Recent content by torus

How is this supposed to work? I thought Instantons (and sphalerons) change baryon and lepton number only in steps of 3 (the number of generations). Therefore the proton can not decay via instantons... Even in 't Hooft's cited paper Phys. Rev. Lett. 37, 8-11 (1976) proton decay is not...

Thanks for all the replies. I found a very nice reference: http://arxiv.org/abs/arXiv:0812.1594 Eq. (2.95) together with (2.64) gives z_1 \sigma^{\mu\nu} z_2 z_3 \sigma_{\mu\nu} z_4 = - (z_1 z_4) (z_2 z_3) - (z_1 z_3) (z_4 z_2) , valid for both commuting and anticommuting spinors z_i, which...

@Bill_K: Thanks a lot, but isn't the singlet in 3x3 symmetric? Then you end up with the symmetry properties I proposed in the first post.

Hm, yes, but I am looking for a representation with spinor indices, i.e. the invariant symbol x_{a b c d} corresponding to this singlet.

Hi, from Srednickis QFT textbook, we know the following coupling of Lorentz group representations: (2,1)\otimes (2,1) = (1,1)_A \oplus (3,1)_S, which yields \epsilon_{a b} as an invariant symbol. Generalising, we can look at (2,1)\otimes (2,1) \otimes (2,1) \otimes (2,1) = (1,1) \oplus...

I am not sure exactly what you mean. It surely depends on the rep of the quarks, right? Let's say QCD is governed by SU(2), call is colorspin. Let's stick to a 3-d rep for the quarks, i.e. put quarks in colorspin-1 reps. Then you can form mesons (couple two colorspins to 0) and baryons (there is...

Not sure if I understand what you mean. The Yukawa coupling constant is just an arbitrary number at this point, it does not matter which sign it has. Its not like the kinetic terms where the prefactor has a specific fixed value. The Yukawa interaction is parameterized by one parameter, you can...

Why does it have to vanish? The diagram is just the finite light-light-scattering one, with one external leg switched from into out.

No, there is no difference as they are connected by just raising/lowering the index nu.

Hi, you need to raise and lower the indices so they match your derivative-operator, i.e. write \partial^\mu A^\nu = g^{\mu \alpha} \partial_\alpha A^\nu then you can use \frac{\partial}{\partial (\partial_\alpha A^\beta)} \partial_\mu A^\nu = \delta^\alpha_\mu \delta^\nu_\beta Hope this...

Okay, I finally found a way to make it clear for myself: The comoving distance a lightray travels from a_1 to a_2 is given by \int_{a_1}^{a_2} \frac{1}{aH} d\ln a = \frac{1}{H a_1} - \frac{1}{H a_2} so let's say in the first half of inflation: a_1 = a_i, a_2=a_f/2 with a_f/a_i being the huge...

Well, the question is exactly: Why does the Hubble radius set the interaction length? Sure, it has the right dimension, but we could still multiply by a or something. Or it could be an integral.

Hi, I'm trying to figure out how inflation (just deSitter) solves the horizon problem, but I am stuck. I understand the solution in terms of conformal coordinates, allowing for a negative conformal time let's the lightcones of CMB intersect. Fine. But how do I see "physically" what is going on...

Hi, I was wondering why phonons should have spin zero, too. After all, the quantized system looks just like QED, only with more (!) polarizations, so even in the simplest acoustic case we have one longitudinal and two transversal modes. This looks like spin 1. On the other hand, spin is...

Ah, thanks a lot.