# Search results

1. ### I About Lie group product ($U(1)\times U(1)$ ex.)

I recently got confused about Lie group products. Say, I have a group U(1)\times U(1)'. Is this group reducible into two U(1)'s, i.e. possible to resepent with a matrix \rho(U(1)\times U(1)')=\rho_{1}(U(1))\oplus\rho_{1}(U(1)')=e^{i\theta_{1}}\oplus e^{i\theta_{2}}=\begin{pmatrix}e^{i\theta_{1}}...
2. ### What function I'm looking at?

Hey all What's could the function be on the plot (see the attachment)?
3. ### Mass dimension of coupling constant -- always an integer?

Just a simple question -- can the dimension of coupling constant be a rational number or should it always be an integer? The question arose when I was trying to construct a Lagrangian with an interaction term involving two spin-1 particles and a fermion. The dimensions add up to 7/2, which...
4. ### Momentum space measure -- change of variables

I'm working through an article called "Cosmic abundances of stable particles -- improved analysis" (link -- viewable only in Firefox afaik), the result of which, equation (3.8), is cited quite a lot. I'm more interested in how they arrived there. Particularly, how come momentum space measure...
5. ### Feynman rules - where do the imaginary numbers come from?

I'm trying to learn how to derive Feynman rules (what else to do during xmas, lol). The book I'm using is QFT 2nd ed by Mandl&Shaw. On p 428 they're trying to show how to derive a Feynman rule for W W^\dagger Z^2 interaction term g^2 \cos^2\theta_W\left[W_\alpha W_\beta^\dagger Z^\alpha Z^\beta...
6. ### Relative velocity of beams

How come relative velocity of the beams can be expressed by v_{12} = \left| \vec{v}_1 - \vec{v}_2 \right| = \left|\frac{\vec{p}_1}{E_1} - \frac{\vec{p}_2}{E_2}\right| where \vec{p}_{1,2} and E_{1,2} is the momenta and energies of incoming particles, respectively? Similar equation is in Peskin &...
7. ### Plane wave expansion of massive vector boson

I'm trying to derive Feynman rules for massive vector boson and its antiparticle. It all boils down to plane wave expansion of the bosons which atm is a little bit confusing. Should I account for two different set of ladder operators (as in the case of complex KG or spinors, cf Peskin&Schröder...
8. ### UDP multiclient server in C

Hi all I'm trying to create a server that uses UDP and is capable of handling multiple clients. The server should 'stream' content to the clients only on the initial request (i.e. when starting the client). The general idea is to test how many clients is the server able to 'serve' before the...
9. ### Dual tensors in Lagrangian

Why is it the case that dual field tensors, e.g. \widetilde{F}^{\mu\nu}=\frac{1}{2}\epsilon^{\mu\nu\rho\sigma}F_{\rho \sigma}, aren't being included in the Lagrangian? For example, one doesn't encounter terms like -\frac{1}{4}\widetilde{F}^{\mu\nu}\widetilde{F}_{\mu\nu} in QED or...
10. ### Mega quick question: must the Lagrangian (density) be real valued?

Mega quick question: must the Lagrangian (density) be hermitian? In other words, must \mathcal{L}=\mathcal{L}^\dagger always be valid?
11. ### Thermalization of photons

Quick question: what is meant by thermalization of photons? And how could this effect distort CMB spectrum (anisotropies)? Any references are also welcome.
12. ### The origin of baryon number

I'm doing a little research on quarks on stumbled upon the baryon number. Wikipedia says that the notion predates the quark model. I have no clue why particle physicists were motivated to introduce it before flavour quantum numbers came to the scene. So... What's the (historical) origin of...
13. ### C/++/# C++ pause and resume while(true) loop

How to design a class, that executes an infinite loop, but may be paused, resumed and stopped by user input? I'll give you an example class Simulation { public: void start(){ m.init(); while(!m.isFinished()) m.update(); // computationally expensive calculation } void pause(); void...
14. ### Applicability of Ising model on real materials

Hi I chose a Monte Carlo simulation of the 2D Ising model as my Computational Physics course project. Unfortunately, I ran intro problems when formulating the exact problem since my professor probably wants me to simulate a real life material and extract magnetization curve M(T,H) out of it...
15. ### Calculating Helmholtz energy (incl degeneracy)

Hi I'm trying to calculate Helmholtz free energy from definition \frac{\partial A}{\partial T}=\frac{\partial}{\partial T}(kT\mathrm{ln}\; Z). First \langle E\rangle=\sum\limits_i p_i E_i=\sum\limits_i E_i\frac{g_i e^{-\beta E_i}}{Z}=\frac{1}{Z}\sum\limits_i E_i g_i e^{-\beta...
16. ### Density and muon

In my (very brief) lecturer's notes there's written that ρ~mp~me3 (*). So.. when (hypothetically) replacing every electron with a muon (around 200me), could the density increase 2003 times? Where comes that (*) relation (in Bohr's model)? Just in case: it's not a homework question
17. ### How to show that a trajectory is closed?

Main question in the title. I did a group work in analytic mechanics about pendulum in rotating reference frame and stumbled upon this one: link, page 22. Does this always hold? In my solution, there are two coordinates with the same frequency (different amplitudes) but asymmetrical in...
18. ### Solving system of 2nd order coupled ODEs

I have to derive equations of motion from Lagrangian and stumbled upon the following system of equations (constants are simplified, that information is unneeded) \begin{cases} \ddot{x}-A\dot{y}+Bx=0 \\ \ddot{y}+A\dot{x}+Dy=0 \end{cases} This is an extension of a simpler problem where B=D...
19. ### Does logarithmic decrement depend on initial amplitude?

Lets say we have a weight attached to a spring. When releasing it under water (whole motion occours under water) at different initial amplitudes, will logarithmic decrement be the same? And will the period change? I think it should be the same for different initial amplitudes. But...
20. ### Resonance in LC(R) parallel circuit

Homework Statement I did an experiment as followed Given following circuit I measured voltage across capacitor and reciprocal frequency of the generator. espilon, L, C, R, and R_E were given. In one configuration, R was shorted. I have to calculate relative frequencies (gamma)...