Understanding Peskin's QFT: Deriving Equations (2.35) and (2.54)

[Beginner_2010]

Homework Statement

Hi,

I have two stupid questions about Peskin's QFT book.

(1) P23, How to derive from (2.35) to (2.36)
(2) P30, How to derive (2.54)

Homework Equations

(1)

(2)

The Attempt at a Solution

(1) If I consider the dual-space vector, [tex] \langle \mathbf{q} | = \sqrt{2 E_{\mathbf{q} }} \langle 0 | a_{\mathbf{q}} [/tex]

Combine with the ket (2.35), obtain
[tex]

\langle\mathbf{q} | \mathbf{p} \rangle = 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } \langle 0 | a_{\mathbf{q}} a_{\mathbf{p}}^{\dag} | 0 \rangle
= 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } (2 \pi)^{3} \delta^{(3)} (\mathbf{p} - \mathbf{q})
[/tex]

Therefore
[tex]
\langle \mathbf{p} | \mathbf{q} \rangle = \langle \mathbf{q} | \mathbf{p} \rangle^* = 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } (2 \pi)^{3} \delta^{(3)} (\mathbf{p} - \mathbf{q})
[/tex]

But Peskin's (2.36) has a prefactor [tex] 2 E_{\mathbf{p}} [/tex] instead of [tex] 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } [/tex], is that made to be the convention?

(2) Is that the principal value of integral [tex] \int_{- \infty}^{+\infty} d p^0 [/tex] or including the little semi-cycles around [tex] -E_{\mathbf{p}} [/tex] and[tex] +E_{\mathbf{p}} [/tex] ? If includes the semi-cycles, i can get the result

Thank you ^_^

 

[vela]

The Attempt at a Solution

(1) If I consider the dual-space vector, [tex] \langle \mathbf{q} | = \sqrt{2 E_{\mathbf{q} }} \langle 0 | a_{\mathbf{q}} [/tex]

Combine with the ket (2.35), obtain
[tex]

\langle\mathbf{q} | \mathbf{p} \rangle = 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } \langle 0 | a_{\mathbf{q}} a_{\mathbf{p}}^{\dag} | 0 \rangle
= 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } (2 \pi)^{3} \delta^{(3)} (\mathbf{p} - \mathbf{q})
[/tex]

Therefore
[tex]
\langle \mathbf{p} | \mathbf{q} \rangle = \langle \mathbf{q} | \mathbf{p} \rangle^* = 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } (2 \pi)^{3} \delta^{(3)} (\mathbf{p} - \mathbf{q})
[/tex]

But Peskin's (2.36) has a prefactor [tex] 2 E_{\mathbf{p}} [/tex] instead of [tex] 2 \sqrt{ E_{\mathbf{q}} E_{\mathbf{p} } } [/tex], is that made to be the convention?

The delta function is non-zero only when p=q, so E_p=E_q.

(2) Is that the principal value of integral [tex] \int_{- \infty}^{+\infty} d p^0 [/tex] or including the little semi-cycles around [tex] -E_{\mathbf{p}} [/tex] and[tex] +E_{\mathbf{p}} [/tex]? If it includes the semi-cycles, I can get the result.

 

[Beginner_2010]

Thank you!