Can symmetries in GUTs explain the occurrence of FCNCs in particle interactions?

In summary: FCNC decays could occur even though they are forbidden by the gauge symmetries?Not nessecarily. The tree level mode \Lambda_{b}->p^{+}k^{-}\mu^{+}\mu^{-} is supressed by the small CKM element V_{bu} Where the FCNC is a b->s transition which is only moderatley supressed due to the large top yukawa. Moreover, this FCNC decay doesn't have helicity supression. Helicity supression happens when a scalar decays to light fermions through a vector coupling, like the decay B_{s}->\mu^{+}\mu^{-}
  • #1
Chain
35
3
Hi, I'm trying to understand the process in the Feynman diagram below:

K6OO2jp.png
[/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 conservation given that the photon / Z boson has spin 1 and the di-lepton pair will have opposite spins cancelling each other out.

I'd much appreciate any help :)
 
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  • #2
You can have an external lepton flip spin states, so that they add to what the decaying particle is (spin 0 or 1), in order to conserve angular momentum. But this will give you an extra factor of lepton mass. Its called "Helicity Suppression".

As for "has to emit a photon / Z boson", what else would it emit to get the in state to the out state? Actually I think you could also draw a box diagram:

Code:
b--o--c--o--s
   |     |  
   W     W
   |     |
e--o-nu--o--e

as well as the one where the gamma/Z comes off of the W in the loop, though these contribute less I believe.
 
  • #3
Ah okay I'll look into that. When you say "what else would it emit to get the in state to the out state?" do you mean what else would it emit to have a di-lepton pair in the final state? Because what I'm asking is does there have have to be a photon or di-lepton pair in the final state or is a decay like Lambda_b -> pK possible without emiting a photon?

Thank you for the response!
 
  • #4
Chain said:
Ah okay I'll look into that. When you say "what else would it emit to get the in state to the out state?" do you mean what else would it emit to have a di-lepton pair in the final state? Because what I'm asking is does there have have to be a photon or di-lepton pair in the final state or is a decay like Lambda_b -> pK possible without emiting a photon?

Thank you for the response!

##\Lambda_b \rightarrow p K^{-}## is possible alone, and has been measured :

http://pdg.lbl.gov/2013/listings/rpp2013-list-lambdab-zero.pdf

Scroll down to ##\Gamma_{20}##
 
  • #5
Ah >__< okay thank you for the help :)
 
  • #6
But also remember without the muon pair the leading diagram to the decal L->pK is NOT a FCNC with a loop. It would be the W emission diagram, where
Code:
u-----u
d-----d
b--w--u

w---> (s ubar)

This will be proportional to Gf Vub Vus, where the FCNC one is a sum over the up-types in the loop, so Vub Vus , Vcb Vcs, Vtb Vts, times the loop factors, and an extra alpha_EM.
 
  • #7
That's interesting, according to the pdg its branching fraction is still of the same order as the FCNC decay modes which occur at the one-loop level. Surely if it occurs at the tree level it should have a higher branching fraction?
 
  • #8
massive particles can be both in RH and LH states (because their LH and RH components are coupled through higg's vev)... only the neutrinos which are considered massless the antiparticle and particle exist in one or the other (the coupling to the higgs vev is very weak)...
Am I wrong?
 
  • #9
That's interesting, according to the pdg its branching fraction is still of the same order as the FCNC decay modes which occur at the one-loop level. Surely if it occurs at the tree level it should have a higher branching fraction?

Not nessecarily. The tree level mode [itex]\Lambda_{b}->p^{+}k^{-}\mu^{+}\mu^{-}[/itex] is supressed by the small CKM element [itex]V_{bu}[/itex] Where the FCNC is a b->s transition which is only moderatley supressed due to the large top yukawa.

Moreover, this FCNC decay doesn't have helicity supression. Helicity supression happens when a scalar decays to light fermions through a vector coupling, like the decay [itex]B_{s}->\mu^{+}\mu^{-}[/itex]
 
  • #10
  • #11
which symmetries produce FCNC in GUTs? and in general, how it possible, if all GUTs up to E6 describe only one generation (family) of particles
 

1. What are FCNCs?

FCNC stands for Flavor Changing Neutral Currents, which are interactions between particles that involve the exchange of a neutral gauge boson and result in a change in the flavor of the particles involved.

2. Why are FCNCs important in physics?

FCNCs provide a way to study the fundamental interactions between subatomic particles and can help us understand the nature of the universe at a deeper level.

3. How do scientists study FCNCs?

Scientists study FCNCs using high energy particle colliders, such as the Large Hadron Collider (LHC), to create collisions that produce rare FCNC events. They also use theoretical models and simulations to understand and predict these interactions.

4. What is the significance of FCNCs in the Standard Model?

FCNCs are forbidden in the Standard Model, which is the most widely accepted theory of particle physics. The observation of FCNCs would indicate the presence of new physics beyond the Standard Model.

5. What are some potential implications of understanding FCNCs?

Understanding FCNCs could lead to the discovery of new particles and interactions, which could help us solve some of the biggest mysteries in physics, such as the nature of dark matter and the unification of the fundamental forces.

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