IR fixed points in SU(3)SU(3) gauge Theories

We propose a novel RG method to specify the location of the IR fixed point in lattice gauge theories and apply it to the $SU(3)$ gauge theories with $N_f$ fundamental fermions. It is based on the scaling behavior of the propagator through the RG analysis with a finite IR cut-off, which we cannot remove in the conformal field theories in sharp contrast with the confining theories. The method also enables us to estimate the anomalous mass dimension in the continuum limit at the IR fixed point. We perform the program for $N_f=16, 12, 8$ and $N_f=7$ and indeed identify the location of the IR fixed points in all cases.Comment: 7 pages, 7 figures, 1 table: the scale of the y axis in Figs..1-4 change; minor modifications as appear in PL

The square-lattice spiral magnet Ba_2CuGe_2O_7 in an in-plane magnetic field

The magnetic structure of Ba_2CuGe_2O_7 is investigated by neutron diffraction in magnetic fields applied along several directions in the $(a,b)$ plane of the crystal. In relatively weak fields, $H\lesssim 0.5$~T, the propagation vector of the spin-spiral rotates to form a finite angle with the field direction. This angle depends on the orientation of $H$ itself. The rotation of the propagation vector is accompanied by a re-orientation of the plane of spin rotation in the spiral. The observed behaviour is well described by a continuous-limit form of a free energy functional that includes exchange and Dzyaloshinskii-Moriya interactions, as well as the Zeeman energy and an empirical anisotropy term.Comment: 7 pages, 6 figure

Four-quark final state in W-pair production: Case of signal and background

We discuss theoretical predictions for W-pair production and decay at LEP2 and higher energies in a form suitable for comparison with raw data. We present a practical framework for calculating uncertainties of predictions given by the KORALW and grc4f Monte Carlo programs. As an example we use observables in the $s\bar s c\bar c$ decay channel: the total four-quark (four-jet) cross section and two-quark/jet invariant-mass distribution and cross section, in the case when the other two may escape detection. Effects of QED bremsstrahlung, effective couplings, running W and Z widths, Coulomb interaction and the complete tree level set of diagrams are discussed. We also revisit the question of technical precision of the new version 1.21 of the KORALW Monte Carlo code as well as of version 1.2(26) of the grc4f one. Finally we find predictions of the two programs to have an overall physical uncertainty of 2%. As a side result we show, on the example of an $s\bar s$ invariant mass distribution, the strong interplay of spin correlations and detector cut-offs in the case of four-fermion final states.Comment: 26 pages, LaTe

Non-trivial Center Dominance in High Temperature QCD

We investigate the properties of quarks and gluons above the chiral phase transition temperature $T_c,$ using the RG improved gauge action and the Wilson quark action with two degenerate quarks mainly on a $32^3\times 16$ lattice. In the one-loop perturbation theory, the thermal ensemble is dominated by the gauge configurations with effectively $Z(3)$ center twisted boundary conditions, making the thermal expectation value of the spatial Polyakov loop take a non-trivial $Z(3)$ center. This is in agreement with our lattice simulation of high temperature QCD. We further observe that the temporal propagator of massless quarks at extremely high temperature $\beta=100.0 \, (T \simeq10^{58} T_c)$ remarkably agrees with the temporal propagator of free quarks with the $Z(3)$ twisted boundary condition for $t/L_t \geq 0.2$, but differs from that with the $Z(3)$ trivial boundary condition. As we increase the mass of quarks $m_q$, we find that the thermal ensemble continues to be dominated by the $Z(3)$ twisted gauge field configurations as long as $m_q \le 3.0 \, T$ and above that the $Z(3)$ trivial configurations come in. The transition is essentially identical to what we found in the departure from the conformal region in the zero-temperature many-flavor conformal QCD on a finite lattice by increasing the mass of quarks. We argue that the behavior is consistent with the renormalization group analysis at finite temperature.Comment: 16 pages, 9 figures; 4 tables, an appendix adde