I work on theoretical particle physics focusing on model-building 'beyond the Standard Model' (BSM).
The Large Hadron Collider is expected to directly probe many of the natural UV completions of the Standard Model. The discovery of a light Higgs and non-observation of light sparticles present an ongoing challenge to how we understand fundamental physics.
Meanwhile, we have found hints for new physics from the sky (astro/cosmo), underground (neutrinos and dark matter), and at high intensity (flavor). How can we use these clues to complement the data gleaned from collider experiments?
My research focused on building and testing new models of terascale physics that are motivated simultaneously by theoretical principles and experimental signatures. I have focused on weakly coupled descriptions of strongly coupled physics: warped extra dimensions, nonlinear realizations, and Seiberg duality.
With C. Csáki, R. Houtz, and J. Terning. Early LHC data restricts the form of any natural realization of supersymmetry to have light stops and otherwise decoupled scalars, as can be realized through compositeness. This scenario presents challenges in both model building and collider phenomenology. One way to generate a naturally light composite stop is within the framework of emergent electroweak symmetry. In this scenario, SU(2)L is identified with the magnetic Seiberg dual gauge group of a confining supersymmetric gauge theory. The minimal realization of this model can generate stops that are nearly degenerate with the top quark. In ongoing work, we are exploring the phenomenological constraints on this model as a representative UV realization of the stealth stop scenario. Sbottom decays provide a particularly promising multilepton signature which can be probed at the 14 TeV LHC.
With J. Berger, M. Perelstein, and M. Saelim. The same-sign di-lepton (SS2L) channel is a rich place to look for new Majorana fermions such as the superpartners of the Standard Model gauge bosons. Motivated by natural SUSY with R-parity violation, one may consider effective theories where a gluino decays into a boosted stop which, in turn, decays into a pair of collimated jets. We are currently exploring the use of jet substructure to determine the stop/gluino spectrum and improve the SS2L reach for these models.
With B. Bellazzini, C. Csáki, J. Hubisz, and J. Shao. The MSSM `WIMP miracle' is in trouble. Direct detection bounds push the spin-independent WIMP-nucleon cross section to 10-8 pb, nearly 100 million times larger than the typical `weak scale' cross section that is necessary to generate the correct relic abundance. MSSM neutralino dark matter typically requires severe tuning (through co-annihilations, resonances, or `well-tempering') to simultaneously generate the correct relic abundance and avoid direct detection bounds. We explore a class of models in which the dark matter particle is the fermionic superpartner of a Goldstone boson associated with a global symmetry broken at the TeV scale. This `Goldstone fermion' is a weakly interacting particle with weak-scale mass that, by virtue of suppressed Standard Model couplings, is naturally able to furnish the correct abundance and evade direct detection constraints. Further, these models open up the possibility of novel collider signatures, including `partially buried' Higgs decay channels. [arXiv:1106.2162, JHEP 09 (2011) 035]
With B. Bellazzini and M. Cliche I am currently exploring extensions of this model by exploring the consequences of pseudoscalar Sommerfeld enhancement and non-Abelian Goldstone sectors.
With Don Bunk, Jay Hubisz, and Jing Shao. A re-examination of the Nambu-Jona-Lasinio model of top condensation in an extra dimension. Previous studies of this framework have relied on a Kaluza-Klein decomposition and assumed that the lightest bound state is well-approximated by a bound state of the lightest KK modes. By working in a manifestly 5D formalism we found that the lightest bound state is, in fact, generically heavy and comes from mixing with states from the entire KK tower. We found that electroweak symmetry breaking occurs due to brane-localized terms which are generated from the renormalization of the UV theory down to the IR. We also extended the Georgi-Grant-Hailu formalism for orbifolded fermions to the case where the fermions have a 5D mass. One may also explore a more natural version of this scenario on a warped extra dimension so that the Higgs is `doubly composite,' i.e. it is a bound state of top quarks which are, in turn, composite states via the AdS/CFT correspondence. [1111.3951]
One of the surprising developments in Randall-Sundrum models is the ability to use the same warping that stabilizes the electroweak scale to simultaneously explain the hierarchical flavor structure of the Standard Model. One particularly subtle point, however, is the calculation of loop-induced flavor-changing transitions such as μ→eγ and b→sγ in the 5D framework. With Csaba Csáki, Yuval Grossman, and Yuhsin Tsai, I elucidated the structure of μ→eγ and showed that they are finite and calculable [arXiv:1004.2037].
More recently, Bibhushan Shakya, Yuhsin Tsai, Monika Blanke and I have generalized our result to b→sγ to give a more complete description of the flavor constraints in RS coming from the C7,8 operators [1203.6650].
The LHC will be the first machine to be able to probe the 'very rare' decay Bs→μ+μ- to a level that is sensitive to the Standard Model prediction. This particular mode can be a discovery channel for new physics after only 10/fb of data and is particularly promising for the MSSM which predicts tan6β enhancements to the cross section.
With Sakis Dedes and Janusz Rosiek, I explored the low tanβ region of the MSSM and found that it is possible to suppress this decay rate below the Standard Model prediction [arXiv:0812.4320, PRD]. This result would be particularly important to understand the constraints on parameter space if this transition is not observed at the LHC.
Our code for numerical scans of the MSSM parameter space is now publicly available as SUSY_FLAVOR [arXiv:1003.4260], prepared in collaboration with Janusz Rosiek, Piotr Chankowski, Sakis Dedes, and Sebastian Jäeger. This program calculates a set of important flavor and CP-violating observables in the framework of the general MSSM.