Pion

This article explains the particle Pion. The artillery piece is explained under 2S7.

Pions or π $\pi$ -mesons (formerly also called Yukawa particles, since predicted by Hideki Yukawa) are the lightest mesons. According to the Standard Model of particle physics, they contain two valence quarks and are therefore not usually considered elementary particles today. Like all mesons, they are bosons, so they have integer spin. Their parity is negative.

There is a neutral pion π $\pi^0$ and two charged pions: $\pi ^{+}$ and its antiparticle π $\pi ^{-}$ . All three are unstable and decay by weak or electromagnetic interaction.

Structure

The π $\pi ^{+}$ is a combination of an up quark and an anti-down quark $\bar d$ (antiquarks are indicated by an overstrike):

$|\pi ^{+}\rangle =|u{\bar d}\rangle$ ,

its antiparticle π $\pi ^{-}$ a combination of a down quark and an anti-up quark ${\bar u}$ :

$|\pi ^{-}\rangle =|d{\bar u}\rangle$ .

Both have a mass of 139.6 MeV/c². Currently, the most accurate measurements of its mass are based on X-ray transitions in exoticatoms that have a π $\pi ^{-}$ instead of an electron. The lifetime of the π $\pi ^{\pm }$ is 2.6 - 10-8 s.

The π $\pi^0$ is a quantum mechanical superposition state of a $u{\bar u}$ - and a $d{\bar d}$ -combination, i.e. two quarkonia. It holds:

$|\pi ^{0}\rangle ={\frac {1}{{\sqrt {2}}}}{\Big [}|u{\bar u}\rangle -|d{\bar d}\rangle {\Big ]}$

while the state orthogonal to it, ${\frac {1}{\sqrt {2}}}{\Big [}|u{\bar {u}}\rangle +|d{\bar {d}}\rangle {\Big ]}$ , with $|s{\bar {s}}\rangle$ mixing to the eta mesons.

Its mass of 135.0 MeV/c² is only slightly smaller than that of the charged pions. Since it decays via the much stronger electromagnetic interaction, its lifetime of 8.5 - 10-17 s is about 10 orders of magnitude shorter.

Due to a freely selectable phase, the three wave functions can also be expressed in the less frequently used form $|\pi ^{+}\rangle =|u{\bar d}\rangle$ , $|\pi ^{-}\rangle =-|{\bar {u}}d\rangle$ and $|\pi ^{0}\rangle ={\frac {1}{{\sqrt {2}}}}{\Big [}|d{\bar d}\rangle -|u{\bar u}\rangle {\Big ]}$ can be written. This then conforms to the Condon-Shortley convention.

Decays

The different lifetimes are due to the different decay channels:

the charged pions decay to 99.98770(4) % by the weak interaction into a muon and a muon neutrino:

$\,\pi ^{+}\to \mu ^{+}+\nu _{{\mu }}$

$\,\pi ^{-}\to \mu ^{-}+\overline {\nu }_{{\mu }}$

The actually energetically more favorable decay into an electron and the associated electron-neutrino is strongly suppressed for helicity reasons (see: helicity#decay of the pion).

In contrast, the decay of the neutral pion takes place by means of the stronger and thus faster electromagnetic interaction. The end products here are usually two photons γ $\gamma$

$\pi ^{0}\to 2\gamma$ →

with a probability of 98,823(32) % or a positron e+, an electron e- and a photon

$\pi ^{0}\to e^{+}+e^{-}+\gamma$

with a probability of 1.174(35) %.

Because of its short lifetime of 8.5 - 10-17 s, the neutral pion is detected in experiments by observing the two decay photons in coincidence.

Questions and Answers

Q: What is a pion?

A: A pion is a meson, which is a subatomic particle made of one quark and one antiquark.

Q: How many types of quarks are there?

A: There are six types of quarks (called flavours).

Q: What two flavours go together to make a pion?

A: The two flavours that go together to make a pion are called up and down.

Q: Does the charge of the pion depend on the type of quarks it contains?

A: Yes, the charge of the pion depends on the type of quarks it contains. When two quarks have different flavours (up and down), the pion will have a charge. This charge is positive when an up quark pairs with a down antiquark and negative when a down quark pairs with an up antiquark.

Q: How long do charged pions exist for?

A: Charged pions exist for around 26 nanoseconds on average. Neutral pions last for only a tiny fraction of this time.

Q: Why are pions significant to our lives?

A: Pions are significant to our lives because they are one of the ways for strong force interactions to take place between nucleons like protons and neutrons in ordinary matter, which hold the nucleus together.

Q: What makes charged or neutral mesons with longest mean lifetime?

A: Charged or neutral mesons with longest mean lifetime are those made up by positive or negative charged particles called hadrons (particles made up by quarks).