Pion

Pion ( π )

π0

Pions or mesons (formerly referred to as Yukawa particles, as predicted by Hideki Yukawa ) are the lightest mesons in particle physics. Since they contain according to the standard model, two valence quarks, they are now usually no longer considered to be elementary particles. Like all mesons are bosons, ie have integer spin. Your parity is negative.

There is a neutral pion and two charged pions: and its antiparticle. All three are unstable and decay by weak or electromagnetic interactions.

• 5.1 range
• 5.2 Example process

Construction

This is a combination of an up quark and an anti- down quark (shown painted over anti-quarks ):

Its antiparticle a combination of a down quark and an anti- up quark:

Both have a rest mass of 139.6 MeV / c ². The current best mass measurements based on X-ray transitions in exotic atoms, which have instead of an electron a. The life of the is 2.6 · 10-8 s

This is a quantum mechanical superposition state of a - and a combination, ie two quarkonia. As it is its own antiparticle, must apply:

Its rest mass is 135.0 MeV / c ² is only slightly smaller than that of the charged pions, but his life is 8.4 · 10-17 s much shorter.

Due to an arbitrary phase, the three wave functions can also be, rarely used, shape, and written. This then corresponds to the Condon - Shortley convention.

Decays

The different lifetimes are due to the different decay channels:

The charged pions decay to ( 99.98770 ± 0.00004 ) % by the weak interaction into a muon and a muon neutrino:

( The actually energetically more favorable decay into an electron and the corresponding electron neutrino is strongly suppressed from Helizitätsgründen. )

In contrast, the decay of the neutral pion by the stronger and faster so that electromagnetic interaction takes place. End products are generally in the range of two photons:

Or a positron e , an electron e and a photon:

Opening angle of the disintegration

In experiments, the neutral pion is detected by its short lifetime due to observation of the two decay photons in coincidence. The flight directions of the photons are described in the laboratory system, within a cone around the direction of flight of the pion. The opening angle of this cone can be determined by the relativistic kinematics calculate:

And are the energies of the two photons. The speed of light is set equal to 1 in the following. The square of the four-momentum of the pion is:

(1):

(2):

(3):

After the energy-momentum relation is valid for the massless photon

(4).

Substituting the relations (2) - (4 ) in equation (1), one obtains:

Now, according to an addition. This relationship is now sets up a, formed according to and receives for the opening angle of the two photons:

History of Research

Cecil Powell, César Lattes, and Giuseppe Occhialini pions and muons discovered in 1947 at the HH Wills Physical Laboratory in Bristol in the cosmic radiation, investigated their properties, and Powell was awarded the 1950 Nobel Prize in Physics. This was, however, as only later became known, was discovered in 1947 a little earlier by Donald H. Perkins in the cosmic radiation. Was Predicted the pion in 1935 as exchange particles of nuclear power by Hideki Yukawa in Japan. The decay of the said Richard Dalitz 1951.

Mass compared with nucleons

When comparing the masses of the pions, each consisting of two quarks ( mesons ), with the masses of the proton and the neutron ( nucleon ), both of which consist of three quarks ( baryons ), notice that the proton and neutron are each far about 50% heavier than the pions; the proton mass is well six times as large as the Pionenmasse. The mass of a proton or a neutron, therefore results not merely by adding the masses of their three current quarks, but also by the presence of the responsible for the binding of quarks and gluons, the so-called sea quarks. These virtual quark-antiquark pairs in the nucleon arise and pass away within the limits of the Heisenberg uncertainty principle and contribute to the observed Konstituentenquarkmasse at.

The pion - exchange model

The pions can take over the role of the exchange particles in a so-called effective theory of the strong interaction ( sigma- model) that describes the binding of the nucleons in the nucleus. ( This is analogous to the van der Waals forces acting between neutral molecules, but are also no elemental power itself, but rather they are based on the electromagnetic interaction is based. )

This theory first proposed by Hideki Yukawa and Ernst Stueckelberg is valid only within a limited energy range, but it allows simpler calculations and more graphic representations. For example, you can represent the mediated by the pion nuclear forces by the Yukawa potential compact: this has potential at small distances repulsive character (mainly via ω - mesons mediated), at medium distances, it is highly attractive ( due to two -meson exchange, analogous to the two -photon exchange of the van der Waals forces ), and at large distances it shows exponentially decaying character ( replacement of individual mesons ).

Range

In this exchange model the finite range of the interaction between the nucleons from the finite mass of the pions follows. The maximum range of the interaction can be estimated by:

• The relationship
• The energy-time uncertainty relation
• And Einstein's famous equivalence between energy and mass

It is of the order of the Compton wavelength of Austauschteilens. In the case of pions one comes to values ​​from a few Fermi (10-15 m). This short compared to the core range expansion is reflected in the constant binding energy per nucleon, which in turn is the basis for the droplet model.

Example process

As an example, the replacement of a charged pion is described between a proton and a neutron: