Magnetic and superconducting instabilities of the Hubbard model at the van Hove filling

We use a novel temperature-flow renormalization group technique to analyze magnetic and superconducting instabilities in the two-dimensional t-t' Hubbard model for particle densities close to the van Hove filling as a function of the next-nearest neighbor hopping t'. In the one-loop flow at the van Hove filling, the characteristic temperature for the flow to strong coupling is suppressed drastically around t'_c approx. -0.33t, suggesting a quantum critical point between d-wave pairing at moderate t'>t'_c and ferromagnetism for t'<t'_c. Upon increasing the particle density in the latter regime the leading instability occurs in the triplet pairing channel.Comment: 4 pages, to appear in Physical Review Letter

Possible ΔΔ\Delta\Delta dibaryons in the quark cluster model

In the framework of RGM, the binding energy of one channel $\Delta\Delta_{(3,0)}$($d^*$) and $\Delta\Delta_{(0,3)}$ are studied in the chiral SU(3) quark cluster model. It is shown that the binding energies of the systems are a few tens of MeV. The behavior of the chiral field is also investigated by comparing the results with those in the SU(2) and the extended SU(2) chiral quark models. It is found that the symmetry property of the $\Delta\Delta$ system makes the contribution of the relative kinetic energy operator between two clusters attractive. This is very beneficial for forming the bound dibaryon. Meanwhile the chiral-quark field coupling also plays a very important role on binding. The S-wave phase shifts and the corresponding scattering lengths of the systems are also given.Comment: LeTex with 2 ps figure

Sterile Neutrinos in E_6 and a Natural Understanding of Vacuum Oscillation Solution to the Solar Neutrino Puzzle

If Nature has chosen the vacuum oscillation solution to the Solar neutrino puzzle, a key theoretical challenge is to understand the extreme smallness of the $\Delta m^2_{\nu_e-\nu_X}$ ($\sim 10^{-10} eV^2$) required for the purpose. We find that in a class of models such as [SU(3)]^3 or its parent group E_6, which contain one sterile neutrino, $\nu_{is}$ for each family, the $\Delta m^2_{\nu_i-\nu_{is}}$ is proportional to the cube of the lepton Yukawa coupling. Therefore fitting the atmospheric neutrino data then predicts the $\nu_e-\nu_{es}$ mass difference square to be $\sim (\frac{m_e}{m_{\mu}})^3 \Delta m^2_{atmos}$, where the atmospheric neutrino data is assumed to be solved via the $\nu_{\mu}-\nu_{\mu s}$ oscillation. This provides a natural explanation of the vacuum oscillation solution to the solar neutrino problem.Comment: 7 pages, UMD-PP-99-109; new references added; no other chang

A Rigourous Treatment of the Lattice Renormalization Problem of F_B

The $B$-meson decay constant can be measured on the lattice using a $1/m_b$ expansion. To relate the physical quantity to Monte Carlo data one has to know the renormalization coefficient, $Z$, between the lattice operators and their continuum counterparts. We come back to this computation to resolve discrepancies found in previous calculations. We define and discuss in detail the renormalization procedure that allows the (perturbative) computation of $Z$. Comparing the one-loop calculations in the effective Lagrangian approach with the direct two-loop calculation of the two-point $B$-meson correlator in the limit of large $b$-quark mass, we prove that the two schemes give consistent results to order $\alpha_s$. We show that there is, however, a renormalization prescription ambiguity that can have sizeable numerical consequences. This ambiguity can be resolved in the framework of an $O(a)$ improved calculation, and we describe the correct prescription in that case. Finally we give the numerical values of $Z$ that correspond to the different types of lattice approximations discussed in the paper.Comment: 27 pages, 2 figures (Plain TeX, figures in an appended postscript file

Is a combination of varenicline and nicotine patch more effective in helping smokers quit than varenicline alone? A randomised controlled trial

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