Search

We present first results for $B_c$ spectroscopy using Lattice Non-Relativistic QCD (NRQCD). For the NRQCD action the leading order spin-dependent and next to leading order spin-independent interactions have been included with tadpole-improved coefficients. We use multi-exponential fits to multiple correlation functions to extract ground and excited $S$ states and give accurate values for the $S$ state hyperfine splitting and the P state ($B^{**}_c$) fine structure, including the effects of $^1P_1/^3P_1$ mixing., 12 pages uuencoded latex file + 1 postscript figure

We present a new determination of the QCD strong coupling constant based on precise lattice calculations of the $\Upsilon$ spectrum. The largest systematic uncertainty in previous such determinations resulted from the absence of vacuum polarization from light quarks. We substantially reduce this error by including two flavors of dynamical light quarks and extrapolating to three. We find $\alphathree_\V(8.2~\GeV) = 0.196(3)$ for three light flavors, corresponding to $\alphafive_\msbar(M_Z) = 0.115(2)$. This is significantly more accurate than previous determinations using this or any other technique.

Using analytic methods and dynamical fermion simulations, we study the properties of two nonperturbative continuum limits of a lattice Yukawa model with a real scalar field, at the (a) ferromagnetic-ferrimagnetic and (b) ferrimagnetic-antiferromagnetic phase boundaries. This work makes use of a new analytic result that the fermion mass vanishes identically in the antiferromagnetic phase.

We study the global limit of the hypercharge U(1){sub Y} sector of the standard lattice electroweak theory. We show that for fermions with both Y{sub R} = 0 and Y{sub R} {ne} 0 the doubler modes can be removed in the continuum limit while the physical fermion masses can be tuned to agree with physical values. 18 refs., 1 fig., 1 tab.

We obtain the ratio R 1 R 1 2 of chiral symmetry breaking scales for quarks in the l = 1 and l = 1 2 representations of SU(2). 〈 ψ ψ〉 is computed for the zero temperature theory in the quenched approximation on a 10 4 lattice and the value R 1 R 1 2 = 6.6 ± 1.0 is extracted. This result is consistent with a previous estimate of the ratio of relevant scales which compared the chiral symmetry restoration temperatures T 1 and T 1 2 for the two representations and found T 1 T 1 2 = 8.6 ± 4.5 . The new l = 1 data scales according to asymptotic freedom.

Yukawa couplings are studied on the latice for fermion coupled to scalr fields of a single component λϕ 4 tgheory. quenchd calculation of the physical fermion mass M F are carried out in the broken phase at zero bare fermion mass and for Yukawa coupling ranging between 1 and 2.5. Our results, when combined with recent nonperturbative determinations of the scalar field wave function renormalization Z by Huang, Manousakis and Polonyi, indicate that the ratio M F / ϕ ren vanishes as one approaches the critical point. This lends support to the idea that Yukawa-coupling-=generated fermion masses cannot become arbitrarily large without, at the same time, the model ceasing to be a valid effective low-energy theory.

Spontaneous symmetry breaking is studied in an SU(5) model of adjoint scalars with quartic couplings λ1 (tr φ2)2 + λ2 trφ4 and with a negative mass term. The breaking for λ2 > 0 is to SU(3)⊗SU(2)⊗U(1), and for λ2 > 0 to SU(3)⊗SU(2)⊗U(1), in agreement with the standard tree-level analysis of the scalar potential.

Mass measurements are carried out in a lattice U(1) gauge-Higgs system with the Higgs fields transforming as a doublet under a global SU(2) symmetry. The model exhibits three distinct phases: Coulomb, Higgs and confining. Evidence is found for a massless vector state in the Coulomb phase, and massless Goldstone scalars in the Higgs phase. Also the behavior is studied of the massive states near the transitions between the Coulomb and Higgs phases and between the confining and Higgs phases.

The phase diagram of lattice SU(2) gauge theory with fundamental representation Higgs fields is probed by l= 1 2 and l =1 fermions. Quenched calculations of 〈 Ψ Ψ〉 l show two distinct regions characterized by broken or unbroken chiral symmetry. The location of the boundary between the two regions depends on the representation l of the fermions. For l= 1 2 fermions the presence is verified of Nambu-Goldstone bosons (“pions”) in the broken region and evidence is found for massless physical fermions in the chirally symmetric region. Effects of boundary conditions and the possibility of observing two particle cuts in finite lattice spectrum calculations are commented on also.

Renormalized Yukawa couplings, yR, are evaluated numerically in the single component lattice λϕ4 theory coupled to staggered fermions. For a wide range of bare couplings, the renormalized yR is found to be “small” (⪅1.5) whenever physical scalar and fermion masses have dropped below about one half the cutoff scale. In these cases Yukawa coupling generated fermion masses are never much larger than the symmetry breaking scale ν = 〈ϕR〉. The simulations have been performed using the hybrid Monte Carlo algorithm for dynamical fermions.

We present calculations of v W 2 and of massive muon pair production cross sections in the kinematic ranges of recent experiments. These calculations test the asymptotically free parton model and excellent agreement with the data is found. Estimates of the transverse momentum 〈 q ⊥ 〉 μμ expected in massive dimuon experiments are made. For s = 750 GeV 2 and M μμ ≈ 10 GeV, we predict 〈 q ⊥ 〉 μμ ≈ 1.4 GeV/ c .

The finite temperature chiral transition is studied in the quenched approximation for octet and sextet quarks. The ratios of critical temperatures are estimated to be T (8) c / T (3) c ∼ 7.5 ± 1.5 and T (6) c / T (3) c ∼ 12.0 ± 2.5.