# Search results

1. ### Question on Tachyon Correlator (Green Schwarz Witten)

I'm reading through chapter 7 of Green-Schwarz-Witten and I have a problem with the derivation of the M-tachyon correlation function. Basically I'm trying to get 7.A.17 from 7.A.12 and eq 7.A.22 in the appendix of the first volume. Basically I want to prove...
2. ### Amplitudes in CFT vanish. Why does N=4 have them?

Hi guys, one question in this growing amplitude business that i don't understand. Usually one says that conformal field theories do not have any non-trivial scattering amplitudes because one cannot define asymptotic states. But say we consider now N=4 Super Yang Mills and moreover treelevel...
3. ### Mathematica Streamlining Replace in Mathematica 8

Hi guys, i have expressions of the type (X*a)(X*b). I want replace this by X^2(a*b). So i tried building a block which does nothing but %//.(X*a_)(X*b_)->X^2(a*b). However, this works only if a is distinct from b. If a and b are equal if HAVE to use the replacement command...
4. ### Loop Integration in Spinor Language

Hi guys, i'm looking at one-loop calculations in terms of helicity spinor (basically a paper by Brandhuber, Travglini and others) language but i have no idea how to integrate them :) For instance \int FeynParam\int d^D L \frac{\langle a|L|b]^2}{(L^2-\Delta^2)^3} How would I do...
5. ### Self-Dual Field Strength in complex coordinates

Hi guys, I have to brush up my knowledge about self-dual Yang Mills and I'm reading an ancient paper by Yang about it...and of course I'm stuck...although Yang writes 'it is easy to see that'... Ok, so the self-duality condition of the YM field strength tensor is defined as...
6. ### 2Point Correlator in CFTs

Hi guys, i'm studying Conformal Field Theory using the big yellow book by Senechal et al. So far everything has been a smooth ride. I'm a bit stuck at the point where they derive the 2- and 3-point correlator for spinless fields. Based on invariance under rotations and translations the...
7. ### N=4 Susy for dummies

Hi guys, I'm under the impression that nowadays everything going on in hep-th is N=4 Susy stuff and that it has 20384093 awesome properties. Are there lecture notes where I can learn about all of that? I mean I know buzzwords but I don't see the whole picture. So, as I've said, are there...
8. ### Wick contraction and derivatives

Hey guys, one quick question about Wick contractions and derivatives: Suppose I want to write down all (non-vacuum) Wick contractions of two fields a, and one field A into a cubic QCD-like interaction term of the form \partial^\nu A^{\mu, a}(p_1) a_\mu^b(p_2) a_\nu^c(p_3) f^{abc} to...
9. ### Old vs new unitarity cut methods

Hey folks, I've been stumbeling recently about new unitarity methods to obtain one-loop amplitudes by cutting them in all possible channels thereby reducing the full amplitude to products of tree amplitudes (pioneered by Bern, Dixon, Kosower, Dunbar). From what I understand from my QFT...
10. ### Four vector made of Pauli matrices commute?

Hey guys There are those vectors made of Pauli matrices like \bar{\sigma}^\mu and {\sigma}^\mu. So if I have the product \bar{\sigma}^\mu {\sigma}^\nu I wonder if it is commutative? And if not, what is the commutator? Cheers, earth2
11. ### [Book?] Scattering processes in Condensed Matter

Hi guys, i know that there are Feynman diagrams in condensed matter around. As far as i understood it they are used to calculate Green's functions but as we all know from qft Feynman diagrams can also be used to calculated scattering amplitudes...so, i wonder if there is a book around with...
12. ### Real scalars for complex scalar results

Hi guys, I'm a bit puzzled. I'm just reading some offline lecture notes where the Feynman rules of real (!) scalars coupled to gluons are given. However, with these rules the amplitude for phi g -> \bar{phi} g is considered. There are no further instructions. I'm just wondering how one can...
13. ### Feynman trick for linear propagators?

Hey guys, say i have some standard propagators then I know how to combine them using Feynman's parameter method. But what do I do if one of these propagators is linear? For instance: \int d^Dk \frac{1}{(k-p)^2(k \cdot q)} where q and p are some momenta. How do I combine them? Does...
14. ### LSZ reduction and Renormalization constants

Hey folks, i have a question about LSZ and how to take into account the renormalization constants of the theory in question. In the derivation, only the field strength renormalization enters as a factor of Z (or square root thereof) but some mates said that also the vertex renormalization...
15. ### Closed Strings and Virasoro algebra

Hey guys, i just started taking a course on ST and so far we discuss the closed string following Green, Schwarz, Witten. I don't see the point of constructing the Virasoro algebra (formula 2.1.85 in GSW) if the corresponding generators are zero due to the constraints. Or to put it differently...
16. ### Feynman rules of scalar QCD (colour ordered)

Hey guys, does any of you know where I could find the Feynman rules for scalar QCD? If they where colour ordered, even better! Cheers, earth2
17. ### Weyl Spinors, SO(1,3) algebra and calculations

Hey guys, something that puzzles me everytime I stumble across spinors is the following: I know that i can express Dirac spinors in terms of2-component Weyl spinors (dotted/undotted spinors). Now, if i do that, i can reexpress for instance the Lorentz or conformal algebra in terms of Weyl...
18. ### Dirac spinors and commutation

Hey guys, i'm stuck (yet again! :) ) I am somewhat confused by Dirac spinors u,\bar{u}. Take the product (where Einstein summation convention is assumed): u^r u^s\bar{u}^s Is this the same as u^s\bar{u}^s u^r? Probably not cuz u^r is a vector while the other thing is a matrix, right...
19. ### Dimensional Regularization and epsilon

Hi! Quick question: Does it make a difference if i choose my dim reg. to be D=4-2epsilon or D=4+2epsilon (i suppose in both cases epsilon >0). I mean opinion i should not matter but standard qft books normally don't touch this question very deeply... Cheers, earth2
20. ### Question about loop integrals

Hey, I've been reading a bit in the QFT book by Pierre Ramond about loops and i stumbled about some derivations in appendix B that I can't follow. First Ramond derives a formula for integration of momenta in arbitrary dimensions \int \frac{d^Nl}{(l^2+2p\cdot...
21. ### Symmetry factor of a bubble in QCD

Homework Statement Calculate the symmetry factor of a 4-point, 1-loop diagram in QCD. Two legs are external, two are not Homework Equations I don't know how to include pictures here, but lets try to describe it: -It basically consists of 2 four-vertices connected by two propagators...

Hey guys, I have a question about how to incorporate ghosts in practical calculations. So, from what I understand I have to add a factor of -1 for each closed gluon loop, right? But there are two ghost fields, c and \bar{c} appearing when I quantize the path integral. Does that mean that I also...
23. ### Mathematica [Mathematica] How to calculate residues if poles not simple?

Hey guys, i have the following situation: I have a function which looks like \frac{(a+bx)^3}{(x-y)^6(x-z)^6} As one can easily see this function has poles at y and z of order 6. Now, I know how to calculate the residue of this function for instance at y, but how do I implement this into...
24. ### Table of Loop Integrals

Hi guys, i was just wondering if there is some book or whatever that has a list of loop integrals. More specifically, I am looking for one-loop integrals in the lightcone gauge with an arbitrary number of propagators. I found a paper by Leibbrandt which tells me how to calculate them, but I...
25. ### Primer on conformal / dual conformal symmetry

Hey guys, i am looking for some primer on conformal, dual conformal symmetry, respectively. I have to read a lot of stuff about scattering amplitudes for uni and in recent papers people talk a lot about these symmetries.... unfortunately i am not so familiar with them, so does any of you know...
26. ### Question on Witten's paper: Perturbative Gauge Theory As A String Theory

Question on Witten's paper: "Perturbative Gauge Theory As A String Theory..." Hi guys, I have a question regarding formula 2.12 of Witten's paper hep-th/0312171 "Perturbative Gauge Theory As A String Theory In Twistor Space". He just states this formula but i don't really understand his...
27. ### Gluon Wave function

Hi there, I have question about a gluon's wave function. First of all, I thought it is just the vector potential A_\mu but I read several papers and they keep refering to the helicity \epsilon_\mu as the wave function. At least this is what I understand from the context. See for instance...
28. ### Superfield Expansion

Hi folks, I just read some stuff about Susy and encountered superfields and their expansion in terms of the supercoords x^\mu, \theta, \bar{\theta}. Reading that (e.g. in the script of Lykken), I found general expansions like S(x,\theta,\bar{\theta})=...+ \theta\theta \psi +...
29. ### Weyl Spinors

Hey guys, I have a question about said spinors. In supersymmetry introductions one finds (e.g. for two left-handed spinors \eta , \nu ) that \eta\nu=\nu\eta due to their Grassmannian character and the antisymmetry of the spinor product. If I look, however, at modern field theoretical...
30. ### Reggezation. How does it come about?

Hey folks, i am reading about the BFKL equation and came across the reggezation of the exchanged gluons. Now, i understand what reggezation means but i don't see where it enters. Especially since most people simple write "virtual corrections enter via replacing the propagators by bla bla...