Talk:Pion

Problems about color

My understanding is that a pion is composed of a quark and an antiquark of the same color, for example, a red up quark and a red antidown quark. But both quarks are red (they have to be to give overall white color). But there appears to be no mention of color in the text, which seems like a serious omission. Not only that, but the animation shows pions composed of two different colors, which appears to be wrong. Andrewthomas10 (talk) 13:51, 21 October 2015 (UTC)[reply]

Your understanding is wrong. It is composed of a quark of a color and an antiquark of the anticolor of the quark. Think of it as a Color charge and its anticharge, the meson is color neutral. The representation of antiblue as yellow, antigreen as magenta and antired as cyan is one option for that. It gives the "white" color (via additive color mixing). — Preceding unsigned comment added by 147.142.17.186 (talk) 17:18, 23 February 2017 (UTC)[reply]

ToDo List

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

pions mediate the nucleon-nucleon force.
The Yukawa potential is a non-relativistic description for them,
The relativistic equation for pions is the Klein-Gordon equation
They are pseudoscalars under parity inversion,
They couple to the axial vector current, the couple is the axial coupling constant which is a "fundamental"/"important" parameter of the nucleon.
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.
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.
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.
Pronunciation-key for "pion?" Koyae (talk) 23:58, 23 March 2010 (UTC) ...Nothing's worse that physicists (or aspiring ones) who can't talk.[reply]

linas 01:00, 24 May 2005 (UTC)[reply]

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)[reply]

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

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

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

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)[reply]

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)[reply]

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)[reply]

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

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

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. $J^{\mathrm {P} }=0^{-}$ 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)[reply]

Hadron overhaul

Please give input at Talk:Hadron#Hadron overhaul. Thanks. Headbomb {^ταλκ_{κοντριβς} – WP Physics} 01:59, 24 January 2010 (UTC)[reply]

π⁰
(often
π
⁰)

The artcle begins with:

In particle physics, a pion (short for pi meson; denoted
π
) is any of three subatomic particles:
π⁰
(often
π
⁰),
π⁺
and
π⁻
. 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
π⁰
(often
π
⁰) 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
π⁰
(often
π
⁰), someone needs to clarify for ignorant laymen like me what the difference is between
π⁰
and
π
⁰.--Jim10701 (talk) 16:53, 22 July 2010 (UTC)[reply]

Flavorful pions

Why are the
π^±
not classified as flavorful mesons? Up and down are flavors, are they not? --Michael C. Price ^talk 10:07, 14 October 2010 (UTC)[reply]

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 B_s, rather than K_b. A top/antibottom mix would be T_b, 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 Landry, G. (2009). "A proposal for a new hadron nomenclature" (PDF). Physics in Canada. 69 (2, suppl.). 2009 CAP Congress: 96. (conference PDF). Headbomb {talk / contribs / physics / books} 13:08, 14 October 2010 (UTC)[reply]

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)[reply]

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. Headbomb {talk / contribs / physics / books} 12:27, 15 October 2010 (UTC)[reply]

So the (uctdsb) group has no collective name? --Michael C. Price ^talk 21:46, 15 October 2010 (UTC)[reply]

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)[reply]

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. Headbomb {talk / contribs / physics / books} 07:08, 16 October 2010 (UTC)[reply]

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)[reply]

π (pi)

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

New reference for π^(+0-)

Lists new values for

mean-life
decay-modes

http://pdg.lbl.gov/2011/listings/rpp2011-list-pi-zero.pdf

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

pseudo-Nambu-Goldstone boson?

The text mentions the pion as a pseudo-Nambu-Goldstone boson, however the linked article is to Goldstone boson. This is somewhat confusing, as I believe the this obscures the pseudoscalar implications. — Preceding unsigned comment added by 70.247.168.41 (talk) 13:17, 8 May 2015 (UTC)[reply]

Helicity suppression

The explanation of the helicity suppression indicates that it is caused by parity violation, i.e. the coupling to left-handed particles and right-handed anti-particles. Supposed the weak interaction would conserve parity, i.e. it couples with the same probability to left-handed and right-handed particles. The vector-line structure of the interaction (W has Spin 1) forces a coupling to a left-handed particle and a right handed particle at a given vertex. Thus, the helicity suppression in the pion decay is caused by the vector-like structure of the interaction plus conservation of angular momentum. Pen88 (talk) 16:41, 7 September 2016 (UTC)[reply]

Why is there only one neutral pion?

To the layman, it seems odd that there is only one neutral pion — shouldn't there be one up+antiup and one down+antidown? Well, the math doesn't come out that way (short remarks above suggest earlier versions of the article explicitly described the $\pi ^{0}$ as an equal superposition of $u{\bar {u}}$ and $d{\bar {d}}$ ), but the article could do with an explicit explanation of how this comes about. (Possibly point 6 in the above todo list is a more technical way of saying the same thing.) 130.243.68.180 (talk) 18:12, 11 May 2018 (UTC)[reply]

I was about to ask this. I suspect it should be on or the other linear combination of the two. (That is, plus or minus.) Gah4 (talk) 01:13, 17 November 2022 (UTC)[reply]

The reason there's only one neutral Pion, is that the total isospin of a symmetric superposition of

u{\bar {u}}

and

d{\bar {d}}

is zero. When it is said that the charged Pions have a isospin of +1 and -1 and the neutral Pion has an isospin of 0, that is the projection of isospin in all directions. The total isospin of all three is 1 (or rather, square root of three divided by two). This is in perfect analogy with a particle with spin 1, which has three possible eigenvalues in the z component of spin, +1, -1, and 0, but always has total spin angular momentum of square root of three divided by two. Unfortunately I can't think of a simple non-technical way to explain this and why an antisymmetric superposition has total spin 1 and a symmetric superposition has total spin 0. So there sorta is a second neutral pion, but it actually isn't a pion, but a superposition of the eta and eta prime mesons. I guess point 6 is what I just described, but in representation theory terms (i.e. if you have a rank 2 tensor over a

\mathbb {C} ^{2}

vector space, the trace is invariant under

SU(2)

, but the traceless component transforms like the triplet representation). --Lukflug (talk) 20:01, 22 November 2022 (UTC)[reply]

This one says that it is the

{\frac {u{\bar {u}}-d{\bar {d}}}{\sqrt {2}}}

. That seems like what I would expect, though I wasn't sure if it was the symmetric or antisymmetric case. Gah4 (talk) 06:09, 23 November 2022 (UTC)[reply]

The counterintuitive - sign is an artifact of the conjugate SU(2) representation acting on the antiquark. If you were composing spins, it would correspond to a + sign, of the spin triplet. The isoscalar, by contrast, has a + sign, corresponding to the - sign of the spin singlet. Cuzkatzimhut (talk) 14:25, 23 November 2022 (UTC)[reply]

Seems that there should be mention of the eta meson, a different linear combination, and the eightfold way describing how they come together. Gah4 (talk) 09:24, 30 September 2024 (UTC)[reply]

Correct information looks misleading

The information that π0 → 3γ decay is forbidden looks like it is a description of decay which takes place but forbidden and may be not correcly understood. It happens because the information on prohibited decay is mixed with information on real decay pathways. Probably, if this info would be given after real decay pathways, it would be great, as it is important.

The decay π0 → 3γ (as well as decays into any odd number of photons) is forbidden by the C-symmetry of the electromagnetic interaction. The intrinsic C-parity of the π0 is +1, while the C-parity of a system of n photons is (−1)n. Unsigned comment by User:81.211.8.86 .

Please sign your comments. What is your point? The decay is forbidden and impossible. Cuzkatzimhut (talk) 18:53, 22 May 2019 (UTC)[reply]

In many cases, forbidden means allowed using some other mechanism, often that is much slower. See Forbidden mechanism. Gah4 (talk) 19:55, 24 November 2022 (UTC)[reply]

What about spin?

Lack of spin 2600:100F:B1B8:A729:0:36:90BF:BF01 (talk) 22:02, 19 December 2023 (UTC)[reply]