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Interview of Gerson Goldhaber by Ursula Pavlish on 2006 February 28, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/34508-5
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Gerson Goldhaber (1924- ). Early training at Hebrew University in Jerusalem (1942-1947). Graduate work at The University of Wisconsin. Research at Columbia University (1950-1953); memories of Rabi. Research at The University of California at Berkeley and at Lawrence Berkeley Laboratory (LBL). Took part in first experiment at Bevatron. Worked on K-mesons, the tau and theta mesons; measured the radius of interaction, and the difference between the behavior of like and unlike pion pairs; contributed to the design effort of SPEAR (Stanford Positron-Electron Asymmetric Ring) accelerator at SLAC; co-discovered, and named the Psi particle with George Trilling; Goldhaber found the naked-charm mesons; with colleagues, measured the lifetimes of a number of particles, including the tau lepton, the D mesons, and the B meson; at SLC (Stanford Linear Collider) did measurements of the mass and width of the Z particle, measuring how much of the Z decay went into neutral leptons. With colleague Robert Cahn, he wrote and published the textbook “The Experimental Foundations of Particle Physics.” Part of The Supernova Cosmology Project (SCP) since 1989. 2004, ‘Gersonfest’ held in his honor.
Well, this is the talk I gave at the recent Symposium.
Entitled, “The Observation of Antiproton Annihilation.”
In the Segrè group, we decided on two approaches. One was the counter experiment, and one was my emulsion experiment as a part of the Segrè group. Actually, as Chamberlain and Wiegand started building the counter experiment, setting it up, when they reached the first focus, I already exposed some emulsions. Here is something about emulsion stacks. There are two hundred sheets of emulsions and you can easily see if something is minimum ionizing or twice minimum ionizing.
How do you easily see that?
You see those two tracks, they look different. There are twice as many grains in twice minimum. It’s the number of grains of developed silver. So, in the second experiment, we used this to follow the antiproton track from the beginning. They entered here and then we followed it all along. So that’s just some statement about the emulsion stack.
Was this common, for people to use so many emulsions, one hundred to two hundred?
Yes. At that time, people had developed these ideas of emulsion stacks. We made quite thick emulsion stacks. The point is, with the emulsion stack you expose it as one unit but then, to develop it you take all these different emulsions apart and stick them on glass and develop them individually. And then, you need a way to follow a track from one emulsion to the next, track following. That, many people have done. I developed the system that we used for that purpose.
For following from one emulsion to the next?
Yes. If it was a steep track, it went on and on and on and you had to follow it. Now, just an amusing thing, what we did with Amaldi, when we split a stack, we kept half the emulsions and we gave them the other half. So when we found something which went down into the Rome part of the stack, we had to write them a letter and tell them where to look and then they sent us back a letter with what they found. This is an example of such a letter from Amaldi. They followed five different tracks, which I had sent them. Most of them were just protons. A proton and an antiproton look the same in the emulsion, until it annihilates. When it comes to rest, then it looks different. The antiproton annihilates, while the proton just stops. Here, there was one case where one disappeared in flight, which could have been a charge exchange in making an antineutron. An antiproton making an antineutron, but could also have been a proton making a neutron. We couldn’t tell since we didn’t see the annihilation. Now, all that work produced one antiproton. And this is the first antiproton event observed. This is how they built the beam. This was the first focus, and then they went on with the second focus. We found the antiproton within two weeks after exposing for lower momentum antiprotons. Here we used higher momentum antiprotons, 1090 MeV/c. We somehow convinced ourselves that they should be higher momentum and so then we had to slow them down. And that’s what killed the experiment. We had to put five inches of copper in front of the stack and that killed all the antiprotons. They have a larger cross section than ordinary particles. Also, it produced lots of junk, which went into the emulsion and made it very hard to scan.
What does that mean exactly, when you have a 1090 MeV/c beam?
That’s a certain momentum of the antiproton beam, not of the Bevatron. It’s the antiproton which has this momentum.
So you calculate that, that’s a theoretical calculation?
Yes, you set your magnets. You can dial the current in your magnets and then you know what momentum you get. We also did one where we put a clearing magnet in the way. But even that didn’t help that much because we had this absorber here which killed the antiprotons. We found this one event, it was found in Rome. Here it is. However, the visible energy was 826 MeV, which is less than the mass of the proton, which is 938 MeV, which meant there was energy going off in neutrals but we couldn’t see that. So the conclusion we drew, that this event is corroborative evidence but not final proof for the interpretation given in reference one, that the new particles observed at the Bevatron are antiprotons. So you see, Segrè and Chamberlain jumped the gun a little bit. I’ll come to that here. Then there was another experiment, which put a big Cerenkov counter and triggered on the protonic mass particles that they saw in their beam. But here again, the energy they observed was less than one proton mass. So they did not prove annihilation. And again, here’s their conclusion from that paper: not inconsistent with expected behavior of antiprotons. Also, they indicated that it wasn’t really established yet that it was antiprotons. Let’s continue. Now, comes the paper. This is where Ekspong worked with me. “An Example of Antiproton Nucleon Annihilation.” Now this paper gave the proof. See, here now we exposed at a lower momentum, 700 MeV/c, which was chosen so that the antiprotons could stop in the emulsion. See, when they stop then they have to annihilate. They come to rest and they meet an atom and annihilate with one of the protons or neutrons. And here, this is the event then, which was the proof because the visible energy was 1300 MeV, which is greater than the mass of a proton. And here I have how we arrived at 1300, how the energy of each of the tracks is in that.
So how does that work? You add up all those?
You add up all the energies. And you add the rest energy. If you make a pion, there’s also 140 MeV for the mass of the pion. This is the kinetic energy but you have to add the rest to get the total energy. And that adds up to 1300. You can check it. Then we showed that this particle is really of protonic mass. We measured the mass and it was 97%, 93%, 102%. There were three different methods. Now here I discuss this. So the experiment of Chamberlain etc gave three necessary conditions for antiprotons: negative charge, mass within 5% of the proton mass, and pair production from the excitation function.
What is that?
To produce an antiproton, you have to produce an antiproton together with a proton to conserve baryon number. The antiproton has negative baryon number. So you have to produce two particles of protonic mass. So you really need more energy than just the mass of an antiproton. Then there was this lead counter experiment which again was a necessary condition.
Now where was this experiment?
Also at the Bevatron just behind the Chamberlain Segrè experiment. Now the sufficient condition was observed in our emulsion experiment. And then I have a quote from the Nobel Prize talk by Chamberlain. “That star gave the final visual proof through the phenomenon of annihilation that we were dealing with antiprotons.” See now, they can say that they did not have the final proof initially. And there is Segrè’s Nobel Prize talk and he says essentially the same thing. This is my sketch in Time magazine. Now I should give you the reference, actually you have it.
I have it in one of your papers.
If you mention it, you should quote that reference. This is Gosta Ekspong and myself when we celebrated the 25th anniversary of the discovery of the annihilation process when we were in Sweden in midsummer. After that (1956-1957), we made a collaboration of all the emulsion groups at the lab. There were several emulsion groups. They all participated in that exposure and we found a total of thirty-five annihilation events. This is the energy released in units of twice the mass of the proton. And you see that more than half of them have visible energy released greater than the mass of the proton. That’s the proof. We proved that with this one event, but we found lots of them afterwards. We had a puzzle that we got a lot of pions and this would mean that the interaction volume was 10 to 15 times of geometric. When there’s a cross section there is an interaction volume. On Fermi’s statistical model there was a way to calculate the interaction volume. It came out very large. 10 to 15 times bigger than expected. But then later on, the GGLP experiment gave a geometric interaction volume. So the answer to this puzzle was that it can be understood that we didn’t make only pions but we made pion resonances. So we made a resonance which decayed into several pions. In other words, the pion multiplicity was too high for just making pions directly. If you made a rho, the rho then decays into two pions. An omega zero decays into three pions. And so this Fermi statistical model just counts particles. It doesn’t say that they are pions. What I’m trying to say here is that we indirectly obtained evidence for resonances which were later observed by Alvarez and company.
So you had this puzzle and you didn’t know…
What it’s due to, yes. In hindsight we can see that it meant that pion resonances were produced. In some sense it was the first evidence for resonances. Here is just this anti Omega minus event which you read a whole book about. So this was all just done to clarify for you how this went with the antiproton discoveries. I happen to have given this talk. Okay. Now, your turn.
My first question for you today, Dr. Goldhaber, is which group work has been your favorite of your various collaborations?
I think the work with SLAC where our group, Trilling and myself, worked with Burt Richter and Martin Perl, which was a very good collaboration in the sense that it was so successful. We found so many things. There was no problem about who gives a talk. There was a lot to talk about. So it was a harmonious collaboration. Now I’m also of course enjoying my present collaboration with Perlmutter. But the earlier one was exceptional in the sense that there was a discovery every week, practically, at the very beginning at SPEAR. There was something new that came out every few weeks.
Could you talk a little bit more about those discoveries?
Yes. Let’s see. Now I gave you a paper about that, “The First Three Years.”
Yes, I have that. Should I take it out?
Okay. “Three Years with the SLAC LBL Detector.” The first discovery was the psi and here we are discussing the psi with Willy Chinowsky, Martin Perl, myself…let’s see if I have this special drawing here. Now, I called it the psi. I was on the phone with George Trilling and we decided on the psi. I was thinking of a letter that wasn’t used yet, in Greek, and this came to mind. Later on it turned out that we found events which looked like a psi. Let’s see, there’s something specific I was looking for. Here. We were originally measuring every 200 MeV and we measured here at 3.0 and 3.2 GeV and missed the psi when we first measured the cross section. However, here is this cross section and it looked kind of flat. These are the 200 MeV intervals. Nothing was seen.
You should have seen a peak here?
Yes, there would have been an enormous peak if we had measured every 100 MeV. Anyhow, this point, however, this point was about 30% high so we decided to go back and investigate it.
And that was your idea, right? You pushed for that.
It was not my idea only. But yes, I pushed for it. Because we had just rebuilt the accelerator to go to higher energies so one had to go back so it took some persuading. I had to persuade Burt Richter and others. But there were other people also pushing for that. I don’t want you to make it seem like I did everything. I did some things but not everything. So in January 1974, John Kadike who is a coworker of mine, noticed this 30% increase. Then in June, Burt Richter presented that there’s a flat cross section at a London conference. However, also in June, Marty Breidenbach went back to measure at 3.1, 3.2 GeV, he went back to measure what’s going on there, why was there that high point? At first, nobody worked on it carefully. But then Roy Schwitters, in October, decided that we have to write a paper. And he looked at all these cross sections at 3.1 GeV and there were some individual runs… six runs were normal, but one run gave three times the cross section and another run gave five times the cross section. These so-called runs are small intervals of data taking, over about one hour of observation. So this was very surprising. Gerry Abrams in our group here confirmed indeed that these cross sections are higher.
He redid the measurements.
Yes. I don’t have that here. There’s another paper in which I show, it’s in there.
“The Adventures in Particle Physics.”
Yes. Anyway, I show how it is that they saw that there was a larger cross section. Then, in addition, my student Scott Whittaker and myself, we found that there seemed to be an excess of K mesons. I found an excess of K zeros which probably was partly correct and partly a statistical fluctuation. But it had an influence on us. It made us say, we must go back and look at this region. So, the week of November 4th, we then had discussions with Burt Richter leading to the decision to go back to 3.1 GeV. And on that weekend, we made a discovery and I started writing the paper while sitting there. And then on the Monday, Roy Schwitters reported at SLAC and I reported here at LBL this result. And then we heard that Sam Ting also had found something, what he called a J.
There was a conference or something, right?
There was a meeting at SLAC that he came to. It wasn’t a conference. It was an advisory committee. He was on the advisory committee for SLAC. When he saw our results he told them about his results. He had observed them earlier but he was cautious about it. Then, finally somebody told The Daily Cal, the Berkeley student paper about this news. So they called me and asked, “Is it true?” And I said, “Yes it’s true but you mustn’t publish it because The Physical Review will not take our paper if it gets published.” But they published it anyhow. You know, journalists don’t listen to requests to hold up news. Then The New York Times said, “Why on earth did you give it to them, why didn’t you give it to us?” So there was a big frantic scramble. Finally, there was a joint press release on the J and the psi. All this, before it was published which was supposedly a no no, that you don’t go to the newspapers before you publish. And then, the group at Frascatti was able to confirm the J/psi.
They were able to confirm your result.
They had this machine and it went up to 3 GeV. They designed it to go to 3 GeV. Well, they ran all their magnets hot and went up to 3.1 GeV and were able to confirm the result. Then we all published in the same issue of Physical Review Letters. So this was Ting, the J, this was our data, and this was Frascatti. We all saw it. Then there was another amusing thing here. Our energy scale was off by about 10 MeV. This is the old scale. So what we called 3.1 was just below the resonance but the cross section was kind of increasing slightly. In the fluctuation of the beam energy, some of the runs went up to high a little bit. In fact, there was some kind of strike of operators and some of the physicists were operating the beam so they didn’t hold it as steady as they should have. So it ran into the resonance. That’s how we discovered it. On the other hand, if we had had the correct energy scale, we would have seen it immediately. We would have seen that it was up by a factor of 50 or so. So this energy scale discrepancy was very important which nearly made us miss it, but we did find it. Ok, so that’s a little background. Another amusing thing here. So this is the article that came out in the Daily Cal so then the Physical Review Letters had to say, “must certainly be one of the most unusual in our history with not just one, but three extremely stimulating reports of a new discovery.” Then they say, “Don’t do that again, that you publish in the newspaper first.” Anyway, so they had to write a special editorial, which I show here. This is why I think, including something like this, if we do write a book, will be of general interest. Then, a week or two later we found the second resonance, the psi prime. And then, however, we went on and on and on and on and on, this took many months, and didn’t find anything. But finally there was something else, which was then discovered later. This resonance is when you begin to produce pairs of charmed particles, charmed mesons, which I later discovered.
So that was a resonance that you discovered there?
The charmed mesons. This is the peak. Together with a colleague, Francois Pierre. Anyhow, so that was an exceptionally exciting time. There have been other exciting times: when we found the antiproton annihilation event, the GGLP effect was developing slowly, it wasn’t clear what it was until we did this theoretical paper with Pais. We were looking for the rho meson but we missed it because we did not have enough data, not enough statistics. We were doing the right thing with the calculations. We would have found the first meson resonance but we missed it. But we did find this other effect, the Bose Einstein effect. Then, in terms of excitement, there was the excitement when I found the dark energy. Now, here again, I was the first to point it out but it needed to be measured much more carefully. I did a quick check. My colleagues then measured it very carefully and agreed. So I can’t take all the credit. And besides, that’s what we were looking for. We were looking for what’s the value of Omega Mass, the mass density of the universe. We were expecting it to be one or greater than one because of gravity which is attractive. Instead, we found the acceleration. So we expected deceleration and found acceleration instead. Those are some highlights.
This is another question about your being a group leader. You were the group leader of an accomplished group from 1962 to 1990. What was it like to be group leader of the Goldhaber-Trilling group? Was there rivalry with other groups at Berkeley?
Not particularly. By the way, it was called Trilling-Goldhaber for very peculiar reasons. When we formed the group, McMillan who was then the director of the lab said, well we were two Goldhabers, it was Sula and I, that we would completely overwhelm Trilling and so he convinced us that we should call it Trilling-Goldhaber. Now, what was it like? It was fun. We had several postdocs: John Kadike and Gerry Abrams and at one time, Wonyong Lee for a while. Wonyong Lee was my student before the group was formed. You see, for seven years I worked in the Segrè group. Then we formed our own group. I guess I told you about that.
Yes, how there’s a law in the Bible that after seven years you have to release the slaves.
Yes. So I have nothing special to add. We took it in stride.
I’m going to turn over the cassette now. This is a technical question from my reading. What is the Fermi 3-3 resonance and how does it relate to the work you did of measuring the phase shift of the 3-3 resonance with Lederman?
Well, the Fermi 3-3 resonance is the first resonance ever discovered. Before that one didn’t know about resonances. Fermi discovered this. What it is, it’s the cross section when pions scatter on protons. Pion proton interaction. What they found is that the cross section was going up. They measured the cross section, pi plus proton elastic scattering, pi minus proton, charge exchange, pi zero neutron, and elastic scattering. They measured the pi minus proton. It turns out that the pi plus proton is the larger cross sections, three times as large. It’s 3-3 because it’s spin 3/2 and isospin 3/2. So that’s just a name that refers to it. It’s referred to by its quantum numbers. They measured pi minus proton and they measured pi plus proton. It stopped here at 150 MeV because they couldn’t get higher energy pi plus mesons. Fermi said, “Oh this must be a resonance.” Then Frank Yang, who was a student of Fermi’s, said, “Wait, how do you know it’s a resonance? I can calculate a phase shift which shows this is not necessarily a resonance. So there’s another solution.” So Lederman and I set out to go to higher energy. How did we go to higher energy? That was nontrivial. If you have a cyclotron, the magnetic field makes protons rotate in the cyclotron. So the magnetic field spits out the pi minuses. They go away in the opposite direction. They’re of opposite charge so they’re bent the other way. But the pi pluses are bent into the cyclotron. So what we did is we put emulsions inside the cyclotron to get to the higher energies and we measured this peak but we measured it in two places and we missed the peak. So we concluded that it’s not a resonance. This is where I’m referring to, that “it got away,” that we missed that one. That’s the story.
Another technical question is how did you show that the Bevatron had reached 6 billion electron volts in that first experiment?
That’s an overstatement. I should modify that. I showed that there were energetic protons. I actually couldn’t make an exact measurement of the momentum. Basically, I demonstrated that there were energetic protons consistent with 6 GeV. Yes, you should change that. I didn’t really demonstrate that they were at 6 GeV. They were consistent with 6 GeV. From the ionization, you can tell the momentum of a proton. But it’s not an accurate measurement.
I want to make sure that I get to all the questions. I guess we can leave some of them for tomorrow if we don’t get through them all. I was asking you to please elaborate on the GGLP effect, the difference in behavior of like and unlike pion pairs.
Yes. I set up that experiment to look for the rho. We expected to see a peak in the pi plus pi minus mass distribution. As I mentioned, there was not enough statistics to see that. So I said, well what we’re looking for is a difference between like pairs and unlike pairs. The unlike pairs were supposed to have the resonance and the others, nothing. As an alternative we measured the opening angle of like pairs and unlike pairs.
That’s the angle between them.
Yes. Here is that paper. Actually, this is already a theoretical paper but it does show the data. The unlike pairs’ angular distribution looked like this and the like pairs looked like that. You can see they look very different. At first this was a mystery. We didn’t know why but we published that result. It came about because we were looking for the rho, we were looking for a difference between the two. Also, see this was the first time that the mass distribution was calculated and I didn’t have full trust in that formula yet. I said, I can look at the angle and see what the difference is. In other words, this is a more down to earth quantity. To my surprise, I found that there was an effect. Later on we explained this as the Bose Einstein effect. That’s what this paper is about. This is not the discovery paper this is the interpretation paper. That’s the one that got the main play. Does that explain?
Yes, that helps. My next question is what was it like to work at CERN? When exactly did you go to CERN? You went twice?
What years, do you remember?
I went around 1960. We had just discovered this Bose Einstein effect. We didn’t quite know what it was. I was trying to convince them to follow up on this but somehow…
The people at CERN?
The people at CERN. But they didn’t. If we had followed up, we would have discovered the resonances. Instead, they worked on other things. What I worked on there was to try and make a separated beam separating pions from kaons and kaons from protons. We had a method which was interesting but which is not the final method that people adopted. So that’s what I did the first time at CERN. That time I had gone there with Sula and my son Nat went to the International School there. The second time, I went with Judy and that was about in 1972. We already had our daughter, Michaela. She was a baby then. What did I work on then? I helped in various experiments but nothing earth shaking. It was a non-earth shaking year. Then, the third time I went again with Judy. By that time we had two daughters, and they both went to the International School in Geneva. Michaela and Shaya. On that occasion, I first learned about colliding beams. I worked in Rubbia’s group.
At CERN. I worked in Rubbia’s group, essentially working at the ISR which was a colliding beam, colliding proton beams.
That was also in the 1970s?
Yes. That was about 1972. I took every one of my sabbaticals at CERN except for the one when we took a trip around the world that you know about already. So this was my fourth sabbatical. Now I am confused about the timing. 1972 or so, that’s when I worked at the ISR. I have to think about it. I forget.
I don’t know if you have a few more minutes or if we should save the other questions for next time.
I have two minutes.
Two minutes, ok. Peter Galison calls the tradition of physics at Berkeley to be patterned on industry, the Military, and large scale engineering. Did you notice this to be so?
Yes. In the sense that we built these big bubble chambers which were major undertakings. Ok. We could meet this afternoon still. You’re seeing Judy at 2 pm. From 3 to 3:30 when there’s coffee. Let’s try that.