Louis S. Osborne, “Mesons & Baryons” - LNS46 Symposium: On the Matter of Particles
OSBORNE: Yes, the explanation is that under time duress, someone that phones up and says, what's the title of your talk? I just couldn't think of anything, Mesons and Baryons. And now I'll explain my change of mind later-- or, rather, shortly. I was going to start out with a fable or a bad dream that I thought of in terms of mesons and baryons, which takes you back to Schrodinger and his equation. Whereby, he wrote down the equation, as I saw it, and then was embarrassed to find he couldn't solve the equation. But worse than that, none of his colleagues, nor anybody else could do it. But came a comforting person with a futuristic point of view. Patience. In 80 years, we'll have a hyper, super cyber computer, which will solve your equation, and you will get a binding energy of the hydrogen atom.
Why is that a story? Because that's a way we're caught, in a way, in terms of QCD in the not preturbitive case. So I have a tendency to move away from mesons and baryons. And then also, in terms of cutting down a 30 minute talk on 40 or six years to manageable proportions, I looked for another theme, and I found it. And you'll see that-- as a matter of fact-- it's naturally taken. And that is, it's photons. Both, real and virtual, and time like and space like. Derek actually went through the rationale, and in fact, more detail than I'd planned to, as to why photons are so important. Because we think we understand them, and then the other part of it is because they're flavor free. That is to say, they will latch on to anything with a charge, and they don't care. Provided that you have the energy to provide the mass, they don't care what flavor you are, what family you are. Anything like that.
Now, of course, the prime illustration of the beauty of photons, is also taken up in other talks at these sessions today this morning and this afternoon. Namely the Friedman, Kendall, Taylor experiments that slack at MIT, which of course make use of the space-like photon, and in fact, condemned those of us who were working on meson and baryon to realize that we weren't dealing with elementary particles. But we're dealing with molecular chemistry, if you please. And then, also, in the time-like region, the experiments that-- I'm sure Sam will cover-- both first experiments on the production of vector mesons, of Daisy, and then following the e-plus e-minus experiments.
So that eliminates part of my problems. And now I'll go-- and I want to do-- I want to essentially do a chronology of some of the things that have happened in the laboratory, both emphasizing mostly the physics, but occasionally putting a little twist on it in terms of nostalgia and whatnot. And there's also an apology to the younger members of this audience, who probably-- it's difficult to conceive-- of why we were perplexed by a certain point at a certain time when the answer was so obvious. Well, I can only tell you that it wasn't obvious at the time. One of the first experiments-- actually Fred alluded to-- and this is the using photons to produce, in this case, pi 0 mesons. And this was, in fact, one of the first experiment that was done with the 300 MEV synchrotron here at MIT. Essentially confirmed what you could do by geometry. Namely, that they were dealing, mostly, with a J equal 3 has residence. And that also, that there's an s-wave production of pi 0s.
At that time, somewhat mysterious. There was another thing that's now-- was obvious, but I know it led to some controversy at the time. Is that, is the coherent production of pi 0s from nuclei, which sounds like perfectly reasonable quantum mechanics-- leaving the nucleus in the ground state. But at that time was controversial because, yes, everybody knew about elastic scattering of things going coherently, but producing pi 0s seemed to be another thing. But here was the experiment, then gilled Davidsons thesis, on producing pi zeros. And you'll notice as you go to heavier and heavier nuclei, that the production peaks forward more and more. Just a fun thing.
Here's a combination of illustrating what would become part of my thesis, if you please. Namely, we were producing, we were producing both physics, and we were also producing physicists. So I label some of these things with people's-- whoever it was thesis, and here's Burt Richter's thesis where the interest there was the expected-- let me do it this way. The expected from what you would expect from the 3 halves residents, but on top of that, a very-- actually very conventional simple photoelectric effect for producing a pi plus meson. And as is evidenced here by the interference term between the two. All of this was very, very, very new.
Now, there was an experiment that was done, and we called it, then, four Daves and a Bob. The four Dave's being Dave Caldwell, Dave Frish, Dave Hill, Dave Ritson, and Bob [? Schlueter ?] being the Bob. This again and it's history, up to that time had been some measurements of pi proton scattering in this energy range, but it sort of made a mush out of this thing. There was a bump there, but it was not very clear. And here, they did a very careful job and saw that these were the two. And in fact, those and the next two, I equal-- t equal-- one half resonances in a pion nucleon system. I added Frank Genovese name to this because it was Frank, who with me-- we were making a differential Cherenkov counter-- and it was he who pointed out to me-- because he was-- and this was part of this experiment-- it was he who pointed out to me that a beam of particles producing Cherenkov light, gives light that looks like a ring of stars. And he said, you know we ought to do? Put a lens in front of it and look at the focal plane and you'll see a ring, which will depend upon the velocity of particles. Cute. That's an undergraduate student.
OK. Now, moving on. And here's some bubble chamber work. And now-- you probably-- I should have outlined here, but I want to emphasize, of course, the-- and now even I can't see it. But, is the contributions from the MIT, and you'll recognize the names of Rosenson, and Yamamoto, and Pless, who are, of course, still with us. But Yamamoto, at that time, was a graduate student. And what was this experiment was in a bubble chamber and was the production of eta zeros. But the most important thing there was it was a mystery still. The eta zero had been found, but there were a neutral decay that was unexplained, that is to say couldn't be found. And it was they who found that the eta zero decayed into two gammas, which was rather extraordinary because it was competing with the three-pi decay which was a strong interaction. And of course the reason behind that was the violation of G-parity, which was prohibiting the three-pi decay of the eta zero. So it unrravelled the mystery at that time.
Then continuing on, and now to the work done at the Cambridge electron accelerator. And here is Byrons V, Baryon's thesis, where one of the outputs of that was the discovery of a resonance by looking at high q square. High momentum transfer photo pi zero production, here. And in fact, this is the I equals the isotopic spin, three halves. The highest baryonic resonance, I think, still extant. And one of the amusing things is if you do a Chew-Frautschi plot with these things, is that all those resonances fall on a nice straight line, if you plot j versus m squared, which seems extraordinarily simple and goes back to my comments about QCD and my prejudice that somehow the molecular physics of quarks may turn out to be simpler than at least would be indicated by the computers that have been brought to bear on that problem. OK.
Then next was also an amusing byproduct of the experiments we did in producing food pi zero and pi plus photo production. And here are just for amusement. We took the ratio of the photo production cross-sections to the pi scattering-- single pi scattering cross sections, and we discovered that even over a very large range of cross-sections, that is from small, forward angle cross-sections to backward, these points all centered about a ratio of 0.85. I roughly 2 pi over alpha. Well that was just thrown out as an amusing thing but also clearly the implication of it was that the photon is acting very much like a strongly interacting particle, simply with a reduced coupling. And indeed I'd believe this was part of the inspiration for Sakurai to invent the vector dominance model.
So continuing with that and continuing on vector dominance, here's another bubble chamber. Again, this was called the cambridge bubble chamber group. And again, the names Pless, Rosenson, and Yamamoto. And of course this typical of bubble chambers, you don't get many events, but what you get are fairly clear. And that is a bump where the rho zero should be. A production cross-section indicating a threshold for it. And the important thing was that indeed confirming the vector dominance model, the cross section for conversion of gamma rays to rhos was very large. In other words, easy to do.
Now, then because, see, they didn't really have the energy. It was a prejudice-- I guess it was a prejudice of mine. There was something smelled wrong with Reggie. Some of you haven't had the benefit of education in the Reggie-- in Reggie models, but they were supposed to be, at one time, a nice explanation, or at least a parameterization that [INAUDIBLE] possibly even included some physics. And it was our feeling that it was a little bit artificial. Here was an experiment where we thought we'd test it rather severely because we manufactured polarized photons with a crystal. And then measured pi plus photo production with polarized photons, and this is a symmetry between one polarization and the other.
That's the severest test you can put a model to when you start taking account the spins of the photon and the polarizations. And indeed we are gratified to be able to eliminate at least two models. And I think this third model has had so many parameters in it that it sort of sunk on its own, under its own weight. So important fact, I hope we helped to bury the Reggie theory. Oh, I should say, by the way-- and here's an aside-- that this was also the occasion of what we call a perfect diamond caper.
To produce the polarized photons required a perfect crystal, and we'd been disappointed by the presumed suppliers in presuming a berillium crystal. And so we had everything going, including a very elaborate goniometer. We had everything ready to go and even had the experiment was all planned out and scheduled. There was only one thing missing. We didn't have the perfect crystal. And so we phoned-- among others-- we phoned, of course, Debeers. We phoned the Rutherford lab. We phoned various solid-state laboratories. We've phoned the Aga Khan's diamond laboratory in India. All looking for a perfect crystal. And we did get, from Johns Hopkins, we found a man who had a crystal, one corner of which, one millimeter, was in fact perfect.
Well with that inspired that nature could do such a thing, I went and got a hold of Harry Winston, who runs a diamond-- a jewelry store on Fifth Avenue in New York. Very elegant. His son had been in the Rocket Society at MIT, and indeed he let me run through all the crystals. I found a lovely four carat crystal, and it turned out-- and measuring it on the outside it was superb. And we cut it up and sure enough it was perfect. Not quite, but I won't go into that.
In any event, now I'm going over into time-like photons as opposed to real. And I want to remind something that Martin [INAUDIBLE] mentioned, but I would like to restate. Namely, if you're one of the first e-plus, e-minus, if you please, experiments was done by Martin. And he, in fact, and this was the experimental apparatus, here, where the positrons went into a gas and then the photons were detected here. But the important point here is one of the results, which was published in '51. This is the drop-off, in counts, due to the lifetime of the triplet state. And then the clever thing that Martin had done was to show that he could get rid of the triplet state by adding gases, which would convert triplet to single, and therefore get rid of some of the counts that he was getting from the triplet state. And so there, indeed, was the first, if you please, e-plus e-minus annihilation experiment.
Now the blaze of in glory, or glory-- whichever you want to think-- of CEA was the last experiment done in terms of the storage ring there, which of course, unfortunately lacked luminosity and therefore was shut down. Here's Harvey Newman's thesis showing that e-plus, e-minus minus scattering was perfectly good and classical, as it should be. But a rather strange result-- a rather unexpected result-- was that the ratio of hydron production to muon pair production was much larger than expected by the quart theories extant at that time. And in fact had risen to much larger values. The fact of the matter is the points here from Joe's thesis had shown this before, in fact, Slack got on the air. But Joe being polite, or urged probably by me since the results were out when he did the thesis, put in the later Slack results into his thesis, which confirmed it.
Now I turn back to space-like photons where we did an experiment at slack, which was probably one of the first, at least the first with a reasonable amount of hydronic results, of looking at the hydronic final state in deep and elastic electron scattering. And I'll only quote-- show you one result-- oh no two results. But one was by looking at the state and just simply saying-- making very simple assumptions like isotopic spin and variance. You could measure, from the hydronic charge ratios-- in a rather complex way, which I won't go into-- something which would give you the absolute value of the up quark charge to the down quark charge. Now you might say, I thought everybody knew that. Well, no, not everybody knew that. In fact, it wasn't too clear that there were quarks. You have to put you in that mindset in order to appreciate some of the silly things we did at that time.
And so, of course, we got, as you should, a factor of two. That is in retrospect. Another result of that experiment, was simply looking at the plus to minus ratio and comparing it, in a funny way, with what was available from neutrinos. That is to say, once a quark has emitted a leading positive meson, it presumably leaves the leading quark in the down state, in which case you'd expect to get a minus meson as the leading quark. Or you would expect to get the same thing you get from a neutrino experiment. Okay. So we looked at that. And indeed, here, the neutrino points and then here's the minus to plus ratio for the mesons coming off that thing. And indeed that was pretty. It was nice.
By the way that line People who know me know that I always lapse into a mood of explaining hydronization model of my own. And this line is the prediction for that ratio. I gave it originally-- the history that I gave it originally-- as a homework problem in 805, The Quantum Mechanics course, because it's very easy to do. Okay. And so this is what you get out of an 805 explanation for hydronization. Works very nicely, by the way. Oh and it's the granddaddy of those models that came before Feynman field and then clearly before [? Lund ?]
Now [INAUDIBLE] and now I depart and make an exception. This is not about photons. [INAUDIBLE] [? Boucher ?] had a nice idea about hydronization. He said, you should be able to find out about hydronization if you look at how it occurs in nuclear material. That is to say, the nuclear medium, somehow you expect, will effect it and that should reveal something about the hydronization mechanism. And in fact, here's a nice, simple experiment done at [INAUDIBLE] me by [? Boucher ?] and other colleagues and my counterpart chamber group and you see that the end result was as follows. But if you look at the forward going particles-- this is plotted versus rapidity-- that you cannot very readily distinguish the difference between the forward-- both the number and the distribution of the high rapidity particles.
Indicating therefore, that in some sense, the hydronization must be taking place outside the nuclear materials since it seems to be unaffected by it. And that actually, for the time being, and possibly [INAUDIBLE] created an industry in the use of nuclei, and production off nuclei. Both as an indication of the hydronization process, but also, as we shall see, an indication of how the quark, itself, interacts with nuclear material. Well actually won't see because I don't have any material on that. We did the same thing with electro production on nuclei. And all I can say about that, because this is a little bit too complicated.
But essentially, these are different elements and the distribution in momentum and the distribution in [INAUDIBLE] And as a way of summarizing that result is that-- in point of fact-- in the same way that was found in the nuclear material at the Fermilab experiment, you cannot distinguish a jet that has been produced in lead or heavy nuclei from the jet of hydrons produced in deuterium. Again, indicating that the hydronization takes place outside.
What's the time here? Okay. Long ago was-- it was found that photons-- and again because of a vector dominance, a photon is frequently a row. And we'll hear more about that. And therefore, when you bombard nuclei with photons, you find indeed that there is what's called a shadowing. Namely, the photon on lead does not look like photons times 208 nucleons but is less. And here's an experiment, which we just finished at e665 at Fermilab indicating the same thing. That is to say here it's done with deep inelastic scattering from muons and you see thought the ratio is around one here as you go to small x. Find xbj.
Indeed, finally, the numbers of shadowing from virtual photons begins to agree at very small x with what you get from photons. All right. And now, I shift. In praise of gluon. And virtual and real, whatever that means with respect to gluons. And why praise of? Well, here's a transparency of something that Fred Epling showed. Namely, the gem detector for the super collider. And it was by way of introduction to the fact that having told you something of the past, let me tell you a little about the present. And of course the challenge here is for you to find the person in this picture. So, let me say that we're coming along.
Our group is primarily interested in doing the muon part of the chambers for that experiment, and we've started on that. And indeed we've just made-- and are about to test one chamber, which is real but approximately the kind of lengths we talk about making, except that we'll have to make something like 900 of these. And this is a first test. And the data reduction here for such a thing. But then, that's pretty far in advance. So, how to amuse ourselves in the meantime? Well, we do have various projects, both CDF and the Gran Sasso project, and so on. And I'm clearly, by the way, for the time being,
I am letting-- Sam is going to take care of himself. I'm talking about I'm not covering the Kendall Friedman experiments were all the nice work that Sam Ting has done. But in the meantime, here, with [INAUDIBLE] [? Boucher, ?] Steve Steadman, Bernie Wadsworth, and whatnot have put in a letter of intent for an experiment at [INAUDIBLE] and that thing is busily going along. And he expects to be running sometime in '97. And let's see-- and punctuate, if you notice, that one of the implications of the transparency's I was showing was that we were not only producing Physics but we were producing physicists.
Yes, even laboratory directors. And so it's rather topical that I present as the last, something to do with Kobe. Namely, the hero of that particular result was George Smoot. And George Smoot was one of Dave Fritches students-- PhD students. And I'd like to say that the reason George saw the light was by virtue of his great training under Dave's leadership. By the way, this is not to be confused, as I was for a while, with those of you who live around here. That is not to be confused with the smooth, which is a unit of measurement for the Massachusetts Avenue Bridge, and refers to an Oliver Smoot who is apparently very stiff enough one evening to have been used as a measuring stick. So I'll end on that note.