# Talk:Pion

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## ToDo List

The following article enhancements should be dealt with at some point:

1. pions mediate the nucleon-nucleon force.
2. The Yukawa potential is a non-relativistic description for them,
3. The relativistic equation for pions is the Klein-Gordon equation
4. They are pseudoscalars under parity inversion,
5. They couple to the axial vector current, the couple is the axial coupling constant which is a "fundamental"/"important" parameter of the nucleon.
6. pion is part of the triplet representation aka adjoint representation of SU(2) flavour symmetry, because the up and down quarks belong doublet rep of SU(2) and a pion is a quark and anti-quark, and su(2) cross su(2)-bar is the adjoint rep.
7. The SU(2) flavour symmetry allows fun games with chiral symmetry and in particular, since su(2) also describes spatial rottations, one can play games with non-trivial topological mappings of the embedding of pion fields into 3D space. i.e. mappings because of the non-trivial fundamental group.
8. Skyrme's topological soliton aka skyrmion aka 'cloud of pions' aka 'chiral model' is a decent one-parameter model of the nucleon (proton/neutron), which predicts a variety of nuclear properties fairly accurately with only one free parameter.
9. Pronunciation-key for "pion?" Koyae (talk) 23:58, 23 March 2010 (UTC) ...Nothing's worse that physicists (or aspiring ones) who can't talk.

linas 01:00, 24 May 2005 (UTC)

Do pions mediate the strong force? gluons do this job! Lluis M. 12:00, 24-12-2005
No and Yes. Pions mediate what is now called the inter-nucleon force, but used to be called the strong force before quarks were discovered. linas 15:55, 23 December 2005 (UTC)

It should be said somewhere that pions are the (pseudo-)Goldstone bosons associated to the chiral SU(2) spontaneus symmetry breaking of QCD.

${\displaystyle SU(2)\otimes {\overline {SU(2)}}\approx SO(3)\oplus U(1)}$

doesn't look right. I'm not even sure what ${\displaystyle {\overline {SU(2)}}}$ means, but if we ignore the overline, the LHS isn't the same as the RHS. Not that I know what ${\displaystyle \approx }$ means in this context.

Agreed. It didn't make any sense (what on earth is the direct sum of two lie groups?). I've removed it. It doesn't detract from
the section. Below is exactly what I removed.
Thus, one has
${\displaystyle SU(2)\otimes {\overline {SU(2)}}\approx SO(3)\oplus U(1)}$
which is one of the many relationships which lends weight to the quark model of pions and nucleons.
Shambolic Entity 04:30, 9 November 2006 (UTC)

errm...the feynman diagram for the pion decay is not correct: the W boson is represented by a dashed line, not a wavy one (which represents the photon). unfortunately atm. i do neither have time nor the experience to create a new one. anyone?! cheers. Diekuhmachtmuh 11:48, 17 November 2006 (UTC)

ok, edit: i just recognized that in respect to the electroweak unification, W- and Z-bosons are represented by the same symbolic propagator as the photon, that is, a wavy line. however, im almost all books and sources the like that i came across, W and Z are still represented by dashed lines. so much about consistency and convention, argl ;-). cheers. Diekuhmachtmuh 12:03, 17 November 2006 (UTC)

pions are not leptons, right? (see "History")

Pions are Bosons. Also, the Pi-Zero quark make-up below the image is wrong. Should be: ${\displaystyle \mathrm {\frac {u{\bar {u}}-d{\bar {d}}}{\sqrt {2}}} }$. I'll change it later today. Arthur (Lazyrussian) (talk) 13:01, 3 June 2008 (UTC)

I think the spin given for the pion in the box is confusing, even if it is correct and the nomenclature is different from others I've seen. ${\displaystyle J^{\mathrm {P} }=0^{-}}$ for all pions (0, - & +) (kind of by definition?), so I'm not sure what the 1(±1) notation is all about... —Preceding unsigned comment added by 86.26.51.55 (talk) 21:57, 24 March 2009 (UTC)

## Template:SubatomicParticle (often Template:SubatomicParticle0)

The artcle begins with:

In particle physics, a pion (short for pi meson; denoted Template:SubatomicParticle) is any of three subatomic particles: Template:SubatomicParticle (often Template:SubatomicParticle0), Template:SubatomicParticle and Template:SubatomicParticle. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force.

What does the parenthesis in Template:SubatomicParticle (often Template:SubatomicParticle0) signify? Although the zero is added through a different mechanism, the text displayed is identical. It ends up looking like apple (often apple), which adds no information. I know absolutely nothing about the subject of this article, but if there is important information in Template:SubatomicParticle (often Template:SubatomicParticle0), someone needs to clarify for ignorant laymen like me what the difference is between Template:SubatomicParticle and Template:SubatomicParticle0.--Jim10701 (talk) 16:53, 22 July 2010 (UTC)

## Flavorful pions

Why are the Template:SubatomicParticle not classified as flavorful mesons? Up and down are flavors, are they not? --Michael C. Price talk 10:07, 14 October 2010 (UTC)

That is because the nomenclature is based on isospin + "flavour quantum numbers" (strangeness S, charm C, bottomness B, topness T). Since they have S, C, B, T = 0, they are flavourless. You could easily assign them upness U and downness D, but this isn't done, and the rules would still be based on isospin + SCBT, rather than being based on UDSCBT.
The current naming scheme was adopted in the 1986 Review of Particle Physics by the PDG (before that it was a complete mess). There is some discussion found in there, but the basic idea is that since nomenclature for the most well-known states were based on isospin + strangeness, and only the hadrons with some C and B in them being inconsistent, formalizing nomenclature to be based on a isospin + flavour, following the mass hierarchy
• Topness (T)
• Bottomness (B)
• Charm (D)
• Strangeness (K)
• Isospin (i.e. the "flavourless")
This is why for example, a strange/bottom mix is named Bs, rather than Kb. A top/antibottom mix would be Tb, and a charm/antiup mix is a D. And down the line is the isospin chunk of mesons, which have horribly complex naming rules based on isospin, spin, and flavourless mixing.
I'm working on a new naming scheme for hadrons for my thesis (one based directly on quark content rather than going through isospin), but it certainly isn't ready for Wikipedia yet (amongst others it's not published, and it not adopted either). I gave a preview of it at the 2009 CAP Congress if you are interested. See {{#invoke:Citation/CS1|citation

|CitationClass=journal }} (conference PDF). 13:08, 14 October 2010 (UTC)

Thanks, that was very interesting. C. Amsler et al. (2008): Naming scheme for hadrons seems to make a distinction between "heavy flavours" (SCTB) and "light flavours" (ud). Perhaps we should adopt this here.--Michael C. Price talk 09:08, 15 October 2010 (UTC)
The problem with that is that strangeness is considered to be a light flavour. From that very PDF you can read "Since the strangeness or a heavy flavor of these mesons is nonzero..." So calling S a heavy flavour is non-standard. Calling ud and du flavourless is a bit of a misnomer if you think of things in terms of quark constituent, but as I said, nomenclature is based on isospin and SCBT, and not on quark constituents directly. 12:27, 15 October 2010 (UTC)
So the (uctdsb) group has no collective name? --Michael C. Price talk 21:46, 15 October 2010 (UTC)
A little googling reveals that collective noun is "flavor". See, for example, this physics dictionary, this 1980 paper.
Whether you call strangeness a heavy or light flavor is not important. Isospin is just a shorthand for the up or down flavors.
--Michael C. Price talk 05:43, 16 October 2010 (UTC)
The collective name for udscbt (in lowercase) is "quarks". There is no U and D, even if you can easily conceptualize them, only a linear combination of them (isospin projection), on which isospin is based (amongst other things, like spin and non-up/down quark content). The flavour quantum numbers are SCBT. Those with SCBT = 0 are said to be flavourless, those with SCBT ≠ 0 are said to be flavourful/flavored.
However, I don't see where this is relevant to the article, so at this point we're getting into WP:FORUM territory. 07:08, 16 October 2010 (UTC)
No, the collective name is quark flavors, as opposed to quark colors.
Relevance is that the pion should be in the flavorful meson table.--Michael C. Price talk 08:06, 16 October 2010 (UTC)

## π (pi)

The usage of Π is under discussion, see Talk:Pi. 65.93.12.101 (talk) 01:29, 4 April 2011 (UTC)

## New reference for π(+0-)

Lists new values for

• mean-life
• decay-modes

do we have a go to update these values Abyssoft (talk) 17:52, 27 July 2011 (UTC)