Dec 01, 2015
As we were climbing the stairs on our way back to our hotel room for the night, my wife, Angela, said: “why don’t you apply for a Fulbright.” Earlier that evening in late May of 2013, we had been enjoying an excellent meal at the hotel after a full and fruitful day at the NSTAR 2013 conference (http://ific.uv.es/nucth/nstar/) in the Valencian city of Peñiscola. Seated around the table were the two spokespersons of the BGO-OD experiment at the Electron Stretcher Accelerator (ELSA), my wife, and me. As Prof. Paolo Levi-Sandri (INFN Rome) and Prof. Hartmut Schmieden (University of Bonn) were discussing their experimental work over the fine meal, it struck me how very similar and complementary our research is and that we should really collaborate more. At the table, Paolo asked me if I would be interested in joining their experiment. I could only answer, “in principle, yes, very much so,” but I honestly did not know how to find the time or money to participate in any meaningful way, for my National Science Foundation grant covered my research at Jefferson Lab in the U.S., not abroad at ELSA.
Upon returning home to Idaho in mid June, I immediately began working on the Fulbright application. The Fulbright liaison at Idaho State University, Spanish Prof. Sharon Sieber, gave me lots of helpful advice and expert pointers. It was this Fulbright scholarship (July 15, 2014 through January 5, 2015) that made my work possible at the University of Bonn. And it has been a whirlwind of wonderful experiences.
I will start off by explaining the nature of this research. Imagine that you could not open a wristwatch with a screwdriver, but still wished to understand how this time-keeping device was put together. One way would be to aim well-defined, fast-moving tiny projectiles at a vast number of identical wristwatches, one at a time, and at many different angles, and repeat the process over and over again. After impact, the cogs, wheels, and springs, along with some background debris, would fly out in various directions and at various speeds. By identifying the trajectories of the outgoing particles and correctly connecting them with the internal structure, we could reconstruct how the watch was assembled in the first place.
Clearly protons are not made of cogs and wheels – and certainly they cannot be opened with screwdrivers – but the principle of revealing the internal structure of a proton still follows along similar lines of this particle-aiming technique on wristwatches. First, you need a particle accelerator for generating a well-defined, energetic beam of particles, which can then be precisely directed onto a target of protons. Working in concert, in turn, are many dedicated detectors that surround the target for measuring the directions, speeds, and types of the outgoing particles.
I became a member of the BGO-OD collaboration in early October 2014, and since that time, I have given four colloquia to general physics audiences at the University of Gießen, Commissariat à l'Energie Atomique of Saclay, the University of Mainz, and the Institute for Nuclear Physics at the Jülich Research Center. The abstract below should shed some more light, so to speak, on the subject.
Studying Baryon Resonances with the CLAS (JLab) and BGO-OD (ELSA) Detectors
An energetic photon incident on a nucleon can interact directly with one of the quarks inside, causing the quark to undergo a flip in spin or endowing the quark with an orbital or radial excitation, and thus, by exciting the quarks to a higher energy state, the nucleon becomes more energetic. These excited states are called baryon resonances (N*s) and are short lived (~10-24 s.) These N*s will dominantly decay into a ground-state nucleon and one or more mesons. The types of mesons produced and how they are distributed in space in the decay process provide key information on the internal symmetries of the quarks in the nucleon. The study of these excited states is called spectroscopy. And just as ordinary optical spectroscopy proved to be the incisive tool for understanding the electronic structure of the elements, we expect nucleon spectroscopy will reveal many of the basic features of the quark substructure of matter, and, in turn, it will provide a critical testing ground for theoretical models describing these systems.
In this talk I will discuss the underlying physics ideas of baryon resonances within the context of the complementary transatlantic experiments CLAS (JLab) and BGO-OD (ELSA).
The Electron Stretcher Accelerator (ELSA) in Bonn and Jefferson Lab (JLab) in Virginia are among the few nuclear physics laboratories in the world that have a high-energy, linearly-polarized photon beam facility. Both experiments are studying the excited states of protons and neutrons by using a beam of linearly-polarized photons. Through identifying these excited states from their subsequent decay into outgoing final-state particles, we seek to understand the nature of strong force as expressed by the structure of protons and neutrons. The strong-force substructure of protons and neutrons gives rise to the nuclear forces that bind protons and neutrons together inside the nucleus, and mesons are the force particles responsible for this nuclear binding.
I mentioned that the experiments at ELSA and JLab are complementary. The CLAS (CEBAF Large Acceptance Spectrometer) (https://www.jlab.org/experimental-hallB) detector is excellent at measuring charged particles at central angles, but is not so good at measuring neutrons or photons in this central region or spotting charged particles in the forward region. The BGO-OD set of detectors, on the other hand, (https://bgo-od.physik.uni-bonn.de/) can measure photons at the central region and charged particles in the forward region, but is not that sensitive to charged particles elsewhere. The name BGO-OD is physics-speak for this capability. (I personally prefer calling BGO-OD the Johnny B. Goode experiment in reference to Chuck Berry’s song, which was the only rock song to be placed on Voyager 1; a spacecraft that left the solar system over two years ago, see: https://www.youtube.com/watch?v=FboXO1FtbvE).
The experiment will run for most of 2015 and that means there was still much to do in preparing for the run when I arrived in mid July 2014. We needed a higher-resolution electron detector so that we could measure the photons very precisely. Electron linear accelerators are the standard means of producing the photon beams. The electron beam is directed onto a thin diamond wafer and as the electron passes through the medium, it loses energy or “brakes” as it traverses the wafer. And as this electron brakes, it will radiate away photons and undergo bremsstrahlung or braking radiation. The post-bremsstrahlung electrons are then deflected in a magnetic field and where they land gives information on the energy and polarization of the bremsstrahlung photon.
I took on the task of designing a three-layered, multi-stranded, scintillating-fiber detector for ensuring the quality of the linearly polarization of the photon beam. Each fiber is very thin (2 mm). A deflected electron will go through a few of these fibers. I have been mentoring two Master’s students (Stefan Alef and Björn-Eric Reitz) at the University of Bonn, whose theses regard the design, construction, and calibration of the particle radiation detectors comprising ARGUS. The device has over 500 channels and the name ARGUS refers to the mythological all-seeing creature Argus Panoptes, the giant with a 100 eyes.
I now have a mind’s eye view of the experiment. This six-month on-site experience has enabled me to become a productive BGO-OD collaborator as well as enabling me to bring new ideas to the JLab CLAS collaboration and to my home institution in Idaho. In keeping with the Fulbright mission, this opportunity has given me a broader perspective in mentoring both graduate students in Germany as well as at my home institution. It all serves in encouraging us to develop an international approach to physics.
My Fulbright experience can be summed up succinctly in three German words: Wanderjahr, Wunderjahr, Wendejahr. This is exactly the kind of experience that Senator J. William Fulbright envisioned for the Fulbright international exchange program towards fostering mutual understanding through a community of nations. A year abroad, a year of wonder, a sea-change year; a sentiment, I presume, that most Fulbrighters would definitely share.
Philip Cole is a Professor of Experimental Nuclear Physics at Idaho State University. He spent his Fulbright at the Rheinische Friedrich-Wilhelms-Universität Bonn. Prof. Cole is a cospokesperson on several Hall-B (CLAS) experiments at Jefferson Lab in Newport News, Virginia. The CLAS collaboration represents more than 60 institutions and 26 countries in Asia, Europe, North America, and South America. Prof. Cole became a member of the BGO-OD collaboration in October 2014, marking the first American to join this experimental nuclear physics group formed of physicists from Germany, Italy, Russia, Switzerland, Ukraine, and the United Kingdom.