New fermion discretizations and their applications

We review the recent progress in new lattice fermion formulations. We focus on the following three types which have possibility of improving lattice simulations. (1) Flavored-mass fermions are a generalization of Wilson fermions with species-splitting mass terms. In particular, staggered-Wilson fermions initiated by Adams have possibilities of reducing numerical costs in overlap fermions and the influence of taste-breaking in staggered fermions. (2) Central-branch Wilson fermions, in which additive mass renormalization is forbidden by extra axial symmetry, could enable us to perform Wilson-fermion lattice QCD without fine-tuning. (3) Minimally doubled fermions, which reduce the number of species by species-dependent chemical potential terms, realizes a ultra-local chiral fermion at the price of hypercubic symmetry. These setups reveal unknown aspects of lattice fermions, and we obtain a deeper understanding of lattice field theory.Comment: 15 pages, 9 figures, plenary talk presented at the 30th International Symposium on Lattice Field Theory - Lattice 2012, June 24-29, 2012 Cairns, Australi

Spin-1/2 fermions with attractive interaction in an optical lattice

We study attractive fermions in an optical lattice superimposed by a trapping potential, such that fermions may form bosonic molecules. We map the model onto nonlinear field equations depending on the Nambu-Gor'kov propagator. The resulting field equations where solved numerically by a relaxation technique that allowed us to calculate the inhomogeneous densities of fermions and condensed molecules at zero temperature. When the interactions between fermions are strong there is a competition between unbound fermions and bound molecules leading to an unexpected reduction of the non-homogeneous density of fermions at the center of the trap.Comment: 10 pages, 7 figures; revise

The Sphaleron Barrier in the Presence of Fermions

We calculate the minimal energy path over the sphaleron barrier in the pre\-sen\-ce of fermions, assuming that the fermions of a doublet are degenerate in mass. This allows for spherically symmetric ans\"atze for the fields, when the mixing angle dependence is neglected. While light fermions have little influence on the barrier, the presence of heavy fermions ($M_F \sim$ TeV) strongly deforms the barrier, giving rise to additional sphalerons for very heavy fermions ($M_F \sim$ 10 TeV). Heavy fermions form non-topological solitons in the vacuum sector.Comment: 19 pages, latex, 18 figures in 3 seperate uuencoded postscript files THU-93/1

An alternative to domain wall fermions

We define a sparse hermitian lattice Dirac matrix, $H$, coupling $2n+1$ Dirac fermions. When $2n$ fermions are integrated out the induced action for the last fermion is a rational approximation to the hermitian overlap Dirac operator. We provide rigorous bounds on the condition number of $H$ and compare them to bounds for the higher dimensional Dirac operator of domain wall fermions. Our main conclusion is that overlap fermions should be taken seriously as a practical alternative to domain wall fermions in the context of numerical QCD.Comment: Revtex Latex, 26 pages, 1 figure, a few minor change

Large N Expansion for Strongly-coupled Boson-Fermion Mixtures

We study a many-body mixture of an equal number of bosons and two-component fermions with a strong contact attraction. In this system bosons and fermions can be paired into composite fermions. We construct a large N extension where both bosons and fermions have the extra large N degrees of freedom and the boson-fermion interaction is extended to a four-point contact interaction which is invariant under the O(N) group transformation, so that the composite fermions become singlet in terms of the O(N) group. It is shown that such O(N) singlet fields have controllable quantum fluctuations suppressed by 1/N factors and yield a systematic 1/N-expansion in terms of composite fermions. We derive an effective action described by composite fermions up to the next-to-leading-order terms in the large N expansion, and show that there can be the BCS superfluidity of composite fermions at sufficiently low temperatures.Comment: 26 pages, 11 figure

Fermion masses and quantum numbers from extra dimensions

We study the localization of fermions on a brane embedded in a space-time with $AdS_n \times M^k$ geometry. Quantum numbers of localized fermions are associated with their rotation momenta around the brane. Fermions with different quantum numbers have different higher-dimensional profiles. Fermion masses and mixings, which are proportional to the overlap of higher-dimensional profiles of the fermions, depend on the fermion quantum numbers.Comment: 14 page