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The American Institute of Physics developed the Physics and Astronomy Classification Scheme (usually abbreviated PACS) in order to catagorize the many fields and subfields of physics. PACS numbers have been used in Physical Review since 1975.

When an article is submitted to Physical Review the author(s) also supply the PACS numbers relevant to the paper. This facilitates the distribution of the paper to editors and peer referees. Later, upon publication, it may help readers to search by subject matter.

At the highest level of the PACS hierarchy, we have

 00. GENERAL
 10. THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
 20. NUCLEAR PHYSICS
 30. ATOMIC AND MOLECULAR PHYSICS
 40. ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER,
     CLASSICAL MECHANICS AND FLUID MECHANICS
 50. PHYSICS OF GASES, PLASMAS AND ELECTRIC DISCHARGES
 60. CONDENSED MATTER: STRUCTURAL, MECHANICAL AND THERMAL
     PROPERTIES
 70. CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL,
     MAGNETIC, AND OPTICAL PROPERTIES
 80. INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF 
     SCIENCE AND TECHNOLOGY
 90. GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

Under these 10 broad subjects are 66 major topics. Each of these contains far too many subcatagories to list here. The full list may be found at http://publish.aps.org/PACS/. To give you an idea of what detail the PACS goes into, here is an example, the first two pages of the particle physics PACS numbers...

10.      THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

11.10.-z Field theory (for gauge field theories, see 11.15)
11.10.Cd Axiomatic approach
11.10.Ef Lagrangian and Hamiltonian approach
11.10.Gh Renormalization
11.10.Hi Renormalization group evolution of parameters
11.10.Jj Asymptotic problems and properties
11.10.Kk Field theories in dimensions other than four (see also 
          04.50 Gravity in more than four dimensions; 04.60.Kz 
          Lower dimensional models in quantum gravity)
11.10.Lm Nonlinear or nonlocal theories and models (see also 
          11.27 Extended classical solutions; cosmic strings, 
          domain walls, texture)
11.10.St Bound and unstable states; Bethe-Salpeter equations
11.10.Wx Finite-temperature field theory 
 .....   Relativistic wave equations, see 03.65.Pm

11.15.-q Gauge field theories
11.15.Bt General properties of perturbation theory
11.15.Ex Spontaneous breaking of gauge symmetries
11.15.Ha Lattice gauge theory (see also 12.38.Gc Lattice QCD 
          calculations)
11.15.Kc Classical and semiclassical techniques
11.15.Me Strong-coupling expansions
11.15.Pg Expansions for large numbers of components (e.g., 1/Nc 
          expansions)
11.15.Tk Other nonperturbative techniques

11.25.-w Theory of fundamental strings
11.25.Db Properties of perturbation theory
11.25.Hf Conformal field theory, algebraic structures
11.25.Mj Compactification and four-dimensional models
11.25.Pm Noncritical string theory
11.25.Sq Nonperturbative techniques; string field theory

11.27.+d Extended classical solutions; cosmic strings, domain 
          walls, texture (see also 98.80.Cq in cosmology)

11.30.-j Symmetry and conservation laws (see also 02.20 Group 
          theory)
11.30.Cp Lorentz and Poincare invariance
11.30.Er Charge conjugation, parity, time reversal, and other 
          discrete symmetries
11.30.Fs Global symmetries (e.g., baryon number, lepton number)
11.30.Hv Flavor symmetries
11.30.Ly Other internal and higher symmetries
11.30.Na Nonlinear and dynamical symmetries (spectrum-generating 
          symmetries)
11.30.Pb Supersymmetry (see also 12.60.Jv Supersymmetric models)
11.30.Qc Spontaneous and radiative symmetry breaking
11.30.Rd Chiral symmetries

11.40.-q Currents and their properties
11.40.Dw General theory of currents
11.40.Ex Formal properties of current algebras (see also 12.39.Fe 
          Chiral Lagrangians)
11.40.Ha Partially conserved axial-vector currents

11.55.-m S-matrix theory; analytic structure of amplitudes
11.55.Bq Analytic properties of S matrix
11.55.Ds Exact S matrices
11.55.Fv Dispersion relations
11.55.Hx Sum rules
11.55.Jy Regge formalism (see also 12.40.Nn in strong interactions)

11.80.-m Relativistic scattering theory
11.80.Cr Kinematical properties (helicity and invariant amplitudes, 
          kinematic singularities, etc.)
11.80.Et Partial-wave analysis
11.80.Fv Approximations (eikonal approximation, variational 
          principles, etc.)
11.80.Gw Multichannel scattering
11.80.Jy Many-body scattering and Faddeev equation
11.80.La Multiple scattering

11.90.+t Other topics in general theory of fields and particles 
          (restricted to new topics in section 11)

12.      Specific theories and interaction models; particle 
         systematics

12.10.-g Unified field theories and models (see also 04.50-in 
          general relativity and gravitation, 11.25.Mj 
          Compactification and four-dimensional models)
12.10.Dm Unified theories and models of strong and electroweak 
          interactions
12.10.Kt Unification of couplings; mass relations

12.15.-y Electroweak interactions for extensions of gauge or 
          Higgs sector, see 12.60.Cn or 12.60.Fr
12.15.Ff Quark and lepton masses and mixing (see also 14.60.Pq 
          Neutrino mass and mixing)
12.15.Hh Determination of Kobayashi-Maskawa matrix elements
12.15.Ji Applications of electroweak models to specific processes
12.15.Lk Electroweak radiative corrections (see also 13.40.Ks 
          Electromagnetic corrections to strong- and 
          weak-interaction processes)
12.15.Mm Neutral currents

12.20.-m Quantum electrodynamics
12.20.Ds Specific calculations
12.20.Fv Experimental tests

12.38.-t Quantum chromodynamics see also 24.85 Quarks, gluons, 
          and QCD in nuclei and nuclear processes
12.38.Aw General properties of QCD (dynamics, confinement, etc.)
12.38.Bx Perturbative calculations
12.38.Cy Summation of perturbation theory
12.38.Gc Lattice QCD calculations (see also 11.15.Ha Lattice 
          gauge theory)
12.38.Lg Other nonperturbative calculations
12.38.Mh Quark gluon plasma
12.38.Qk Experimental tests

12.39.-x Phenomenological quark models
12.39.Ba Bag model
12.39.Dc Skyrmions
12.39.Fe Chiral Lagrangians
12.39.Hg Heavy quark effective theory
12.39.Jh Nonrelativistic quark model
12.39.Ki Relativistic quark model
12.39.Mk Glueball and nonstandard multi-quark/gluon states
12.39.Pn Potential models

12.40.-y Other models for strong interactions
12.40.Ee Statistical models
12.40.Nn Regge theory, duality, absorptive/optical models 
          (see also 11.55.Jy Regge formalism)
12.40.Vv Vector-meson dominance
12.40.Yx Hadron mass models and calculations
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