Diagrammatic Exponentiation for Products of Wilson Lines

We provide a recursive diagrammatic prescription for the exponentiation of gauge theory amplitudes involving products of Wilson lines and loops. This construction generalizes the concept of webs, originally developed for eikonal form factors and cross sections with two eikonal lines, to general soft functions in QCD and related gauge theories. Our coordinate space arguments apply to arbitrary paths for the lines.Comment: 16 pages, 4 figures; minor corrections, version to appear in Phys. Rev.

Top Jets at the LHC

We study the reconstruction of high p_T hadronically-decaying top quarks at the LHC. The main challenge in identifying energetic top quarks is that the decay products become increasingly collimated. This reduces the efficacy of conventional methods that exploit the topology of the top decay chain. We focus on the cases where the decay products of the top quark are reconstructed as a single jet, a "top-jet". The most basic "top-tag" method based on jet mass measurement is considered in detail. To analyze the top-tagging method, theoretical and experimental aspects of the QCD jet background are examined. Based on QCD factorization, we derive a simple analytic approximation for the shape of the QCD jet mass spectrum. We observe a good agreement with the Monte Carlo simulation. We consider high-p_T t\bar{t} production in the Standard Model as an example, and show that our theoretical QCD jet mass distributions can efficiently characterize the background via sideband analyses. We show that with 25 fb^{-1} of data, our approach allows us to resolve top-jets with p_T $\ge$ 1 TeV from the QCD background, and about 1.5 TeV top-jets with 100 fb^{-1}, without relying on b-tagging. To further improve the significance we consider jet shapes (analyzed in 0807.0234 [hep-ph]), which resolve the substructure of energy flow inside cone jets. A method of measuring top quark polarization by using the b-quark p_T is also presented. The main advantages of our approach are: (i) the mass distributions are driven by first principle calculations, instead of relying solely on Monte Carlo simulation; (ii) for high p_T jets (p_T $\ge$1 TeV), IR-safe jet shape variables are robust against detector resolution effects. Our analysis is applicable to other boosted massive particles such as the electroweak gauge bosons and the Higgs.Comment: 46 pages, 23 figure

Gauged Flavor Group with Left-Right Symmetry

We construct an anomaly-free extension of the left-right symmetric model, where the maximal flavor group is gauged and anomaly cancellation is guaranteed by adding new vectorlike fermion states. We address the question of the lowest allowed flavor symmetry scale consistent with data. Because of the mechanism recently pointed out by Grinstein et al. tree-level flavor changing neutral currents turn out to play a very weak constraining role. The same occurs, in our model, for electroweak precision observables. The main constraint turns out to come from WR-mediated flavor changing neutral current box diagrams, primarily K - Kbar mixing. In the case where discrete parity symmetry is present at the TeV scale, this constraint implies lower bounds on the mass of vectorlike fermions and flavor bosons of 5 and 10 TeV respectively. However, these limits are weakened under the condition that only SU(2)_R x U(1)_{B-L} is restored at the TeV scale, but not parity. For example, assuming the SU(2) gauge couplings in the ratio gR/gL approx 0.7 allows the above limits to go down by half for both vectorlike fermions and flavor bosons. Our model provides a framework for accommodating neutrino masses and, in the parity symmetric case, provides a solution to the strong CP problem. The bound on the lepton flavor gauging scale is somewhat stronger, because of Big Bang Nucleosynthesis constraints. We argue, however, that the applicability of these constraints depends on the mechanism at work for the generation of neutrino masses.Comment: 1+23 pages, 1 table, 5 figures. v3: some more textual fixes (main change: discussion of Lepton Flavor Violating observables rephrased). Matches journal versio