Effects of QCD Resummation on W+hW^+h and tbˉt\bar b Production at the Tevatron

The resummation of multiple soft gluon emission affects the production rate and kinematic distributions of $W^+h$ (where h is a Higgs boson) and $t \bar b$ pairs at the Tevatron with $\sqrt{s}=2$ TeV. Using the Collins-Soper-Sterman resummation formalism, the production rate is enhanced over the next-to-leading-order (NLO) prediction by 2-3% for the $W^+h$ process, for Higgs boson masses between 80-120 GeV, and over 3% for the $t\bar b$ process for $m_t=175$ GeV. After resummation, the $t\bar b$ rate changes by 12-13% when $m_t$ is varied by $\pm 5$ GeV. Various kinematic distributions are presented for the individual final state particles and for the pair. The explicit radiation of hard gluons in NLO QCD is included also for the $t\bar b$ final state.Comment: 9 pages, 12 Postscript figures, in RevTeX format, uses epsf.te

Doping dependence of the electron-doped cuprate superconductors from the antiferromagnetic properties of the Hubbard model

Within the Kotliar-Ruckenstein slave-boson approach, we have studied the antiferromagnetic (AF) properties for the $t$-$t'$-$t''$-$U$ model applied to electron-doped cuprate superconductors. Due to inclusion of spin fluctuations the AF order decreases with doping much faster than obtained in the Hartree-Fock theory. Under an intermediate {\it constant} $U$ the calculated doping evolution of the spectral intensity has satisfactorily reproduced the experimental results, without need of a strongly doping-dependent $U$ as argued earlier. This may reconcile a discrepancy suggested in recent studies on photoemission and optical conductivity.Comment: 5 pages, 4 eps figures, minor improvement, references added, to appear in Phys. Rev.

Study of gossamer superconductivity and antiferromagnetism in the t-J-U model

The d-wave superconductivity (dSC) and antiferromagnetism are analytically studied in a renormalized mean field theory for a two dimensional t-J model plus an on-site repulsive Hubbard interaction $U$. The purpose of introducing the $U$ term is to partially impose the no double occupancy constraint by employing the Gutzwiller approximation. The phase diagrams as functions of doping $\delta$ and $U$ are studied. Using the standard value of $t/J=3.0$ and in the large $U$ limit, we show that the antiferromagnetic (AF) order emerges and coexists with the dSC in the underdoped region below the doping $\delta\sim0.1$. The dSC order parameter increases from zero as the doping increases and reaches a maximum near the optimal doping $\delta\sim0.15$. In the small $U$ limit, only the dSC order survives while the AF order disappears. As $U$ increased to a critical value, the AF order shows up and coexists with the dSC in the underdoped regime. At half filing, the system is in the dSC state for small $U$ and becomes an AF insulator for large $U$. Within the present mean field approach, We show that the ground state energy of the coexistent state is always lower than that of the pure dSC state.Comment: 7 pages, 8 figure

Origin of the X-ray Emission in the Nuclei of FR Is

We investigate the X-ray origin in FRIs using the multi-waveband high resolution data of eight FR I sources, which have very low Eddington ratios. We fit their multi-waveband spectrum using a coupled accretion-jet model. We find that X-ray emission in the source with the highest L_X (~1.8*10^-4 L_Edd) is from the advection-dominated accretion flow (ADAF). Four sources with moderate L_X(~several*10^-6 L_Edd) are complicated. The X-ray emission of one FR I is from the jet, and the other three is from the sum of the jet and ADAF. The X-ray emission in the three least luminous sources (L_X<1.0*10^-6L_Edd) is dominated by the jet. These results roughly support the predictions of Yuan and Cui(2005) where they predict that when the X-ray luminosity of the system is below a critical value, the X-radiation will not be dominated by the emission from the ADAF any longer, but by the jet. We also find that the accretion rates in four sources must be higher than the Bondi rates, which implies that other fuel supply (e.g., stellar winds) inside the Bondi radius should be important.Comment: 6 pages. To published in Journal of Physics, in proceedings of "The Universe under the Microscope - Astrophysics at High Angular Resolution" (Bad Honnef, Germany, April 2008), eds. R. Schoedel, A. Eckart, S. Pfalzner, and E. Ro