Particle Theory In and Beyond the Standard Model

The goal of particle theory research in the CTP is to enable discoveries of physics beyond the standard model (BSM), both through precision tests of the standard model itself and through detailed studies of possible new phenomena. With the momentous discovery of the Higgs boson at the LHC in 2012, the standard model is now complete, yet its shortcomings loom larger than ever. The standard model cannot account for the nature and origin of dark matter, nor does it address the puzzling hierarchy between the electroweak and Planck scales. For this reason, particle theorists in the CTP are developing new theoretical frameworks to address physics in and beyond the standard model, and studying the resulting experimental signatures at dark matter detection experiments, high intensity experiments, and colliders like the LHC.

The CTP has a long history of leadership in particle theory. Emeritus faculty Dan Freedman, Jeffrey Goldstone, and Roman Jackiw are responsible for some of the fundamental theoretical ideas – especially those associated with symmetries and symmetry breaking – which lie at the heart of the standard model and its extensions. Frank Wilczek is one of the authors of the standard model and is regarded as luminary in particle theory, with long-standing interests in axions, unification, and supersymmetry. Eddie Farhi and Robert Jaffe have taken techniques developed in particle theory and applied them to the fields of quantum computation and fluctuation physics, respectively. Tracy Slatyer and Jesse Thaler represent the next generation of particle theorists, whose work draws on experimental and theoretical developments in areas ranging from dark matter detection to formal supergravity.

The current particle theory effort in the CTP includes research that has a direct impact on experiments as well as research that pursues more formal theoretical directions. Successful particle theorists have an appreciation and understanding of experimental methods, and the CTP prides itself on maintaining close connections to experimental research conducted in the LNS. At the same time, research in particle theory offers opportunities to push the boundaries of knowledge in quantum field theory (QFT), and excellence and creativity in QFT has long been a theme that unites the research conducted in the CTP.

Higgs physics will become increasingly important with the upgraded 14 TeV LHC run starting in 2015. Precent-level measurements of the Higgs boson couplings are needed to test the Higgs boson's role in generating fundamental particle masses. Wilczek has long emphasized that BSM scenarios such as supersymmetry predict small deviations in these couplings as well as additional Higgs particles. More recently, Wilczek has shown how the Higgs boson can act as the portal to dark matter, and Thaler has shown how presence of Higgs bosons can be used to tag new physics signals at the LHC. The connection between Higgs physics and BSM physics remains an active area of research. Precision calculations are crucial for studying the detailed characteristics of the Higgs boson, and Iain Stewart has applied effective field theory methods to calculate key Higgs cross sections and thus reduce theory uncertainties in Higgs measurements.

Another area of increasing importance for the 14 TeV LHC is jet physics. Jets are collimated sprays of particles that arise when quarks and gluons are produced at the LHC, and copious jet production is a potential smoking gun for supersymmetric theories. Jesse Thaler has been at the forefront of the emerging field of jet substructure, developing new jet analysis techniques to capitalize on the exceptional ability of the LHC experiments to resolve jet constituents. These jet substructure methods can enhance BSM signals above standard model backgrounds, and they are currently being implemented in new physics searches by the MIT CMS $pp$ group. Stewart and Thaler have also developed new techniques to perform precision jet calculations, capitalizing on recent development on applying resummation techniques to hadronic collisions. Jet substructure may also offer new probes of the phenomena of jet quenching in the quark/gluon plasma, an area of considerable interest to the MIT CMS heavy ion group.

Dark matter is a key research direction in the CTP, bridging particle physics and astroparticle physics (see Cosmology page). The gravitational evidence for dark matter is overwhelming, but the nature and origin of dark matter is still unknown. The two leading paradigms for dark matter are axions and stable relics (possibly of supersymmetric origin), but given the lack of any conclusive dark matter signals to date, CTP researchers are taking imaginative approaches to dark matter and its potential signatures.

Particle theory also connects to more formal developments in QFT (as well as string theory). Almost all collider studies involve the calculation of scattering amplitudes, but independent of collider applications, Freedman has shown that scattering amplitudes themselves have a rich mathematical structure with hidden symmetries. Inspired by potential LHC signatures of supersymmetry, Thaler has shown that the dynamics of supersymmetry breaking can be richer than previously thought, leading to new results in formal supergravity. Strong dynamics is a feature of many extensions of the standard model, and one can gain some analytic handles on these scenarios by treating them as if they were conformal field theories (i.e.~special QFTs with a scaling symmetry). Conformal field theories may also be relevant for understanding jet physics, since the interactions of quarks and gluons can sometimes be approximated as having a scaling symmetry. More generally, techniques developed in particle theory have the potential to offer new insights in other fields, especially condensed matter physics.

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