# Understanding the W Decay Branching Ratio to Quarks & Gluons

• I
• kelly0303
In summary, the conversation discusses the concept of branching ratio in the Electroweak Unification chapter of Modern Particle Physics by Mark Thomson. The branching ratio of the W decay to quarks is discussed, including the inclusion of a gluon and two quarks in the final state. The possibility of photon emission is also mentioned. The confusion arises regarding the concept of "later" in the same Feynman diagram and the use of 1PI diagrams in understanding the process. Ultimately, the conversation concludes that understanding the process depends on whether one wants to focus on W vertices or W decays specifically.
kelly0303
Hello! In Modern Particle Physics by Mark Thomson, in the Electroweak Unification chapter, pg. 412 he talks about the branching ration of the W decay to quarks. And for this he includes both the ##W\to q \bar{q'}## and ##W\to q \bar{q'}g## i.e. the state with a gluon and 2 quarks in the final state. I am not sure I understand this. Isn't the decay defined just at the W vertex? The gluon is produced later by the quark and it has nothing to do with the W properties. Also, one can have a photon, too coming out of the quark and even in the leptonic case, one can have a photon emission by one of the leptons resulting from the decay. Why is it just this gluon case considered? Thank you!

"Later" is not a well-defined concept if it happens in the same Feynman diagram. If you calculate it your integral will also run over an "earlier" gluon emission.
The quarks will hadronize in some way afterwards anyway, but you can also have the emission of a gluon with a high energy - in that case you get three jets instead of two. Photon emission is possible as well, it is relatively rare as the electromagnetic interaction is much weaker.

mfb said:
"Later" is not a well-defined concept if it happens in the same Feynman diagram. If you calculate it your integral will also run over an "earlier" gluon emission.
The quarks will hadronize in some way afterwards anyway, but you can also have the emission of a gluon with a high energy - in that case you get three jets instead of two. Photon emission is possible as well, it is relatively rare as the electromagnetic interaction is much weaker.
I am sorry, I am still confused. The diagram he shows, is not "1 particle irreducible" (1PI). I remember from my QFT class that, when trying to understand a process, one looks only at diagrams that't can't be split into 2 or more diagrams (and this one can). For example, one can add lots of self interactions on the quark propagator, but that doesn't contribute to the Wqq vertex. Of course Wqq is just a first order approximation, but but higher order must still be 1PI, while W->qqg is not.

kelly0303 said:
I am sorry, I am still confused. The diagram he shows, is not "1 particle irreducible" (1PI). I remember from my QFT class that, when trying to understand a process, one looks only at diagrams that't can't be split into 2 or more diagrams (and this one can). For example, one can add lots of self interactions on the quark propagator, but that doesn't contribute to the Wqq vertex. Of course Wqq is just a first order approximation, but but higher order must still be 1PI, while W->qqg is not.
I do not think you have understood what 1PI means and when it is used. It is used in connection to propagators, not interaction diagrams. What you want is the interaction diagram with no 1PIs on the external legs. The leg with the extra gluon is not a 1PI because it has three legs. A 1PI only has two legs, the ”incoming” and ”outgoing” particle.

vanhees71
That depends on the process you want to understand. Do you want to understand vertices with W or do you want to understand W decays?

## 1. What is the W decay branching ratio to quarks and gluons?

The W decay branching ratio to quarks and gluons refers to the probability of a W boson decaying into either a quark-antiquark pair or a gluon pair. This ratio is determined by the strength of the W boson's interaction with different types of particles and is an important factor in understanding the behavior of the W boson.

## 2. How is the W decay branching ratio to quarks and gluons measured?

The W decay branching ratio to quarks and gluons is measured through experiments at particle colliders, such as the Large Hadron Collider (LHC). By colliding high-energy particles and analyzing the resulting decay products, scientists can determine the relative probabilities of the W boson decaying into different types of particles.

## 3. Why is understanding the W decay branching ratio important?

Understanding the W decay branching ratio to quarks and gluons is important because it provides insight into the fundamental interactions between particles in the Standard Model of particle physics. It also helps us to better understand the behavior of the W boson and its role in processes such as particle decay and the production of new particles.

## 4. How does the W decay branching ratio to quarks and gluons relate to the Higgs boson?

The W decay branching ratio to quarks and gluons is closely related to the Higgs boson, as the Higgs boson is responsible for giving particles their mass through interactions with the Higgs field. The W boson's decay branching ratio is affected by the Higgs boson's mass, and studying this ratio can provide valuable information about the Higgs boson and its properties.

## 5. Can the W decay branching ratio to quarks and gluons change over time?

The W decay branching ratio to quarks and gluons is a fundamental property of the W boson and is not expected to change over time. However, it can be affected by certain conditions, such as the energy of the particle collisions or the presence of other particles, which can alter the relative probabilities of different decay channels. Scientists continue to study the W decay branching ratio to better understand its behavior and any potential changes.

• High Energy, Nuclear, Particle Physics
Replies
13
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
17
Views
5K
• High Energy, Nuclear, Particle Physics
Replies
4
Views
2K
• Introductory Physics Homework Help
Replies
8
Views
404
• High Energy, Nuclear, Particle Physics
Replies
2
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
35
Views
7K
• High Energy, Nuclear, Particle Physics
Replies
1
Views
3K
• High Energy, Nuclear, Particle Physics
Replies
1
Views
3K
• High Energy, Nuclear, Particle Physics
Replies
2
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
24
Views
4K