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Sum Rules (quantum Field Theory)
In quantum field theory, a ''sum rule'' is a relation between a static quantity and an integral over a dynamical quantity. Therefore, they have a form such as: \int A(x) dx = B where A(x) is the dynamical quantity, for example a structure function characterizing a particle, and B is the static quantity, for example the mass or the charge of that particle. Quantum field theory sum rules should not be confused with sum rules in quantum chromodynamics or quantum mechanics. Properties Many sum rules exist. The validity of a particular sum rule can be sound if its derivation is based on solid assumptions, or on the contrary, some sum rules have been shown experimentally to be incorrect, due to unwarranted assumptions made in their derivation. The list of sum rules below illustrate this. Sum rules are usually obtained by combining a dispersion relation with the optical theorem, [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quantum Field Theory
In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and in condensed matter physics to construct models of quasiparticles. QFT treats particles as excited states (also called Quantum, quanta) of their underlying quantum field (physics), fields, which are more fundamental than the particles. The equation of motion of the particle is determined by minimization of the Lagrangian, a functional of fields associated with the particle. Interactions between particles are described by interaction terms in the Lagrangian (field theory), Lagrangian involving their corresponding quantum fields. Each interaction can be visually represented by Feynman diagrams according to perturbation theory (quantum mechanics), perturbation theory in quantum mechanics. History Quantum field theory emerged from the wo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Compton Scattering
Compton scattering, discovered by Arthur Holly Compton, is the scattering of a high frequency photon after an interaction with a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon (which may be an X-ray or gamma ray photon), it is called the Compton effect. Part of the energy of the photon is transferred to the recoiling electron. Inverse Compton scattering occurs when a charged particle transfers part of its energy to a photon. Introduction Compton scattering is an example of elastic scattering of light by a free charged particle, where the wavelength of the scattered light is different from that of the incident radiation. In Compton's original experiment (see Fig. 1), the energy of the X ray photon (≈17 keV) was significantly larger than the binding energy of the atomic electron, so the electrons could be treated as being free after scattering. The amount by which the light's wavelength changes is called the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fine-structure Constant
In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by (the Greek letter ''alpha''), is a fundamental physical constant which quantifies the strength of the electromagnetic interaction between elementary charged particles. It is a dimensionless quantity, independent of the system of units used, which is related to the strength of the coupling of an elementary charge ''e'' with the electromagnetic field, by the formula . Its numerical value is approximately , with a relative uncertainty of The constant was named by Arnold Sommerfeld, who introduced it in 1916 Equation 12a, ''"rund 7·" (about ...)'' when extending the Bohr model of the atom. quantified the gap in the fine structure of the spectral lines of the hydrogen atom, which had been measured precisely by Michelson and Morley in 1887. Definition In terms of other fundamental physical constants, may be defined as: \alpha = \frac = \frac , where * is the elementary char ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sidney Drell
Sidney David Drell (September 13, 1926 – December 21, 2016) was an American theoretical physicist and arms control expert. At the time of his death, he was professor emeritus at the Stanford Linear Accelerator Center (SLAC) and senior fellow at Stanford University's Hoover Institution. Drell was a noted contributor in the fields of quantum electrodynamics and high-energy particle physics. The Drell–Yan process is partially named for him. Biography Born in Atlantic City, New Jersey, Drell graduated from Atlantic City High School. He earned his undergraduate degree in physics from Princeton University in 1946, having been admitted at the age of 16. Grimes, William"Sidney Drell, Who Advised Presidents on Nuclear Weapons, Dies at 90" ''The New York Times'', December 22, 2016. Accessed December 22, 2016. "Sidney David Drell was born on Sept. 13, 1926, in Atlantic City, to Jewish immigrants from the Russian empire." He was awarded a masters in physics in 1947 and received ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Proton Spin Crisis
The proton spin crisis (sometimes called the "proton spin puzzle") is a theoretical crisis precipitated by a 1987 experiment by the European Muon Collaboration (EMC), which tried to determine the distribution of spin within the proton. Physicists expected that the quarks carry all the protons' spin. However, not only was the total proton spin carried by quarks far smaller than 100%, these results were consistent with almost zero (4~24%) proton spin being carried by quarks. This surprising and puzzling result was termed the "proton spin crisis". The problem is considered one of the important unsolved problems in physics. Background A key question is how the nucleons' spins are distributed amongst their constituent parts ( "partons": quarks and gluons). Components of proton's spin are expectation values of individual sources of angular momentum. These values depend on the renormalization scale, because their operators are not separately conserved. Physicists originally expecte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Free Neutron Decay
When embedded in an atomic nucleus, neutrons are (usually) stable particles. Outside the nucleus, free neutrons are unstable and have a mean lifetime of (about , ). Therefore, the half-life for this process (which differs from the mean lifetime by a factor of ) is (about , ). (An article published in October 2021, arrives at for the mean lifetime). The beta decay of the neutron described in this article can be notated at four slightly different levels of detail, as shown in four layers of Feynman diagrams in a section below. : The hard-to-observe quickly decays into an electron and its matching antineutrino. The subatomic reaction shown immediately above depicts the process as it was first understood, in the first half of the 20th century. The boson () vanished so quickly that it was not detected until much later. Later, beta decay was understood to occur by the emission of a weak boson (), sometimes called a charged weak current. Beta decay specifically involves ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Nucleon
In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons were thought to be elementary particles, not made up of smaller parts. Now they are known to be composite particles, made of three quarks bound together by the strong interaction. The interaction between two or more nucleons is called internucleon interaction or nuclear force, which is also ultimately caused by the strong interaction. (Before the discovery of quarks, the term "strong interaction" referred to just internucleon interactions.) Nucleons sit at the boundary where particle physics and nuclear physics overlap. Particle physics, particularly quantum chromodynamics, provides the fundamental equations that describe the properties of quarks and of the strong interaction. These equations describe quantitatively how quarks can bind toget ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one atomic mass unit, they are both referred to as nucleons. Their properties and interactions are described by nuclear physics. Protons and neutrons are not elementary particles; each is composed of three quarks. The chemical properties of an atom are mostly determined by the configuration of electrons that orbit the atom's heavy nucleus. The electron configuration is determined by the charge of the nucleus, which is determined by the number of protons, or atomic number. The number of neutrons is the neutron number. Neutrons do not affect the electron configuration, but the sum of atomic and neutron numbers is the mass of the nucleus. Atoms of a chemical element t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
James Bjorken
James Daniel "BJ" Bjorken (born 1934) is an American theoretical physicist. He was a Putnam Fellow in 1954, received a BS in physics from MIT in 1956, and obtained his PhD from Stanford University in 1959. He was a visiting scholar at the Institute for Advanced Study in the fall of 1962. Bjorken is Emeritus Professor in the SLAC Theory Group at the Stanford Linear Accelerator Center, and was a member of the Theory Department of the Fermi National Accelerator Laboratory (1979–1989). He was awarded the Dirac Medal of the ICTP in 2004; and, in 2015, the Wolf Prize in Physics and the EPS High Energy and Particle Physics Prize. Early life and education James Bjorken's father, J. Daniel Bjorken, was an immigrant from Sweden near Lake Siljan. He changed his surname from "Björkén" to Bjorken upon arriving in the US; he moved to Chicago to work as an electrical engineer, which is where he met his future wife, Edith. James Bjorken grew up in Chicago and enjoyed mathematics, chemi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pion
In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more generally, the lightest hadrons. They are unstable, with the charged pions and decaying after a mean lifetime of 26.033 nanoseconds ( seconds), and the neutral pion decaying after a much shorter lifetime of 85 attoseconds ( seconds). Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays. The exchange of virtual pions, along with vector, rho and omega mesons, provides an explanation for the residual strong force between nucleons. Pions are not produced in radioactive decay, but commonly are in high-energy collisions between hadrons. Pions also result from some matter–antimatter annihilation events. All types of pions are also produced in natural processes wh ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Polarizability
Polarizability usually refers to the tendency of matter, when subjected to an electric field, to acquire an electric dipole moment in proportion to that applied field. It is a property of all matter, considering that matter is made up of elementary particles which have an electric charge, namely protons and electrons. When subject to an electric field, the negatively charged electrons and positively charged atomic nuclei are subject to opposite forces and undergo charge separation. Polarizability is responsible for a material's dielectric constant and, at high (optical) frequencies, its refractive index. The polarizability of an atom or molecule is defined as the ratio of its induced dipole moment to the local electric field; in a crystalline solid, one considers the dipole moment per unit cell. Note that the local electric field seen by a molecule is generally different from the macroscopic electric field that would be measured externally. This discrepancy is taken into account by ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cross Section (physics)
In physics, the cross section is a measure of the probability that a specific process will take place when some kind of radiant excitation (e.g. a particle beam, sound wave, light, or an X-ray) intersects a localized phenomenon (e.g. a particle or density fluctuation). For example, the Rutherford cross-section is a measure of probability that an alpha particle will be deflected by a given angle during an interaction with an atomic nucleus. Cross section is typically denoted ( sigma) and is expressed in units of area, more specifically in barns. In a way, it can be thought of as the size of the object that the excitation must hit in order for the process to occur, but more exactly, it is a parameter of a stochastic process. In classical physics, this probability often converges to a deterministic proportion of excitation energy involved in the process, so that, for example, with light scattering off of a particle, the cross section specifies the amount of optical power scattere ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
