Particle Physics

The top quark is the heaviest known type of quark, and possibly the last. Particle physicists sometimes refer to it as the "truth” quark, not always with tongue in cheek. The top quark might be just an ordinary quark, no stranger than the "strange" one, but it might hold the key to major questions of Nature through its connection to the origin of mass, the Higgs boson, and cosmic dark matter. At the Fermi National Accelerator Laboratory outside Chicago, hundreds of these heavy quarks have been observed and some first snapshots of their behavior have been obtained. At the Large Hadron Collider at CERN, millions of the heavy quarks will be produced. This lecture will review current knowledge of the top quark and explain how this knowledge has been obtained through experiments at the giant particle accelerators. Future experiments, which might reveal the top quark's deeper mysteries, will also be described.

The BCFW recursion relations define Yang-Mills and gravity amplitudes in terms of lower-point amplitudes. I will discuss several connections between the internal consistency of this recursive definition and the allowed interactions of massless, higher-spin particles.

Recent data from the PAMELA, Fermi/LAT and INTEGRAL/SPI experiments, among others, give evidence of excess electrons and positrons in the galaxy, which might be due to annihilation of dark matter. Models in which the dark matter transforms under a hidden nonabelian gauge symmetry can naturally account for the unusual features needed to fit these data. I will discuss generic features of such models, some of their distinctive consequences for cosmology, and new results for reconciling their predictions with the anomalous observations. Special attention will be given to the 511 keV gamma ray excess observed by INTEGRAL, and to the possibility that high energy lepton excesses get significant contributions from dark matter annihilation in subhalos rather than the main halo of our galaxy.

We explore a new scenario explaining mass origin of standard model (SM) particles without a Higgs boson. In this framework SM W, Z gauge bosons and fermions are composites getting masses from confinement of substructure at IR (conformal symmetry breaking). Therefore here SM electroweak gauge symmetry and its breaking are IR emergent phenomena. Using AdS/CFT we build a calculable warped 5D model. Realistic mass spectrum and good fit to electroweak precision data (S, T parameters) can be obtained. Furthermore the composite nature of W,Z may offer novel solution for WW scattering unitarization and predicts deviation from SM which can lead to distinctive signatures at the LHC.

We derive constraints on the sign of couplings in an effective Higgs Lagrangian using prime principles such as the naturalness principle, global symmetries, and unitarity. Specifically, we study four dimension-six operators, O_H, O_y, O_g, and O_gamma, which contribute to the production and decay of the Higgs boson at the Large Hadron Collider (LHC), among other things. Assuming the Higgs is a fundamental scalar, we find: 1) the coefficient of O_H is positive except when there are triplet scalars, resulting in a reduction in the Higgs on-shell coupling from their standard model (SM) expectations if no other operators contribute, 2) the linear combination of O_H and O_y controlling the overall Higgs coupling to fermion is always reduced, 3) the sign of O_g induced by a new colored fermion is such that it interferes destructively with the SM top contribution in the gluon fusion production of the Higgs, if the new fermion cancels the top quadratic divergence in the Higgs mass, and 4) the correlation between naturalness and the sign of O_gamma is similar to that of O_g, when there is a new set of heavy electroweak gauge bosons. Next considering a composite scalar for the Higgs, we find the reduction in the on-shell Higgs couplings persists. If further assuming a collective breaking mechanism as in little Higgs theories, the coefficient of O_H remains positive even in the presence of triplet scalars. In the end, we conclude that the gluon fusion production of the Higgs boson is reduced from the SM rate in all composite Higgs models. Our study suggests a wealth of information could be revealed by precise measurements of the Higgs couplings, providing strong motivations for both improving on measurements at the LHC and building a precision machine such as the linear collider.