In physics Asymmetry

1 in physics

1.1 thermodynamics
1.2 particle physics

1.2.1 parity violation
1.2.2 cp violation
1.2.3 baryon asymmetry of universe
1.2.4 isospin violation
1.2.5 in collider experiments

in physics

asymmetry arises in physics in number of different realms.

thermodynamics

the original non-statistical formulation of thermodynamics asymmetrical in time: claimed entropy in closed system can increase time. derived second law (any of two, clausius or lord kelvin s statement can used since equivalent) , using clausius theorem (see [[kerson huang isbn 978-0471815181]]). later theory of statistical mechanics, however, symmetric in time. although states system below maximum entropy evolve towards higher entropy, states such system have evolved higher entropy.

particle physics

symmetry 1 of powerful tools in particle physics, because has become evident practically laws of nature originate in symmetries. violations of symmetry therefore present theoretical , experimental puzzles lead deeper understanding of nature. asymmetries in experimental measurements provide powerful handles relatively free background or systematic uncertainties.

parity violation

until 1950s, believed fundamental physics left-right symmetric; i.e., interactions invariant under parity. although parity conserved in electromagnetism, strong interactions , gravity, turns out violated in weak interactions. standard model incorporates parity violation expressing weak interaction chiral gauge interaction. left-handed components of particles , right-handed components of antiparticles participate in weak interactions in standard model. consequence of parity violation in particle physics neutrinos have been observed left-handed particles (and antineutrinos right-handed particles).

in 1956-1957 chien-shiung wu, e. ambler, r. w. hayward, d. d. hoppes, , r. p. hudson found clear violation of parity conservation in beta decay of cobalt-60. simultaneously, r. l. garwin, leon lederman, , r. weinrich modified existing cyclotron experiment , verified parity violation.

cp violation

after discovery of violation of parity in 1956-57, believed combined symmetry of parity (p) , simultaneous charge conjugation (c), called cp, preserved. example, cp transforms left-handed neutrino right-handed antineutrino. in 1964, however, james cronin , val fitch provided clear evidence cp symmetry violated in experiment neutral kaons.

cp violation 1 of necessary conditions generation of baryon asymmetry in universe.

combining cp symmetry simultaneous time reversal (t) produces combined symmetry called cpt symmetry. cpt symmetry must preserved in lorentz invariant local quantum field theory hermitian hamiltonian. of 2006, no violations of cpt symmetry have been observed.

baryon asymmetry of universe

the baryons (i.e., protons , neutrons , atoms comprise) observed far in universe overwhelmingly matter opposed anti-matter. asymmetry called baryon asymmetry of universe.

isospin violation

isospin symmetry transformation of weak interactions. concept first introduced werner heisenberg in nuclear physics based on observations masses of neutron , proton identical , strength of strong interaction between pair of nucleons same, independent of whether protons or neutrons. symmetry arises @ more fundamental level symmetry between up-type , down-type quarks. isospin symmetry in strong interactions can considered subset of larger flavor symmetry group, in strong interactions invariant under interchange of different types of quarks. including strange quark in scheme gives rise eightfold way scheme classifying mesons , baryons.

isospin violated fact masses of , down quarks different, different electric charges. because violation small effect in processes involve strong interactions, isospin symmetry remains useful calculational tool, , violation introduces corrections isospin-symmetric results.

in collider experiments

because weak interactions violate parity, collider processes can involve weak interactions typically exhibit asymmetries in distributions of final-state particles. these asymmetries typically sensitive difference in interaction between particles , antiparticles, or between left-handed , right-handed particles. can used sensitive measurement of differences in interaction strength and/or distinguish small asymmetric signal large symmetric background.

a forward-backward asymmetry defined afb=(nf-nb)/(nf+nb), nf number of events in particular final-state particle moving forward respect chosen direction (e.g., final-state electron moving in same direction initial-state electron beam in electron-positron collisions), while nb number of events final-state particle moving backward . forward-backward asymmetries used lep experiments measure difference in interaction strength of z boson between left-handed , right-handed fermions, provides precision measurement of weak mixing angle.
a left-right asymmetry defined alr=(nl-nr)/(nl+nr), nl number of events in initial- or final-state particle left-polarized, while nr corresponding number of right-polarized events. left-right asymmetries in z boson production , decay measured @ stanford linear collider using event rates obtained left-polarized versus right-polarized initial electron beams. left-right asymmetries can defined asymmetries in polarization of final-state particles polarizations can measured; e.g., tau leptons.
a charge asymmetry or particle-antiparticle asymmetry defined in similar way. type of asymmetry has been used constrain parton distribution functions of protons @ tevatron events in produced w boson decays charged lepton. asymmetry between positively , negatively charged leptons function of direction of w boson relative proton beam provides information on relative distributions of , down quarks in proton. particle-antiparticle asymmetries used extract measurements of cp violation b meson , anti-b meson production @ babar , belle experiments.