Shortly after the Big Bang, the Universe was composed of a plasma of free particles called quarks and gluons. As the Universe expanded and cooled, the quarks and gluons condensed into the protons and neutrons which now make up the cores of all atoms. This phase transition from a hot, dense plasma to a gas of normal matter is similar to that of steam condensing into liquid water. Using large particle accelerators, scientists are recreating this subatomic soup in the laboratory by colliding gold or lead atoms at very high energies and studying the intriguing properties of this unusual state of matter. One of the most important discoveries to come from experiments at the Relativistic Heavy Ion Collider (RHIC) is that the dense matter is opaque to very high energy quarks and gluons produced in the collisions. These `partons' suffer tremendous energy loss as they make their way out of the collision zone, depositing a large fraction of their initial energy in the medium. The strength of this energy loss is directly related to the density of the quark gluon plasma and varies according to the type of parton. From observations at RHIC, the heavier quarks appear to be more strongly coupled to the quark gluon plasma than was previously expected. High energy electrons are produced by the decays of particles made from heavy quarks, so their presence within the cluster of particles that comprise an observed jet is strong evidence for the jet to have originated from a heavy quark. The energy loss of these electron-jets can be used to investigate the properties of the quark gluon plasma.
The CERN Large Hadron Collider (LHC) in Switzerland will collide nuclei at energies 30 times higher than at RHIC, producing an even hotter, denser, longer-lived medium. This RUI project will utilize the ALICE experiment to explore the properties of this matter using the energy loss of heavy quark jets as a probe. Undergraduate students working on this experiment will be trained in the most advanced data collection, reduction and analysis techniques at the cutting edge of high energy nuclear physics. They will be able to apply these skills in a wide variety of careers, whether they choose to pursue an advanced degree in basic science or not. Their contributions will also help us develop new insights into the fundamental interactions among the basic building blocks of the universe, leading to a deeper understanding of the world around us.
For four years, from 2010-2014, the Cal Poly Nuclear Physics Group, composed of PI Jennifer Klay, and 13 different undergraduate students, conducted simulations, calibrated detectors, and collected and analyzed data from the ALICE experiment at the CERN Large Hadron Collider (LHC) in pursuit of a deeper understanding of the properties of the strong nuclear force and the evolution of the early Universe. ALICE is the only dedicated heavy ion experiment at the LHC. The data collected by ALICE since the LHC began operating in 2009 has been analyzed and results published in over 90 peer-reviewed journal articles to date. These results have shed light not only on the principles of the strong nuclear force and aided deeper understanding of the primordial composition of matter but may also provide insight into the cosmological evolution of the universe. The students in our group collected data at CERN, worked on calibrating the ALICE electromagnetic calorimeter, wrote computer simulations of nuclear collisions, and developed computer algorithms to sift through large collections of nuclear collision events to search for signatures of high momentum electrons correlated with jets of energetic particles. These electrons are the result of the presence of heavy quarks, fundamental particles created in the collisions. By analyzing the electrons' energy and momenta, the properties of the quark-gluon plasma can be determined from the energy loss of their parent heavy quarks as they traverse the plasma. The students who participated in this project learned valuable transferable skills that enabled them to graduate and continue to graduate school or obtain employment in technical fields. Each of them was mentored in the full scientific process, from the formation of a research goal, the development of a plan to pursue that goal, and the communication of the results of the research to the broader scientific community. Four Cal Poly senior projects on ALICE physics topics were completed during the course of this award, and six presentations by students at local and regional conferences were given. One participating student, Kevin Coulombe, who earned his teaching credential after graduating in 2010, became a high school teacher and returned to the project to resume his research with ALICE during the summer of 2013. He has carried his enthusiasm for scientific discovery into his classroom and is inspiring a new generation of high school students to pursue careers in STEM. Recruiting, retaining, and educating science, technology, engineering and mathematics students and teachers are critical in advancing scientific literacy, maintaining economic growth, and continuing scientific discoveries. The involvement of Cal Poly undergraduates in the study of heavy quark energy loss in ALICE at the LHC has given them a unique chance to be part of a large, global, collaborative experiment, where they contributed to advancing our understanding of nuclear matter in extremis. These students all graduated better prepared to confront challenging problems in a broad array of fields, including education, fundamental research, and technology development.
