# Electron Positron Annihilation

1. Aug 7, 2014

### Rob Hoff

What kinds of particles do e- and e+ annihilation make? Where does the higgs boson come into play? And why are the electromagnetic and the weak force so closely related?

Thanks peeps

2. Aug 7, 2014

### Orodruin

Staff Emeritus
Electron-positron annihilation can result in essentially any particles that interact through the electromagnetic or weak forces - or through Higgs interactions. The requirement is that there is enough energy available in the center of mass system.

The Higgs boson could be acting both as a mediator and a final state of the annihilation, although the Higgs coupling to electrons is very small. Higgs production has so far not been observed in electron-positron colliders.

The electromagnetic and weak forces are closely related because they are both different facets of the unified electroweak interaction. The symmetry of electroweak interaction is spontaneously broken and the result of to separate it into what we know as weak and electromagnetic interactions.

3. Aug 7, 2014

### mathman

Electron-positron annihilation almost always results in a pair of gamma rays, usually 511 kev (rest mass of the electron) each.

4. Aug 7, 2014

### Orodruin

Staff Emeritus
Note that what mathman is saying is true at low energies, where the kinetic energy is not sufficient to produce any other particles (neutrino production is suppressed due to only happening through the weak interaction). At higher energies, such as available in particle colliders - LEP in the case of electron-positron annihilation - the possibilities are much more varied.

5. Aug 8, 2014

### Rob Hoff

Very interesting. Thanks guys!

6. Aug 8, 2014

### haael

Speaking of Higgs: what quantum numbers does Higgs have, exactly?

Higgs has nonzero hypercharge and isospin and it is not its own antiparticle, so it would have to be produced in pairs Higgs-antiHiggs, right?

Also, Higgs is spinless, so its production in electron-positon annihilation would depend on the particle's spins.
If the electron and positon have opposite spins, then they form an intermediate parapositonium and decay into 2 photons.
If they have parallel spins, they form an intermediate ortopositonium and decay into 3 photons. The 3rd photon carries the nonzero net spin of the ortopositonium.

How would decay of ortopositonium look like when Higgs is one of the results? Would it be: Higgs, antiHiggs, photon?

7. Aug 8, 2014

### Orodruin

Staff Emeritus
Energies in colliders are way too high for bound states to form. The typical thing to do when computing cross-sections is to assume unpolarized beams and average over the incoming spins. The Higgs could never be the result of a decay of positronium simply because the bound positronium state does not possess enough energy to produce it.

The Higgs is a part of an SU(2) doublet scalar field with hypercharge 1 before EW symmetry breaking. Upon EW symmetry breaking, what we call the Higgs boson has a remaining electric charge of zero and is its own antipartcle. It would be fully possible to have the following reaction in a collider:
$$e^+ + e^- \to h + \gamma$$
LEP, the most powerful electron-positron collider so far, was able to put lower bounds on the Higgs mass but never observed Higgs production.

8. Aug 14, 2014

### Rob Hoff

Why would the higgs boson show up if the other output was a photon? Photons don't have mass I thought.

9. Aug 14, 2014

### Drakkith

Staff Emeritus
The photon has nothing to do with the higgs boson in that reaction. The higgs was created at the same time as the photon. As long as the conservation laws are followed, the collision can create almost any particle.

10. Aug 17, 2014

### Rob Hoff

I can't wrap my head around how the annihilation could pop out almost anything as an output, there has to be a method to the madness. Is there any way to accurately predict what will result?

11. Aug 17, 2014

### Drakkith

Staff Emeritus
The best we can do is give probabilities based on what type of particles are colliding and at what energy. Some particles can't be created in lower energy collisions since the combined kinetic energy and rest mass of the particles are not enough.

12. Aug 17, 2014

### Rob Hoff

Hey thanks Drakkith

13. Aug 17, 2014

### Orodruin

Staff Emeritus
You can compute the probabilities of each particular annihilation (we call these branching ratios) in the framework of quantum field theory and they will typically depend on the incoming particle energies. As usual in quantum physics, you cannot say what a particular annihilation will result in, just the probabilities.