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In this note we compute the flavor-dependent chiral-logarithmic corrections to the decay Z to b bbar in the three site Higgsless model. We compute these corrections diagrammatically in the gaugeless limit in which the electroweak couplings vanish. We also compute the chiral-logarithmic corrections to the decay Z to b bbar using an RGE analysis in effective field theory, and show that the results agree. In the process of this computation, we compute the form of the chiral current in the gaugeless limit of the three-site model, and consider the generalization to the N-site case. We elucidate the Ward-Takahashi identities which underlie the gaugeless limit calculation in the three-site model, and describe how the result for the Z to b bbar amplitude is obtained in unitary gauge in the full theory. We find that the phenomenological constraints on the three-site Higgsless model arising from measurements of Z to b bbar are relatively mild, requiring only that the heavy Dirac fermion be heavier than 1 TeV or so, and are satisfied automatically in the range of parameters allowed by other precision electroweak data.
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We present NLO QCD results for W/Z gauge boson production with bottom quark pairs at the Tevatron including full bottom-quark mass effects. We study the impact of QCD corrections on both total cross-section and invariant mass distribution of the bottom-quark pair. Including NLO QCD corrections greatly reduces the dependence of the tree-level cross-section on the renormalization and factorization scales. We also compare our calculation to a calculation that considers massless bottom quarks and find that the bottom-quark mass effects amount to about 8-10% of the total NLO QCD cross-section and can impact the shape of the bottom-quark pair invariant mass distribution, in particular in the low invariant mass region.
Recently, Higgsless models have proven to be viable alternatives to the Standard Model (SM) and supersymmetric models in describing the breaking of the electroweak symmetry. Whether extra-dimensional in nature or their deconstructed counterparts, the physical spectrum of these models typically consists of ``towers of massive vector gauge bosons which carry the same quantum numbers as the SM W and Z. In this paper, we calculate the one-loop, chiral-logarithmic corrections to the S and T parameters from the lightest (i.e. SM) and the next-to-lightest gauge bosons using a novel application of the Pinch Technique. We perform our calculation using generic Feynman rules with generic couplings such that our results can be applied to various models. To demonstrate how to use our results, we calculate the leading chiral-logarithmic corrections to the S and T parameters in the deconstructed three site Higgsless model. As we point out, however, our results are not exclusive to Higgsless models and may, in fact, be used to calculate the one-loop corrections from additional gauge bosons in models with fundamental (or composite) Higgs bosons.
The forward-backward (FB) asymmetry of $b$ quarks in $e^+e^-$ collisions at the Z pole measured at LEP, $A_{FB}^{0,b} = 0.0992pm0.0016$, remains today the electroweak precision observable with the largest disagreement (2.4$sigma$) with respect to the Standard Model prediction, $(A_{FB}^{0,b})_{_{rm th}} = 0.1030 pm 0.0002$. Beyond the dominant statistical uncertainties, QCD effects, such as $b$-quark showering and hadronization, are the leading sources of $A_{FB}^{0,b}$ systematic uncertainty, and have not been revised in the last twenty years. We reassess the QCD uncertainties of the eight original $A_{FB}^{0,b}$ LEP measurements, using modern parton shower PYTHIA-8 and PYTHIA-8 + VINCIA simulations with nine different implementations of soft and collinear radiation as well as of parton fragmentation. Our analysis, combined with NNLO massive $b$-quark corrections independently computed recently, indicates total propagated QCD uncertainties of $sim$0.7% and $sim$0.3% for the lepton-charge and jet-charge analyses, respectively, that are about a factor of two smaller than those of the original LEP results. Accounting for such updated QCD effects leads to a new $A_{FB}^{0,b} = 0.0996pm0.0016$ average, with a data-theory tension slightly reduced from 2.4$sigma$ to 2.1$sigma$. Confirmation or resolution of this long-term discrepancy requires a new high-luminosity $e^+e^-$ collider collecting orders-of-magnitude more data at the Z pole to significantly reduce the $A_{FB}^{0,b}$ statistical uncertainties.
The study of the Higgs boson properties is one of the most important tasks to be accomplished in the next years, at the Large Hadron Collider (LHC) and at future colliders such as the Future Circular Collider in hadron-hadron mode (FCC-hh), the potential 100 TeV follow-up of the LHC machine. In this view the precise study of the Higgs couplings to weak gauge bosons is crucial and requires as much information as possible. After the recent calculation of the next-to-leading order QCD corrections to the production cross sections and differential distributions of a Standard Model Higgs boson in association with a pair of weak bosons, matched with parton shower in the POWHEG-BOX framework, we present the gluon fusion correction $g gto H W^+_{} W^-_{} ( H Z Z)$ to the process $p p to H W^+_{} W^-_{} (H Z Z)$. This correction can be sizeable and amounts to $+3,%$ ($+10,%$) in the $H W^+_{} W^-_{}$ process and $+5,%$ ($+18,%$) in the $H Z Z$ process at the LHC (FCC-hh). We also present the first study of the impact of the bottom--quark initiated channels $bbar{b}to H W^+_{} W^-_{} / H Z Z$ and find that they induce a significant $+18,%$ correction in the $H W^+_{} W^-_{}$ channel at the FCC-hh. We present results on total cross sections and distributions at the LHC and at the FCC-hh.
We study the Yukawa corrections to the gamma gamma --> bbar{b} corss section in the topcolor assisted technicolor models at the photon-photon colliders. We find that, for the favorable parameters, the relative corrections from pseudo Goldstone bosons give out a 3.2%--5.9% decrement of the cross section from the tree level when sqrt{s}=500 GeV, the contributions from new extended technicolor gauge bosons Z* and colored gauge bosons B are negligibly small, and the relative correction arising from new color-singlet heavy gauge boson Z is less than -3.2%. Therefore, the total relative corrections are significantly larger than the corresponding corrections in the standard model, the general two Higgs doublet model and the minimal supersymmetric standard model. Since these corrections are obvious for the International Linear Colliders, the process gamma gamma --> bbar{b} is really interesting in testing the standard model and searching for the signs of technicolor.
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