# Questions tagged [pions]

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103 questions
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### CP-symmetery conservation

How can one show that for a given decay, say $\pi^+ \rightarrow \nu_\mu + \mu^+$, CP-symmetry is conserved or violated? Is there a visual way of doing such?
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### Why can the pion decay into two photons?

The neutral pion belongs to the pseudoscalar meson octet, so it has, in the ground state ($L=0$): \begin{align} P_{\pi^0}&=-1 \\ C_{\pi^0}&=+1. \end{align} And the photon has \begin{align} ...
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### Why is the relative angular momentum of quark+antiquark in a pion equal to zero?

In a set of lecture notes provided by my lecturer, it says that a pion consists of a quark ($q$) and an antiquark ($\bar{q_2}$) with relative $L=0$. The negative parity and the fact that $S=0$ makes ...
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### Transformation of opening angle in different frame of rerefence in special relativity

I have to find the minimum opening angle for the decay $\pi^0 \rightarrow \gamma \gamma$ given the energy and rest mass of the $\pi^0$. My question is that: since the center-of-mass frame's opening ...
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### How does the exchange of pions result in the strong force?

I understand that the residual strong force is a result of an exchange of pions. But I fail to understand how this exchange results in a force that holds nuclei together! May this query please be ...
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### Experimental proof pions are pseudoscalar particles

In nuclear physics lessons I've been taught that pions are pseudoscalar particles and thus their intrinsic parity is odd. The professor said that an experimental proof of this can be derived observing ...
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### Feynman Diagrams for pion production in proton-proton collisions

I have a conceptual question about the Feynman Diagrams representing pion production at the LHC. When trying to draw the FD for this process, I have drawn two possibilities, and I'm not sure which is ...
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### $\rho\to \pi\pi$ parity

I'm having some difficulty understanding why the $\rho^0$$\rightarrow$$\pi^+ \pi^-$ decay conserves parity. By my understanding, the parity of a meson is given by $P_aP_b(-1)^{L+1}$ and the parity of ...
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### Form of the one-pion exchange potential

Part of the one pion exchange potential (OPEP) contains the following term $$V(r) \propto (\vec{\sigma}^{(1)} . \nabla)(\vec{\sigma}^{(2)} . \nabla) \frac{\exp\left(- \frac{r}{R}\right)}{r}$$ And I ...
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### Tree Order Feynman Diagram with neutral pion

I am trying to draw the tree-order Feynman diagram for the process $\gamma + p \rightarrow \pi^0 + p$. My question regards the quark content (thus the quark lines of the diagram) for the neutral pion....
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### What does it mean to subtract these quantum states?

For example, in particle physics the neutral pion $π^0$ exists in two quantum states $u\bar{u}$, and $d\bar{d}$; and is displayed in the form: $$\frac{u\bar{u}-d\bar{d}}{\sqrt2}$$ I understand that ...
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### Pion interaction in vacuum [closed]

Does pion zero particle $\pi^0$ (zero charge and null spin) experience casimir force? If i wonder how pion particle with quantum electrodynamic field.
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### Neutral meson weak interaction

Suppose the quark neutral current weak interaction lagrangian: $$\tag 1 L = \bar{q}\gamma_{\mu}(g_{V}+g_{A}\gamma_{5})qZ^{\mu}, \quad q = u,d$$ How to derive from this Lagrangian the neutral ...
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### Electromagnetic decay of a neutral pion

Why do neutral pions decay electromagnetically? Other neutral particles such as the neutron decay weakly, and charged pions decay weakly.