History of High Energy electron-positron collisions

[dseabroo]

Main Question or Discussion Point

Can someone please tell me the history of high-energy electron-positron collisions? In specific when was it discovered that such collisions can create hadrons? Any information would be useful. Thanks.

 

[hamster143]

According to Wikipedia, the first electron-positron collider was called AdA, it was constructed in Italy in the early 60's. Its energy and luminosity were insufficient to observe hadron production. Soviet collider called VEPP-2 was launched in Novosibirsk in 1966. They were the ones who first observed pion pair production in in e+e- collisions in 1967. By the early 70's, Americans got seriously involved too, SLAC in Stanford was up and running by 1972 and made a couple of major discoveries in 1974-76.

http://accelconf.web.cern.ch/AccelConf/p73/PDF/PAC1973_0756.PDF

That does not answer your exact question - when was it discovered that you could make hadrons in electron-positron collisions - because, even before these colliders went online, physicists knew that you could produce pion pairs that way, and they were even able to estimate production cross sections. How they got there, I don't know, maybe it involved observing cosmic ray positrons somehow.

 

[Bob_for_short]

That does not answer your exact question - when was it discovered that you could make hadrons in electron-positron collisions - because, even before these colliders went online, physicists knew that you could produce pion pairs that way, and they were even able to estimate production cross sections. How they got there, I don't know, maybe it involved observing cosmic ray positrons somehow.

The hadron pair production e^-+e⁺ → h^-+h⁺ can be considered and calculated as a cross channel of the known elastic electron scattering off hadrons e^-+h⁺ → e^-+h⁺.

Bob_for_short.

 

[hamster143]

The hadron pair production e^-+e⁺ → h^-+h⁺ can be considered and calculated as a cross channel of the known elastic electron scattering off hadrons e^-+h⁺ → e^-+h⁺.

Bob_for_short.

That is a good point - crossing symmetry was already known in the 60's. Regarding the cross process, to get pion and kaon pair production amplitudes, physicists must have measured pion scattering on electrons, not vice versa.

 

[dseabroo]

Thanks for your quick reply guys. A few more questions: In addition to creation via collisions are you aware of any other way that hadrons come about. For instance do charged proton-anti-proton pairs appear, theoretically, in electric fields via the Schwinger Effect? If you would provide any citations to important papers about the creation of hadron pairs via Schwinger or electron-positron collisions I would be charmed. Any insight at all into hadron creation is welcome. Thanks again.

 

[hamster143]

Thanks for your quick reply guys. A few more questions: In addition to creation via collisions are you aware of any other way that hadrons come about. For instance do charged proton-anti-proton pairs appear, theoretically, in electric fields via the Schwinger Effect?

Theoretically yes, but electric fields required are immense. Even electron-positron pair production via Schwinger Effect has not been observed yet (to my knowledge) and energies required for pp are three orders of magnitude higher. There is potential to achieve those effects with high-powered lasers.

Most hadrons in the Universe came about through condensation of primordial quark-gluon plasma as it cooled shortly after the Big Bang.

Can't help you with papers, sorry ...

 

[dseabroo]

Thanks again Hamster143. Could you answer me this: When you say that pp require a field three orders of magnitude greater than ee pairs are you saying that there is a certain threshold to reach before *any* pp pairs appear or will you see a smaller number of pp pairs formed in lesser fields. Also in the more powerful field will you still see ee pairs and is there some way to calculate what the ratio of pp/ee pairs would be?

 

[hamster143]

First of all I must correct myself - the field for pp is not three but six orders of magnitude greater than the field for ee. And the field needed for ee pairs is on the order of 10^18 V/m. Below that, pair production is exponentially suppressed by a factor $$(E_{crit}/E)^2 exp(-E_{crit}/E)$$. You still see pairs, but, the lower the field, the less likely it is to create particle pairs. Already at the field 1% of critical the rate is so low that you're exceedingly unlikely to detect any pairs created during your lifetime.

The field needed can be estimated by a simple semiclassical argument. In the vacuum, virtual particle pairs appear and disappear all the time. A typical separation between two particles in such a pair is on the order of their Compton wavelength. You will see significant pair production if the field is so strong that the difference of potentials over the Compton wavelength is comparable to the rest energy of the pair.

If the field is strong enough, you will see ee pairs, pp pairs, all sorts of other pairs, muons, pions, etc. etc. The ratio of pp/ee in a strong enough field would probably be on the order of 1 - but protons and antiprotons are not elementary particles and the dynamic of their creation would be much more complicated.

Also, there's a big assumption we're making, namely that it's even possible to sustain a field of that magnitude. A system with field strength above ee creation threshold would be constantly losing energy to pair creation.

 

[dseabroo]

Thanks again for the further insight hamster143. You have been incredibly helpful. Yes I am aware that sustaining a field of this strength would be next to impossible, but I am only speaking of the theoretical here. I would just like to ask how it is that you know all this good stuff? I certainly couldn't find this stuff doing an internet search. Are you sure there are no citations out there to back any of this up?

 

[hamster143]

You're underestimating the power of internet search ... If you search for "Schwinger effect" in Google, link #5 gives you a recent article about a proposal to reproduce the effect with high-powered lasers, and there's a formula for the critical energy and the pair creation rate in the first paragraph. Link #3 in the result for "Schwinger critical field" takes you to a serious book on high-field electrodynamics, with some more information on the subject.

Some of it is just general knowledge of QFT.

 

[dseabroo]

Thanks hamster. I do not see the link #5 you speak of. I have been searching the internet for some time on this subject with little luck, so its not as if I haven't been trying it! As for "Schwinger critical field" I simply never knew to search for that term. BTW when you say that the production of protons is "much more complicated" I assume that you are referring to the fact that protons are made from quarks. Would it then be correct to say that the real question is the ratio of ee pairs to quark pairs? Given that quarks are presumably less massive than protons, would this mean that the required field would actually be less than what you would expect doing the math for complete protons? (I hope this makes any sense)

 

[hamster143]

Link number 5 in google search for schwinger effect:

http://arxiv.org/abs/0811.3570?context=physics.optics

click on "PDF".

re: quarks, that is a good observation, but here's the thing ... quarks are strong-interacting. In order to pull apart a pair of virtual quarks, you must overcome strong interaction, and to do that, you need a field of ~ 10^24 V/m - which is the same order of magnitude as the field needed to create complete proton-antiproton pairs according to Schwinger formula.

More generally, ee pair creation is pure quantum electrodynamics (thus relatively simple) and pp pair creation involves QCD (thus much more complicated).

 

[dseabroo]

Hamster - so I guess the question remains, has anyone actually written a paper about the theoretical creation of pp pairs via Schwinger? Furthermore does anyone have a citation to back up the idea that quark production requires the same field strength as complete protons? I find that very interesting.