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Theory Seminar
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 Theory Seminars Spring 2008 View Printable Version

## January 28, 2008

Speaker: Professor Michael Creutz, BNL

Abstract:

Motivated by the description of the graphene electronic structure in terms of the relativistic Dirac equation, a generalization to four dimensions yields a strictly local fermion action describing two species and possessing an exact chiral symmetry. This is the minimum number of species required by the well known no-go'' theorems.

## February 4, 2008

Speaker: Professor Weonjong Lee, Seoul University

Abstract:

A recent progress in calculating B_K using staggered fermions are reviewed. In particular, there has been a series of progress in understanding the power counting rules through numerical study on pion spectrum in parallel with a theoretical development of staggered chiral perturbation theory. This progress is essential to calculate B_K with extremely high precision.

## February 11, 2008

Speaker: Professor Lisa Randall, Harvard University

Title: "GIM in the Bulk"

Abstract:

We show how extra-dimensional theories can naturally explain the flavor structure of the Standard Model and also how they can shed light on CP violation.

## February 18, 2008

Speaker: Stanley Brodsky, Stanford University and SLAC

Abstract:

The AdS/CFT correspondence between string theory in AdS space and conformal field theories in physical space-time leads to an analytic, semi-classical model for strongly-coupled QCD which has scale invariance and dimensional counting at short distances and color confinement at large distances. Although QCD is not conformally invariant, one can nevertheless use the mathematical representation of the conformal group in five-dimensional anti-de Sitter space to construct an analytic approximation to the theory. The AdS/CFT correspondence also provides a mapping between the fifth-dimensional coordinate of AdS space and a specific impact variable which measures the separation of the quark and gluonic constituents within the hadron. This connection allows one to compute the analytic form of the frame-independent light-front wavefunctions of mesons and baryons, the fundamental entities which encode hadron properties and which allow the computation of exclusive scattering amplitudes. A new integrable relativistic light-front equation in ordinary space-time is found which reproduces the results obtained using the fifth-dimensional theory. An extension for massive quarks will also be discussed. Since they are complete and orthonormal, the AdS/CFT wavefunctions can also be used as a basis for the diagonalization of the full light-front QCD Hamiltonian, thus systematically improving the AdS/CFT approximation.

## February 25, 2008

Speaker: Raphael Bousso, University of California - Berkeley

## March 10, 2008

Speaker: Professor Sinya Aoki, Tsukuba University

Abstract:

Nucleon-nucleon (NN) potential is studied by lattice QCD simulations in the quenched approximation. An NN potential $V_{\rm NN}(r)$ isdefined from the equal-time Bethe-Salpeter amplitude with alocal interpolating operator for the nucleon. By studying the NN interaction in the $^1$S$_0$ and $^3$S$_1$ channels, we show thatthe central part of $V_{\rm NN}(r)$ has a strong repulsive core of a few hundred MeV at short distances ($r \alt 0.5$ fm) surrounded by anattractive well at medium and long distances.These features are consistent with the known phenomenological features of the nuclear force. More recent progresses are alsodiscussed.

## March 24, 2008

Speaker: Patrick Fox, Fermilab National Accelerator Laboratory

Abstract:

Assuming that the leptons and quarks other than top are massless at tree level, we show that their masses may be induced by loops involving the top quark. As a result, the generic features of the fermion mass spectrum arise from certain combinations of loop factors. Explicitly, we construct a renormalizable model involving some colored scalar fields, which leads to 1-loop bottom and tau masses, a 2-loop charm mass, 3-loop muon and strange masses, 4-loop masses for up and down quarks, and a 5-loop electron mass. This realistic pattern of masses does not require any symmetry to differentiate the three generations of fermions. The scalar fields may lead to observable effects in future experiments searching for $\mu \to e$ conversion in nuclei, lepton-flavor violating kaon decays, and other flavor-changing processes.

## April 21, 2008

Speaker: Dr. Yang Bai, Fermilab National Accelerator Laboratory

Abstract:

We construct a little Higgs model with the most minimal extension of the standard model gauge group by an extra U(1) gauge symmetry. For specific charge assignments of scalars, an approximate U(3) global symmetry appears in the cutoff-squared scalar mass terms generated from gauge bosons at one-loop level. Hence, the Higgs boson, identified as a pseudo-Goldstone boson of the broken global symmetry, has its mass radiatively protected up to scales of 5-10 TeV. In this model, a Z2 symmetry, ensuring the two U(1) gauge groups to be identical, also makes the extra massive neutral gauge boson stable and a viable dark matter candidate with a promising prospect of direct detection.

## April 28, 2008

Speaker: Prof. Gilad Perez, Stony Brook University

Abstract:

We briefly describe the important and intriguing role played by the top within the standard model (SM) of elementary particles. We are thus motivated to improve our (rather poor) knowledge of the top fundamental parameter and couplings. It is very exciting that such a study will be possible, very soon, when the LHC experiment will start running. We demonstrate how the special features of the top quark will allow us to perform precision tests of the SM and open a window to discover beyond the SM physics, focusing on warped extra dimension models as a concrete example.

## May 5, 2008

Speaker: Dr. Hooman Davoudiasl, Brookhaven

Abstract:

We present a predictive warped model of flavor that is cut off at an ultraviolet scale of order 1000 TeV. This Little Randall-Sundrum (LRS)" model is a volume-truncation, by a factor y~6, of the RS scenario and is holographically dual to dynamics with number of colors larger by y. The LRS couplings between Kaluza-Klein states and the Standard Model fields are explicitly calculable. Assuming separate gauge and flavor dynamics, a number of unwanted contributions to precision electroweak, Z b b\bar, and flavor observables are suppressed in the LRS framework, compared with the RS counterparts. An important consequence of the LRS truncation, independent of precise details, is a significant enhancement of the clean (golden) di-lepton LHC signals, by order y^3, due to a larger rho-photon" mixing and a smaller inter-composite coupling.