Pions, or pi mesons, are particles that consist of a quark and an antiquark of either up or down flavour. The exchange of virtual pions provides an explanation for the residual strong force between nucleons.
Questions tagged [pions]
217 questions
40
votes
4 answers
Is it pions or gluons that mediate the strong force between nucleons?
From my recent experience teaching high school students I've found that they are taught that the strong force between nucleons is mediated by virtual-pion exchange, whereas between quarks it's gluons. They are not, however, taught anything about…
qftme
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39
votes
3 answers
The exchange of photons gives rise to the electromagnetic force
Pardon me for my stubborn classical/semiclassical brain. But I bet I am not the only one finding such description confusing.
If EM force is caused by the exchange of photons, does that mean only when there are photons exchanged shall there be a…
skywaddler
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votes
3 answers
What does it mean that the neutral pion is a mixture of quarks?
The quark composition of the neutral pion ($\pi^0$) is $\frac{u\bar{u} - d\bar{d}}{\sqrt{2}}$. What does this actually mean?
I think it's bizarre that a particle doesn't have a definite composition. There's a difference of 2 MeV between the quark…
Kasper
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How do $\pi^0$ particles exist?
I have been taught that the $\pi^0$ particle contains either an up quark and an anti-up quark or a down and an anti-down. How can these exist without annihilating?
Also, it is its own antiparticle, but it doesn't make sense that the up version and…
david4dev
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2 answers
Chiral anomaly and decay of the pion
I am told that if all classical symmetries were reflected as quantum symmetries, the decay of the neutral pion $$\pi^0 ~\longrightarrow~ \gamma\gamma$$ would not happen. Why would the conservation of the axial current in QED prevent the decay of the…
Whelp
- 4,176
15
votes
1 answer
Physical meaning of the chiral condensate in QCD
Considering the QCD Lagrangian in the chiral limit, where all the quarks masses are set to zero. Then the Lagrangian has the following chiral symmetry:
$$
SU(L)_{V} \times SU(L)_{A} \times U(1)_{V} \times U(1)_{A}
$$
As it is known, this symmetry…
ottavio
- 151
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3 answers
Is the neutral pion a singlet?
In Griffiths' Introduction to Elementary Particles, it is mentioned p. 179 that the $\pi^0$ is a singlet under $SU(2)$ isospin. But it is also part of the $\pi^-,\pi^0,\pi^+$ isospin triplet. How can it be both?
Don't particles of a given $SU(2)$…
Whelp
- 4,176
11
votes
5 answers
Why is the decay of a neutral $\rho$ meson into two neutral pions forbidden?
Why is the decay of a neutral $\rho$ meson into two neutral pions forbidden? (Other modes of decay are possible though.)
Is it something with conservation of isospin symmetry or something else? Please explain in a bit more detail.
Adeel
- 219
11
votes
1 answer
What's the heuristic reasoning for Chiral Symmetry breaking
In Peskin and Schroeder (page 669), and other references, the heuristic reasoning for why one would expect chiral symmetry breaking at low energies is that quark masses are small and hence it's not very energetically costly to create quark-antiquark…
user26866
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How is the pion related to spontaneous symmetry breaking in QCD?
In chapter 19 of An Introduction to Quantum Field Theory by Peskin & Schroeder, they discuss spontaneous symmetry breaking (SSB) at low energies in massless (or nearly massless) QCD, given by
$$\mathcal{L}_{\text{QCD}} = \bar{Q}…
Flynn Linton
- 121
- 5
11
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2 answers
Why the pion does not get mass under Spontaneus breaking of chiral symmetry, but the quarks do?
Some sources state that when the mass of a quark goes to zero, it allows for Spontaneous Breaking of Chiral Symmetry and gets a constituent mass of about $200\, \mathrm{MeV}$.
Other sources state that when the masses of the light quarks go to zero,…
arivero
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1 answer
$\Delta^+$ decay in GZK process
The dominant channels in the GZK process are
$$p+\gamma_{\rm CMB}\to\Delta^+\to p+\pi^0,$$
$$p+\gamma_{\rm CMB}\to\Delta^+\to n+\pi^+.$$
According to the pdg, $\Delta\to N+\pi$ makes up essentially 100% of the branching ratio (BR). It doesn't,…
jazzwhiz
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1 answer
Up and down quark content of neutral pion
If isospin were a perfect symmetry, then the neutral pion would have equal $u\bar u$ and $d\bar d$ content, but since up quarks are slightly lighter than down quarks, the neutral pion, being the lightest neutral meson, should have a slightly higher…
Bert Barrois
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9
votes
1 answer
Pion production in proton-proton collision
Why is $\pi^0$ created in the high-energy collision $p+p\to p+p+\pi^0$?
Kyle Kanos
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9
votes
2 answers
How do we measure meson decay constants?
I'm trying to understand how people actually measure decay constants that are discussed in meson decays. As a concrete example lets consider the pion decay constant. The amplitude for $\pi ^-$ decay is given by,
\begin{equation}
\big\langle 0 | T …
JeffDror
- 9,093