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D. Zeppenfeld

Publications and source records attributed to D. Zeppenfeld.

At least 91 records · Page 5Linked to original sources
arXiv1997

$Wγγ$ Production at the Fermilab Tevatron Collider: Gauge Invariance and Radiation Amplitude Zero

The electroweak process $p\bar p \to \ell^\pmνγγ$ is calculated at tree level, including finite W width effects. In order to obtain a gauge invariant amplitude, the imaginary parts of $WWγ$ triangle graphs and $WWγγ$ box diagrams have to be included, in addition to resumming the imaginary contributions to the W polarization. We demonstrate the existence of a radiation amplitude zero in $p\bar p \to W^\pm γγ\to \ell^\pmνγγ$, and discuss how it may be observed in correlations of the $γγ$ and lepton rapidities at the Fermilab Tevatron.

hep-ph↗
arXiv1996

Multijet Structure of High E_T Hadronic Collisions

Multijet events at large transverse energy (sum E_T > 420 GeV) and large multijet invariant mass (M_jets > 600 GeV) have been studied by the CDF Collaboration at the Fermilab Tevatron. The observed jet multiplicity distribution can be understood in a QCD inspired exponentiation model, in regions of phase space which require going beyond fixed order perturbation theory.

hep-ph↗
arXiv1996

Anomalous Gauge Boson Couplings

The measurement of anomalous gauge boson self couplings is reviewed for a variety of present and planned accelerators. Sensitivities are compared for these accelerators using models based on the effective Lagrangian approach. The sensitivities described here are for measurement of generic parameters kappa_v, lambda_v, etc., defined in the text. Pre-LHC measurements will not probe these couplings to precision better than O(1/10). The LHC should be sensitive to better than O(1/100), while a future NLC should achieve sensitivity of O(1/1000) to O(1/10000) for center of mass energies ranging from 0.5 to 1.5 TeV.

hep-ph↗
arXiv1996

Probing color-singlet exchange in $Z+2$-jet events at the LHC

The purely electroweak process $qq\to qqZ$ (via $t$-channel $γ/Z$ or $W$ exchange) provides a copious and fairly clean source of color-singlet exchange events in $pp$ collisions at the LHC. A judicious choice of phase-space region allows the suppression of QCD backgrounds to the level of the signal. The color-singlet-exchange signal can be distinguished from QCD backgrounds by the radiation patterns of additional minijets in individual events. A rapidity-gap trigger at the minijet level substantially enhances the signal versus the background. Analogous features of weak boson scattering events make $Z+2$-jet events at the LHC an ideal laboratory for investigation of the soft-jet activity expected in weak-boson scattering events.

hep-ph↗
arXiv1996

Developments in Perturbative QCD: Challenges from Collider Physics

The search for new phenomena at hadron colliders requires a good understanding of QCD processes. The analysis of multi-jet signatures in the top quark search at the Tevatron is one example, forward jet-tagging and rapidity gap techniques in the analysis of weak boson scattering events at the LHC will be another important application. These topics are discussed in the context of multi-parton/multi-jet QCD processes. Also described are some of the calculational tools, like amplitude techniques and automatic code generation for tree level processes.

hep-ph↗
arXiv1996

Triple Gauge Boson Couplings

We present the results obtained by the "Triple Gauge Couplings" working group during the LEP2 Workshop (1994-1995). The report concentrates on the measurement of $WWγ$ and $WWZ$ couplings in $e^-e^+\to W^-W^+$ or, more generally, four-fermion production at LEP2. In addition the detection of new interactions in the bosonic sector via other production channels is discussed.

hep-ph↗
arXiv1995

Probing Trilinear Gauge Boson Couplings at Colliders

A direct measurement of the trilinear $WWγ$ and $WWZ$ couplings is possible in the pair production of electroweak bosons at $e^+e^-$ and hadron colliders. This talk addresses some of the theoretical issues: the parameterization of ``anomalous couplings'' in terms of form factors and effective Lagrangians, the complementary information which can be obtained in $e^+e^-$ vs. hadron collider experiments, and a novel way to implement finite $W$-width effects in a gauge invariant manner.

hep-ph↗
arXiv1995

ANOMALOUS GAUGE BOSON INTERACTIONS

We discuss the direct measurement of the trilinear vector boson couplings in present and future collider experiments. The major goals of such experiments will be the confirmation of the Standard Model (SM) predictions and the search for signals of new physics. We review our current theoretical understanding of anomalous trilinear gauge boson self-interactions. If the energy scale of the new physics is $\sim 1$ TeV, these low energy anomalous couplings are expected to be no larger than ${\cal O}(10^{-2})$. Constraints from high precision measurements at LEP and low energy charged and neutral current processes are critically reviewed.

hep-ph↗
arXiv1995

Finite Width Effects and Gauge Invariance in Radiative $W$ Production and Decay

The naive implementation of finite width effects in processes involving unstable particles can violate gauge invariance. For the example of radiative $W$ production and decay, $q\bar q' \to \ellνγ$, at tree level, it is demonstrated how gauge invariance is restored by including the imaginary part of triangle graphs in addition to resumming the imaginary contributions to the $W$ vacuum polarization. Monte Carlo results are presented for the Fermilab Tevatron.

hep-ph↗
arXiv1994

Minijet veto: a tool for the heavy Higgs search at the LHC

The distinct color flow of the $qq\to qqH,\; H\to W^+W^-$ process leads to suppressed radiation of soft gluons in the central region, a feature which is not shared by major background processes like $t\bar t$ production or $q\bar q \to W^+W^-$. For the leptonic decay of a heavy Higgs boson, $H\to W^+W^- \to \ell^+ν\ell^-\barν$, it is shown that these backgrounds are typically accompanied by minijet emission in the 20--40 GeV range. A central minijet veto thus constitutes a powerful background rejection tool. It may be regarded as a rapidity gap trigger at the semihard parton level which should work even at high luminosities.

hep-ph↗
arXiv1994

A determination of the QED coupling at the $Z$ pole

We critically examine the evaluation of the hadronic contribution to the running of the QED coupling, $α$, from $Q^2=0$ to $Q^2=M_Z^2$. Using data for $e^+e^-\to{}$hadrons we find that $α(M_Z^2)^{-1} = 128.99\pm 0.06$, as compared to the existing value of $128.87\pm 0.12$.

hep-ph↗
arXiv1994

Gluon Radiation Patterns in Pomeron Exchange Events

Color singlet two gluon exchange provides a perturbative model for the pomeron. This mechanism is thought to explain the production of rapidity gaps in hard dijet events at the Tevatron. It is shown that in $qQ$ scattering via two gluon color singlet exchange the emission of soft gluons follows closely the pattern found for $t$-channel photon exchange. Gluon emission is strongly suppressed between the two quark jets. After hadronization this leads to a depressed level of hadronic activity between the jets and thus allows the formation of rapidity gaps.

hep-ph↗
arXiv1994

Evidence for a Hard Pomeron in Perturbative QCD

The $t$-channel exchange of two gluons in a color singlet state represents the lowest order approximation to the Pomeron. This exchange mechanism is thought to also explain the formation of rapidity gaps in dijet events at the Tevatron. At the perturbative level this requires suppressed gluon emission in the rapidity interval between widely separated jets, analogous to color coherence effects in $t$-channel photon exchange. By calculating the imaginary part of the two gluon, color singlet exchange amplitude we show how this pattern does emerge for gluon emission at small transverse momenta. At large $p_T$ the radiation pattern characteristic for color octet single gluon exchange is reproduced.

hep-ph↗
arXiv1993

Heavy Higgs Boson Production in Association with Three Jets at Hadron Supercolliders

We consider the real emission QCD correction to heavy Higgs boson production via weak boson fusion in high energy $pp$ collisions. The ${\cal O}(α_s)$ corrections are determined for the complete electroweak $q q \rightarrow q q W^+ W^-$ process. The presence of a third parton in the final state affects the formation of rapidity gaps only slightly. In particular, soft emission into the gap region is severely suppressed. Also, we investigate how the additional hard emission affects forward-jet-tagging and central-jet-vetoing efficiencies in the search for $H \rightarrow W^+ W^- \rightarrow \ell^+ ν\ell^- \barν$ decays.

hep-ph↗
arXiv1993

Measuring Three Vector Boson Couplings in $qq\to qqW$ at the SSC

We investigate the electroweak process $qq\to qqW$, {\it i.e.} $W$ production via $Wγ$ and $WZ$ fusion, as a probe for nonstandard $WWγ$ and $WWZ$ vertices at hadron supercolliders. Triggering on events with one very forward and one very backward jet while requiring the $W$ decay lepton to be central, strongly enhances the triple gauge boson vertex contribution and suppresses backgrounds from QCD $Wjj$ events and $t\bar tj$ production below the signal level. At the SSC, the process is sensitive to $WWV$, $V=γ\, ,Z$ couplings $κ_V -1$, $λ_V$, and $g_1^Z$ in the $0.03\dots 0.1$ range.

hep-ph↗
arXiv1993

Anomalous Higgs Boson Production and Decay

A non-standard symmetry breaking sector may lead to derivative couplings of the Higgs boson and thereby to anomalous interactions with the electroweak gauge bosons. In the framework of gauge-invariant effective Lagrangians the resulting changes in Higgs boson production and decay mechanisms are related to anomalous triple vector boson couplings. Using low energy constraints on the dimension-six operators in the effective Lagrangian, we discuss the size of deviations from SM predictions which may be expected in Higgs boson production and decay rates. Large enhancements are allowed {\it e.g.} in the $Z \to H γ$ and $H\to γγ$ partial decay widths, leading to $Z\to γγγ$ events at LEP.

hep-ph↗
arXiv1993

Low Energy Constraints and Anomalous Triple Gauge Boson Couplings

Low energy (1-loop) constraints on anomalous triple gauge boson vertices (TGV's) are revisited and compared to the sensitivity achievable at LEP II and at future linear $e^+e^-$ colliders. The analysis is performed within the framework of an effective Lagrangian of gauge invariant dimension six operators with the gauge bosons and a single Higgs doublet field as the low energy degrees of freedom. The low energy data do not directly bound TGV's but they provide strong constraints on models which lead to anomalous gauge boson interactions in addition to other low energy effects.

hep-ph↗