Particle-antiparticle annihilation and spin

In summary, the particle and antiparticle will annihilate into two photons. Because of spin conservation (one photon has spin of 1 and particle antiparticle has in your sense net spin of 0), two photons also with net spin zero are created. For positronium (electron-positron bound state), the state with total spin 0 annihilates much faster than the state with spin 1, because the spin 1 state must annihilate into three photons, with 1+1+1=1.
  • #1
Nicky
54
0
I have a question regarding particle-antiparticle annihilation, such as electron-positron, proton-antiproton, etc. Can the annihilation still occur if the two particles are in opposite spin eigenstates, i.e. if the pair has zero net spin?
 
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  • #2
The spin of the whole system has to be conserved.

The particle and the antiparticle will annihilate into two photons. Because of spin conservation (one photon has spin of 1 and particle antiparticle has in your sense net spin of 0). You see? Two photons also with net spin zero.
 
  • #3
For positronium (electron-positron bound state), the state with total spin 0 annihilates much faster than the state with spin 1, because the spin 1 state must annihilate into three photons, with 1+1+1=1.
Incidentally, the two particles are NOT in opposite spin eigenstates.
If they are in an eigenstate of total spin (1 or 0), the individual particles can not be in spin eigenstates.
 
  • #4
Sorry, I'm missing something very obvious that I do know, but can't put my finger on right now (note to self - drinking heavily the night before doing Physics doesn't work...). Why can't the spin 0 positronium state decay to two photons with spin +1, -1 respectivaly? That conserves the spin surely?
 
  • #5
James - who said it doesn't?
 
  • #6
Meir Achuz said:
For positronium (electron-positron bound state), the state with total spin 0 annihilates much faster than the state with spin 1, because the spin 1 state must annihilate into three photons, with 1+1+1=1.

Does the spin 1 state decay in two stages? Maybe spin 1 positronium -> spin 0 positronium + photon, then spin 0 positronium -> two photons?
 
  • #7
Kruger said:
The spin of the whole system has to be conserved.

The particle and the antiparticle will annihilate into two photons. Because of spin conservation (one photon has spin of 1 and particle antiparticle has in your sense net spin of 0). You see? Two photons also with net spin zero.

Can u prove it?

Meir Achuz said:
For positronium (electron-positron bound state), the state with total spin 0 annihilates much faster than the state with spin 1, because the spin 1 state must annihilate into three photons, with 1+1+1=1.

Why?

juvenal said:
James - who said it doesn't?

Umm,Quantum Mechanics...? :rolleyes:

Daniel.
 
  • #8
Can u prove it?

hehe. No. You learned me that spin has not to be conserved. Only total angular momentum has to be conserved.
 
  • #9
oh oh, this is a dangerous one


drop it like it's hot...

marlon
 
  • #10
dextercioby said:
Umm,Quantum Mechanics...? :rolleyes:

Daniel.

Maybe you're misunderstanding my point, and I'm not sure why you are since it's pretty explicit.

You're saying that spin 0 positronium CANNOT decay to two photons? That is what I'm referring to.

If so, I beg to differ:

http://rockpile.phys.virginia.edu/mmod27.pdf

Bottom of page 3. (Further up may lie the answer to your inquiry of Meir Achuz's post).
 
Last edited by a moderator:
  • #11
I didn't say that,i missinterpreted your question,since you simply asked it without quoting what in James Jackson's post you were referring to.

Daniel.
 
  • #12
So far we have left out charge conjugation invariance, which was tacitly assumed in my original answer. Photons have C=-1. Positronium of spin 0 has C=+1, and so can decay into two photons. Positronium of spin 1 has C=-1, and cannot decay into two photons. Three is the next lowest number. If this leads to more questions, I will try to answer them as asked.
 

What is particle-antiparticle annihilation?

Particle-antiparticle annihilation is a process in which a particle and its corresponding antiparticle collide and are converted into other particles or forms of energy.

What is spin in the context of particle-antiparticle annihilation?

In particle physics, spin refers to the intrinsic angular momentum of a particle. It is a fundamental property of particles and can be either half-integer (such as 1/2 or 3/2) or integer (such as 0 or 1). In particle-antiparticle annihilation, the spin of the particles involved plays a crucial role in determining the products of the annihilation process.

Why is particle-antiparticle annihilation important in understanding the universe?

Particle-antiparticle annihilation is important because it helps us understand the fundamental building blocks of the universe. By studying the particles and energies produced during annihilation, scientists can gain insights into the nature of matter, antimatter, and the forces that govern their interactions. Additionally, particle-antiparticle annihilation played a crucial role in the early stages of the universe's formation, as it helped determine the balance between matter and antimatter.

Can particle-antiparticle annihilation be observed in everyday life?

No, particle-antiparticle annihilation is not something that can be observed in everyday life. It typically occurs in extreme environments, such as particle accelerators or in the high-energy collisions of cosmic rays. However, its effects can be seen in the form of the products produced by the annihilation process.

What are some practical applications of particle-antiparticle annihilation?

Particle-antiparticle annihilation has several practical applications, particularly in medical imaging and cancer treatment. Positron emission tomography (PET) uses the annihilation of positrons (the antiparticle of electrons) with electrons in the body to produce images of the body's internal functions. In cancer treatment, positron-emitting isotopes can be used to target and destroy cancer cells through the process of annihilation. Additionally, the study of particle-antiparticle annihilation has led to advancements in particle accelerators and nuclear technology.

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