Weak decay, decay rate, colour-supressed and colour-allowed

In summary: So for example, a proton is made up of two up quarks and one down quark, with each quark being a different colour.
  • #1
binbagsss
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My textbook says:

One of the decays occurs via a ## b \to c ## quark transition and is 'colour-allowed'.(Left of diagram).

The other decay has a of ## b \to c ## and is 'colour-suppressed'.(Right of diagram)

I'm unsure of what is meant by these terms, it doesn't really explain.

I've attached the Feynman diagrams .

Could anyone explain these to me or send me a good link?
Thanks very much.
 

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  • #2
The left diagram works automatically as long is colour is conserved in the upper W vertex. The right only works if the upper vertex connects to quarks of the same colour as the original b quark.
 
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  • #3
Orodruin said:
The left diagram works automatically as long is colour is conserved in the upper W vertex. The right only works if the upper vertex connects to quarks of the same colour as the original b quark.

Thanks. I've tried to have a look into what you are saying, but my textbook doesn't explain it.

I don't understand what you've said because I don't think I understand some basic concepts- I don't understand why the right diagram can't be drawn in the same form as the left in terms of the W boson. Why is this?
 
  • #4
binbagsss said:
I don't understand why the right diagram can't be drawn in the same form as the left in terms of the W boson. Why is this?
There is no b->s transition that could emit a W, and also no W -> c cbar process. Both would violate electric charge conservation.
 
  • #5
Orodruin said:
The left diagram works automatically as long is colour is conserved in the upper W vertex. The right only works if the upper vertex connects to quarks of the same colour as the original b quark.

Ok, so I know that colour must be conserved at a vertex. And that quarks come in 3 colours. As the W boson is colourless, am I correct in thinking that in the left diagram, the anti bottom quark and anti charm quark must be of the same colour, as must the anti down and up, and on the right diagram the anti bottom and anti charm must be, and the anti strange and charm?
 
  • #6
Right.

And to form the hadrons, the right diagram has the additional requirement that charm and anti-charm have to have opposite colors (also down and anti-strange but that is equivalent to the previous requirement). The left diagram has this automatically as the original two quarks have opposite colors.
 
  • #7
mfb said:
Right.

And to form the hadrons, the right diagram has the additional requirement that charm and anti-charm have to have opposite colors (also down and anti-strange but that is equivalent to the previous requirement). The left diagram has this automatically as the original two quarks have opposite colors.

Opposite as in anti colour/colour?
 
  • #9
Ahh okay thanks I think I've got it. Mesons have the additional criteria that the quark and antiquark have to be the same colour. (For a baryon does anything go? )

So in the right diagram I've concluded all quarks are of the same type of colour.
But the left diagram there is more freedom of choice as the upper vertex two quarks can be either r,g,b, whereas all other quarks in that diagram are of the same colour?
 
  • #10
binbagsss said:
For a baryon does anything go?
No, a baryon has to have one quark of each colour.
 

1. What is weak decay and how does it differ from other types of decay?

Weak decay is a type of nuclear decay that occurs in unstable nuclei. It is characterized by the emission of particles, such as beta particles, and the conversion of a neutron into a proton or vice versa. Unlike other types of decay, such as alpha or gamma decay, weak decay involves the transformation of particles within the nucleus, rather than the emission of particles from the nucleus.

2. What is the decay rate and how is it measured?

The decay rate is a measure of how quickly a radioactive substance decays. It is typically expressed as the number of decays per unit time, such as decays per second or decays per year. The decay rate can be measured using various methods, such as counting the number of particles emitted from the substance over a period of time or using a Geiger counter to detect the radiation emitted by the substance.

3. What is the difference between colour-suppressed and colour-allowed decay?

Colour-suppressed decay is a type of weak decay that occurs in particles that have a strong interaction with the strong nuclear force. In this type of decay, the strong interaction suppresses the weak interaction, resulting in a slower decay rate. Colour-allowed decay, on the other hand, occurs in particles that have a weak interaction with the strong nuclear force. In this type of decay, the weak interaction is not suppressed, leading to a faster decay rate.

4. How does the concept of colour in particle physics relate to decay?

In particle physics, colour is a property that describes the strong interaction between particles. It is not related to the visual color we see with our eyes. When particles decay, they can change their colour state, which can affect the strength of the strong interaction and therefore the rate of decay. This is why we have terms like "colour-suppressed" and "colour-allowed" decay in nuclear physics.

5. How does the study of weak decay contribute to our understanding of the universe?

The study of weak decay is important in understanding the fundamental properties of matter and the forces that govern the behavior of particles. It also plays a crucial role in nuclear physics and astrophysics, as many processes in stars and other astronomical objects involve weak decay. By studying weak decay, we can gain insights into the structure of matter and the evolution of the universe.

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