A Higgs Conundrum with Vector Fermions

Many models of Beyond the Standard Model physics involve heavy colored fermions. We study models where the new fermions have vector interactions and examine the connection between electroweak precision measurements and Higgs production. In particular, for parameters which are allowed by precision measurements, we show that the gluon fusion Higgs cross section and the Higgs decay branching ratios must be close to those predicted by the Standard Model. The models we discuss thus represent scenarios with new physics which will be extremely difficult to distinguish from the minimal Standard Model. We pay particular attention to the decoupling properties of the vector fermions.Comment: 34 pages, 15 figures. Version accepted for publication in Phys. Rev.

Hadronic Contributions to the Photon Vacuum Polarization and their Role in Precision Physics

I review recent evaluations of the hadronic contribution to the shift in the fine structure constant and to the anomalous magnetic moment of the muon. Substantial progress in a precise determination of these important observables is a consequence of substantially improved total cross section measurement by the CMD-2 and BES II collaborations and an improved theoretical understanding. Prospects for further possible progress is discussed.Comment: 17 pages 7 figures 2 tables, update: incl. CMD-2 data, reference

Four-fermion production with RACOONWW

RACOONWW is an event generator for e+e- --> WW --> 4fermions(+gamma) that includes full tree-level predictions for e+e- --> 4f and e+e- --> 4f+gamma as well as O(alpha) corrections to e+e- --> 4f in the so-called double-pole approximation. We briefly sketch the concept of the calculation on which this generator is based and present some numerical results.Comment: 9 pages, latex, 6 postscript files, to appear in the proceedings of the UK Phenomenology Workshop on Collider Physics, Durham, UK, 19-24 September, 199

Two-flavour Schwinger model with dynamical fermions in the L\"uscher formalism

We report preliminary results for 2D massive QED with two flavours of Wilson fermions, using the Hermitean variant of L\"uscher's bosonization technique. The chiral condensate and meson masses are obtained. The simplicity of the model allows for high statistics simulations close to the chiral and continuum limit, both in the quenched approximation and with dynamical fermions.Comment: Talk presented at LATTICE96(algorithms), 3 pages, 3 Postscript figures, uses twoside, fleqn, espcrc2, epsf, revised version (details of approx. polynomial

Beautiful Baryons from Lattice QCD

We perform a lattice study of heavy baryons, containing one ($\Lambda_b$) or two $b$-quarks ($\Xi_b$). Using the quenched approximation we obtain for the mass of $\Lambda_b$ $$M_{\Lambda_b}= 5.728 \pm 0.144 \pm 0.018 {\rm GeV}.$$ The mass splitting between the $\Lambda_b$ and the B-meson is found to increase by about 20\% if the light quark mass is varied from the chiral limit to the strange quark mass.Comment: 11 pages, Figures obtained upon request from [email protected]

New GUT predictions for quark and lepton mass ratios confronted with phenomenology

Group theoretical factors from GUT symmetry breaking can lead to predictions for the ratios of quark and lepton masses (or Yukawa couplings) at the unification scale. Due to supersymmetric (SUSY) threshold corrections the viability of such predictions can depend strongly on the SUSY parameters. For three common minimal SUSY breaking scenarios with anomaly, gauge and gravity mediation we investigate which GUT scale ratios $m_e/m_d$, $m_\mu/m_s$, $y_\tau/y_b$ and $y_t/y_b$ are allowed when phenomenological constraints from electroweak precision observables, $B$ physics, $(g-2)_\mu$, mass-limits on sparticles from direct searches as well as, optionally, constraints from the observed dark matter density are taken into account. We derive possible new predictions for the GUT scale mass ratios and compare them with the phenomenologically allowed ranges. We find that new GUT scale predictions such as $m_\mu/m_s = 9/2$ or 6 and $y_\tau/y_b = 3/2$ or 2 are often favoured compared to the ubiquitous relations $m_\mu/m_s = 3$ or $y_\tau/y_b =1$. They are viable for characteristic SUSY scenarios, testable at the CERN LHC and future colliders.Comment: 33 pages, 5 figures; references added; version to appear in Phys. Rev.

Adaptive Step Size for Hybrid Monte Carlo Algorithm

We implement an adaptive step size method for the Hybrid Monte Carlo a lgorithm. The adaptive step size is given by solving a symmetric error equation. An integr ator with such an adaptive step size is reversible. Although we observe appreciable variations of the step size, the overhead of the method exceeds its benefits. We propose an explanation for this phenomenon.Comment: 13 pages, 5 Postscript figures, late

Measuring the FSR--inclusive pi+pi- cross section

Final state radiation (FSR) in pion--pair production cannot be calculated reliably because of the composite structure of the pions. However, FSR corrections have to be taken into account for a precise evaluation of the hadronic contribution to g-2 of the muon. The role of FSR in both energy scan and radiative return experiments is discussed. It is shown how FSR influences the pion form factor extraction from experimental data and, as a consequence, the evaluation of a_mu^had. In fact the O(alpha) FSR corrections should be included to reach the precision we are aiming at. We argue that for an extraction of the desired FSR--inclusive cross section sigma^(gamma)_had a photon--inclusive scan measurement of the ``e+e- to pi+pi- + photons'' cross section is needed. For exclusive scan and radiative return measurements in contrast we have to rely on ad hoc FSR models if we want to obtain either sigma^(gamma)_had or the FSR--exclusive cross section sigma^(0)_had. We thus advocate to consider seriously precise photon--inclusive energy scan measurements at present and future low energy e+e- facilities. Then together with radiative return measurements from DAFNE and BABAR and forthcoming scan measurements at VEPP-2000 we have a good chance to substantially improve the evaluation of a_mu^had in the future.Comment: 18 pages, 13 Figure

Pion pole light-by-light contribution to g-2 of the muon in a nonlocal chiral quark model

We calculate the pion pole term of the light-by-light contribution to the $g-2$ of the muon in the framework of an effective chiral quark model with instanton-like nonlocal quark--quark interaction. The full kinematic dependence of the pion-photon transition form factors is taken into account. The dependence of form factors on the pion virtuality decreases the result by about 15% in comparison to the calculation where this dependence is neglected. Further, it is demonstrated that the QCD constraints suggested by Melnikov and Vainshtein are satisfied within the model. The corresponding contributions originate from the box diagram as well from the pion-pole term. Our chiral nonlocal model result for the pion-pole light-by-light contribution to $(g-2)/2$ of the muon is $(6.3-6.7) \cdot10^{-10}$, which is in the ball park of other effective-model calculations.Comment: 15 pages, 7 figure

Leading-order calculation of hadronic contributions to the muon g−2g-2 using the Dyson-Schwinger approach

We present a calculation of the hadronic vacuum polarization (HVP) tensor within the framework of Dyson--Schwinger equations. To this end we use a well-established phenomenological model for the quark-gluon interaction with parameters fixed to reproduce hadronic observables. From the HVP tensor we compute both the Adler function and the HVP contribution to the anomalous magnetic moment of the muon, $a_\mu$. We find $a_\mu^{HVP}= 6760\times 10^{-11}$ which deviates about two percent from the value extracted from experiment. Additionally, we make comparison with a recent lattice determination of $a_\mu^{HVP}$ and find good agreement within our approach. We also discuss the implications of our result for a corresponding calculation of the hadronic light-by-light scattering contribution to $a_\mu$.Comment: 9 pages, 8 figure