- #1

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## Main Question or Discussion Point

How to use the standard techniques of projection operators to obtain the equation (1) by the first formula? Thanks

- Thread starter Thor Shen
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- #1

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How to use the standard techniques of projection operators to obtain the equation (1) by the first formula? Thanks

- #2

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In this case it's simpler to work with a real representation of the pion fields ##\vec{\pi} \in \mathbb{R}^3##. Then the isospin rotations are simply described by real ##\mathrm{SO}(3)## matrices. The split into irreducible parts of the tensor product is then straight forward.

The ##s=0## part of the product of two pion fields is found by looking for a scalar built as a bilinear form of ##\vec{\pi}##. That's uniquely given by the scalar product ##\vec{\pi} \cdot \vec{\pi}=\delta_{ab} \pi^a \pi^b##. To find the corresponding amplitude for this channel thus you simply trace over the index pair ##(a,b)##

$$\delta_{ab} T^{abcd}=(3A+B+C) \delta^{cd}.$$

This gives you the first equation in (1).

The ##s=1## piece is given by the vector product ##\epsilon_{abe} \pi^a \pi^b##. Thus you have to contract the amplitude with ##\epsilon_{abe}## leading to

$$\epsilon_{abd} T^{abcd}=B \epsilon_{cde}+C \epsilon_{dce}=(B-C) \epsilon_{cde},$$

giving you the 2nd line of Eq. (1).

Finally the ##s=2## part is represented by the traceless symmetric part of the tensor ##\vec{\pi} \otimes \vec{\pi}##, i.e., ##3 \pi^a \pi^b -\vec{\pi}^2 \delta^{ab}##. So you just have to symmetrize your amplitude wrt. the index pair ##(a,b)## and subtract the trace, leading to

$$T^{(ab)cd}=2A \delta^{ab} \delta^{cd}+ (C+D) \delta^{ac} \delta^{bd}$$

The trace is

$$\delta_{ab} T^{(ab)cd}=(6A+C+D)\delta_{cd}$$

and thus you get

$$T^{(ab)cd}-\frac{1}{3} (6A+C+D) \delta^{ab} \delta^{cd} = \frac{2}{3} (C+D) \delta^{ac} \delta^{bd},$$

which gives you the corresponding amplitude in the 3rd line of Eq. (1).

- #3

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I am a newcomer in this field, so there are many definitions and notions I cannot catch up. Firstly I still cannot understand how to get the tensor [tex]3\pi^a\pi^b-\vec{\pi}^2\delta^{ab}[/tex] and find the way that symmetric amplitude subtracts its trace? Second problem is the symmetric amplitude

[tex] T^{(ab)cd}=\frac{1}{2}(A\delta^{ab}\delta^{cd}+B\delta^{ac}\delta^{bd}+C\delta^{ad}\delta^{bc}+A\delta^{ba}\delta^{cd}+B\delta^{bc}\delta^{ad}+C\delta^{bd}\delta^{ac})

[/tex].

Could you point out where is my error, because the definition I remembered is not the same as yours. Last but not least, [tex]2A\delta^{ab}\delta^{cd}+(B+C)\delta^{ac}\delta^{bd}-\frac{1}{3}(6A+B+C)\delta^{ab}\delta^{cd}=(B+C)\delta^{ac}\delta^{bd}-\frac{1}{3}(B+C)\delta^{ab}\delta^{cd}[/tex], then how to deal with the different between [tex]\delta^{ab}\delta^{cd}~and~\delta^{ac}\delta^{bd}[/tex] and the factor [tex]\frac{2}{3}[/tex]

By the way, which book can tell me more details about this notion? Thanks again!

- #4

ChrisVer

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In general these type of combinations can be easier obtained (I think) from Young Tableaux...

For the other question, you just have:

[itex] (C+D) \delta^{ac} \delta^{bd} - \frac{1}{3} (C+D) \delta^{ab} \delta^{cd} [/itex]

[itex] \frac{1}{3}(C+D) [3\delta^{ac} \delta^{bd} - \delta^{ab} \delta^{cd} ] =\frac{1}{3}(C+D) [2\delta^{ac} \delta^{bd} + \delta^{ac} \delta^{bd} - \delta^{ab} \delta^{cd} ] [/itex]

The last two terms in the brackets cancel out....

- #5

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Thank you for your reply. The traceless and symmetric for the tensor like the Young using in meson decomposed into octet and decuplet, right?

In general these type of combinations can be easier obtained (I think) from Young Tableaux...

For the other question, you just have:

[itex] (C+D) \delta^{ac} \delta^{bd} - \frac{1}{3} (C+D) \delta^{ab} \delta^{cd} [/itex]

[itex] \frac{1}{3}(C+D) [3\delta^{ac} \delta^{bd} - \delta^{ab} \delta^{cd} ] =\frac{1}{3}(C+D) [2\delta^{ac} \delta^{bd} + \delta^{ac} \delta^{bd} - \delta^{ab} \delta^{cd} ] [/itex]

The last two terms in the brackets cancel out....

Well, why the procedure that symmetrizing amplitude with subtracting its trace can stand for T

- #6

ChrisVer

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the cancelation of deltas you give are a general feature and doesn't apply for a given representation...

A fast way is to use mathematica to show you that the combination is zero for all a,b,c,d which was the way I used to give a fast answer... As for how to see that, well you could play with symmetries? or try to put some given values on a,b,c,d.... ?

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- #8

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Thank you for your recommendation. By the way , [itex]T^{I=2}(s,t)=<I=2,I_3=0|T^{abcd}|I=2,I_3=0> [/itex]? I and I

- #9

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Thanks again firstly.

the cancelation of deltas you give are a general feature and doesn't apply for a given representation...

A fast way is to use mathematica to show you that the combination is zero for all a,b,c,d which was the way I used to give a fast answer... As for how to see that, well you could play with symmetries? or try to put some given values on a,b,c,d.... ?

In general, [itex]\delta^{ac}\delta^{bd}-\delta^{ab}\delta^{cd}=\varepsilon_{dae}\varepsilon_{bce}\neq 0[/itex], and a,b,c,d,e=1,2,3.

Only when a=d or c=b are above deltas equal to 0.

- #10

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Sure, you can as well treat the entire business also with the usual isospin states. Then you represent your pions with ##\pi^{\pm}## and ##\pi^0## fields rather than in the real SO(3) representation of isospin. Of course, this is entirely equivalent. The relation between the fields isThank you for your recommendation. By the way , [itex]T^{I=2}(s,t)=<I=2,I_3=0|T^{abcd}|I=2,I_3=0> [/itex]? I and I_{3}are isospin and component of isospin for initial final state,respectively

##\pi^{\pm}=\frac{1}{\sqrt{2}} (\pi^1 +\pm \mathrm{i} \pi^2), \quad \pi^{0}=\pi^3.##

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