Feynman diagram Definition and 167 Threads

I'm reading Srednicki's Quantum Field Theory. I 'm trying to read Srednicki's presentation of Feynman Diagrams in the chapter Path Integral for the Interacting Field Theory. Link to the book: The path integral for the phi-cubed theory is equation 9.11 in the book. Please read that. I get the...

I am trying to calculate box diagram of Kaon mixing by follow the "CP Violation" book. Now, I arrived at equation (B.8) and I have problem with getting equation (B.12). F(x_\alpha,x_\beta)=\dfrac{1}{(1-x_\alpha)(1-x_\beta)}(\dfrac{7x_\alpha...

Hi everyone! In an exam on particle physics there has been a problem where for a number of decays we were asked to either reason their non-existence (i.e. name a conservation law that it contradicts to) or draw a Feynman diagram. However, with one of those decays I have a problem: \gamma + p...

Homework Statement For the reaction below draw three Feynman diagrams, one that proceeds through exchange of a gluon, one through a photon and one exchanging a weak W-Boson. ##\pi^0+\pi^0\rightarrow \pi^++\pi^-## Which diagram provides the dominant contribution for this reaction? Explain how...

What is the intuition behind divergences for the sunset diagram? I know that there is quadratic divergence by why no quartic divergence or higher?

For quartic scalar field theory these are some of the lowest order diagrams (taken from the solutions to 9.2 srednicki). I'm wondering if someone can give me an intuition of how to actually calculate them. What I'm thinking is that vertices are $$\int \frac{d^{4}x}{(2\pi)^{4}}$$ and for the...

Could someone please tell me the difference between tree diagrams and loop diagrams? If I'm thinking correctly tree diagrams are before contracting? Also how do vacuum diagrams fit into the picture? Thanks!

On page 60 of srednicki (72 for online version) for the $$\phi^{3}$$ interaction for scalar fields he defines $$Z_{1}(J) \propto exp\left[\frac{i}{6}Z_{g}g\int d^{4}x(\frac{1}{i}\frac{\delta}{\delta J})^{3}\right]Z_0(J)$$ Where does this come from? I.e for the quartic interaction does this...

Please check my logic. 1. Feynman diagram is a tool to calculate 'final state' from an 'original state'. It is mostly used for simple processes, like scattering. 2. 'Final state' usually have a precise meaning due to observation of particles trajectories. 3. However, in MWI there is nothing...

Consider the lowest order interaction e.g. e- e+ -> virtual photon-> muon anti muon. I appreciate that the electron-positron pair cannot annihilate into a real photon due to conservation of 4-momentum, but why is the pair permitted to produce a virtual photon? I know that virtual particles...

Consider a ##j## point all massive leg one loop polygonal Feynman diagram ##P## representing some scattering process cut on a particular mass channel ##s_i##. Invoking the relevant Feynman rules and proceeding with the integration via dimensional regularisation for example gives me an expression...

Hi everyone, sorry if this is not the right place to post that question but I'm new to this forum, i'll delete if necessary. I am currently trying to learn QFT from Matthew Schwartz's "Quantum field theory and the standard model", quite clear during the first chapters, but i have been...

Hello all, If I am having the the effective lagrangian which is actually free + interaction lagrangian (obtained from the minimal substitution for pseudoscalar and vector mesons). then how to compute the vertices of the interaction ? I have taken into consideration of all symmetry breaking...

Hi there, my question is the following. If an electron and positron annihilate, how can they result in ZZ? The issue I'm having is that due to charge conservation, the exchange particle can't be W- or W+. It also can't be a photon since the Z's don't have electrical charge to couple to. It...

Nonlinear sigma models are particular field theories in which the fields take values in some nontrivial manifold. In the simplest cases this is equivalent to saying that the fields appearing in the lagrangian are subject to a number of constraints. Since the lagrangian fields are not independent...

Hi. Do you know any book/paper/lecture notes where I can find complete derivation of Feynman rules for both scalar and pseudo-scalar Yukawa theory, and maybe an example of application to decay of fermion?

Homework Statement :[/B] For the decay of the ∆+ → n + π+, sketch an appropriate quark-level Feynman diagram. State which interaction is responsible for this decay. 3. The Attempt at a Solution This is more of a question to see if I'm doing it right more than anything. The first Feynman...

I was studying my notes and specifically for the ##e^+e^- \rightarrow \mu^+ \mu^-## process, cross section is given by \sigma = \frac{4\pi}{3} \left( \frac{\alpha \hbar c}{W} \right)^2 where ##\alpha = \frac{g_{EM}^2}{4\pi}## and ##W## is the centre of mass energy. Is this the same for...

As I understand it, the fundamental unit of interaction in QED is a term with a pair of (spinor) electron factors and a (vector) photon factor, represented in a Feynman diagram as two (anti-)electron lines and one photon line meeting at a vertex. I get the case where the photon and electron...

Homework Statement (a) What is a meson? (b) State what these mesons are made up of and explain their quantum numbers and interactions. (c) Find the lifetimes of rho meson and kaon. (d) Find the dominant decay mode and explain why. (e) Why is decay to 2 neutral pions forbidden? (f) Explain how...

Homework Statement [/B] (a) Explain lepton universality. (b) Explain why decay mode is forbidden and find hadronic branching ratios. (c) Find the lifetime of tau lepton. (d) What tau decay mode would be suitable? (e) Find the precision. (f) How do you improve the results? (g) Why is it much...

Supposed to represent a relativistic proton colliding with a stationary proton, leading to changes in the momentum of both and the production of a neutral pion. The pion then decays into two photons. No clue if this is right. I've never drawn anything much more complicated than...

The arrows in a Feynman diagram represent electric current. right? If this is the case then why do neutrons and neutrinos have arrows. How do they have an electric current?

Exchange Particles are to show the transfer of (for example) +/- charge to the other side so the charges balance. But I don't understand... Beta plus decay: p → n + e+ + νe. This is just an example, the Feynman diagram shows a W+ boson transferring the positive charge to the right hand side...

Consider ##\Sigma^0 \rightarrow \Lambda + \pi^0##. (Not Allowed) According to griffiths, this strong interaction is not allowed by 'Conservation of Energy'. I'm not sure why, as this simply shows an up and anti-up quark coming together, producing a gluon, where mass of gluon is mass of up and...

I thought the fundamental electromagnetic vertex is Why is the following diagram below allowed? The 'special' feynman diagram for compton scattering ##e^- + \gamma \rightarrow e^- + \gamma## is

Dear community! I watched The Big Bang Theory and watched this episode. If you click on it you would get to a homepage - if you scroll down you would get to a section called Episode Questions -> Physics Bowl. According to the newspapers The Big Bang Theory have an employee who is scientist...

Homework Statement (a) e- + e+ -> e- + e+ (b) e- + e- -> e- + e- c) e- + e- -> e- + e- + u+ + u- d) y -> e+ + e- e) y + y -> y + y Homework EquationsThe Attempt at a Solution Part (a)[/B] Part (b) Part (c) Part (d) Part (e) Not sure what to do with this, since usually the...

Homework Statement Hey guys! So basically in the question I'm given the action S=\int d^{d}x \left[ \frac{1}{2}\partial_{\mu}\phi\partial_{\nu}\phi\eta^{\mu\nu} - \frac{m^{2}}{2}\phi^{2} -\frac{\lambda}{4!}\phi^{4}\right]. I have use the feynman rules to calculate the tree level diagram with...

Hi all, Is anyone familiar with FeynArts and FormCalc programmes? When I calculate a process having ## V_{CKM} ## in the couplings like ## b \to s \gamma ##, and use Insertion Level of Classes, after calculating the amplitude I get some parameters I don't understand, like CKM[Gen4, 3]...

Hello ! In the book Quantum Field for mathematician, there is this Wick diagram as an example to understand how to compute the symmetry factor (I am sorry, I draw it with paint...) This is about a hermitian field interacting with a complex field. The book says it has a symmetry factor of 2...

If I have a scalar field $$\alpha$$ and a Dirac particle $$\beta$$ and its anti particle $$\overline{\beta}$$ such that the three couple to give a vertex factor of $$-ik$$ when evaluating the Feynman diagram (where k is an arbitrary constant). How do I evaluate the first order diagram of...

I am reading Srednicki's book and I am stuck at this point of calculating symmetry factors for Feynman's Graph in the context of dealing with interacting scalar field. First of all my question is what is the standard procedure to calculate it. The way Srednicki has talked about it is that if...

Hello folks, I've just started to read "An Introduction to Quantum Field Theory by Peskin and Schroeder" and at the end of 6th page I could read: "... Since H_I should conserve angular momentum, the photon to which these particles couple must have the correct polarization vector to give it...

With $$ H = \alpha \left(x^{2}\partial_{x} - x \partial_{x} \right) + \beta\left(x \partial_{x}^{2} - x^{2}\partial_{x}^{2}\right) $$ Sorry for the messed up tex. I don't know how to fix it. It works in my editor. Hallo PF! This might become a somewhat long post so for the...

One of the fundamental predictions of QCD that has not yet been definitively observed is that there exist bound states of two or three gluons that are color charge neutral. In principle, every property of every excited states of every possible glueball can be determined precisely from first...

In Introduction to QFT (peskin) 4.5, he writes: The computation for M, of course, will be quite different when identical particles are present. However, I have finished reading the first part of the book and found no special treatment for identical particles. Can anybody tell me how to...

Homework Statement For the following reactions draw Feynman diagrams, clearly labelling all the quarks, leptons and exchanged particles and stating the type of interaction involved (if more than one interaction could be involved give the most likely): (i) ##\pi^0 \to \gamma + \gamma##...

Hi guys. I'm working on a model described by a non-local QFT. I think I got the Feynman rules right, but I get divergences from ##\delta(0)##-like factors.Homework Statement It's a QFT for a complex scalar field ##\psi(x)=\psi(\mathbf{x},t)## with action $$S= \int dx...

Continue reading...

Definition/Summary Feynman diagrams are a combinatorial device for enumerating a sum of integrals of products of delta functions. Feynman diagrams are a mathematical tool with no physical significance. In particular, no single Feynman diagram is intended to represent any physical...

Above is Figure 65 from Feynman's book QED. It shows the bidirectional stream of photons between an electron and a nucleus, that Feynman says "keeps the electron within a certain range." I can visualize repulsive forces, but I'm having trouble with attractive forces. I understand that force...

If one has a 4-fermion vertex, like in Fermi theory : ##G_f (f_1 \Gamma f_2)(l_1 \Gamma l_2)## And you are calculating a one-loop diagram where you have the diagram of : f1-> f2,l1,l2 -> f1 (used for dispersive/unitarity approach) Where in the end you'll use Cutkosky rules to calculate...

I was curious as to the difference in free neutron decay and the lack of decay when bound in a nucleus. I found the following statement here http://en.m.wikipedia.org/wiki/Neutron#Free_neutron_decay I don't understand that. Is the wave function of the neutron different when free or bound...

In a metal, can one electron decay into one lower-energy electron plus one phonon? (i.e., can the attached Feynman diagram occur?) If we replace phonons by photons and consider the process in a vacuum, I guess this is prohibited because you can always boost to a frame where the incoming and...

Hi folks, I'm assured that scattering cross-sections in QFT computed at tree level correspond to cross-sections in the classical theory. For example the tree level cross-section for electron-electron scaterring in QED corresponds to scattering of classical point charges. But I'm not sure I...

Hi all, I am currently trying to calculate the beta function for scalar QCD theory (one loop for general su(n)). I therefore need to calculate the Feynman rules in order to apply them to the one loop diagrams. Unfortunately I am getting very confused with what the Lagrangian for scalar QCD...

All the Feynman diagrams I have seen so far for a neutron colliding with a neutrino have a w+ with an arrow from the neutrino to the neutron. Would it not also be possible with a W- leaving the neutron taking away negative charge for it to become a positive proton or is there some quantum rule...

Hi, I was just trying to draw a Feynman diagram for ψ' to χ_{c} states. So we have a electromagnetic transition. I feel tempted to just have c and \bar{c} coming in and letting a γ leave from one of the legs. But that looks like an incomplete Feynman diagram to me. Maybe I have to get two...

Hi, I'm trying to understand the process in the Feynman diagram below: [/PLAIN] Specifically I'm wondering if the virtual quark has to emit a photon / Z boson and if so why? Also I don't understand how the photon / Z boson decays to a di-lepton pair since surely this violates spin...