Parity (physics)#Parity violation
Parity violation referred to in physics in 1956 discovered the fact that there are physical processes that occur in a mirror built up world differently than in a mirror image of the normal world. In other words, such a parity-violating process, which is observed in the mirror is different from the process that actually takes place in a mirror-inverted, but otherwise identically constructed Versuchansordnung.
The name of parity violation is precisely violation of the conservation of parity. The size parity is defined in quantum mechanics to describe the symmetry character of the wave function with respect to space inversion. This " space reflection " is meant when one speaks in this context the fact that physical laws or procedures " mirror symmetry " are.
Parity Hurtful processes are known only in the weak interaction. The other fundamental forces of physics ( gravity, electromagnetic interaction, strong interaction ) are parity preserving. However, these three basic forces determine the processes of everyday life, so it is not so easy to observe parity violation. So then for a long time was scientific doctrine that nature is governed exclusively by mirror-symmetric laws, parity violation was maintained until the mid 20th century for completely excluded. Evidence to the contrary, succeeded in 1956 the groups of CS Wu on the example of radioactivity and, almost simultaneously, by Leon Lederman on the example of the decay of polarized muons.
As eigenvalues of the quantum is only the parity quantum numbers 1 ( symmetrically ), and -1 ( antisymmetric ). Almost always have the energy levels of atoms, molecules, etc. in a very good approximation a certain parity ( 1 or -1 ), some often used wave functions (eg, the plane wave ) but. As long as a process of the weak interaction is not involved, the symmetry character to remain so, as it exists at the beginning ( parity conservation of the wave function of the entire system ). For example, if an excited atom by the electromagnetic interaction produces a photon, which has for the parity -1, then the atom must be in its final state of opposite parity to the initial state have ( parity selection rule ). Have combined photon and atom in the final state have the same parity as the initial state. In processes of the weak interaction, however (eg, radioactivity, weak decay of unstable elementary particles) arises from an initial state with pure parity a final state with equal proportions of both parities, so maximum mixture. Therefore, the parity violation is called maximal by the weak interaction.
Discovery of parity violation
The parity violation has already been discovered in 1928 and published properly, but viewed as a measurement error because they did not fit at that doctrine of parity conservation. For the same reason then also proposed by Hermann Weyl in use today and description of electrons ( and neutrinos ) in the form of the 2-component spinor was rejected.
1956 published Tsung- Dao Lee and Chen Ning Yang, the presumption that the " τ - θ puzzle " was thus elucidate the decay of the kaon that in the weak interaction, as opposed to gravity, the strong and electromagnetic interactions, the parity is not retained. They pointed out ( in ignorance of the work of Cox et al. 1928 ) that this issue had never been carefully examined, and could suggest a number of specific experiments for it. For this they received in 1957 the Nobel Prize for Physics after Chien- Shiung Wu was confirmed in a pioneering experiment this suspicion: has the angular distribution of rays ( electrons) in a mirrored built apparatus actually not the same shape as the mirror image of the original structure.
The principle of the Wu experiment is illustrated in the figure: In the original design ( left in the figure), a radioactive 60Co source is observed from above with a detector that counts the flying upwards of electron radiation. The source is located in an electromagnet whose coil is flowed through from bottom to top of electrons. To the right is a plane mirror. In the mirror image of the detector as much as one -electron in the real apparatus left. When choosing the mirror plane (perpendicular and perpendicular to the plane ), the direction from below upwards in the mirror image of the bottom to the top. The current direction in the supply cables, the position of the detector and the direction of flight of the electrons are therefore counted in a mirror image, the same as in the apparatus to the left by the mirror. A second, mirror- apparatus will now be constructed and operated exactly as specified on the mirror image (right in the figure). When parity conservation in the process you would have to run exactly as it shows the mirror image of the original process. However, the detector of the mirror- apparatus counts significantly less electrons. Consequently, the parity conservation is violated.
To check this the radiation is observed with the detector, resulting from the ( parity-conserving ) electromagnetic interaction. Here you can observe in the mirror- apparatus with the mirror image ( and the original apparatus ) matches exactly. ( Formulas are provided below [ Note 1 ] )
In real experiment Wu has the mirror image apparatus not real rebuilt, but simply reversed the current direction in the original apparatus and thus the magnetic field in which was located the source of radiation. The mere reversing means here the same as the mirror-image replica, because the only physical difference between the two structures is that the mirrored magnetic coil has the opposite screw sense and, therefore, produces the opposite magnetic field for a given current. It is the observed parity violation, therefore, solely by means of the two directions of motion of the electrons expressed in the feeder cables of the coil or in the preferred direction of the rays: In the original building they are parallel and in its mirror image therefore, in reality, the mirror -built apparatus but opposite.
Further details will be treated under Wu- experiment. Inspired by the Wu experiment succeeded Richard L. Garwin, Leon M. Lederman and Marcel Weinrich within one month after a much simpler proof of parity violation by the weak interaction, this time based on the non-mirror symmetric angular distribution of the electrons arising polarized the decay muons. They were to draw even more ready than the group CSWu, you could but in the publication precedence.
Explanation of parity violation
Problem of perception
The parity violation contradicts the immediate intuition, as a mechanical apparatus, which would not work in mirror image replica exactly like the original, is far beyond imagination possibilities. [Note 2] These are, however, in line with all the practical experience in the macroscopic world, which are completely determined by the parity-conserving interactions of gravity and electromagnetism.
Chirality
The physical explanation of parity violation is based on the chirality, that is, the possibility of following the Dirac theory with each fermion (eg electron, proton, neutron, neutrino ) a right-handed and left-handed to identify a share. The parity violation is explained by the fact that the weak interaction of the fermions is not equally strong tackles two units, but generally only on the left - chiral ( at antifermions only on the right - chiral ). Since a space reflection in the particles and the antiparticles these two chiral units are interchanged, a mitgespiegelte weak interaction would now start at the other component ( for particles at the right-handed, with antiparticles at the left-handed ), the real weak interaction in the mirror mock experiment but not. A process in mirror-reversed replica may therefore differ from the mirror image of the original process.
For fermions with ( almost ) the speed of light, the chirality is (nearly) identical to the helicity. This is also called longitudinal polarization, because it measures the degree of orientation of the spin along the direction of flight: When the spin is in the direction of flight in the opposite direction. General speed at a right-handed chiral particle has helicity, a chiral left-handed. Each particle that is not moving at light speed, consists of proportions from two chiral components. When both components are the same size, with proceeds of one to zero, the other against the 1 As a consequence, for example, have high energy electrons with "spin forward " (ie positive helicity ) only a small left - chiral component, with they can participate in the weak interaction.
Chirality in the beta radioactivity
The beta-minus decay of an atomic nucleus based on the conversion of a neutron into a proton, one electron, and an antineutrino emerge. The left hand portion of a neutron emits a virtual W - boson from, making it the proton, while the W - boson annihilated immediately in a left-handed electron and a right-handed antineutrino. Since the antineutrino is emitted practically only with the speed of light, it has always maximum longitudinal polarization. Measurements of the polarization of electrons and neutrinos from the radioactivity have confirmed this picture ( see, eg, Goldhaber experiment).
Chirality in the decay of the charged pion
A negatively charged pion decays almost exclusively into a muon and a muon antineutrino
And only 0.0123 % in an electron and an electron antineutrino
Although this second decay pathway alone due to the higher kinetic energy of the electron and the neutrino would be more likely than the first.
The easiest way to explain this branching ratio is based on the parity violation. The pion has spin 0, so the spins of the muon (or electron) and antineutrino must be opposed. Here, the nearly massless antineutrino has a right-handed antiparticles its spin practically parallel to the direction of flight. Since the flight directions of both particles are opposite because of the conservation of momentum, the spin must be parallel to its flight direction at the muon (or electron), so be positive helicity. With a positive helicity but is the left - chiral share to zero, the closer the velocity of the particle velocity of light. Now the kinetic energy of the electron is due to its small mass is many times higher than its rest energy, it has so very different than the muon, already nearly the speed of light. Consequently, the electron is the left - chiral component, by which alone its production by the weak interaction depends to suppress approximately five orders of magnitude. This explains the observed frequency ratio of the two decay modes.
This reproduced in many textbooks argument is been criticized that the parity violation is not accounted for by the weak interaction for the branching ratio, but to be their character a vectorial interaction. Assuming that the weak interaction would not only generate particles and antiparticles as left - and right - chiral, but equally often in the opposite direction, then they would get the parity, but due to their vector character nevertheless a particle together with an antiparticle with opposite can generate chiralities. As before, however, write the momentum and angular momentum conservation during the resulting decay particles and antiparticles same helicities before; it is obtained for the decay pathway for electron therefore definitely the same disability as described above. The vector character of the weak interaction, in turn, follows from its construction in the form of a gauge theory. There are two possible vectorial subtypes that have vector ( V) or axialvektoriellen (A) character (see VA theory). The parity violation comes through a specific interaction of the two into being in the weak interaction. In contrast, based the electromagnetic interaction and the strong interaction, which are formulated as a gauge theory, and consequently also have vector character, only on the vector portion. Therefore, in these two parity is conserved. Noteworthy on the vector character is also that can only lead the two vectorial shapes to a parity violation of the as part of the Dirac theory in principle possible five forms of interaction.
Chirality in the decay of the muon
Muons resulting from the decay of a pion, are fully polarized in the direction of flight (see previous section ) on the basis of parity violation by the weak interaction. If they ( after complete deceleration in matter ) according to
Decay, the weak interaction causes an asymmetric angular distribution of the electrons. For example, high energy electrons are preferably opposite to the direction of the muon spins emitted. This can be explained by the fact that a high energy electron can only arise if the other two particles for the conservation of the total momentum to fly parallel to the electron in the opposite direction. Since they have as neutrino and antineutrino opposite helicities, her two angular momenta are opposite and add therefore zero. The direction of the electron spins is fixed so that it is the initial angular momentum of the muon. Since the weak interaction gives rise to only the left-handed chiral component of the electron in the decay, it produces most likely the electrons are emitted opposite to its spin, so here is opposite to the original direction of flight of the muon. The same phenomenon occurs with the opposite sign and the decay of the positive pion, followed by. Here the positron fly so primarily in the direction of the spin, which is opposite to its original direction of flight on his part.