Ital ZZ Research Articles

Using neural networks to enhance the Higgs boson signal at hadron colliders.

Neural networks are used to help distinguish the {ital ZZ}{r_arrow}{ital l}{sup +}{ital l}{sup {minus}}-jet-jet signal produced by the decay of a 400 GeV Higgs boson at a proton-proton collider energy of 15 TeV from the {open_quote}{open_quote}ordinary{close_quote}{close_quote} QCD {ital Z}+jets background. The ideal case where only one event at a time enters the detector (no pileup) and the case of multiple interactions per beam crossing (pileup) are examined. In both cases, when used in conjunction with the standard cuts, neural networks provide an additional signal-to-background enhancement. {copyright} {ital 1996 The American Physical Society.}

Limits on Z-photon couplings from p-p-bar interactions at sqrt s=1.8TeV.

We report limits on anomalous [ital ZZ][gamma] and [ital Z][gamma][gamma] couplings derived from an analysis of [ital Z]+photon production based upon approximately 20pb[sup [minus]1] of the [bar p] collision data recorded by the Collider Detector at Fermilab. We observe good agreement with the standard model expectation that all [ital Z][gamma] couplings are zero. Confidence level intervals for individual couplings and contours for pairs of [ital CP]-conserving/violating [ital ZZ][gamma] and [ital Z][gamma][gamma] couplings are presented.

W-boson-loop versus ultraheavy-fermion-loop contributions to the process gamma gamma -->ZZ.

The computation of the cross section for [gamma][gamma][r arrow][ital ZZ] to one-loop order in the standard model with [ital m][sub Higgs]=100 GeV shows that the [ital W]-loop contribution dominates the unpolarized cross section. A signal of a hypothetical fourth family of ultraheavy particles could be detected at energies below the fourth-family mass only if both final [ital Z] are longitudinal. The mass, however, could not be determined because the results are insensitive to its value.

Detection of the heavy Higgs boson at gamma gamma colliders.

We consider the possibility of detecting a heavy Higgs boson (${m}_{H}>2{m}_{Z}$) in proposed $\ensuremath{\gamma}\ensuremath{\gamma}$ colliders through the semileptonic mode $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}H\ensuremath{\rightarrow}\mathrm{ZZ}\ensuremath{\rightarrow}q\overline{q}{l}^{+}{l}^{\ensuremath{-}}$. We show that due to the nonmonochromatic nature of the photon beams produced by the laser-backscattering method, the resultant cross section for Higgs production is much smaller than the on-resonance cross section, and generally decreases with increasing collider energy. Although continuum $\mathrm{ZZ}$ production is expected to be negligible, we demonstrate the presence of, and calculate sizable backgrounds from, $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}Z$, $q\overline{q}Z$, with $Z\ensuremath{\rightarrow}q\overline{q}, {l}^{+}{l}^{\ensuremath{-}}$ respectively, and $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}t\overline{t}\ensuremath{\rightarrow}b\overline{b}{l}^{+}{l}^{\ensuremath{-}}\ensuremath{\nu}\overline{\ensuremath{\nu}}$. This channel may be used to detect a Higgs boson of mass ${m}_{H}$ up to around 350 GeV at a 0.5 TeV ${e}^{+}{e}^{\ensuremath{-}}$, collider, assuming a nominal yearly luminosity of 10-20 ${\mathrm{fb}}^{\ensuremath{-}1}$.

Majoron models and the Higgs boson search.

If the SU(2)[times]U(1) model is extended by adding a complex singlet spin-zero field and a spontaneously broken global U(1) symmetry, a Higgs scalar can decay into a pair of Goldstone bosons. This decay could dominate over the conventional decay modes into fermion-antifermion pairs. In that case, the Higgs boson may escape detection in the standard search through [ital ZZ][sup *][ital H] coupling and hence could be lighter than implied by the CERN LEP data. Restrictions on the Higgs boson mass and other parameters of the model are derived using the data on the standard model Higgs boson search in the mass region 10--50 GeV.

Searching for the CP-odd Higgs boson of the minimal supersymmetric model at hadron supercolliders.

The {ital CP}-odd Higgs boson {ital A}{sup 0} of a nonminimal Higgs model possesses no tree-level couplings to vector-boson pairs. As a result, many of the techniques employed to search for the standard model Higgs boson at the Superconducting Super Collider (SSC) and CERN Large Hadron Collider (LHC) are not likely to be useful in searches for the {ital A}{sup 0}. This paper focuses on the phenomenology of {ital A}{sup 0} production and decay in the minimal supersymmetric model. We evaluate a comprehensive set of branching ratios for {ital A}{sup 0} (which includes all two-body tree-level and one-loop decays into nonsupersymmetric final states), including the effects of leading-log radiative corrections. The one-loop decay {ital A}{sup 0}{r arrow}{ital ZZ} leads to the gold-plated'' signature {ital ZZ}{r arrow}{ital l}{sup +}{ital l{minus}}{ital l+}{ital l{minus}} and provides an observable signature for {ital A}{sup 0} production at the SSC and LHC only over a very narrow region of parameter space. More promising signatures for the {ital A}{sup 0} are also discussed.

Searching for CP-even Higgs bosons of the minimal supersymmetric model at hadron supercolliders.

The {ital CP}-odd Higgs boson {ital A}{sup 0} of a nonminimal Higgs model possesses no tree-level couplings to vector-boson pairs. As a result, many of the techniques employed to search for the standard model Higgs boson at the Superconducting Super Collider (SSC) and CERN Large Hadron Collider (LHC) are not likely to be useful in searches for the {ital A}{sup 0}. This paper focuses on the phenomenology of {ital A}{sup 0} production and decay in the minimal supersymmetric model. We evaluate a comprehensive set of branching ratios for {ital A}{sup 0} (which includes all two-body tree-level and one-loop decays into nonsupersymmetric final states), including the effects of leading-log radiative corrections. The one-loop decay {ital A}{sup 0}{r arrow}{ital ZZ} leads to the gold-plated'' signature {ital ZZ}{r arrow}{ital l}{sup +}{ital l{minus}}{ital l+}{ital l{minus}} and provides an observable signature for {ital A}{sup 0} production at the SSC and LHC only over a very narrow region of parameter space. More promising signatures for the {ital A}{sup 0} are also discussed.

Production cross sections for the multi-Higgs-boson process e+e--->Hi0H+H-

The production cross sections for the process {ital e}{sup +}{ital e{minus}}{r arrow}{ital H}{sub {ital i}}{sup 0}{ital H+}{ital H{minus}} are calculated in the two-Higgs-doublet model. We consider the situations in which {ital H}{sub {ital i}}{sup 0} could be produced either from the propagator exchange or from the initial legs or the final legs. At {radical}{ital s} =300 GeV, this process can be detected if the mass of the Higgs boson {ital H}{sub {ital i}}{sup 0} is smaller than {ital M}{sub {ital Z}}. Our calculations show that the situation in which the propagator exchange is {ital ZZ} becomes the dominant one, while the other situations are negligible.

Order- alpha s calculation of hadronic ZZ production.

An order-{alpha}{sub {ital s}} calculation of {ital pp}{sup ({minus})}{r arrow}{ital ZZ}+{ital X} is presented. Results are given for the total cross section and differential distributions for Fermilab Tevatron, CERN Large Hadron and Collider, and Superconducting Super Collider energies. The calculation utilizes a combination of analytic and Monte Carlo integration methods which makes it easy to calculate a variety of observables and to impose experimental cuts.

Resonant Raman scattering in YBa2Cu3O7: Band theory and experiment.

We have measured and calculated the dependence of the Raman phonon intensities of YBa{sub 2}Cu{sub 3}O{sub 7} on laser frequency. The local-density approximation and the linear muffin-tin-orbital method were used for the calculations. We find good agreement not only in the absolute phonon intensities in {ital zz} or {ital xx}/{ital yy} polarization but also in the shape of the resonance profiles. These results indicate that the local-density approximation gives an essentially correct description of the band structure and of the electron-phonon interaction {plus minus}2 eV around the Fermi energy. We interpret our data in terms of specific interband transitions and estimate the mode mixing.

Production of top quarks via vector-boson fusion in e+e- collisions.

The production of top quarks at an {ital e}{sup +}{ital e}{sup {minus}} collider with a center-of-mass energy of 2 TeV is discussed. Production cross sections for vector-boson fusion processes are calculated using the effective-boson approximation. Included are {gamma}{gamma}, {ital WW}, {gamma}{ital Z}, and {ital ZZ} fusion production of top-quark pairs and production of {ital t}-{ital {bar b}} pairs by {gamma}{ital W} and {ital ZW} fusion. The cross sections for top-quark production from the interactions of beamstrahlung photons are presented.