Nuclear & Particle Physics
Overview
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 at the 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 postdocs. 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.
The theory group leverages effective field theory techniques to pursue a broad research agenda that blends together aspects of high energy, gravitational, and condensed matter physics with topics involving black holes, scattering amplitudes, and collective excitations in condensed matter systems. Studies of critical behavior in random quantum systems and the mechanism of color confinement are also being pursued. Another thrust concerns questions at the intersection of quantum field theory and gravity, examining the behavior of gravitational scattering amplitudes and consistency conditions for field theories in de Sitter spacetime and for theories in which Lorentz invariance is spontaneously broken. Nuclear and hadron physics involving large-scale computing resources that implement calculations of quantum chromodynamics (QCD) on a space-time lattice is also a research focus.
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.
Leonard Kisslinger (Emeritus) uses a variety of techniques, such as QCD sum rules, Dyson-Schwinger equations, and light-front field theory, to study QCD and electroweak processes for deducing the structure of hadrons and nuclei, early universe phase transitions, cosmic electroweak bubbles, and pulsar kicks.
Curtis Meyer is the spokesperson of the GlueX experiment which is currently being built at JLab. The primary goal of the experiment is to search for a theoretically expected exotic form of matter in which the gluonic fields of QCD directly impact the observable properties of matter. It is believed that the spectrum of these exotic hybrid mesons will help to elucidate the nature of confinement in QCD. In order to carry out this analysis, large-scale, sophisticated analyses need to be performed on big data sets. We have developed a significant set of software tools that are currently being applied to data on excited protons from the CLAS at Jefferson Lab. These analyses hope to resolve a long standing issue in QCD known as the missing baryon problem by uncovering new excited states. Results from this program are just starting to come out, where we are seeing some exciting hints of new states.
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
John Alison's research focuses on the study of Higgs-boson pair production. His work directly probes models extending the SM and will constrain - and ultimately measure - one of the most distinctive and elusive properties of the Higgs boson: its interaction with itself. This self-interaction has deep connections with the cosmology of the early Universe and will allow us to understand how fundamental particles acquired mass.
Alison is also heavily involved in detector design and development. CMU is building detector modules for the High-Granularity Calorimeter, an upgrade to the existing CMS detector. Automated module assembly, wire bonding, and testing for over 5000 silicon modules, will all be done in-house on CMU's campus.
Another major focus of Alison group is using recent advances in machine learning to extract more physics from the data provided by the LHC. This includes both improving the algorithms that run in real-time to select which data to record for further analysis and improving classification and regression problems associated with the most common decays of the Higgs boson.
Roy Briere studies heavy quarks produced at electron-positron colliders. Current efforts include ongoing work with the BES III detector in Beijing, and preparations for data-taking with BELLE-II in Japan. The main physics thrust is weak decay "flavor physics" based on decays of D and B mesons, which contain the heavy quark types (flavors) charm and bottom, respectively. Their decays allow studies of CP violation and other rare phenomena used to search for hints of new physics. His other contributions to experiments include drift-chamber calibration and simulation, analysis tools, and various management roles.
Tom Ferguson's research interests as an Emeritus are in experimental high-energy particle physics and, in particular, the use of electron-positron and proton-proton colliding beam accelerators. A member of the Compact Muon Solenoid (CMS) collaboration, which has constructed a massive detector for the Large Hadron Collider (LHC). The LHC is a 14 TeV proton-proton colliding beam accelerator, situated at the European accelerator center, CERN, in Geneva, Switzerland. Since beginning to run in 2010, the LHC has reached 8 TeV in center-of-mass energy and is already close to its design intensity. The best-known result from the LHC so far is the discovery of the Standard Model Higgs boson in 2012 by the CMS and ATLAS experiments. In the future, we hope to discover entirely new families of particles beyond the Standard Model, such as supersymmetry.
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.
Riccardo Penco uses effective field theory (EFT) techniques to study a variety of phenomena in gravitational, high energy, and condensed matter physics. In the context of gravitational physics he has been studying on formal and phenomenological aspects of black hole physics. In high energy physics, he has been interested in exploring the relation between scattering amplitudes of different quantum field theories, and understanding their behavior at low energies. Finally, within the realm of condensed matter physics he has been focusing his attention on the dynamics and (self-)interactions of collective excitations.
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 used 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's 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.