Nuclear & Particle Physics
Our group seeks to understand the building blocks of the universe. We study elementary particles that form matter as well as those that mediate the interactions between them. Known matter consists of the quarks and leptons of the standard model, while force carriers include the familiar photon along with gluons, mediating the strong force, and weak bosons.
Our research aims to improve the understanding of several mysteries within the otherwise very successful standard model of particle physics. For example, the matter-antimatter asymmetry of the universe is related to CP violation, but detailed understanding is lacking. The origins of mass have been partly explained with the recent discovery of the Higgs boson, but many questions remain. We are also searching for new physics beyond the standard model, including the possible direct production of
dark matter particles at high-energy accelerators. Cosmological studies find that dark matter makes up one quarter of the energy-matter content of the universe. Both dark matter and the matter asymmetry relate to issues studied in our Department's McWilliams Center for Cosmology.
The internal structure of protons and neutrons, including both quarks and the gluonic field and their confinement, can be studied with direct probes in scattering experiments. In addition, the spectrum and behavior of large numbers of baryon and meson states give insight into non-perturbative strong interactions. Finally, while neutrinos are known to have mass, directly measuring their mass is a crucial task for particle physics.
We carry out these studies with experiments at international laboratories, including CMS at at the Large Hadron Collider at CERN in Switzerland, GLUEX and the SUPERBIGBITE program at the Thomas Jefferson National Accelerator Facility (JLAB), the BESIII experiment in Beijing, China, the BELLE-II experiment at KEK in Japan and KATRIN in Germany. Although our experiments are located around the world, the development and construction of many components of these experiments takes place here at Carnegie Mellon. We maintain our own on-campus shop for the development and construction of detectors, and have space for assembly and testing. This provides excellent training for our graduate students and post docs. We also maintain a large local computer structure where analysis is carried out in addition to the work done at the host laboratories or utilizing grid computing.
Complementing the experimental work above, our theoretical effort centers on the study of the strong and electroweak forces. The Particle Theory group has been active in weak-interaction topics relevant to heavy-quark physics and lattice QCD efforts related to the strong interaction. Other topics range from theories of extra dimensions to applications of effective field theory, which may be used to calculate gravity wave profiles for inspiraling black holes as well as providing systematic methods of calculating strong interaction observables at high energies. Topics in lattice QCD include the spectroscopy of bound states of confined quarks, including exotic forms of matter involving excitations of gluons. This work relies on intensive Markov-chain Monte Carlo path integrations on massively parallel supercomputers. Lattice QCD, along with other techniques, is also critical to untangling strong interaction effects from observables used to study weak interaction physics.
Research in Quark Interaction Physics
Gregg Franklin’s research probes the proton structure using experiments, based at the Jefferson Lab electron accelerator, which take advantage of recent developments in polarized target and polarized beam technologies. For example, the Jefferson Lab G0experiment directly measured the distribution of “strange” quarks and anti-quarks in the proton using spin-dependent (parity violating) elastic scattering as a signature. It required an accuracy significantly better than one part in a million. The group has also probed the electric form-factor of the neutron using a polarized 3He target. As the design goals of future experiments put increasing demand on obtaining precise measurements of the actual beam polarization, the group will be implementing a new beam polarimeter based on helicity-dependent Compton scattering of beam electrons off of photons in a resonant laser cavity. On a longer time-frame, the group is involved in a suite of newly approved experiments which will utilize a new spectrometer dubbed “SuperBigbite”. Dr. Franklin will be overseeing the development of a hadron calorimeter and is co-spokesperson of an experiment which will user Super Bigbite to measure transverse quark-spin distributions.
Colin Morningstar uses Markov-chain Monte Carlo computations in quantum chromodynamics (QCD) with a space-time lattice regulator to investigate hadron formation and quark confinement. He has computed the mass spectrum of glueballs in the Yang-Mills theory of gluons, studied the excitation spectrum of the effective QCD string between a static quark-antiquark pair, and obtained the first glimpse of the nucleon excitation mass spectrum from QCD. He is a member of a large nationwide COLLABORATION of lattice QCD theorists dedicated to Monte Carlo calculations of QCD observables on large-scale computing clusters. He and Curtis Meyer built and maintain the CMU QCD cluster.
Diana Parno studies the physics of the neutrino, a fundamental particle that is generated in particle decays and nuclear reactions. The discovery of neutrino mass provided the first evidence of physics beyond the Standard Model in the electroweak sector, and tests of the neutrino mass scale offer a rare opportunity to probe a cosmological parameter in the laboratory. Parno is Analysis Co-Coordinator for the KATRIN experiment in Karlsruhe, Germany, which is designed to improve on existing direct neutrino-mass limits by an order of magnitude. KATRIN will use the kinematics of tritium beta decay to extract the neutrino-mass scale with an anticipated sensitivity of 0.2 eV/c2 (90% confidence level), a value more than 2.5 million times lighter than the electron mass! Parno's group is involved with the main KATRIN detector system, data-quality assurance, background studies, and the molecular physics of gaseous tritium sources more generally.
Brian Quinn is involved in several experiments which probe the structure of the nucleon (neutron and proton) by using the electron beam available at Jefferson Lab. Recently he led the effort to design and propose an experiment to measure the magnetic form factor of the neutron at high momentum transfer. That experiment has been approved and will run after the 12 GeV upgrade. The apparatus for that experiment with be constructed by reconfiguring pieces of the SuperBigBite spectrometer, leading to his involvement in that project. He led CMU's development of specialized very high rate electronics for the G-zero experiment which used parity-violating electron scattering as a probe of the weak structure of the nucleon, which can then serve to measure the contribution of strange quarks to the electromagnetic structure of the nucleon. This line of research continues with the HAPPEX III experiment which uses a very different technique to make the same measurements. The PREX experiment aims to measure quite different physics, the neutron radius of lead, but makes clever use of the parity-violation technique to probe the neutron distribution.
Reinhard Schumacher does research in the electromagnetic production of strange particles (kaons and hyperons). The production of strange-quark pairs via the well-known electromagnetic interaction (via photon and electron-induced reactions) gives us an avenue for refining our knowledge of baryon resonances, coupling constants, and polarization observables; the results are compared with modern quark models and with unitarized chiral perturbation theory. For example, he and his co-workers established properties of the elusive Lambda(1405) baryon, including its spin, parity, and aspects of its two-pole structure at the JLab's CEBAF Large Acceptance Spectrometer (CLAS) in Hall B and using the GlueX detector in Hall D. He is a former Chairman of the CLAS Collaboration.
Research in High Energy Physics
Fred Gilman's research is in theoretical particle physics, particularly in understanding the nature of CP violation. He looks to the LHC to provide us with additional sources of CP violation that would explain the dominance of matter over antimatter in the universe and to produce in the laboratory the particles that make up the dark matter. Gilman is leading efforts to create and expand the McWilliams Center for Cosmology. He is a member of the Board of Directors of the Large Synoptic Survey Telescope Corporation.
Manfred Paulini studies questions connecting particle physics phenomena to issues relevant to cosmology. One of those questions concerns the fact that our visible universe exhibits a predominance of matter over antimatter. A process in particle physics called CP violation is considered relevant for breaking the symmetry between matter and anti-matter particles. Paulini has studied CP violation in mesons containing b-quarks with the CDF experiment at Fermilab. Another question concerns the nature of dark matter that makes up about one quarter of the content of the universe. Paulini currently searches for the production of dark matter particles with the CMS experiment at CERN. He is in particular interested in supersymmetric models involving the decay of dark matter particles to photons in the final state.
Ira Rothstein uses the data from the LHC to explain the origin of mass and the nature of the dark matter. He has worked on various topics in this field ranging from theories of extra dimensions to calculating Higgs boson production rates. He has also using quantum field theory to calculate classical gravity wave profiles for inspiralling black holes. He also works on effective field theory techniques to find systematic ways of calculating strong interaction observables at high energies.
Jim Russ' research centers on heavy quark physics at CDF and CMS, as well as particle astrophysics studies of ultra-high energy neutrino sources, e.g., Active Galactic Nuclei. At CDF, Russ is involve in B-physics and Quarkonium studies. These lead naturally into similar physics studies in the early running at CMS. Later Russ will work on Exotic searches for New Physics, using muons as probes. The neutrino work is now at the prototype stage and will develop over the next several years.
Helmut Vogel's research interests are in experimental high-energy physics at colliding-beam accelerators, most recently the proton-proton collider, LHC, at CERN, whose center-of-mass energy of 8 TeV is the highest ever achieved. There, the CMS and ATLAS experiments have recently discovered a Higgs boson to complete the Standard Model of the unified electroweak interaction. At the LHC, Vogel is involved in the CMS experiment. His main interests are studies of heavy quarkonium (charmonium and bottomonium) and searches for physics beyond the Standard Model in the form of additional quarks beyond the known three families.