James Bjorken

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James Bjorken
James Bjorken.jpg
Born (1934-06-22) June 22, 1934 (age 87)
NationalityAmerican
Alma mater MIT (B.S., 1956)
Stanford University (Ph.D., 1959)
Known for Bjorken scaling
Intrabeam scattering
Jet quenching
Co-predicting the charm quark
Awards Putnam Fellow (1954)
Heineman Prize (1972)
E. O. Lawrence Award (1977)
Pomeranchuk Prize (2000)
ICTP Dirac Medal (2004)
Wolf Prize in Physics (2015)
EPS High Energy and Particle Physics Prize (2015)
Scientific career
Fields Theoretical physics
Institutions Fermilab, SLAC
Doctoral students Davison Soper
Helen Quinn

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. [1] 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).

Contents

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. [2]

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, chemistry, the french horn, and watching the Chicago Cubs play at Wrigley Field. After graduating from Maine East High School in 1952, he decided to attend Massachusetts Institute of Technology (MIT) over the University of Chicago. Despite being offered more financial aid to attend the University of Chicago, his parents advised him that he should move further away to find his independence. [3]

At MIT, he quickly decided to major in physics; one of the main reasons was his enjoyment of the lectures that Hans Mueller gave. Another of his influences at MIT was Sidney Drell, who became his mentor. After graduating in 1956, he attended Stanford University, graduating with his PhD in 1959 and staying on as a postdoctoral researcher for several years. [3]

Work

Bjorken discovered in 1968 what is known as light-cone scaling (or Bjorken scaling), a phenomenon in the deep inelastic scattering of light on strongly interacting particles, known as hadrons (such as protons and neutrons): Experimentally observed hadrons behave as collections of virtually independent point-like constituents when probed at high energies.

Properties of these hadrons scale, that is, they are determined not by the absolute energy of an experiment, but, instead, by dimensionless kinematic quantities, such as a scattering angle or the ratio of the energy to a momentum transfer. Because increasing energy implies potentially improved spatial resolution, scaling implies independence of the absolute resolution scale, and hence effectively point-like substructure.

This observation was critical to the recognition of quarks as actual elementary particles (rather than just convenient theoretical constructs), and led to the theory of strong interactions known as quantum chromodynamics, where it was understood in terms of the asymptotic freedom property. In Bjorken's picture, the quarks become point-like, observable objects at very short distances (high energies), shorter than the size of the hadrons.

Bjorken also discovered the Bjorken sum rule , [4] [5] the prototypical QCD spin sum rule. It states that in the Bjorken scaling domain, the integral of the spin structure function of the proton minus that of the neutron is proportional to the axial charge of the nucleon. Specially: , where is the Bjorken scaling variable, is the first spin structure function of the proton (neutron), and is the nucleon axial charge that characterizes the neutron β-decay. The sum rule was experimentally verified within better than a 10% precision. [6]

Bjorken was also among the first to point out to the phenomena of jet quenching in heavy ion collisions in 1982.

Richard Feynman subsequently reformulated this concept into the parton model, used to understand the quark composition of hadrons at high energies. [7] The predictions of Bjorken scaling were confirmed in the early late 1960s electroproduction experiments at SLAC, in which quarks were seen for the first time. The general idea, with small logarithmic modifications, is explained in quantum chromodynamics by "asymptotic freedom".

Bjorken co-authored, with Sidney Drell, a classic companion volume textbook on relativistic quantum mechanics and quantum fields.

Publications

Books

Selected papers

Full list of papers

INSPIRE-HEP -- Bjorken

Notes

Related Research Articles

Nucleon

In chemistry and physics, 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 an isotope's mass number.

Quark Elementary particle

A quark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly observable matter is composed of up quarks, down quarks and electrons. Owing to a phenomenon known as color confinement, quarks are never found in isolation; they can be found only within hadrons, which include baryons and mesons, or in quark–gluon plasmas. For this reason, much of what is known about quarks has been drawn from observations of hadrons.

Quantum chromodynamics Theory of the strong nuclear interactions

In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks and gluons, the fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of quantum field theory called a non-abelian gauge theory, with symmetry group SU(3). The QCD analog of electric charge is a property called color. Gluons are the force carrier of the theory, just as photons are for the electromagnetic force in quantum electrodynamics. The theory is an important part of the Standard Model of particle physics. A large body of experimental evidence for QCD has been gathered over the years.

Strong interaction Force binding particles within hadrons

In nuclear physics and particle physics, the strong interaction is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. At the range of 10−15 m (slightly more than the radius of a nucleon), the strong force is approximately 137 times as strong as electromagnetism, 106 times as strong as the weak interaction, and 1038 times as strong as gravitation. The strong nuclear force confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds these neutrons and protons to create atomic nuclei, where it is called the nuclear force. Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton.

The QCD vacuum is the vacuum state of quantum chromodynamics (QCD). It is an example of a non-perturbative vacuum state, characterized by non-vanishing condensates such as the gluon condensate and the quark condensate in the complete theory which includes quarks. The presence of these condensates characterizes the confined phase of quark matter.

Exotic hadron

Exotic hadrons are subatomic particles composed of quarks and gluons, but which — unlike "well-known" hadrons such as protons, neutrons and mesons — consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.

Nuclear force Force that acts between the protons and neutrons of atoms

The nuclear force is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclear force almost identically. Since protons have charge +1 e, they experience an electric force that tends to push them apart, but at short range the attractive nuclear force is strong enough to overcome the electromagnetic force. The nuclear force binds nucleons into atomic nuclei.

Drell–Yan process

The Drell–Yan process occurs in high energy hadron–hadron scattering. It takes place when a quark of one hadron and an antiquark of another hadron annihilate, creating a virtual photon or Z boson which then decays into a pair of oppositely-charged leptons. Importantly, the energy of the colliding quark-antiquark pair can be almost entirely transformed into the mass of new particles. This process was first suggested by Sidney Drell and Tung-Mow Yan in 1970 to describe the production of lepton–antilepton pairs in high-energy hadron collisions. Experimentally, this process was first observed by J.H. Christenson et al. in proton–uranium collisions at the Alternating Gradient Synchrotron.

In particle physics, the parton model is a model of hadrons, such as protons and neutrons, proposed by Richard Feynman. It is useful for interpreting the cascades of radiation produced from QCD processes and interactions in high-energy particle collisions.

The neutron magnetic moment is the intrinsic magnetic dipole moment of the neutron, symbol μn. Protons and neutrons, both nucleons, comprise the nucleus of atoms, and both nucleons behave as small magnets whose strengths are measured by their magnetic moments. The neutron interacts with normal matter through either the nuclear force or its magnetic moment. The neutron's magnetic moment is exploited to probe the atomic structure of materials using scattering methods and to manipulate the properties of neutron beams in particle accelerators. The neutron was determined to have a magnetic moment by indirect methods in the mid 1930s. Luis Alvarez and Felix Bloch made the first accurate, direct measurement of the neutron's magnetic moment in 1940. The existence of the neutron's magnetic moment indicates the neutron is not an elementary particle, as for an elementary particle to have an intrinsic magnetic moment, it must have both spin and electric charge, and the neutron has spin 1/2 ħ, but no net charge. The existence of the neutron's magnetic moment was puzzling and defied a correct explanation until the quark model for particles was developed in the 1960s. The neutron is composed of three quarks, and the magnetic moments of these elementary particles combine to give the neutron its magnetic moment.

The timeline of particle physics lists the sequence of particle physics theories and discoveries in chronological order. The most modern developments follow the scientific development of the discipline of particle physics.

Light front quantization Technique in computational quantum field theory

The light-front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular, it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates, where plays the role of time and the corresponding spatial coordinate is . Here, is the ordinary time, is one Cartesian coordinate, and is the speed of light. The other two Cartesian coordinates, and , are untouched and often called transverse or perpendicular, denoted by symbols of the type . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactly soluble relativistic theory, but in practical calculations some choices may be more suitable than others.

Quark–gluon plasma Phase of quantum chromodynamics (QCD)

Quark–gluon plasma or QGP is an interacting localized assembly of quarks and gluons at thermal and chemical (abundance) equilibrium. The word plasma signals that free color charges are allowed. In a 1987 summary, Léon van Hove pointed out the equivalence of the three terms: quark gluon plasma, quark matter and a new state of matter. Since the temperature is above the Hagedorn temperature—and thus above the scale of light u,d-quark mass—the pressure exhibits the relativistic Stefan-Boltzmann format governed by fourth power of temperature and many practically mass free quark and gluon constituents. We can say that QGP emerges to be the new phase of strongly interacting matter which manifests its physical properties in terms of nearly free dynamics of practically massless gluons and quarks. Both quarks and gluons must be present in conditions near chemical (yield) equilibrium with their colour charge open for a new state of matter to be referred to as QGP.

The EMC effect is the surprising observation that the cross section for deep inelastic scattering from an atomic nucleus is different from that of the same number of free protons and neutrons. From this observation, it can be inferred that the quark momentum distributions in nucleons bound inside nuclei are different from those of free nucleons. This effect was first observed in 1983 at CERN by the European Muon Collaboration, hence the name "EMC effect". It was unexpected, since the average binding energy of protons and neutrons inside nuclei is insignificant when compared to the energy transferred in deep inelastic scattering reactions that probe quark distributions. While over 1000 scientific papers have been written on the topic and numerous hypotheses have been proposed, no definitive explanation for the cause of the effect has been confirmed. Determining the origin of the EMC effect is one of the major unsolved problems in the field of nuclear physics.

Fermilab E-906/SeaQuest

Fermilab E-906/SeaQuest is a particle physics experiment which will use Drell–Yan process to measure the contributions of antiquarks to the structure of the proton or neutron and how this structure is modified when the proton or neutron is included within an atomic nucleus.

Light front holography

In strong interaction physics, light front holography or light front holographic QCD is an approximate version of the theory of quantum chromodynamics (QCD) which results from mapping the gauge theory of QCD to a higher-dimensional anti-de Sitter space (AdS) inspired by the AdS/CFT correspondence proposed for string theory. This procedure makes it possible to find analytic solutions in situations where strong coupling occurs, improving predictions of the masses of hadrons and their internal structure revealed by high-energy accelerator experiments. The most widely used approach to finding approximate solutions to the QCD equations, lattice QCD, has had many successful applications; however, it is a numerical approach formulated in Euclidean space rather than physical Minkowski space-time.

The proton magnetic moment is the magnetic dipole moment of the proton, symbol μp. Protons and neutrons, both nucleons, make up the nucleus of all atoms heavier than protium, and both nucleons act as small magnets whose strength is measured by their magnetic moments. The magnitude of the proton's magnetic moment indicates that the proton is not an elementary particle.

Light-front quantization applications

The light front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular, it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates, where plays the role of time and the corresponding spatial coordinate is . Here, is the ordinary time, is one Cartesian coordinate, and is the speed of light. The other two Cartesian coordinates, and , are untouched and often called transverse or perpendicular, denoted by symbols of the type . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactly soluble relativistic theory, but in practical calculations some choices may be more suitable than others. The basic formalism is discussed elsewhere.

In high energy particle physics, specifically in hadron-beam scattering experiments, the Transverse Momentum Distributions (TMDs) are the distributions of the hadron's quark or gluon momenta that are perpendicular to the momentum transfer between the beam and the hadron. Specifically, they are probability distribution to find inside the hadron a parton with a transverse momentum and longitudinal momentum fraction . TMDs provide information on the confined motion of quarks and gluons inside the hadron and complement the information on the hadron structure provided by parton distributions functions and generalized parton distributions (GPDs).

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:

References

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