Elementary Particle Physics/High Energy Physics
What are the fundamental building blocks that make up the universe we live in? In what ways can those building blocks interact with one another? Cosmological observations show that we can nly explain roughly 5% of the energy content of the universe using known particles and interactions; what makes up the rest? Why is the universe made up of only matter with almost no antimatter? hy does the most massive known particle, the top quark, have a mass comparable to that of a gold nucleus while neutrinos have such small masses that we haven’t been able to measure them yet? The experimental high energy physics (HEP) program at Notre Dame (ND) is focused on answering these questions and others like them using detectors operating at both energy frontier and intensity frontier facilities. We design, construct, and operate detectors for these experiments and develop software and computational facilities for analyzing the data. We apply sophisticated data analysis techniques to extract small signals from challenging backgrounds. Ultimately we hope to uncover evidence of new particles or interactions to work towards resolving the mystery of what particles and interactions make up the other 95% of the universe that we live in.
The ND experimental HEP group has a long history of making important contributions to leading HEP experiments around the world, including numerous fixed-target experiments, the D0 experiment at the Tevatron Collider at Fermilab, and the Babar experiment at SLAC. The current focus of our program centers on the CMS Experiment on the LHC as well as several neutrino experiments.
Compact Muon Solenoid (CMS): The CMS detector records proton-proton collisions generated by the Large Hadron Collider (LHC) located at the CERN laboratory in Geneva, Switzerland. The LHC is the world’s most energetic particle collider, enabling scientists from across the globe to explore phenomena at the highest accessible energy scales—what we call the “energy frontier” of particle physics. We played a leading role in the construction of the electromagnetic calorimeter (ECAL) and hadronic calorimeter (HCAL), and continue to make important contributions to the operation and upgrading of the ECAL, HCAL, and trigger systems. We are also contributing to new detector projects as part of the Phase II upgrade of CMS. These efforts and generic detector R&D is facilitated by on and off campus lab facilities, including a 7,000 sq. ft. detector development lab adjacent to campus and work areas at CERN. A technical staff of three electrical engineers and four technicians assists faculty and graduate students in this work. We also play an important role in the development of software for simulating the detector and analyzing the data. We operate one of the largest Tier-3 computing facilities in the world, and in partnership with the ND Center for Research Computing, provide the opportunity for ND group members to access in bursts extremely large-scale computing resources (on the order of 20,000 CPU cores) for CMS data analysis. ND researchers made key contributions to the discovery of the Higgs Boson, and continue to drive important physics analyses such as the search for supersymmetry and the search for new phenomena involving top quarks or Higgs bosons.
Neutrino Experiments: ND is a member of several collaborations engaged in studies of neutrino physics. These collaborations are all working towards an improved understanding of neutrino oscillations, the mysterious process whereby the flavor (electron, muon, or tau) of a neutrino changes as it travels through space. The Double Chooz experiment is in its completion phase, after successfully measuring a crucial parameter governing neutrino oscillations. The Deep Underground Neutrino Experiment (DUNE), which will commence operations circa 2026, will expand our understanding of neutrino oscillations by looking for evidence that neutrinos and antineutrinos oscillate differently. If they do, this could help to clarify why we live in a universe made up of matter, rather than equal parts matter and antimatter. The IceCube experiment, located at the South Pole, seeks to detect neutrinos traveling to earth from some of the most extreme astrophysical phenomena known, such as supernovae, neutron stars, or black holes. ND also contributes to several experiments aimed at improving the precision of accelerator-based oscillation measurements. The MINERvA experiment at Fermilab is making precise measurements of neutrino-nucleus interactions, and the EMPHATIC and NA61 experiments measure the nuclear interactions that happen in accelerator-based neutrino beamlines, in order to better understand the number of neutrinos in those beams and their energy spectra.
In addition to these ongoing scientific experiments, ND plays an active role in research and development both for detectors and computational technology for future HEP experiments.
We are also founding members of the QuarkNet education and outreach program, established to bring cutting edge particle physics experiences to high school teachers. In addition to our local QuarkNet center, we continue to lead the national QuarkNet program, consisting of 55 centers at universities and labs around the country. Each year we reach 400-500 teachers directly, and through them roughly 60,000 high school students nationwide.
The theoretical high energy physics group at Notre Dame works on a broad range of topics, including model-building beyond the Standard Model, collider phenomenology, and tests and applications of particle physics models within the realm of astrophysics and cosmology.
Historically, the group played a major role in extending our understanding of CP violation, particularly through studies of the b-quark system. Today, in the era of the LHC, the group continues to work on the forefront of phenomenology in and beyond the standard model, including searches for, and tests of, supersymmetry, extra spacetime dimensions, and strong dynamics, as well as model-independent approaches to new physics that rely on the techniques of effective field theory. Studies are motivated both by theoretical drivers (such as the hierarchy problem) and by the most recent results coming from the LHC. Careful phenomenological studies of LHC collider physics allows the group to connect abstract theory to observable signatures and signals. In all of these topics, discussion and collaboration with our experimental colleagues is frequent and ongoing, benefitting both groups.
Finally, the connections to astrophysics (especially in studies of dark matter) and to cosmology (including signatures of new physics in the cosmic microwave background) are deep. Recent and current work includes non-canonical approaches to dark matter creation in the early universe, using dark sectors to ease the Hubble tension, constraints on primordial black holes, and finding ways to test realistic dark matter models at the LHC.