Problem with Feynman rules in QCD -- help understanding please

In summary, Pedro is trying to calculate the cross section of a gluon emission from quarks in an electron-positron annihilation at first order using the Feynman rules. However, he is unsure about a minus sign in his matrix element, as he is getting a plus sign instead. He is seeking help from anyone who can provide insight, and has specified the momenta of the particles in his equation.
  • #1
Aruera
Hi, I'm trying to compute the cross section of a gluon emission from quarks in a electron-positron annihilation at first order (I attach the diagrams below).

upload_2017-10-18_3-34-25.png


Applying the Feynman rules the matrix element should be:
upload_2017-10-18_3-36-42.png

My problem is that I have no idea where this minus sign comes from. (I obtain a plus sign).

If anyone can help me I would be very grateful.

Thanks,
Pedro.

P.S. In the equation I posted, k1, k2 and k3 are the 4-momentum of the quark, antiquark and the gluon respectively and p1 and p2 of the electron and positron.
 
Physics news on Phys.org
  • #2
The momenta ##k_2## and ##k_3## are defined in the direction opposite to the fermion flow. In the other diagram ##k_1## and ##k_3## are in the same direction as the fermion flow.
 
  • Like
Likes Aruera

FAQ: Problem with Feynman rules in QCD -- help understanding please

1. What are Feynman rules in QCD?

Feynman rules in QCD (Quantum Chromodynamics) are a set of mathematical rules used to calculate the probability of interactions between quarks and gluons, the fundamental particles that make up protons and neutrons. These rules involve the use of Feynman diagrams, which visually represent the possible paths of particle interactions.

2. What is the problem with Feynman rules in QCD?

The problem with Feynman rules in QCD is that they become increasingly complex as the number of particles involved in an interaction increases. This can make it difficult to accurately calculate the probability of the interaction, and can also lead to inconsistencies and inaccuracies in the calculations.

3. How can I better understand Feynman rules in QCD?

To better understand Feynman rules in QCD, it is helpful to have a strong understanding of quantum mechanics and particle physics. Additionally, studying and practicing with Feynman diagrams and their corresponding calculations can also improve understanding of these rules.

4. Are there any resources available to help me understand Feynman rules in QCD?

Yes, there are many resources available to help understand Feynman rules in QCD. These include textbooks, online courses, and videos that explain the concepts and provide practice problems. It may also be helpful to seek guidance from a mentor or tutor who is well-versed in this topic.

5. How important are Feynman rules in QCD in the field of particle physics?

Feynman rules in QCD are essential in the field of particle physics, as they allow for the calculation of important quantities such as cross sections and decay rates. They also provide a framework for understanding and predicting the behavior of subatomic particles, which is crucial in furthering our understanding of the fundamental laws of the universe.

Similar threads

I No "i" in the Triple gluon vertex Feynman Rule
Replies
8
Views
2K
I Glueballs Observed For The First Time
Replies
10
Views
1K
I Confused about Feynman diagrams
Replies
10
Views
3K
QCD Feynman Diagrams: Understanding Particle Interactions
Replies
7
Views
3K
Cross Section: Quark-Gluon vs. Quark-Photon
Replies
2
Views
2K
Attractive Forces from Particle Exchange
Replies
2
Views
3K
B How many gluons on average are there in a proton at once?
Replies
9
Views
3K
Calculating Feynman Rules for Effective Electroweak Chiral Lagrangian
Replies
2
Views
5K
QCD Calculation for e+e- Annihilation: Quarks, Gluons, & Hadrons
Replies
24
Views
5K
Constructing Feynman diagrams and understanding conservation rules.
Replies
3
Views
2K

Hot Threads

Recent Insights

Back
Top