Throughout the ages, humans have been curious to find out what holds the world together at its core. More than 99% of the mass of visible matter in the universe is nuclear matter. Protons and neutrons are the building blocks of atomic nuclei. The proton is nowadays known to possess a rich inner structure of elementary particles: gluons bind quarks together via the strong nuclear force. As the field theory of the strong nuclear force, Quantum Chromo Dynamics provides general answers to the question: How do quarks and gluons form the nuclei of matter? To answer specific questions about nuclear structure, we however cannot get by without data from experiments that break open nuclear matter.  

Our research team from the University of Illinois at Urbana-Champaign is particularly interested in experiments that use spin-polarized particle beams or fixed targets. By also considering transverse quark momenta (in addition to the longitudinal momenta along the axis of the incident beam), spin, and orbital angular momenta, proton substructure becomes similarly rich as the substructure of the hydrogen atom, which was first described in the 1930's. During the early decades of the 21st century, proton hyperfine structure has moved into the focus of spin physicists.

Blue Waters enabled us to study the proton at great precision using data from the particle physics experiment COMPASS at CERN, Europe's center for nuclear research. The experiment constitutes a powerful microscope that can look deep inside the proton. A precise measurement of the dynamics and arrangement of quarks inside the proton will provide experimental input needed to improve the quantitative understanding of the strong nuclear force. With its massive data storage capabilities and petascale processing capabilities, Blue Waters will eventually turn the COMPASS data into unique images of quark position and motion inside the proton and will thus refine the theory, Quantum Chromo Dynamics.

In order to explore its structure, we break the proton apart in particle collisions at COMPASS. A beam of high-energetic particles is impinged on a fixed target. The particles emerging from the collision are invisible to the human eye and are instead registered in large detectors, from where the particle information is digitized and stored in a special data format. To interpret the data, they yet have to be processed using complex algorithms and lookup data bases.

The massive computing resources at NCSA's Blue Waters were necessary to extract the desired physics message from our data. The procedure of finding particle tracks emerging from the interaction point and traversing hundreds of COMPASS detector layers is CPU-intensive. The tracking procedure is one of the first steps in the data analysis. An additional highly CPU-expensive task is the sampling of about 2% of the data to determine efficiencies for the 170 detector planes. To provide the data in a timely manner for physics analysis, the challenge consists of parallelizing the submissions of the tracking code on the computing grid while respecting the system in terms of I/O and numbers of requested computing nodes. A typical mass production campaign of COMPASS data on Blue Waters required about 50,000 submissions of the tracking code and was ideally launched in parallel. Detailed and thus CPU-intensive simulations of the experimental data carried out on the Blue-Waters grid allowed us to understand subtle detector effects and to obtain an absolute normalization of the data with the smallest possible uncertainties.

This two-year project provided outstanding educational value for more than a dozen graduate students and half a dozen young postdoctoral researchers, who work, learn and advance at the lively interface between fundamental physics research on the one hand side and computational science on the other side. The involvement in this unique Blue-Waters project intensely prepared these scientists for a job in either academia or industry, while building a community of scientists capable of using petascale computing.

Even while we are not holding an academic discourse about nuclear physics: it is an integral part of our daily life. Nuclear fusion processes at the core of our sun are the source of the vast energy flow that sustains life on earth. Fission of nuclei provides about 20% of the electricity consumed in the United States and propels many naval vessels. The knowledge of nuclear forces and instrumentation developed for the atomic nuclei and its constituents have important applications, such as x-ray and magnetic resonance imaging (MRI), radiation therapies for cancer treatment, materials science, x-ray lithography, as well as propulsion and power generation.

Last Modified: 09/21/2020
Modified by: Caroline Riedl