Why Can't a Top Quark Decay Into a Charm or Up Quark?

In summary, the Standard Model does not have a vertex for a top quark decaying into a charm or up quark, although it can still happen through rare loop processes. This is due to experimental observations and the fact that the strong interaction does not allow for any quark transitions, while the weak interaction only allows for up-type to down-type transitions.
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Paul159
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Hello,

I have a very basic question : why a top quark for example cannot decay into a charm or up quark ?

The fact is that I don't really understand where the concept of up- and down-type quark come from (except that they have the same charge). Why a up-type quark cannot transform into another up-type quark ? Is this only an experimental fact or is there a theoretical result prohibiting it ?

Thanks.
 
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The Standard Model has no vertex with a top quark and charm or up. It can still happen via loop processes, but it's so rare we have never seen it. We do see b->s and b->d transitions, however. This is called flavor-changing neutral current.
Why is there no such vertex? Well, ultimately it's a result of observations. You can write down theories that have such an interaction, but they don't seem to exist, or they must be so rare that we haven't seen them. The strong interaction doesn't allow any quark transitions, and the weak interaction only allows up-type to down-type and vice versa via coupling to the W boson (which has an electric charge).
 
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Thank you very much.
 
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1. Why can't a top quark decay into a charm or up quark?

The top quark is the heaviest of all known quarks, with a mass of approximately 173 GeV. This makes it much heavier than the charm and up quarks, which have masses of only 1.28 GeV and 2.3 MeV, respectively. According to the laws of physics, particles can only decay into lighter particles, so the top quark cannot decay into a charm or up quark.

2. Can a top quark decay into other particles?

Yes, the top quark can decay into other particles that are lighter than itself. The most common decay mode for a top quark is into a bottom quark and a W boson. It can also decay into a strange or down quark, along with a W boson. These decay modes have been observed and studied extensively in experiments at the Large Hadron Collider.

3. How does the top quark decay into a bottom quark and a W boson?

The top quark can decay into a bottom quark and a W boson through the weak interaction. This process is governed by the Standard Model of particle physics, which describes the fundamental particles and their interactions. The W boson carries away the top quark's energy and momentum, while the bottom quark is left behind as the decay product.

4. Are there any exceptions to the rule that particles can only decay into lighter particles?

There are some rare cases where particles can decay into particles that are heavier than themselves. This is possible through processes such as virtual particles or loop diagrams, which involve the creation and annihilation of particles in a very short amount of time. However, these processes have very low probabilities and are not observed as frequently as decays into lighter particles.

5. What is the significance of studying the top quark's decay?

The top quark is the heaviest known fundamental particle, and its decay processes can provide valuable insights into the nature of the fundamental forces and particles in the universe. By studying its decay, scientists can test and refine the predictions of the Standard Model, as well as search for potential new physics beyond the Standard Model. Additionally, the top quark's decay can also help us understand the origin of mass in the universe and the role of the Higgs boson.

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