# Deriving charge for Noether current in free complex scalar field QFT

## Homework Statement

Hi I a attempting to derive the expression for the conserved Noether charge for a free complex scalar field.
The question I have to complete is: " show, by using the mode expansions for the free complex scalar field, that the conserved Noether charge (corresponding to complex phase rotations) is given by
$$Q= \int \frac{\partial^3 p}{(2\pi)^3} ( a_p^* a_p - b_p^* b_p)$$
Where $$a_p, a_p^*$$ and $$b_p, b_p^*$$ are the creation and annihilation operators for 2 kinds of particles respectively"

## Homework Equations

The Lagrangian for a free complex scalar field is;
$$L= \frac{1}{2} \partial_μ \psi \partial^μ \psi- \frac{1}{2}m^2\psi^2$$

I have recognized that there is a symmetry of$$\psi → e^{ i\alpha } \psi$$

which leads to a Noether current of
$$j_μ = i \partial _μ \psi^* \psi - i \partial _μ \psi \psi^*$$
$$Q= \int \partial ^3 x j_0 = i \int \partial ^3 x ( \partial _0 \psi^* \psi - \partial _0 \psi \psi^* )$$
$$Q=i \int \partial ^3 x ( \psi \dot{\psi^*} - \psi^* \dot{\psi} )$$
where
$$\dot{\psi} = \pi ^*$$
$$\dot{\psi^*} = \pi$$
so $$Q=i \int \partial ^3 x ( \psi \pi - \psi^* \pi ^* )$$
And the mode expansions;
$$\psi = \int \frac{\partial^3 P}{(2\pi)^3} \frac{1}{\sqrt{2 ω_p}}( a_p e^{ip.x } + b_p^* e^{-ip.x})$$
$$\psi^* = \int \frac{\partial^3 P}{(2\pi)^3} \frac{1}{\sqrt{2 ω_p}}( a_p^* e^{-ip.x} + b_p e^{ip.x })$$
$$\pi = i\int \frac{\partial^3 P}{(2\pi)^3} \sqrt{ \frac{ω_p}{2}}( a_p^* e^{-ip.x } - b_p e^{ip.x})$$
$$\pi^* = -i\int \frac{\partial^3 P}{(2\pi)^3} \sqrt{\frac{ω_p}{2}}( a_p e^{ip.x } - b_p^* e^{-ip.x})$$

## The Attempt at a Solution

I have then plugged these into the Q equation;
$$Q=i \int \partial ^3 x [ \int \frac{\partial^3 P}{(2\pi)^3} \frac{1}{\sqrt{2 ω_p}}( a_p e^{ip.x } + b_p^* e^{-ip.x}) i\int \frac{\partial^3 P}{(2\pi)^3} \sqrt{ \frac{ω_p}{2}}( a_p^* e^{-ip.x } - b_p e^{ip.x}) - \int \frac{\partial^3 P}{(2\pi)^3} \frac{1}{\sqrt{2 ω_p}}( a_p^* e^{-ip.x} + b_p e^{ip.x }) (-i)\int \frac{\partial^3 P}{(2\pi)^3} \sqrt{\frac{ω_p}{2}}( a_p e^{ip.x } - b_p^* e^{-ip.x}) ]$$
so factoring out the i's and 2 pi's from the integrals;
$$Q=\frac{i^2}{(2\pi)^6} \int \partial ^3 x [ \int \partial^3 P \frac{1}{\sqrt{2 ω_p}}( a_p e^{ip.x } + b_p^* e^{-ip.x}) \int \partial^3 P \sqrt{ \frac{ω_p}{2}}( a_p^* e^{-ip.x } - b_p e^{ip.x}) + \int \partial^3 P \frac{1}{\sqrt{2 ω_p}}( a_p^* e^{-ip.x} + b_p e^{ip.x }) \int \partial^3 P \sqrt{\frac{ω_p}{2}}( a_p e^{ip.x } - b_p^* e^{-ip.x}) ]$$
It's at this point I'm utterly stuck on how to procede, I did think that could cancel the sqrt ω terms but they depend on p: $$ω_p = \sqrt{P^2 + m^2}$$
so as they are inside integrals dependent on P they can't be cancelled.

I am at an utter loss how to proceed from here though. If anyone can offer any pointers or assistance is would be greatly appreciated.
EDIT: made a sign error in exponential's for $$\psi$$ and $$\psi^*$$

Last edited:

TSny
Homework Helper
Gold Member
You can try changing the order of integration and doing the x integration before the p integrations.

Ok, I see how you can change the order of the integration, and changing one of the P's to r to make it clearer to me, to get :
$$Q=\frac{-1}{(2\pi)^6} \int \partial ^3 x [ \int \partial^3 P \frac{1}{\sqrt{2 ω_p}}( a_p e^{ip.x } + b_p^* e^{-ip.x}) \int \partial^3 r \sqrt{ \frac{ω_r}{2}}( a_r^* e^{-ir.x } - b_r e^{ir.x}) + \int \partial^3 P \frac{1}{\sqrt{2 ω_p}}( a_p^* e^{-ip.x} + b_p e^{ip.x }) \int \partial^3 r \sqrt{\frac{ω_r}{2}}( a_r e^{ir.x } - b_r^* e^{-ir.x}) ]$$
$$Q=\frac{-1}{(2\pi)^6} \int \partial ^3 x [ \int \partial^3 P \sqrt{\frac{ω_r}{ ω_p}} ( a_p e^{ip.x } + b_p^* e^{-ip.x}) \int \partial^3 r ( a_r^* e^{-ir.x } - b_r e^{ir.x}) + \int \partial^3 P \sqrt{\frac{ω_r}{ ω_p}}( a_p^* e^{-ip.x} + b_p e^{ip.x }) \int \partial^3 r ( a_r e^{ir.x } - b_r^* e^{-ir.x}) ]$$
$$Q=\frac{-1}{(2\pi)^6} ( \int \partial^3 P \int \partial^3 r \int \partial ^3 x \sqrt{\frac{ω_r}{ ω_p}}( a_p e^{ip.x } + b_p^* e^{-ip.x} ) ( a_r^* e^{-ir.x } - b_r e^{ir.x}) + \int \partial^3 P \int \partial^3 r \int \partial ^3 x \sqrt{\frac{ω_r}{ ω_p}}( a_p^* e^{-ip.x} + b_p e^{ip.x })( a_r e^{ir.x } - b_r^* e^{-ir.x}) )$$

$$Q=\frac{-1}{(2\pi)^6} ( \int \partial^3 P \int \partial^3 r \int \partial ^3 x \sqrt{\frac{ω_r}{ ω_p}} ( a_p a_r^* e^{i( p - r ).x} -a_p b_r e^{i( p + r).x} + b_p^* a_r^* e^{-i( p + r ).x} -b_p^* b_r e^{i( r - p ).x} ) + \int \partial^3 P \int \partial^3 r \int \partial ^3 x \sqrt{\frac{ω_r}{ ω_p}} ( a_p^* a_r e^{i( r - p ).x} -a_p^* b_r^* e^{-i( P + r ).x} + b_p a_re^{i( p + r).x} - b_p b_r^* e^{i( p - r ).x} )$$

I'm not sure how to go about integrating the x components;
I've never integrated anything over $$\partial^3 x$$ and am struggling to find anything that can help explain it.
would;
$$\int \partial ^3 x a_p^* a_r e^{ i(p +r).x} = \frac{1}{ (i(p+r))^3}a_p^* a_r e^{ i(p +r).x}$$
or as its a dot product in the exponential,
$$\int \partial ^3 x a_p^* a_r e^{ i(p +r).x} = \int \partial ^3 x a_p^* a_r e^{ i( (p1 +r1)x1 + (p2+r2)x2 +(p3+r3)x3 )}$$
$$= \frac{1}{i^3(p1+r1)(p2+r2)(p3+r3) }a_p^* a_r e^{ i(p +r).x}$$

But i'm sort of expecting the x integral to give some kind of delta function in r so that the r integral is easy leaving just the p integral ie the required result.
EDIT: maybe the commutation relations for the ap and bp operators will give delta functions, now to get the x integral out of the way first!
EDIT2: corrected sign error in exponential's.

Last edited:
TSny
Homework Helper
Gold Member
The delta function will come in through a well known "identity" that you can find here http://dlmf.nist.gov/1.17 (see equation 1.17.12).

Before going further, make sure you have the correct signs in your exponential functions for ##\pi## and ##\pi^*## (that you used to construct ##Q##).

ah, I think its $$\psi$$ and $$\psi^*$$ that have the incorrect signs.
Will edit through all above to correct this.

Bloody sign errors, when will they end!

ahh finally solved it,
using
$$\int \partial^3 x e^{i(p+r).x} = (2\pi)^3 \delta^3(p+r)$$
and working through.

Thanks for the assitance TSny!