arXiv subjects
Publications and source records attributed to S. Willenbrock.
We study the signal and background for the process tth, h->WW at the LHC and a 100 TeV VLHC. Signals are studied in two-, three-, and four-lepton final states. We find a statistical uncertainty in the top-quark Yukawa coupling at the LHC of 16%, 8%, 12% for m_h = 130, 160, 190 GeV, respectively. The statistical uncertainty at the VLHC is likely to be negligible in comparison with the systematic uncertainty.
Unitarity of longitudinal weak vector boson scattering implies an upper bound on the scale of electroweak symmetry breaking, $Λ_{EWSB}\equiv \sqrt{8π}v\approx$ 1 TeV. Appelquist and Chanowitz have derived an analogous upper bound on the scale of fermion mass generation, proportional to $v^2/m_f$, by considering the scattering of same-helicity fermions into pairs of longitudinal weak vector bosons in a theory without a standard Higgs boson. We show that there is no upper bound, beyond that on the scale of electroweak symmetry breaking, in such a theory. This result is obtained by considering the same process, but with a large number of longitudinal weak vector bosons in the final state. We further argue that there is no scale of (Dirac) fermion mass generation in the standard model. In contrast, there is an upper bound on the scale of Majorana-neutrino mass generation, given by $Λ_{Maj}\equiv 4πv^2/m_ν$. In general, the upper bound on the scale of fermion mass generation depends on the dimensionality of the interaction responsible for generating the fermion mass. We explore the scale of fermion mass generation in a variety of excursions from the standard model: models with fermions in nonstandard representations, a theory with higher-dimension interactions, a two-Higgs-doublet model, and models without a Higgs boson.
We study the production of the Higgs boson in association with a single top quark at hadron colliders. The cross sections for the three production processes (t-channel, s-channel, and W-associated) at both the Tevatron and the LHC are presented. We investigate the possibility of detecting a signal for the largest of these processes, the t-channel process at the LHC, via the Higgs decay into b b-bar. The QCD backgrounds are large and difficult to curb, hindering the extraction of the signal. Extensions of our analysis to the production of supersymmetric Higgs bosons are also addressed. The cross section is enhanced for large values of tangent beta, increasing the prospects for extracting a signal.
I consider the measurement of the top-quark mass, the CKM matrix element V_{tb}, and the top-quark Yukawa coupling to the Higgs boson at the Tevatron, the LHC, and a Linear Collider. The theoretical motivations for these measurements, as well as the experimental possibilities, are discussed.
We derive a model-independent upper bound on the scale of Majorana-neutrino mass generation. The upper bound is $4πv^2/\sqrt 3 m_ν$, where $v \simeq 246$ GeV is the weak scale and $m_ν$ is the Majorana neutrino mass. For neutrino masses implied by neutrino oscillation experiments, all but one of these bounds are less than the Planck scale, and they are all within a few orders of magnitude of the grand-unification scale.
The top quark, discovered at the Fermilab Tevatron collider in 1995, is the heaviest known elementary particle. Its large mass suggests that it may play a special role in nature. It behaves differently from the other known quarks due both to its large mass and its short lifetime. Thus far we have only crude measurements of the properties of the top quark, such as its mass, weak interactions, strong interactions, and decay modes. These measurements will be made more precise when the Tevatron begins operation again in 2001. I review the present status of these measurements, and discuss their anticipated improvement.
I present an overview of standard-model top-quark physics at the Fermilab Tevatron. Topics discussed include the top-quark mass, weak interaction, strong interaction, and rare decays.
The pole mass of a heavy quark is ambiguous by an amount of order $Λ_{QCD}$. We show that the heavy-quark potential, $V(r)$, is similarly ambiguous, but that the total static energy, $2M_{pole}+V(r)$, is unambiguous when expressed in terms of a short-distance mass. This implies that the extraction of a short-distance mass from the quarkonium spectrum is free of an ambiguity of order $Λ_{QCD}$, in contrast with the pole mass.
We argue that the leading-order subprocess for Higgs-boson production in association with bottom quarks is (b\bar{b} -> H). This process is an important source of Higgs bosons with enhanced Yukawa coupling to bottom quarks. We calculate the corrections to this cross section at next-to-leading-order in 1/ln(m_H/m_b) and alpha_s and at next-to-next-to-leading order in 1/ln(m_H/m_b).
Single-top-quark production probes the charged-current weak interaction of the top quark, and provides a direct measurement of the CKM matrix element V_{tb}. We perform two independent analyses to quantify the accuracy with which the W-gluon fusion (gq -> t\bar{b}q) and (q\bar{q} -> t\bar{b}) signals can be extracted from the backgrounds at both the Tevatron and the LHC. Although perturbation theory breaks down at low transverse momentum for the W-gluon fusion \bar{b} differential cross section, we show how to obtain a reliable cross section integrated over low \bar{b} transverse momenta up to a cutoff. We estimate the accuracy with which V_{tb} can be measured in both analyses, including theoretical and statistical uncertainties. We also show that the polarization of the top quark in W-gluon fusion can be detected at the Fermilab Tevatron and the CERN LHC.
We study the upper bound on the scale of fermion mass generation in a two-Higgs-doublet model. If the model is weakly-coupled, the scale of fermion mass generation is much less than the Appelquist-Chanowitz unitarity bound. However, if we allow some dimensionless Higgs self-couplings to become large, the Appelquist-Chanowitz unitarity bound can be saturated. The unitarity bound on the scale of top-quark mass generation is about 3 TeV, which may be within the reach of future colliders.
Single-top-quark production via W-gluon fusion at hadron colliders provides an opportunity to directly probe the charged-current interaction of the top quark. We calculate the next-to-leading-order corrections to this process at the Fermilab Tevatron, the CERN Large Hadron Collider, and DESY HERA. Using a b-quark distribution function to sum collinear logarithms, we show that there are two independent corrections, of order 1/[ln(m_t^2/m_b^2)] and alpha_s. This observation is generic to processes involving a perturbatively derived heavy-quark distribution function at an energy scale large compared with the heavy-quark mass.
The hypercharges of the fermions are not uniquely determined in SO(10) grand unification, but rather depend upon which linear combination of the two U(1) subgroups of SO(10) > SU(3) X SU(2) X U(1) X U(1) remains unbroken. We show that, in general, a given hypercharge assignment can be obtained only with very high-dimensional Higgs representations. The observation that the standard model is obtained with low-dimensional Higgs representations can therefore be regarded as further evidence for SO(10) grand unification. This evidence is independent of the fact that SO(10) > SU(5).
I discuss two aspects of top-quark physics: V_tb and the top-quark mass. The similarities and differences with bottom and charm physics are emphasized.
We show that the perturbation series for quarkonia energies diverges at large orders. This results in a perturbative ambiguity in the energy that scales as e^(-1/a*Lambda) where a is the Bohr radius of quarkonium and Lambda is the QCD scale parameter. This ambiguity is associated with a nonperturbative contribution to the energy from distances of order 1/Lambda and greater. This contribution is separate from that of the gluon condensate.
``Weakly-coupled'' and ``strongly-coupled'' models of electroweak symmetry breaking are introduced by analogy with the Fermi theory of the weak interaction and the low-energy interaction of pions, respectively. The implications of these two classes of models for colliders beyond the LHC and NLC are discussed.
I review recent theoretical work on top-quark physics within the standard model, beyond the standard model, and at the Planck scale.