James Bjorken

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, 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: ${\displaystyle {\int }_0^{1}dx(g_1^p(x)-g_1^n(x))=g_A/6}$, where ${\displaystyle x}$ is the Bjorken scaling variable, ${\displaystyle g_1^{p(n)}(x)}$ is the first spin structure function of the proton (neutron), and ${\displaystyle g_A}$ is the nucleon axial charge that characterizes the neutron β-decay. The sum rule was experimentally verified within better than a 10% precision.

Bjorken was also among the first to point out 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. 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.

Personal life and death

In 1967, Bjorken married Joan Goldthwaite; they had two children and were married until her death in 1983. He lived in Sky Londa, California.

Bjorken died from melanoma at a care facility in Redwood City, California, on August 6, 2024 at the age of 90.