Pair production: Has it been observed experimentally?

[damianpaz]

I was reading about electron-positron annihilation and I got to the part where it says that this is a reversible process, meaning that an electron and a positron could be created from photons.

About this reverse process the article says "In nuclear physics, this occurs when a high-energy photon interacts with a nucleus".

But that doesn't sound like the reverse of annihilating an electron and a positron to produce photons.

Has ever been observed in the laboratory the creation of a "pair" only from photons?

If no, why not? If yes any source to the experiment would be appreciated.

Thanks in advance.

 

[triclon]

I work with gamma ray astronomers and they see it all the time, in fact it creates an annoying background when trying to observe very high energy gamma rays from space. Basically a cosmic ray, a proton or atomic nucleus moving near the speed of light through space strikes the upper atmosphere. It his the nucleus of one one of the atoms in our atmosphere and leads to pair production of an electron and positron (and perhaps other particles as well) which then in turn strike other nuclei creating a shower of electrons and positrons. Even more cool is that these particle showers created by cosmic rays they think seed clouds in thunderstorms with strong enough electrical fields to trigger lightning. So basically:

particle + nucleus --> electron + positron + nucleus

You need the nucleus to conserve energy and momentum, although only it's motion is altered in this reaction usually (ie. it's momentum and kenetic energy).

 

[ZapperZ]

I was reading about electron-positron annihilation and I got to the part where it says that this is a reversible process, meaning that an electron and a positron could be created from photons.

About this reverse process the article says "In nuclear physics, this occurs when a high-energy photon interacts with a nucleus".

But that doesn't sound like the reverse of annihilating an electron and a positron to produce photons.

Has ever been observed in the laboratory the creation of a "pair" only from photons?

If no, why not? If yes any source to the experiment would be appreciated.

Thanks in advance.

This is so well-known and verified that they are being used as positron sources for accelerators. The ILC design used this concept, for example.

Here's an example of the same pair production principle being used to produced polarized positrons:

http://www.slac.stanford.edu/exp/e166/

Zz.

 

[LostConjugate]

So WAIT.

Fermions are made up of bosons? Why didn't anyone say so?

 

[Drakkith]

So WAIT.

Fermions are made up of bosons? Why didn't anyone say so?

Sir you must be joking!

 

[LostConjugate]

Your saying that you can make electrons out of photons and you can make photons out of electrons?

 

[ZapperZ]

Your saying that you can make electrons out of photons and you can make photons out of electrons?

Huh?

Pair production is electron-positron pairs out of photons, with some momentum transfer! That's why they passed gamma photon through high-Z crystals to get e-p pairs! No one said anything about fermions being made out of bosons!

Zz.

 

[LostConjugate]

Huh?

Pair production is electron-positron pairs out of photons, with some momentum transfer! That's why they passed gamma photon through high-Z crystals to get e-p pairs! No one said anything about fermions being made out of bosons!

Zz.

But that doesn't sound like the reverse of annihilating an electron and a positron to produce photons.

The reverse of this is annihilating a photon to produce electrons and positrons.

meaning that an electron and a positron could be created from photons.

Once again mention of fermions (electrons / positrons) being created from bosons (photons).

Now your response to these statements was:

This is so well-known and verified

It is so well-known and verified that electrons and positrons are created from photons? This is new to me.

 

[ZapperZ]

The reverse of this is annihilating a photon to produce electrons and positrons.

Could you show where, for example, in that experiment that I cited that they "annihilate" a photon?

Once again mention of fermions being created from bosons.

Yes, but created using something does NOT mean that fermions are MADE of it! There are EXTRA INGREDIENTS required beyond JUST photons (momentum transfer). And how many bosons can you add and subtract to give you a net spin of 1/2?

Zz.

 

[LostConjugate]

Could you show where, for example, in that experiment that I cited that they "annihilate" a photon?

I just made that up to keep in reference with the "opposite" of. I don't really know what they do.

Yes, but created using something does NOT mean that fermions are MADE of it! There are EXTRA INGREDIENTS required beyond JUST photons (momentum transfer). And how many bosons can you add and subtract to give you a net spin of 1/2?

Zz.

What about this quote: "by converting the positrons back to photons"

What in the world does that mean?

I understand that positrons / electrons are permutations in an electron field and photons are obviously permutations in the electromagnetic field. Two different things entirely.

I also understand that an electron and a proton can become a neutron in the construction of a neutron star, however both those particles are bosons.

 

[ZapperZ]

I just made that up to keep in reference with the "opposite" of. I don't really know what they do.

So you made something up?

What about this quote: "by converting the positrons back to photons"

What in the world does that mean?

I converted $4 today into a burrito. You're saying that my burrito is made up of $4 bills?

Zz.

 

[LostConjugate]

I was reading about electron-positron annihilation and I got to the part where it says that this is a reversible process, meaning that an electron and a positron could be created from photons.

About this reverse process the article says "In nuclear physics, this occurs when a high-energy photon interacts with a nucleus".

But that doesn't sound like the reverse of annihilating an electron and a positron to produce photons.

Has ever been observed in the laboratory the creation of a "pair" only from photons?

If no, why not? If yes any source to the experiment would be appreciated.

Thanks in advance.

Zapper your the one that replied to this post and stated that it has been verified. This post is speaking of annihilation of photon, creating electrons and positrons from photons, and the creation of a "pair" ONLY from photons.

I have never heard of such a thing, so I am confused.

 

[ZapperZ]

Zapper your the one that replied to this post and stated that it has been verified. This post is speaking of annihilation of photon, creating electrons and positrons from photons, and the creation of a "pair" ONLY from photons.

I have never heard of such a thing, so I am confused.

I have no idea what you read in that post that caused you to conclude that fermions are made of bosons.

But that doesn't sound like the reverse of annihilating an electron and a positron to produce photons.

The "reverse" means that it must be photon-photon annihilation? How about reverse as in "run the movie backwards"?

Zz.

 

[LostConjugate]

I have no idea what you read in that post that caused you to conclude that fermions are made of bosons.



The "reverse" means that it must be photon-photon annihilation? How about reverse as in "run the movie backwards"?

Zz.

I read this

meaning that an electron[fermion] and a positron could be created from photons[boson]

 

[ZapperZ]

I read this

meaning that an electron[fermion] and a positron could be created from photons[boson]

... and other stuff, such as a momentum transfer.

But is an electron made up of photons? Nope! Nothing in the conversion process implies that one is made up of the other.

Zz.

 

[LostConjugate]

... and other stuff, such as a momentum transfer.

But is an electron made up of photons? Nope! Nothing in the conversion process implies that one is made up of the other.

Zz.

Ok now we are on the right path.

What do you mean by momentum transfer, certainly electrons are not made up of photons and momentum. What other stuff?

 

[Drakkith]

Ok now we are on the right path.

What do you mean by momentum transfer, certainly electrons are not made up of photons and momentum. What other stuff?

No, he means that electrons can be created by momentum transfer. For example, colliding two protons together at high energies results in the creation of many particles, including electrons, positrons, photons, ETC.

 

[LostConjugate]

Where to they come from?

 

[vkroom]

I think we are messing up something here.

\gamma \rightarrow e^- + e^+ is mass creation from energy: E=m_0 c^2 + p^2c^2. If the photons have less energy than twice the rest mass of electrons this creation process will not occur except maybe as vacuum fluctuations.

 

[Drakkith]

Where to they come from?

The energy of the collisions!

 

[strangerep]

\gamma \rightarrow e^- + e^+

That doesn't conserve energy-momentum.

Did you perhaps mean

\gamma + \gamma \leftrightarrow e^- + e^+

?

 

[Drakkith]

That doesn't conserve energy-momentum.

Did you perhaps mean

\gamma + \gamma \leftrightarrow e^- + e^+

?

It does in pair production thanks to the nucleus the photon interacts with.

 

[Waterfox]

Hopefully someone can help me understand this http://www.hep.ucl.ac.uk/~opal/gammagamma/gg-tutorial.html

Is it saying that due to the uncertainty principle a photon may at anytime fluctuate into a fermion/anti-fermion pair into which another high energy photon can interact with and cause pair production?

 

[ZapperZ]

That doesn't conserve energy-momentum.

Did you perhaps mean

\gamma + \gamma \leftrightarrow e^- + e^+

?

This is why I kept emphasizing the phrase with momentum transfer. If you look at the article that I cited, the high energy gamma have to pass through a crystal. This is because it needs to interact with something "heavy", such as the nucleus, for there to be a momentum transfer to conserve momentum. The gamma doesn't spontaneously produce e-p pair in vacuum!

Zz.

 

[LostConjugate]

The photon is the quanta of the energy transferred in a perturbation of the electromagnetic field.

The momentum of a photon is simply a measurement of the rate of perturbation.

How are you taking these two concepts and creating an electron and a positron which have rest mass.

 

[Dadface]

I think pair production is a well documented and explained event but I think people here are straying from the original question where reference was made to the electron positron pair being created not as an event involving a single photon but as an event involving photon(s) as a reversible process of annihilation.(as exemplified by strangereps equation post 21)

 

[LostConjugate]

Ok I will start a new thread, this pair production thing is new to me.

 

[Drakkith]

The photon is the quanta of the energy transferred in a perturbation of the electromagnetic field.

The momentum of a photon is simply a measurement of the rate of perturbation.

How are you taking these two concepts and creating an electron and a positron which have rest mass.

The key here is that all energy has mass. When the photon was created, an amount of mass was removed from whatever created it and will be transferred to whatever the photon is absorbed by. When we talk about mass we *usually* talk about rest mass. Since a photon cannot be at rest it cannot have rest mass and is considered massless. However, it still is affected by and produces gravity just like things that do have mass. It has momentum and energy though, and these things are conserved in any particle created event.

This is effectively no different really than smashing to electrons together and getting a shower of particles. The total mass of the created particles can be MORE than the mass of the 2 electrons! So they certainly weren't created from JUST the mass, but from the energy!

 

[LostConjugate]

So where did I go wrong when I said that fermions were made from bosons..?

 

[Drakkith]

So where did I go wrong when I said that fermions were made from bosons..?

Because they are not. Just like bosons are not made of fermions. They all have different fundamental properties such as charge, mass, etc. If two electrons collide at 5 ev, they simply bounce off each other. If they collided at 14 TeV, entire nucleons and heavier particles are produced! So are electrons made of protons? No!

Also, consider the fact that accelerating a charge produces EM waves, aka photons. Does this mean that the electron is disappearing and turning into photons? No!

 

[LostConjugate]

Because they are not. Just like bosons are not made of fermions. They all have different fundamental properties such as charge, mass, etc. If two electrons collide at 5 ev, they simply bounce off each other. If they collided at 14 TeV, entire nucleons and heavier particles are produced! So are electrons made of protons? No!

Also, consider the fact that accelerating a charge produces EM waves, aka photons. Does this mean that the electron is disappearing and turning into photons? No!

Exactly. Yet we can get particles from high enough energy photons.

 

[Drakkith]

Exactly. Yet we can get particles from high enough energy photons.

Yes we can. Energy! Momentum! That is what creates things.

 

[LostConjugate]

Yes we can. Energy! Momentum! That is what creates things.

I can resonate with that.

 

[qsa]

http://teachers.web.cern.ch/teachers/archiv/HST2002/Bubblech/mbitu/electron-positron.htm

 

[strangerep]

So where did I go wrong when I said that fermions were made from bosons..?
[...]
we can get particles from high enough energy photons

(Perhaps my answer below should be in a separate thread, but I put it here for now...)

LostConjugate,

Have you come across the notion of the Wigner method for classification of elementary particles? I suspect the answer is "no", so here's the idea (very briefly):

All elementary particle types are classified according to the so-called "unitary irreducible representations of the Poincare group". Another way of saying this is that every type of elementary particle must correspond to one of the physically distinguishable ways that things can transform under rotations, boosts and translations. It turns out that these "ways" can be classified in terms of 2 invariants: the (relativistic) mass^2 and something known as W^2 -- where W is the Pauli-Lubanski vector, which can be regarded as a relativistic generalization of spin.

So (again cutting a longer story short) we can classify elementary particle types according to their mass, spin (and also their component of spin in an arbitrary direction). This classification is not exhaustive of course, because there's also the other so-called "intrinsic" properties such as electric charge, hypercharge, lepton number, baryon number, etc. The set of all these properties is thought of as the set of "quantum numbers" that make one type of particle different from another.

Additionally, we have ordinary energy, momentum, angular momentum, etc. For all these things to be useful in describing processes, we want their total amounts to be conserved during interactions. E.g., the total charge must be conserved, as must the total energy and momentum, the total lepton number, etc, etc.

So how is all this relevant to \gamma + \gamma \leftrightarrow e^- + e^+ ?

Well, basically the elementary particles are not made "from" anything (else they would not be "elementary"). However, we can describe any given particle instance by the set of quantum numbers outlined above and a few more -- which I'll illustrate by re-writing the equation above with some extra arguments:
<br /> \gamma(E_1, p_1, \sigma_1, \dots) + \gamma(E_2, p_2, \sigma_2, \dots) \leftrightarrow e^-(E_3, p_3, \sigma_3, \dots) + e^+(E_4, p_4, \sigma_4, \dots)<br />
where E means energy, P means momentum, \sigma means spin, and the
"..." means all the other quantum numbers.

For conservation of energy in the reaction, we must have
<br /> E_1 + E_2 = E_3 + E_4<br />
and similarly for the other quantum numbers.

So is total electric charge conserved? Yes, because photons have charge=0 and the
sum of electric charge on the rhs is 1 + -1 = 0.

The only reactions that can occur are those that satisfy these conservation laws for
total quantum numbers.

Can you see where I'm going with all this? Bosons are not "made from" fermions, but we
can find reactions such that a collection of bosons can be converted into a collection of
fermions, etc, etc.

HTH.

 

[LostConjugate]

Can you see where I'm going with all this? Bosons are not "made from" fermions, but we
can find reactions such that a collection of bosons can be converted into a collection of
fermions, etc, etc.

HTH.

I understand that charge and energy are conserved in the process. Though because you can convert energy into a massive particle and vice-ver now how can you say that a particle is "elementary". It may have discrete properties but everything can be broken down to energy.

It is the discrete states that are elementary and no longer the particles.

By the way do you learn about all this in QFT? That may be why I have not gotten to it yet, just started taking QFT only finished QM Core and GR.

 

[Drakkith]

I understand that charge and energy are conserved in the process. Though because you can convert energy into a massive particle and vice-ver now how can you say that a particle is "elementary". It may have discrete properties but everything can be broken down to energy.

It is the discrete states that are elementary and no longer the particles.

By the way do you learn about all this in QFT? That may be why I have not gotten to it yet, just started taking QFT only finished QM Core and GR.

Actually, you cannot break things down into energy. Energy does not exist by itself. Particles HAVE energy, but they are not made OF energy. For example, and Electron requires a certain amount of energy to be produced from a collision event. But when we investigate an electron, we have not found it to be anything other than an elementary particle. This is in contrast to a Proton where we have found it to be composed of 3 quarks.

Attempting to say things are made of energy is similar to saying they are made of charge or mass or spin. All of these are merely fundamental properties of the particle.

 

[Dale]

Has ever been observed in the laboratory the creation of a "pair" only from photons?

If no, why not? If yes any source to the experiment would be appreciated.

Here is a relevant wikipedia page. Down at the bottom are some references and links you may want to pursue.
https://www.physicsforums.com/newreply.php?do=newreply&p=3412501

 

[qsa]

(Perhaps my answer below should be in a separate thread, but I put it here for now...)

LostConjugate,

Have you come across the notion of the Wigner method for classification of elementary particles? I suspect the answer is "no", so here's the idea (very briefly):

All elementary particle types are classified according to the so-called "unitary irreducible representations of the Poincare group". Another way of saying this is that every type of elementary particle must correspond to one of the physically distinguishable ways that things can transform under rotations, boosts and translations. It turns out that these "ways" can be classified in terms of 2 invariants: the (relativistic) mass^2 and something known as W^2 -- where W is the Pauli-Lubanski vector, which can be regarded as a relativistic generalization of spin.

So (again cutting a longer story short) we can classify elementary particle types according to their mass, spin (and also their component of spin in an arbitrary direction). This classification is not exhaustive of course, because there's also the other so-called "intrinsic" properties such as electric charge, hypercharge, lepton number, baryon number, etc. The set of all these properties is thought of as the set of "quantum numbers" that make one type of particle different from another.

Additionally, we have ordinary energy, momentum, angular momentum, etc. For all these things to be useful in describing processes, we want their total amounts to be conserved during interactions. E.g., the total charge must be conserved, as must the total energy and momentum, the total lepton number, etc, etc.

So how is all this relevant to \gamma + \gamma \leftrightarrow e^- + e^+ ?

Well, basically the elementary particles are not made "from" anything (else they would not be "elementary"). However, we can describe any given particle instance by the set of quantum numbers outlined above and a few more -- which I'll illustrate by re-writing the equation above with some extra arguments:
<br /> \gamma(E_1, p_1, \sigma_1, \dots) + \gamma(E_2, p_2, \sigma_2, \dots) \leftrightarrow e^-(E_3, p_3, \sigma_3, \dots) + e^+(E_4, p_4, \sigma_4, \dots)<br />
where E means energy, P means momentum, \sigma means spin, and the
"..." means all the other quantum numbers.

For conservation of energy in the reaction, we must have
<br /> E_1 + E_2 = E_3 + E_4<br />
and similarly for the other quantum numbers.

So is total electric charge conserved? Yes, because photons have charge=0 and the
sum of electric charge on the rhs is 1 + -1 = 0.

The only reactions that can occur are those that satisfy these conservation laws for
total quantum numbers.

Can you see where I'm going with all this? Bosons are not "made from" fermions, but we
can find reactions such that a collection of bosons can be converted into a collection of
fermions, etc, etc.

HTH.

you are describing standard textbook stuff ,fine. But there are tons of advanced "mainstream" theories that go beyond this standard picture. I hope you do agree , since the standard picture is lacking in more than one way.

 

[damianpaz]

Ok. So I take that pair production can be obtained just from photons and no nucleus is required.

 

[Drakkith]

Ok. So I take that pair production can be obtained just from photons and no nucleus is required.

It involves two photons and is very very rare.

 

[strangerep]

you are describing standard textbook stuff ,fine. But there are tons of advanced "mainstream" theories that go beyond this standard picture. I hope you do agree , since the standard picture is lacking in more than one way.

If the theories you have in mind are indeed "mainstream" (meaning having lots of publications in peer-reviewed journals, and having empirical evidence that supports them in a way that makes them experimentally distinguishable from "standard textbook stuff"), then there should be no problem mentioning them by name (and quoting references!) here on PF.

 

[strangerep]

Ok. So I take that pair production can be obtained just from photons and no nucleus is required.

What Drakkith said. :-)

It requires very high energy (4th-order QED effect). Doing it in accelerator experiments was only achieved fairly recently, iirc.

Doing it by having a gamma ray interact with the Coulomb field of a nucleus is much easier. (Look up "Delbruck scattering".)
http://en.wikipedia.org/wiki/Delbruck_scattering

 

[qsa]

If the theories you have in mind are indeed "mainstream" (meaning having lots of publications in peer-reviewed journals, and having empirical evidence that supports them in a way that makes them experimentally distinguishable from "standard textbook stuff"), then there should be no problem mentioning them by name (and quoting references!) here on PF.

Do you consider String and LQG as "mainstream" or not.

 

[Dale]

Do you consider String and LQG as "mainstream" or not.

AFAIK, neither of those support your claim that fermions are made of bosons.

 

[qsa]

AFAIK, neither of those support your claim that fermions are made of bosons.

i was just saying that there are other ideas as to the nature of particles by people who are generally not considered crackpots. just bigger picture that's all.


like this one

http://arxiv.org/PS_cache/hep-th/pdf/0507/0507109v4.pdf

also similar idea appears in lqg

http://pirsa.org/11070005/
Spinfoam Fermions
Elena Magliaro
Abstract: A serious shortcoming of spinfoam loop gravity is the absence of matter. I present a minimal and surprisingly simple coupling of a chiral fermion field in the framework of spinfoam quantum gravity. This result resonates with similar ones in early canonical loop theory...
(6 July 2011)

also Torsten.

 

[strangerep]

Do you consider String and LQG as "mainstream" or not.

The lack of experimental results that could establish either of them as superior to SM+GR means they should be discussed in the BTSM forum.

 

[Dale]

i was just saying that there are other ideas as to the nature of particles by people who are generally not considered crackpots. just bigger picture that's all.

I don't think anyone was objecting to the bigger picture, just some specific assertions.

 

[qsa]

I don't think anyone was objecting to the bigger picture, just some specific assertions.

Although I do understand the sentiment, but most people come to PF because they are very smart and curious people. they can figure out equations and what not from textbooks, but largely they want to know more and deep. There is a misunderstanding, some guys like Lostconjugate test the deep water by making there own conjecture , it is a learning process and not a crackpottery. I gave the Lorentz theory (not mine) as an example of the many Ideas already tried and the ongoing asthetics. Even the QCD mass calculation has an element of the theory like was admited by its inventors Wilczek. Anyway, It was just an example.