1990 Nobel Prize in Physics
Reason for Award
for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons, which have been of essential importance for the development of the quark model in particle physics
Laureates
United States of America
United States of America
Canada
Explanation
Everything around us is made of atoms. Three scientists shot very fast electrons at the protons and neutrons in an atom’s center to peek inside. It is like hitting a watermelon with ping-pong balls to learn what is inside. They discovered even smaller particles inside, called “quarks.” Finding quarks helped us understand what matter is really made of.
Related Keywords
deep inelastic scattering
A technique in which high-energy leptons are fired at nuclei or protons to investigate their inner structure. Because the energy transfer is large enough to break the target, the process is called “inelastic.” The larger the momentum transfer Q², the better the spatial resolution, enabling tests for point-like constituents. Since the 1970s, experiments with electrons, muons and neutrinos have exploited this method. DIS data remain essential for determining parton distribution functions used at the LHC and in cosmic-ray physics.
quark
Fundamental particles that feel the strong force and come in six flavors, such as up and down. A proton is made of two up quarks and one down quark; a neutron has one up and two downs. Quarks are never seen alone because of confinement—they always form bound states called hadrons. Their electric charges are fractional, ±2/3e or −1/3e. Deep inelastic scattering provided the first clear experimental evidence of their reality.
parton model
A phenomenological picture proposed by Richard Feynman that treats hadrons as collections of point-like constituents called partons. Assuming partons behave almost independently at high Q² leads naturally to Bjorken scaling. Partons were later identified with quarks and gluons and justified within perturbative QCD. The model evolved into today’s parton distribution functions (PDFs), indispensable for cross-section predictions at the LHC. At extreme kinematics it inspires studies of small-x physics and saturation effects.
Bjorken scaling
An empirical rule stating that nucleon structure functions depend only on the momentum fraction x and not on Q². Predicted by James Bjorken and confirmed by SLAC experiments, it pointed to the existence of point-like partons and supported the quark model. In QCD, small logarithmic violations of scaling are predicted and match data with high precision. The degree to which scaling holds or breaks is a critical test of the strong-interaction theory.
structure function
Quantities, such as F₁(x,Q²) and F₂(x,Q²), that parametrize the deep inelastic scattering cross-section. They encode the density of partons inside hadrons and connect directly to PDFs. Through the Callan–Gross relation they also convey information on parton spin. Because measurements from different experiments are directly comparable, global fits combine world data to extract PDFs. The resulting structure functions are used in cosmic-ray and neutrino flux analyses as well.
SLAC
Abbreviation for the Stanford Linear Accelerator Center in Menlo Park, California. It houses a 3.2 km linear accelerator that delivered the world’s highest-energy electron beams in the late 1960s. Besides deep inelastic scattering, SLAC achieved milestones such as the discovery of the J/ψ particle and operation of a B factory. Today the upgraded linac drives the LCLS X-ray free-electron laser, advancing materials science. SLAC has long been a driving force in particle-physics and accelerator technology.
quantum chromodynamics
The gauge theory of the strong interaction, based on SU(3) color symmetry. Quarks and gluons are its fundamental fields, exhibiting asymptotic freedom and confinement. Small violations of Bjorken scaling are explained by logarithmic QCD corrections, making DIS a prime testing ground. The DGLAP equations governing parton-density evolution derive from perturbative QCD. QCD is central to jet physics at the LHC and to heavy-ion studies at RHIC.
Callan–Gross relation
The formula F₂ = 2xF₁ that holds if electrons scatter from spin-½ point particles in deep inelastic scattering. SLAC data satisfied this relation well, indicating that the partons inside the proton carry spin-½, consistent with quarks. It provides a rare direct probe of quark spin properties. Including QCD corrections introduces slight deviations, whose measurement constrains α_s. An analogous relation (the Paschos-Wolfenstein ratio) is discussed in neutrino scattering.
electron linear accelerator
A device that accelerates electrons in a straight line, suffering far less synchrotron-radiation loss than circular machines. SLAC’s 2-mile linac provided 19 GeV electrons, crucial for the deep inelastic scattering experiments. It consists of a series of accelerating cavities that impart energy via RF electric fields. Linear accelerators now serve as sources for medical radiation and X-ray free-electron lasers. The adoption of superconducting technology enables high duty-cycle beams, supporting future linear-collider projects.
scaling violation
The breakdown of Bjorken scaling observed with high-precision measurements. QCD predicts a logarithmic Q² dependence of structure functions due to gluon emission and quark-pair creation. The observed patterns are described by the DGLAP evolution equations and used to determine the strong coupling α_s. The magnitude of scaling violation is directly linked to the gluon content, making it essential for precise PDFs. It is also a critical input for modeling high-energy cosmic-ray showers and neutrino telescopes.