# Recent content by Silviu

1. ### Early career in math

I read several articles saying that most mathematicians have the peak of their career before 30 and after that they don't do much significant work. Although this is a simplification of the reality, the truth is that in math many people do major breakthroughs before 30, a lot more than in other...
2. ### Casimir effect with Gaussian regulator

I didn't find it there...
3. ### Casimir effect with Gaussian regulator

Homework Statement Calculate the Casimir force in 1D using a Gaussian regulator. Homework Equations The Attempt at a Solution I reached a point where I need to evaluate a sum of the form $$\sum_n n e^{-\epsilon^2n^2}$$ Can someone help me? I didn't really find anything useful online. I...
4. ### A Spin-helicity formalism for gluon-gluon amplitudes

Hello! In Schwarz's QFT he introduces in chapter 27 the Spin-Helicity formalism as a way of calculating gluon-gluon interactions much easier than going through all the Feynman calculus from the beginning to the end. It seems so amazing, but I am not sure I understand what is the fundamental...
5. ### I Path Integral in QFT

But I am not sure how does he get the first and last terms. He gives a formula for ##\Phi_i##, but ##|0>## is not an eigenstate of ##\hat{\Phi}(x,t)##. What confuses me the most is why don't we have boundary terms for the integral. In QM you have boundaries the for the beginning and end points...
6. ### I Path Integral in QFT

I am not sure about anything right now... But as I said in the post I can't see where he is using the fact that the initial and final sates are the vacuum so technically you would get the same answer no matter what your initial and final states are, which doesn't really make sense to me.
7. ### I Path Integral in QFT

Anyone please? I am still not sure I understand what I am missing...
8. ### I Path Integral in QFT

Hello! I am reading from Schwarz book on QFT the Path Integral chapter and I am confused about something. I attached a SS of that part. So we have $$<\Phi_{j+1}|e^{-i\delta H(t_j)}|\Phi_{j}>=N exp(i\delta t \int d^3x L[\Phi_j,\partial_t \Phi_j])$$ What happens when we have the left and right...
9. ### I Spin and helicity conservation in QED

So, if helicity is not a good quantum number for massive particles and in the massless case helicity is the same as chirality, why do we need helicity in the first place. Why don't we just use chirality? Yes, that is the book I am using. So in 5.3 as far as I can tell is mainly using helicity...
10. ### I Spin and helicity conservation in QED

Hi! I am kinda confused about what gets conserved in QED and what not. So the chirality is always conserved, I got that. So in the massless limit, helicity is too. Now in the massive limit. Are spin and helicity conserved? And if they are, are they at each interaction vertex, or just overall...
11. ### I Long distance QED

Hello! We derived the electron proton scattering differential cross section using QED and I noticed that the equation doesn't depend on the impact parameter. Using classical EM one can calculate the deflection of an incoming electron as a function of the impact parameter, so I was wondering how...
12. ### I Why are spinors not observables?

Hello! I am reading some QFT and it is a part about how causality implies spin-statistic theorem. In general, one needs 2 observables to commute outside the light-cone. For scalars, we have $$[\phi(x),\phi(y)]=0$$ outside the light-cone, and by using the operator form of the field you get that...
13. ### I LRZ for scalar QED

I tried, but I am confused at certain steps. I would normally post here what I did so far and ask for help, but I am sure they are derived in some book already (also typing down everything would be quite time consuming as there are many indices).
14. ### I LRZ for scalar QED

Thank you! That is the book I am using actually and I am at that chapter, but in that book the rules are just listed, I would like a full derivation, the same way it is done in previous chapters for the ##\phi^3## interaction.
15. ### I LRZ for scalar QED

Hello! Can someone direct me towards a reading where the Feynman rules for scalar QED are derived? Thank you!