicated hardware facilities at national laboratories and universities, and from national leadership-class supercomputing facilities.
|
From page 53... ...
Science Questions 53 Hot and Dense QCD Transport in QCD (quenched) QCD critical point Quarkonium spectroscopy QCD at T>0 High-T limit of QCD EOS Continuum extrapolated QCD EOS Cold QCD Nucleon Spin Alpha particle Nuclear force Gluon distributions Deuteron Excited hadronspectrum Neutron EDM Nuclear Structure Light nuclei Weakly bound nuclei 0ν ββ rates for 48Ca Light ion reactions Neutron induced fission Triple α process Dynamics of neutron star crust Nuclear Astrophysics Global solar model Precision nuclear network Multienergy neutrino transport Precision neutrino network 3D supernova Accelerator Physics Isotope separator optimization Energy Recovery Linac Electron-cooling design 6D Vlasov 10-1 1 10 102 103 Petaflop-Yrs on Task 2-03-01_Computing.eps FIGURE 2.3.1 Estimates of the computational resources required to make breakthrough pre dictions in key areasbitmaps physics: hot and type and rules of hadrons, nuclear of nuclear with vector dense QCD, structure structure and reactions, nuclear astrophysics, and accelerator physics.
|
From page 54... ...
is a fundamental problem that bridges hadron physics and nuclear structure. While excellent progress has been made in this domain (see the section "The Strong Force and the Internal Structure of Neutrons and Protons")
|
From page 55... ...
Science Questions 55
|
From page 56... ...
While the main energy source of core collapse supernovae and long gamma-ray bursts is gravity, nuclear physics triggers the explosion. Neutron stars are giant nuclei in space, and short gamma-ray bursts are likely created when such gigantic nuclei collide.
|
From page 57... ...
A few key isotopes in the reaction sequence of the rapid neutron capture process (r-process) responsible for the origin of heavy elements in nature have now been produced by rare isotope facilities.
|
From page 58... ...
the study of unstable isotopes that exist in large quantities inside neutron stars and are copiously produced in stellar explosions but difficult to make in laboratories and (2) the determination of extremely slow nuclear reaction rates, which are important for the understanding of stars.
|
From page 59... ...
Existing X-ray observatories will be complemented with new facilities that push observations toward harder X-rays and possibly gamma-rays and will provide new data on neutron stars and stellar explosions. New-generation gravitational wave detectors are expected to detect signals from supernovae and neutron stars for the first time.
|
From page 60... ...
Such interdisciplinary research networks are also needed to attract and educate the next generation of nuclear astrophysicists, who, with emerging new facilities in nuclear physics, astrophysics, and high-performance computing, are likely to make transformational advances in our understanding of the cosmos. Origin of the Elements The complex composition of our world -- some 288 stable or long-lived iso topes of 83 elements -- is the result of an extended chemical evolution process that started with the big bang and was followed by billions of years of nuclear process ing in numerous stars and stellar explosions (see Figure 2.12)
|
From page 61... ...
How were the first heavy elements created by the potentially extremely massive stars formed after the big bang? The pattern of the elements ejected in their deaths might still be observable today in the most iron-poor stars of the galaxy, survivors of an early second generation of stars.
|
From page 62... ...
Addressing this problem will remain a formidable challenge in the coming decade. Advances in experimental techniques such as high-intensity stable beam accelerators in underground laboratories, intense rare isotope beams, and advanced detection and target systems will be needed (see Figure 2.14)
|
From page 63... ...
SOURCE: Courtesy of Richard Cyburt, Michigan State University. side, ab initio calculations of nuclear reactions and models that account for cluster structures in nuclei are particularly promising guides for predicting reaction rates at the energies nuclei have in stars.
|
From page 64... ...
Most of these exotic nuclei have never been made in the laboratory. This will change with the advent of next-generation rare isotope beam facilities like FRIB, which will allow experimental nuclear physicists to produce such nuclei and to determine their properties.
|
From page 65... ...
The most promising ones involve core collapse supernovae and the merging of two neutron stars. As a breakthrough, observations of the surface composition of iron-poor stars have opened an unprecedented window into the gradual enrichment of the early galaxy with r-process elements.
|
From page 66... ...
However, to measure neutron captures on these unstable nuclei, radioactive beam facilities will have to work in concert with neutron beam facilities, where radioactive samples can be quickly irradiated to measure neutron capture rates. Where this is not possible, experimenters and theorists will have to develop new indirect techniques to extract the relevant information from other types of nuclear reactions.
|
From page 67... ...
In the coming decade it will have to be determined if the np-process and the light element primary process are the same, what their contributions to the chemical evolution of the galaxy are, and what the underlying nuclear physics is. The np-process involves extremely neutron-deficient rare isotopes, which need to be studied at rare isotope facilities.
|
From page 68... ...
that have been synthesized by nuclear reactions in the progenitor star and during the supernova explosion and have now been ejected into space. The supernova remnant contains a neutron star that has been formed in the supernova explo sion and can be observed as a radio pulsar.
|
From page 69... ...
Such simulations succeeded in predicting explosions of lighter massive stars, albeit not always with the observed features. These explosions were in most cases achieved with the so-called delayed explosion mechanism, where the explosion energy is provided by the strong flux of neutrinos emerging from the compressed, hot stellar core that ultimately becomes a neutron star.
|
From page 70... ...
Neutrino interactions with exotic phases of nuclear matter during the collapse, for example with "nuclear pasta" (described in more detail in the subsection "Neutron Stars") , might also play a role, as does inelastic neutrino scattering on nuclei, which affects neutrino energy spectra.
|
From page 71... ...
Thermonuclear energy is often released in reactions with rare isotopes that do not exist on Earth but are produced under the extreme temperatures and densities arising during the explosion. X-Ray Bursts: Can They Be Used as Probes of Accreting Neutron Stars?
|
From page 72... ...
the composition of the ashes that accumulate on the neutron star surface over time and affect many neutron star observations (see the subsection "Neutron Stars")
|
From page 73... ...
Unfortunately the rates of a few key reactions that produce such heavier elements in novae are still unknown, preventing a quantitative interpretation of these observations. The reaction rates that are particularly difficult to measure involve rare isotopes and will challenge rare isotope experimentation in the coming decade.
|
From page 74... ...
Measuring the basic properties of neutron stars like mass, radius, and cooling rates is one way to constrain the nature of nuclear matter. Progress in astronomy in the last decade has yielded a range of neutron star masses from timing observations of pulsars.
|
From page 75... ...
Future gravitational wave observations of neutron star mergers, giant flares, and continuous wave emission from spinning neutron stars have the potential to directly probe the properties of matter in the interior of the star. How Do Rare Isotope Crusts Shape Neutron Star Observations?
|
From page 76... ...
The thin crust is mostly made of neutron rich rare isotopes, while the interior consists chiefly of neutrons with small admixtures of protons, electrons, muons, hyperons, and other particles. At the extreme densities in the core, other forms of nuclear matter, such as a quark gluon plasma, might exist.
|
From page 77... ...
The actual crust composition of such mass-accumulating neutron stars depends on the ashes from thermonuclear explosions on the surface, on the rate of electron capture by nuclei, on the properties of the neutron-rich nuclei produced by these captures, and on the rate of a special class of fusion reactions that occur at high density. Fundamental insight into these questions can be gained with the next generation of rare isotope accelerator facilities, which will be able to produce many of the rare isotopes in neutron star crusts.
|
From page 78... ...
The solution of the solar neutrino problem through neutrino detection and through precision laboratory measurements of the nuclear reaction rates powering the sun is a triumph of nuclear astrophysics (see the section "Fundamental Sym metries")
|
From page 79... ...
The extreme numbers of neutrinos streaming out of the hot core of a developing supernova interact with nuclei in the outer shells of the star about to explode. These interactions happen sufficiently frequently to alter the composition significantly, creating a set of rather rare isotopes of boron, fluorine, lanthanum, and tantalum.
|
From page 80... ...
, and reactions where electrons scatter off nuclei probe some of the nuclear physics that determines the rate of neutrino nucleus interactions. These approaches have been successfully used in the past and are expected to be applied to cases of inter est in the coming decade at stable and rare isotope beam accelerator facilities.
|
From page 81... ...
The liquid becomes more "perfect" as the coupling gets stronger, since as the distance that particle-like excitations can travel decreases, a hydrodynamic description becomes ever more accurate and the role of dissipation in damping the flow becomes ever smaller. QGP is a good example of a strongly coupled liquid.
|
From page 82... ...
Gases have imperfection indices in the thousands. The imperfection index is defined as 4πη/s because it is equal to one in any of the many very strongly coupled plasmas that have a dual description via Einstein's theory of general relativity extended to higher dimensions.
|
From page 83... ...
At present, it is not clear whether the qualitative successes of gauge/gravity duality as applied to quark-gluon plasma are just that, or whether they are a sign that QGP itself has a dual gravity description. If the latter were to be the case, quantitative understanding of QGP properties could one day teach us not only about other strongly coupled fluids in condensed matter and atomic physics but also about the nature of the quantum gravitational theory dual to QCD.
|
From page 84... ...
Are very high energy quarks or very heavy "bottom quarks" weakly coupled to the fluid or do they rapidly become part of the soup? Experiments at RHIC and lattice QCD calculations both indicate that as QGP cools, the reassembly of quarks and gluons into hadrons takes place over a broad temperature range.
|
From page 85... ...
One of the inputs to ideal hydrodynamics is an equation of state, which is taken from numerical calculations of QCD thermodynamics on a discrete space-time lattice. The other input is the assumption of a perfect liquid FIGURE 2.21 When the almond-shaped initial geometry in an off-center heavy ion collision at RHIC expands as a nearly perfect fluid, it explodes with greater momentum in the direction in which the initial almond is narrowest.
|
From page 86... ...
Jet quenching refers to a suite of experimental observables that together reveal what happens when a very energetic quark or gluon plows through the strongly coupled plasma. It should be noted that these energetic particles are not external probes; they must be produced within the same collision that produces the strongly coupled plasma itself.
|
From page 87... ...
How much energy is radiated in gluons is determined by a single material property of the strongly coupled liquid, called the jet quenching parameter, which Number of energetic particles Jet quenching at RHIC 0.2 p+p d+Au Au+Au 0.1 0 -1 0 1 2 3 4 5 (radians) FIGURE 2.22 The extinction of the away-side jet.
|
From page 88... ...
It is nevertheless interesting that the jet quenching parameter seems to be larger than it would be in weakly coupled QGP and is comparable to that in the strongly coupled QGP found in QCD-like theories obtained via gauge/gravity duality. QGP Shining Brightly Experimenters at RHIC have recently achieved the long-standing goal of see ing the light (ordinary photons)
|
From page 89... ...
Novel Particle Production Mechanisms Evidence that the droplets of matter formed in heavy ion collisions at RHIC are composed of collectively flowing quarks that are not bound up into protons and neutrons ("deconfined" quarks) comes from detailed measurements of a wide variety of particle species, which reveal surprising patterns in heavy ion collisions
|
From page 90... ...
produced in heavy-ion collisions at RHIC (left-hand panel)
|
From page 91... ...
Finally, it is also worth noting that the RHIC program and its exciting science discoveries have attracted outstanding young physicists into nuclear physics, further cementing its impact. Quantifying QGP Properties and Connecting to the Microscopic Laws of QCD and Its Macroscopic Phase Diagram In the coming decade nuclear physicists will perform experiments at both RHIC and LHC to address the new scientific questions raised by the RHIC discoveries.
|
From page 92... ...
A map of the expected QCD phase diagram (Figure 2.25) predicts that the continuous crossover currently being explored in heavy-ion collisions at the highest RHIC energies will become discontinuous if the excess Early Universe Temperature The Phases of QCD LHC Experiments RHIC Experiments RH IC En er Quark-Gluon Plasma gy Sc a n Crossover ~170 MeV Future FAIR Experiments 1 st or de r ph as et ra n Critical Point sit ion Color Hadron Gas Superconductor Nuclear Vacuum Matter Neutron Stars 0 MeV 0 MeV 900 MeV Baryon Chemical Potential FIGURE 2.25 The phase diagram of QCD is shown as a function of baryon chemical potential (a measure of the matter to antimatter excess)
|
From page 93... ...
Yet data from RHIC on heavy quarks identified indirectly via the isolated electrons produced in their decays show that these "bowling balls" feel the strongly coupled QGP just as much as light quarks do. They not only lose energy, they also seem to be dragged along by the expanding fluid, behaving just like another component of the fluid.
|
From page 94... ...
Neutron stars are magnificent laboratories within which various phases of dense matter are found. Just below the surface of a neutron star, increasingly neutron-rich nuclei form a rigid crust until a critical density of 4.3 × 1011 g/cm3 is reached.
|
From page 95... ...
/ star inner crust Crab Nebula of neutron star 0.6 neutron drip line clustering in neutron skin 0.3 super novae finite nuclei proton rich heavy ion collisions gas of with stable beams 0 deuterons and alphas proton drip line -0.3 saturation density nuclear matter density 2-04-01_right matter3.eps bitmaps with vector ruling & shading, some type outlined
|
From page 96... ...
Lower maximum densities leave less room for exotic matter in the interior and thus constrain the ways in which neutron stars can cool by emitting neutrinos. The observation of high-mass neutron stars, especially when combined with future neutron star cooling data, presents a deep challenge to our understanding of high density interacting nuclear matter, and at the same time points to the directions that must be taken to make significant advances in solving this outstanding problem.
|
From page 97... ...
Studying how the strongly coupled liquid responds to an energetic quark or gluon shooting through it represents a third frontier. There are some indications of a "sonic boom" in the fluid, excited by the supersonic projectile, but the relevant features in the data can also arise from event-by-event fluctuations in the initial shape of the collision zone.
|
From page 98... ...
do the smallest mesons known, the Υ mesons, dissolve. Lattice QCD calculations of the screened quark-antiquark potential support this picture, but a definitive experimental confirmation is not yet in hand.
|
From page 99... ...
Currently, reducing the transverse area by selecting off-center collisions necessarily also reduces the energy density. The comparison between uranium-uranium and gold-gold collisions will allow separate control of the size of the QGP droplet and its energy density, allowing for clean studies of the path-length dependence of jet quenching observables, one of the key discriminants between different theoretical calculations that model the energy loss of energetic quarks moving through the QGP.
|
From page 100... ...
They then developed the codes needed to describe the anisotropic expansion of the exploding droplets of QGP produced in heavy ion collisions. An important microphysical input to these calculations is the equation of state, obtained from the lattice QCD calculations discussed above.
|
From page 101... ...
The SB arrows indicate the energy density and pressure of very weakly coupled QGP.
|
From page 102... ...
One is to develop calculations that will allow measuring particle production at the LHC, at forward angles at RHIC, and, it is hoped, at a future electron ion collider to yield a quantitative determination of Qs that will lead to an understanding of the saturated gluonic matter that is predicted to exist inside every nucleus. Another challenge is to understand how the shards released by shattering these classical gluon fields so quickly form the strongly coupled fluid seen at RHIC.3 3 Portions of this paragraph were adapted from the DOE/NSF, Nuclear Science Advisory Committee (NSAC)
|
From page 103... ...
It has long been known that cold dense quark matter, as may occur at the center of neutron stars, must be a color superconductor, as indicated in the lower right section of the phase diagram displayed in Figure 2.25. Theoretical advances during the last decade have made this subject more quantitative and much richer.
|
From page 104... ...
. The equations of state for ordinary nuclear matter that allow for such a heavy neutron star indicate that the density of the nuclei at its center must be roughly five times that of ordinary nuclei.
|
From page 105... ...
While these phenomena are not obvious in the basic equations that describe QCD, they play a principal role in determining the observable characteristics of atomic nuclei. Yoichiro Nambu was awarded a share of the Nobel prize in physics in 2008 in recognition of his contribution to understanding dynamical chiral symmetry breaking.
|
From page 106... ...
Understanding how nuclei and various phenomena emerge from the underlying theory of QCD and discovering the new phenomena that will guide us to a more complete picture is the challenge being addressed by the current and future experi ments in QCD. The Basic Properties of Protons and Neutrons: Spatial Maps of Charge and Magnetism The fundamental composition of the lightest bound states formed by an inter action can be determined by establishing their internal "landscape." With gravity, for example, an understanding of the solar system can be gained through mapping the locations of the planets of as a function of time.
|
From page 107... ...
Science Questions 107
|
From page 108... ...
This is in stark contrast to what had previously been assumed and written in textbooks based on decades of unpolarized electron scattering measurements. This is illustrated in Figure 2.28, where the ratio of the electric to magnetic form factors is shown as a function of the momentum transferred (Q)
|
From page 109... ...
In parallel with the production of polarized targets, the production of spin-polarized nuclei through optical pump ing has been adopted and further developed for other applications such as medical imaging, precision experiments to test fundamental symmetries and few-nucleon systems, and the study of magnetism in materials using polarized neutron scattering. Magnetic resonance imaging (MRI)
|
From page 110... ...
Examples include profile measurements in thin films and multilayered materials, measurements of magnetic moments, measurements of magnetization density distributions in paramagnets and ferromagnets, and measurements of magnetic domain sizes in spin glasses and amorphous magnets. Neutron spin filters based on polarized helium-3 have some significant advantages over more traditional methods of polarizing neutrons, such as the ability to accommodate large divergence beams and to be effective over a wide range of energies.
|
From page 111... ...
Science Questions 111 FIGURE 2.5.2 The use of polarized helium-3 has also led to a new MRI technique for imaging the gas space of the lungs: noble gas imaging. This is a good illustration of the spin-off appli cations of fundamental research.
|
From page 112... ...
Booth, C.R. Hogg, et al., 2010, Physical Review Letters 104: 207203 (2010)
|
From page 113... ...
Science Questions 113
|
From page 114... ...
. One can see the appar ent development of an electric dipole moment in the y direction. The dipole behav ior is more evident for the neutron since it has no charge, therefore no "monopole" contribution, which acts like a background. This phenomenon is entirely due to the interplay of special relativity and the internal structure of nucleons.
|
From page 115... ...
Numerical lattice QCD calculations of the structure of nucleons, starting from the fundamental degrees of freedom of quarks and gluons on a spatial and temporal grid, have been ongoing for more than two decades. While the calculations have not been able to completely control all systematic errors, they have provided important insight into hadron structure.
|
From page 116... ...
Combining the weak interaction data with the precision electromagnetic data makes it possible to disentangle the pieces of the proton's charge and magnetism coming from up, down, and strange quarks. The most sensitive of such experiments have been carried out at JLAB, but the combination of all the data taken within the last decade from JLAB, the Mainz Microtron facility in Germany, and MIT's Bates Laboratory has now very tightly constrained the possible contributions from strange quarks to a less than 5 percent contribution to the proton's magnetism and a significantly smaller contribution to the proton's charge.
|
From page 117... ...
, using a number of complementary techniques, have established that quark spins account for only about 30 percent of the proton spin, and very little if any comes from the spin of the gluons. By elimination, this means that orbital motion must be a key player after all.
|
From page 118... ...
The red and black points are calculations from lattice QCD. Lower right: A cartoon of the process of polarized proton-proton scattering, showing the inter action of a polarized quark with a gluon.
|
From page 119... ...
Uncovering the high-energy effects requires both ingenious experimental techniques and full control of the theoretical predictions of the Standard Model. Apart from QCD, the other interactions described by the Standard Model are weak, and their consequences are well enough understood to help constrain QCD.
|
From page 120... ...
120 Nuclear Physics 0.3 polarized gluon distribution 0.2 max. expectation 0.1 -0 extracted from measurements min.
|
From page 121... ...
This is one of the main motivations behind the support of the lattice QCD effort in this country and worldwide. If the experimental findings at the LHC were to show that new, strongly coupled QCD-like theories are needed to extend the Standard Model beyond our current understanding, the lattice methodologies developed for these quantitative QCD calculations could prove to be invaluable at a much higher energy scale.
|
From page 122... ...
A central question has been to distinguish conventional nuclear structure effects from processes that are not described by models using only nucleons as the building blocks. One particularly fruitful approach is to focus on light nuclei with between 2 and 12 nucleons, where the nuclear structure is well understood experimentally and well described by exist ing models.
|
From page 123... ...
A lattice QCD calculation shows that the effective quark mass is momentum dependent, rising as the momentum becomes lower. Deep inelastic scattering studies on the proton have shown that the transition from the nonperturbative regime to the perturbative regime occurs when the momentum transferred to the quark is between 1 and 2 GeV.
|
From page 124... ...
The other curves show this effect for quarks that are not quite massless at high energies, which makes the numerical lattice QCD calculations tractable. Right: Lattice computation of the relationship between the mass of the lightest meson, the pion, and the mass of a nucleon.
|
From page 125... ...
Many promising applications are on the horizon, including the use of SCET to systematically study jet quenching and radiation in heavy-ion collisions at RHIC and the LHC and to study the substructure of hadrons inside jets. Can a Nucleus Become Glasslike?
|
From page 126... ...
One approach to systematically tie effective models to QCD, the underlying theory, is to perform lattice QCD calculations for the nucleon-nucleon interaction. During the past decade, work in this direction began, but it is still a daunting chal lenge.
|
From page 127... ...
Calculations of properties of nucleonnucleon interactions and the lightest bound nuclei are still very challenging in lattice QCD, because of the large range of energy scales involved. Lattice QCD calculations are performed in a volume with a well-defined lattice spacing.
|
From page 128... ...
For almost 50 years the Roper resonance has baffled nuclear physicists. Discovered in 1963 by L
|
From page 129... ...
The lattice QCD calculation of the masses of the eight lightest baryons is shown in Figure 2.33, showing remarkably good agreement with experiment. An easier problem is to compute the spectrum of hadrons that contain the heavier charm and bottom quarks on the lattice: the spectrum of bound states with heavy quarks also compares very well with experimental data from the high-energy physics experiments BaBar, Belle, and CLEO.
|
From page 130... ...
Lattice QCD is also being used to understand other aspects of hadronic struc ture. Calculations of electromagnetic transitions between excited states are another useful connection to experiment.
|
From page 131... ...
Richards, and C.E. Thomas, 2011, Isoscalar meson spectroscopy from lattice QCD, Physical Review D 83:111502, Figure 4.
|
From page 132... ...
An EIC would also provide direct access to the dynamics of the complex system of strongly interacting quarks and gluons that result in the proton's spin. This includes orbital motion, the importance of which research at JLAB and RHIC and within the HERMES experiment has made apparent.
|
From page 133... ...
The next decade will find nuclear physicists continuing to look for fingerprints of physics beyond the Standard Model. In addition to exploring the nature of neutrinos, efforts will take place on the precision frontier, where subtle details in the decay patterns of nuclei and the free neutron, in weak interactions between nucleons, and in interactions of electrons in scattering experiments, among others, might signify the presence of new physics.
|
From page 134... ...
Painstaking laboratory measurements of the nuclear reaction rates that determine neutrino production in the sun similarly excluded uncertainties in the nuclear physics of the solar model. On the other hand, neutrinos with mass gave a good account of the observations.
|
From page 135... ...
They show conclusively that, in contradiction to the expectation of the minimal Standard Model, neutrinos do have mass, albeit very small (see Figure 1.5)
|
From page 136... ...
But new challenges have emerged besides the neutrino mass. One of the most spectacular achievements of twentieth-century physics is quantum electrodynamics, now a subset of the Standard Model.
|
From page 137... ...
170 180 190 200 210 aµ × 1010 – 11659000 FIGURE 2.37 Comparison of BNL measurement of the muon anomalous magnetic moment (bottommost points) with several recent theoretical calculations based1.eps Standard Model.
|
From page 138... ...
Among these challenges it is principally the precision frontier -- where exquisitely sensitive mea surements may reveal tiny deviations from Standard Model predictions and point to the fundamental symmetries of the NSM, or directly reveal the interactions of dark-matter particles and neutrinos -- that has attracted the attention and par ticipation of nuclear physicists. Particle physicists are approaching closely related fundamental physics questions with different tools, intense accelerator-produced beams of neutrinos, muons, and kaons.
|
From page 139... ...
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 Standard Model Cabibbo-Kobayashi-Maskawa (CKM)
|
From page 140... ...
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
|
From page 141... ...
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
|
From page 142... ...
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.
|
From page 143... ...
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.
|
From page 144... ...
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
|
From page 145... ...
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.
|
From page 146... ...
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.
|
From page 147... ...
The priority of the research goals that need underground space, including neutrinoless double beta decay, dark matter searches, and solar neutrino physics, 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.
|
From page 148... ...
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.
|
From page 149... ...
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.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.