Quantcast
  • Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.

News

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback

Attributions

(propose a free ad)

Site Statistics

145 submissions , 122 unreviewed
3,930 questions , 1,398 unanswered
4,847 answers , 20,601 comments
1,470 users with positive rep
501 active unimported users
More ...

How to compute amptitude for $\gamma\pi^+\to\pi^+\pi^0$

+ 4 like - 0 dislike
167 views

I wish to find the amplitude for process $\gamma\pi^+\to\pi^+\pi^0$ at low energies. I am familiar with the basic concepts and techniques of QFT but have never dealt with the scattering processes including hadrons. In this case, I do not have a clue where to start. I am basically asking for a reference where this process is computed and/or what are the key points one needs to have in mind thinking about the reaction: appropriate effective Lagrangian, certain selection rules etc.

asked May 29, 2015 in Theoretical Physics by Weather Report (210 points) [ revision history ]

This process generally belongs to the field of electrodynamics of hadrons. There is an old book by R. Feynman about it and, I am sure, there are recent textbooks and monographs on this subject.

2 Answers

+ 3 like - 0 dislike

Here are some references for related processes, using different assumptions:

A 1960 paper on pion-photon scattering

Feynman's 1972 book Photon-Hadron Interactions (see table of contents).

A survey article The hadronic properties of the photon in high-energy interactions

Charged pion-pair production and pion polarizabilities to two loops

Reaction $\gamma\pi\to\pi\pi$ in a confined quark model

answered May 29, 2015 by Arnold Neumaier (12,355 points) [ revision history ]
edited May 29, 2015 by Arnold Neumaier
+ 3 like - 0 dislike

After some time spent with the literature, I've decided to write an answer. Hopefully, this will sharpen my own thoughts and potentially help the others.

Currently a detailed calculation from the first principles of QCD is not available for the hadronic processes. Instead, one has to resort to some effective description. A common tool is the chiral perturbation theory (http://en.wikipedia.org/wiki/Chiral_perturbation_theory) -- effective theory constructed by taking the (approximate) chiral symmetry as well as other symmetries of QCD into account.

The standard chiral Lagrangian is symmetric w.r.t. replacement $\pi\to-\pi$ and therefore forbids processes with the odd number of $\pi$-mesons, like $\pi^0\to\gamma\gamma$ or the process of interest $\gamma\pi^+\to\pi^0\pi^+$. However, this $\pi\to-\pi$ symmetry is not a legitimate symmetry of QCD but only of the simple effective description. One can add a contribution to the chiral Lagrangian known as the Wess-Zumino term (http://www.sciencedirect.com/science/article/pii/037026937190582X) which violates this symmetry and permits the corresponding processes. This anomalous term is very special for it is not a local Lagrangian in 4d. Another side of this peculiarity is that in contrast to the other terms in the effective Lagrangian the anomalous term comes with the predefined coefficient (Witten http://www.sciencedirect.com/science/article/pii/0550321383900639).

Effective theory amplitudes are naturally expanded in powers of $E/\Lambda\ll1$ with the $E$ the energy scale of the process and $\Lambda$ the characteristic scale up to which the effective description holds. The lowest-energy amplitude for reaction $\gamma\pi^+\to\pi^0\pi^+$ is produced by a single tree diagram coming from the Wess-Zumino term and it is elementary to evaluate (for example see  http://www.sciencedirect.com/science/article/pii/037026938490100X).

Unlike the case of $\pi^0\to\gamma\gamma$ the agreement between the theoretical prediction and the experimental value is not excellent: $A_{theor}=9.70\pm0.05 GeV^{-3},\quad A_{exp}=12.9\pm0.9\pm0.5 GeV^{-3}$. The discrepancy is believed to be due to disregarding the higher-energy corrections. However, the task of finding/estimating the corrections to the lowest-order result is not at all simple. One can adopt a variety of approaches on this way. Here my understanding falls rapidly and I will content myself with a reference to paper http://arxiv.org/abs/hep-ph/0107127 where combining a number of higher-energy effects (including loop corrections and previously overlooked electromagnetic corrections due to a single-photon exchange) the authors were able to bring the theory and experiment into agreement.

This summarizes my understanding. Any clarifications and corrections are welcome.

answered Jun 9, 2015 by Weather Report (210 points) [ revision history ]
edited Jun 9, 2015 by Weather Report

Nice summary!

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
p$\hbar\varnothing$sicsOverflow
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
To avoid this verification in future, please log in or register.




user contributions licensed under cc by-sa 3.0 with attribution required

Your rights
...