The LBNL theory group carries out cutting-edge research in a wide range of areas in particle theory and cosmology, centered on models of new physics, precision measurements at high energy colliders, flavor physics, dark matter, and early universe cosmology. An essential aspect of the group’s operation is through the Berkeley Center for Theoretical Physics, a joint research center of the UC Berkeley Physics Department and the LBNL Physics Division, of which all lab and campus theorists are members. This further broadens the scope of the group’s interests to string theory.
To find out more about the various different research directions we are involved in, please continue reading.
New physics at colliders
The Large Hadron Collider (LHC) is currently probing physics at the highest energies produced in a laboratory. Both the increased energy and the considerably larger luminosity may allow the direct production of hypothetical particles beyond the standard model (BSM). One fundamental paradigm that has shaped theoretical particle physics over the past 40 years, namely whether the smallness of the electroweak symmetry breaking scale is due to new dynamics at energy scales around a TeV or not, is currently tested experimentally by the LHC. If there are no new particles or new dynamics up to the energy scale that can be probed by Run II at the LHC, it would imply that the fundamental parameters of the standard model determining the weak scale are tuned to at least the percent level.
The group seeks creative avenues to discover new physics, be it for models solving the hierarchy problem such as weak scale Supersymmetry (SUSY), for dark matter or exotic new physics scenarios. We highlight the signatures that can be expected for such models at the LHC and other experiments. Because of the breadth of our group, we are able to break down the boundaries between astro-particle physics, cosmology and collider physics. Since all of these areas will provide insight into fundamental physics, an inclusive view is crucial to fully interpret the plethora of experimental results in the coming years.
We are also continually looking for new physics in unexpected places: experimental searches are typically focused around a few models of new physics, such as the MSSM. The group interacts with the experiments to broaden the types of analyses that are carried out to ensure that new physics will not be missed.
Precision collider physics
As the luminosity of the LHC experiment is rising, and the experimental community is continuously understanding the detector environment better, the experimental uncertainties are continuously becoming smaller. The theory community is responding with higher precision theoretical calculations of known standard model processes, which are important for measuring SM parameters to an increased precision, and, more importantly, for predicting backgrounds for new physics searches.
The Berkeley theory group has a long tradition of providing precise theoretical predictions, and the expertise in effective field theory techniques and jet physics has provided a unique strength compared to many other theory groups. We are continuing to build on this strength. As the experimental collaborations continue to place more aggressive cuts to enhance signals over backgrounds, theoretical predictions in restricted regions of phase space are becoming ever more important. In these regions of phase space, higher order resummation is typically much more important than higher fixed order calculations, and the Berkeley theory group is working to provide the highest precision calculations, as well as guidance, to the community.
The next 10 years will be critical for tests of theories of WIMP DM. Direct detection experiments have already moved well below the scattering cross-section predicted for a WIMP interacting with a nucleus via the Z boson. Another crucial benchmark will be reached as direct detection experiments reach the cross section at which a neutralino DM candidate would be expected to scatter off the nucleus through the Higgs boson.
While the WIMP will never be completely excluded by the combination of direct and indirect detection experiments, as we push down to the neutrino background in direct detection experiments, if a DM particle is not discovered, an increasingly strong argument will be made for exploring other theories of DM. Conversely, if strong evidence does appear for the discovery of the DM in one of these experiments, we will have good hints from the combination of direct and indirect detection, as well the the LHC, to begin to hone in on the properties of such a particle.
Other theories of DM have already received a great deal of attention and development over the last 5-7 years, and our theoretical focus will continue to be directed towards further development of theories that had previously been little explored. One example of these theories is Asymmetric DM. More broadly, the group is interested in theories of hidden sector DM and ways to detect it by developing new types of experiments.
Indirect searches for new physics
Besides searching for new particles at the LHC (and in dark matter detectors, etc.), yet undiscovered particles can also influence measurements via virtual effects. Such searches for new physics were very successful in the past; e.g., effects of the top quark were measured much before it was directly discovered. Measurements of quark and lepton decays, in particular flavor-changing neutral currents and CP violation, already probe mass scales comparable and possibly well above LHC energies, depending on the details of new physics. A clear empirical evidence for new physics comes from the observed baryon asymmetry of the Universe, which requires CP violation beyond the standard model.
In the next decade the sensitivity of many measurements that can reveal new sources of CP violation and new heavy particles will increase 100-fold or more, enhancing the sensitivity to new phenomena at several times shorter distance scales (i.e., higher energy scales) than we can access today. We pursue many research directions toward better understanding the predictions of the standard model relevant for these upcoming experiments, devising new observables sensitive to new physics, as well as studying the influence of beyond standard model scenarios on these upcoming measurements and their synergies with the LHC new particle searches.
Exclusive event generators
Over the past 4 years the Berkeley theory group has built up a research group with the goal of producing precision exclusive event generators. The theoretical framework of GENEVA was conceived to allow a generic higher order perturbative calculation to be turned into an exclusive event generator. The first processes available with GENEVA highlight this strength by implementing higher order resummed calculations into an event generator framework. In the coming years, we will provide many new types of perturbative calculations in the GENEVA framework, setting GENEVA apart from other available event generators. Examples are electroweak Sudakov logarithms, which are responsible for turning traditionally small electroweak corrections into O(10%) effects at high invariant mass events, or the resummation of logarithms of the heavy quark mass that make fixed order results of heavy quark production at high transverse moment unreliable. Finally, GENEVA will implement recent theoretical results by the Berkeley group on combining NNLO results with parton shower algorithms.
Cosmology has entered a precision era in the last decade. The enormous amount of data pouring in from the cosmic microwave background, from large scale structure, and other early universe probes, will greatly increase our understanding of the universe’s evolution. This is an enormous boon for fundamental physics, as we hope to learn more about the nature of the dark matter and new physics beyond the standard model. As cosmology moves deeper into its precision era, however, better calculational and theoretical control will be necessary for matching observation onto the underlying model of the Universe. With calculation tools and probes from particle physics, and strong ties with the cosmology group, the Berkeley theory group seeks to concretely advance our ability to learn about fundamental physics from astrophysics and cosmology.
Despite its many successes, the standard model does not provide a satisfactory quantum description of gravity. The quantum realm and the geometric spacetime realm of gravity must be reconciled, though the reconciliation, which eluded Bohr and Einstein, may perhaps be glimpsed in string theory. At the same time, theoretical puzzles, such as the black hole information problem, are shedding increasing light on the connections between gauge theories and gravity. These advancements may even have important implications for our understanding of cosmological horizons.
It is commonly believed that our familiar dimensions of space became large during an early era of the universe when length scales underwent a period of exponential inflation. What is the physical theory underlying such catastrophic early cosmic behaviour, and how can it be tested? String theory is also making fruitful contributions to cosmology via theories for this early period of inflation.