Asymmetry arises in physics in a number of different realms.
Thermodynamics The original non-statistical formulation of
thermodynamics was
asymmetrical in time: it claimed that the
entropy in a closed system can only increase with time. This was derived from the Second Law (any of the two,
Clausius' or
Lord Kelvin's statement can be used since they are equivalent) and using the
Clausius' Theorem (see
Kerson Huang ). The later theory of statistical mechanics, however, is symmetric in time. Although it states that a system significantly below maximum entropy is very likely to evolve
towards higher entropy, it also states that such a system is very likely to have evolved
from higher entropy.
Particle physics Symmetry is one of the most powerful tools in
particle physics, because it has become evident that practically all laws of nature originate in symmetries. Violations of symmetry therefore present theoretical and experimental puzzles that lead to a deeper understanding of nature. Asymmetries in experimental measurements also provide powerful handles that are often relatively free from background or systematic uncertainties.
Parity violation Until the 1950s, it was believed that fundamental physics was left-right symmetric; i.e., that interactions were invariant under
parity. Although parity is conserved in
electromagnetism,
strong interactions and
gravity, it turns out to be violated in
weak interactions. The
Standard Model incorporates parity violation by expressing the weak interaction as a
chiral gauge interaction. Only the left-handed components of particles and right-handed components of antiparticles participate in weak interactions in the Standard Model. A consequence of parity violation in particle physics is that
neutrinos have only been observed as left-handed particles (and antineutrinos as right-handed particles). In 1956–1957
Chien-Shiung Wu, E. Ambler, R. W. Hayward, D. D. Hoppes, and R. P. Hudson found a clear violation of parity conservation in the beta decay of cobalt-60. Simultaneously,
R. L. Garwin,
Leon Lederman, and R. Weinrich modified an existing cyclotron experiment and immediately verified parity violation.
CP violation After the discovery of the violation of parity in 1956–57, it was believed that the combined symmetry of parity (P) and simultaneous
charge conjugation (C), called
CP, was preserved. For example, CP transforms a left-handed neutrino into a right-handed antineutrino. In 1964, however,
James Cronin and
Val Fitch provided clear evidence that CP symmetry was also violated in an experiment with neutral
kaons. CP violation is one of the necessary conditions for the generation of a baryon asymmetry in the early universe. Combining the CP symmetry with simultaneous
time reversal (T) produces a combined symmetry called
CPT symmetry. CPT symmetry must be preserved in any
Lorentz invariant local
quantum field theory with a
Hermitian Hamiltonian. As of 2006, no violations of CPT symmetry have been observed.
Baryon asymmetry of the universe The
baryons (i.e., the
protons and
neutrons and the atoms that they comprise) observed so far in the universe are overwhelmingly
matter as opposed to
anti-matter. This asymmetry is called the
baryon asymmetry of the universe.
Isospin violation Isospin is the symmetry transformation of the
weak interactions. The concept was first introduced by
Werner Heisenberg in
nuclear physics based on the observations that the masses of the
neutron and the
proton are almost identical and that the strength of the
strong interaction between any pair of nucleons is the same, independent of whether they are protons or neutrons. This symmetry arises at a more fundamental level as a symmetry between up-type and down-type
quarks. Isospin symmetry in the strong interactions can be considered as a subset of a larger
flavor symmetry group, in which the strong interactions are invariant under interchange of different types of quarks. Including the
strange quark in this scheme gives rise to the
Eightfold Way scheme for classifying mesons and baryons. Isospin is violated by the fact that the masses of the up and down quarks are different, as well as by their different electric charges. Because this violation is only a small effect in most processes that involve the strong interactions, isospin symmetry remains a useful calculational tool, and its violation introduces corrections to the isospin-symmetric results.
In collider experiments Because the
weak interactions violate parity,
collider processes that can involve the weak interactions typically exhibit asymmetries in the distributions of the final-state particles. These asymmetries are typically sensitive to the
difference in the interaction between particles and antiparticles, or between left-handed and right-handed particles. They can thus be used as a sensitive measurement of differences in interaction strength and/or to distinguish a small asymmetric signal from a large but symmetric background. • A
forward-backward asymmetry is defined as AFB=(NF-NB)/(NF+NB), where NF is the number of events in which some particular final-state particle is moving "forward" with respect to some chosen direction (e.g., a final-state electron moving in the same direction as the initial-state electron beam in electron-positron collisions), while NB is the number of events with the final-state particle moving "backward". Forward-backward asymmetries were used by the
LEP experiments to measure the difference in the interaction strength of the
Z boson between left-handed and right-handed fermions, which provides a precision measurement of the
weak mixing angle. • A
left-right asymmetry is defined as ALR=(NL-NR)/(NL+NR), where NL is the number of events in which some initial- or final-state particle is left-polarized, while NR is the corresponding number of right-polarized events. Left-right asymmetries in Z boson production and decay were measured at the
Stanford Linear Collider using the event rates obtained with left-polarized versus right-polarized initial electron beams. Left-right asymmetries can also be defined as asymmetries in the polarization of final-state particles whose polarizations can be measured; e.g.,
tau leptons. • A
charge asymmetry or particle-antiparticle asymmetry is defined in a similar way. This type of asymmetry has been used to constrain the
parton distribution functions of protons at the
Tevatron from events in which a produced
W boson decays to a charged lepton. The asymmetry between positively and negatively charged leptons as a function of the direction of the W boson relative to the proton beam provides information on the relative distributions of up and down quarks in the proton. Particle-antiparticle asymmetries are also used to extract measurements of CP violation from
B meson and anti-B meson production at the
BaBar and
Belle experiments. == See also ==