A Noble Nobel Experiment Finish

A Noble Nobel Finish

Charles Jordan

My PhD thesis subject at Columbia University in New York was how often a set of three particles (vector mesons called the Rho, the Omega and the Phi) decayed into an electron and its anti-particle, a positron.

Their masses range from .760 to 1.004 times the mass of  the proton, the building block along with the neutron of all the elements. It's not that important for the story, but these decay numbers tell you something about how the protons electric charge is constructed which is very important to our understanding of the electric behavior of matter.

According to Einstein's famous law E equals Mc² , your accelerator beam can only produce so much mass in a collision depending on its maximum energy. The energy at DESY could only barely produce the Phi meson.

We made headlines in the Sunday times in Germany due to our first experiment. That experiment made DESY, new in the business, famous, and our support by the German government very solid..

We had embarrassed a Harvard professor with our experiment. A solid state physicist, he had decided to do a high-energy experiment at the new Harvard accelerator and had made some mistakes which led him to claim that a favorite theory in physics, Quantum Electrodynamics or QED, was wrong. Prof. Pipkin was a dedicated scientist and decided to come over to Germany to understand what he had done wrong. I was appointed his guide to inspect our apparatus. If that wasn't embarrassing enough, he raised up too quickly going under a rigid optical bench with a sharp corner and impaled himself causing extensive bleeding.  “He covered the wound with his handkerchief as I asked whether we should go have it taken care of.  He said “No. No. I’m all right.”  But we couldn’t go on.

After my thesis experiment was done, I accepted a position at the Stanford Linear Accelerator Center (SLAC) and my thesis advisor Sam Ting (Ting Chao Cheung) who had been given a tenured position at MIT for the work done in my thesis experiment among other things, decided to reproduce our experimental set up at a higher energy accelerator at Brookhaven National Laboratory on Long Island in New York.

“Why did he decide to do that?” you might ask. Since the results of that decision won him the Nobel Prize, the effect of various considerations  might shed some light on the creative process of the search for elementary particles.  A higher energy will definitely be able to produce heavier particles and for the first time if any are there.  The maximum mass at DESY is 2.8 Gev and at Brookhaven it is 7.2 Gev. 

Sam had experience doing this kind of experiment. Experiments are complicated. It’s easy to make a mistake.  At Brookhaven he needed to make some changes anyway.   Instead of looking for electrons and positrons he looked for muons and anti-muons, which are just like electrons and positrons, just 200 times heavier.  He chose them because they are easier to detect in the the background of particles at Brookhaven which has different beams that DESY.   The grand old man of the Columbia physics department was Isidor Isaac Rabi,  also a Nobel prize winner. When he heard about the discovery of the muon he was quoted as saying "Who ordered that?

Experiments take a lot of time and are costly requiring a lengthy confirmation procedure. Sam advised me not to worry about what I should do for my thesis experiment. He said ("just get out as fast as you can so that you can do what you want to do."

Experience, reading, talking to theoreticians generates the possibility of guessing which research direction has the best chance of being productive. He told me when he took the MIT position rather than staying at Columbia, that he was afraid that the leading theorist at Columbia, another physicist named TD Lee  was so smart that he would dominate Sam's choices.

Sam started the set of experiments with muons mentioned before and sure enough by May there seem to be a peak in the mass spectrum around 3.1 Gev never before seen.

Due to the scattering of the muons after they left the target, the peak was pretty broad and Sam, quite aware of the importance of this new resonance, was anxious that he might lose face if the object was just due to equipment malfunction or something. So he didn't publish the results just yet and took some more data. By the end of this summer, he had his paper written and was scheduled to go to SLAC for the yearly review of planned experiments there. The Guidance Council involves professors working at other accelerator's in order to get the best physics results for the Department of Energy's money.

 Meanwhile Marty Breidenbach, working with Burton Richter (Group C, I was in group A.), was looking at a beam energy scan in steps of about 25 Mev. In Sam's experiment a proton beam hits a proton target and then a muon pair plus other things come out. At SLAC, the electron strikes a positron (antimatter going the other way, two colliding beams) and if the energy is right at 3.1 Gev, every pair will make a new particle. The two methods are exactly opposite.  By now, the theoreticians were also guessing what the particle might be. The East Coast theoreticians called them J particles in the West Coast theoreticians called them ψ particles.

The particles have a width in energy or mass which is related to how long they live after being produced. A long life means a narrow width. And the width is not really measurable by Sam's experiment due to his equipment. But if the SLAC experiment was not exactly at3.1 Gev nothing would happen.  Turns out that the width of the ψ just 5.5 kev which is about a millionth of the mass of the ψ.  It is pretty lucky they could hit it at all. 

Marty saw a hint in their May data that their energy steps, set by magnet currents were too large. So he had all the control units of the magnet power supplies upgraded to make the energy steps much finer.

Meanwhile in September Sam had arrived at SLAC and was looking around for some physicists to talk to. I had already left SLAC.  He couldn't find anybody. The next day he came back again worried that something was a foot. Finally he saw the Director of SLAC, Pief Panofsky, walking into his office.

"Pief," he said ruffling his papers, "I have made the most wonderful discovery, a new particle at 3.1 Gev."

"Sam," he responded, "that's great. So have we!"

The previous evening the SLAC energies in the scan had lined up right at 3.1 Gev and almost every collision produced a ψ. In minutes, thousands of the particles were produced while changing the energy just slightly cause the signal to go away. Sam and Burton Richter shared the Nobel prize, for discovering the J/ψ particle,  the only elementary particle with two names.

Just a while ago, I was attending a memorial service for Marty Perl, also Nobel Prize winner for the discovery of an even heavier electron and a thesis advisor of Sam’s.  Sam was there and we were discussing old times when Burt came in and sat across the entrance hall.  Sam was carrying an old heavy Leica he adores.  He got up and told me, his old graduate student, to take a picture of him and Burt. He must not have had a picture of them together.  He lined up the camera and told me exactly where to stand.  When I showed him the picture, he said “No. No. No.  Let me set it up again.”  That time the picture was acceptable, his old graduate student had followed directions correctly.