arXiv subjects

U. Baur

Publications and source records attributed to U. Baur.

71 records · Page 4Linked to original sources

arXiv1995

QCD Corrections and Non-standard Three Vector Boson Couplings in $W^+W^-$ Production at Hadron Colliders

The process $p\,p\hskip-7pt\hbox{$^{^{(\!-\!)}}$} \rightarrow W^{+} W^{-} + X \rightarrow \ell^+_1 ν_1 \ell^-_2 \bar ν_2 + X$ is calculated to ${\cal O}(α_s)$ for general $C$ and $P$ conserving $WWV$ couplings ($V=γ,\, Z$). The prospects for probing the $WWV$ couplings in this reaction are explored. The impact of ${\cal O}(α_s)$ QCD corrections and various background processes on the observability of non-standard $WWV$ couplings in $W^+ W^-$ production at the Tevatron and the Large Hadron Collider (LHC) is discussed in detail. Sensitivity limits for anomalous $WWV$ couplings are derived at next-to-leading order for the Tevatron and LHC center of mass energies, and are compared to the bounds which can be achieved in other processes. Unless a jet veto or a cut on the total transverse momentum of the hadrons in the event is imposed, the ${\cal O}(α_s)$ QCD corrections and the background from top quark production decrease the sensitivity of $p\,p\hskip-7pt\hbox{$^{^{(\!-\!)}}$} \rightarrow W^{+} W^{-} + X \rightarrow \ell^+_1 ν_1 \ell^-_2 \bar ν_2 + X$ to anomalous $WWV$ couplings by a factor two to five.

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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.

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arXiv1994

$WZ$ Production at Hadron Colliders: Effects of Non-Standard $WWZ$ Couplings and QCD Corrections

The process $p\,\pbar \rightarrow W^{\pm}Z + X \rightarrow \ell^\pm_1 ν_1 \ell_2^+ \ell_2^- + X$ is calculated to ${\cal O}(α_s)$ for general $C$ and $P$ conserving $WWZ$ couplings. At the Tevatron center of mass energy, the QCD corrections to $WZ$ production are modest. At Large Hadron Collider (LHC) energies, the inclusive QCD corrections are large, but can be reduced significantly if a jet veto is imposed. Sensitivity limits for the anomalous $WWZ$ couplings are derived from the next-to-leading order $Z$ boson transverse momentum distribution for Tevatron and LHC energies. Unless a jet veto is imposed, next-to-leading order QCD corrections decrease the sensitivity to anomalous $WWZ$ couplings considerably at LHC energies, but have little influence at the Tevatron. We also study, at next-to-leading order, rapidity correlations between the $W$ and $Z$ decay products, and the $ZZ/WZ$ and $WZ/Wγ$ cross section ratios. These quantities are found to be useful tools in searching for the approximate zero present in the Standard Model $WZ$ helicity amplitudes. The prospects for observing the approximate amplitude zero at the Tevatron and the LHC are critically assessed.

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arXiv1994

Amplitude Zeros and Rapidity Correlations in $Wγ$ and $WZ$ Production in Hadronic Collisions

A comparative study of amplitude zeros in $Wγ$ and $WZ$ production in hadronic collisions is presented. The Standard Model amplitude for $q_1 \bar q_2 \rightarrow W^\pm Z $ at the Born-level is shown to exhibit an approximate zero located at $\cosθ= (g^{q_1}_{-} + g^{q_2}_{-}) / (g^{q_1}_{-} - g^{q_2}_{-})$ at high energies, where the $g^{q_i}_{-}$ ($i=1,2$) are the left-handed couplings of the $Z$-boson to quarks and $θ$ is the center of mass scattering angle of the $W$-boson. This approximate zero is similar to the well-known radiation zero in $Wγ$ production. Prospects to observe the amplitude zeros using rapidity correlations between the final state particles are explored.

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arXiv1994

Amplitude Zeros in $W^\pm Z$ Production

We demonstrate that the Standard Model amplitude for $f_1 \bar f_2 \rightarrow W^\pm Z $ at the Born-level exhibits an approximate zero located at $\cosθ= (g^{f_1}_{-} + g^{f_2}_{-}) / (g^{f_1}_{-} - g^{f_2}_{-})$ at high energies, where the $g^{f_i}_{-}$ ($i=1,2$) are the left-handed couplings of the $Z$-boson to fermions and $θ$ is the center of mass scattering angle of the $W$-boson. The approximate zero is the combined result of an exact zero in the dominant helicity amplitudes ${\cal M}(\pm,\mp)$ and strong gauge cancelations in the remaining amplitudes. For non-standard $WWZ$ couplings these cancelations no longer occur and the approximate amplitude zero is eliminated.

hep-ph↗

arXiv1994

Rapidity Correlations in $Wγ$ Production at Hadron Colliders

We study the correlation of photon and charged lepton pseudorapidities, $η(γ)$ and $η(\ell)$, $\ell=e,\,μ$, in $p\,\pbar \rightarrow W^\pmγ+X\rightarrow \ell^\pm p\llap/_Tγ+X$. In the Standard Model, the $Δη(γ,\ell)= η(γ) - η(\ell)$ differential cross section is found to exhibit a pronounced dip at $Δη(γ,\ell) \approx \mp 0.3$ ($=0$)in $p\bar p$ ($pp$) collisions, which originates from the radiation zero present in $q\bar q'\rightarrow Wγ$. The sensitivity of the $Δη(γ,\ell)$ distribution to higher order QCD corrections, non-standard $WWγ$ couplings, the $W+$~jet ``fake'' background and the cuts imposed is explored. At hadron supercolliders, next-to-leading order QCD corrections are found to considerably obscure the radiation zero. The advantages of the $Δη(γ,\ell)$ distribution over other quantities which are sensitive to the radiation zero are discussed. We conclude that photon lepton rapidity correlations at the Tevatron offer a {\it unique} opportunity to search for the Standard Model radiation zero in hadronic $Wγ$ production.

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arXiv1993

Electroweak Physics at Current Accelerators and the Supercollider

The activities of the Electroweak Physics Working Group are summarized. Three main issues are addressed: 1) prospects for measuring the $W$ mass at the Tevatron using the inclusive electron energy spectrum, 2) constraining the strange quark distribution function in $W$ + charm production, and 3) possibilities to determine the three vector boson couplings at the Tevatron and SSC.

hep-ph↗

arXiv1993

THE $t\bar tγ$ BACKGROUND TO $pp\rightarrow Wγ+ X$ AT THE SSC

We calculate the $pp\rightarrow t\bar tγ+X\rightarrow Wγ+ X$ cross section at SSC energies. Approximately 40\% of the total cross section originates from photon bremsstrahlung off the final state jet in $t\bar tj$ production. Without cuts restricting the hadronic activity, the $t\bar tγ$ rate is a factor 10 (2) larger than the tree level $Wγ$ cross section for a top quark mass of 110~GeV (200~GeV). Imposing a jet veto cut, the $t\bar tγ$ rate can be suppressed to a level well below the $Wγ+0$~jet signal cross section.

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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

The Charm Content of W+1 Jet Events as a Probe of the Strange Quark Distribution Function

We investigate the prospects for measuring the strange quark distribution function of the proton in associated $W$ plus charm quark production at the Tevatron. The $W+c$ quark signal produced by strange quark -- gluon fusion, $sg\rightarrow W^-c$ and $\bar sg\rightarrow W^+\bar c$, is approximately 5\% of the inclusive $W+1$ jet cross section for jets with a transverse momentum $p_T(j)>10$~GeV. We study the sensitivity of the $W$ plus charm quark cross section to the parametrization of the strange quark distribution function, and evaluate the various background processes. Strategies to identify charm quarks in CDF and DØ\ are discussed. For a charm tagging efficiency of about 10\% and an integrated luminosity of 30~pb$^{-1}$ or more, it should be possible to constrain the strange quark distribution function from $W+c$ production at the Tevatron.

hep-ph↗

arXiv1993

Rapidity Correlations in $Wγ$ Production at the Tevatron

We study the correlation of photon and charged lepton pseudorapidities, $η(γ)$ and $η(\ell)$, $\ell=e,\,μ$, in $p\bar p\rightarrow W^\pmγ+X\rightarrow \ell^\pm p\llap/_Tγ+X$ at the Tevatron. In the Standard Model, the $Δη(γ,\ell)= η(γ) - η(\ell)$ differential cross section is found to exhibit a pronounced dip at $Δη(γ,\ell) \approx \mp 0.4$, which originates from the radiation zero present in $q\bar q'\rightarrow Wγ$. The sensitivity of the $Δη(γ,\ell)$ distribution to higher order QCD corrections, non-standard $WWγ$ couplings, and the cuts imposed is explored. The $Δη(γ,\ell)$ distribution is compared with other quantities which are sensitive to the radiation zero.

hep-ph↗

arXiv1993

Probing the $WWγ$ Vertex in Radiative $b$-Quark Decays

The recent CLEO results on on radiative $b$-quark decays are used to derive constraints on anomalous $WWγ$ couplings. These constraints are compared with expectations from $p\bar p\rightarrow e^\pm p\llap/_Tγ+X$ at the Tevatron. The usefulness of exclusive radiative $B$ meson decay channels in probing the $WWγ$ vertex is largely limited by present theoretical uncertainties in the calculation of hadronic matrix elements.

hep-ph↗

arXiv1993

Measurement of the Strange Quark Distribution Function in W + Charm Quark Events

We investigate the prospects of measuring the strange quark distribution function at the Tevatron, using $W$ plus charm quark events. The $W$ plus charm quark signal produced by strange quark--gluon fusion, $sg\rightarrow W^-c$ and $\bar sg\rightarrow W^+\bar c$, is approximately 5\% of the inclusive $W+1$ jet cross section for jets with a transverse momentum $p_T(j)>10$~GeV. We study the sensitivity of the $W$ plus charm quark cross section to the parametrization of the strange quark distribution function, and evaluate the various background processes. Strategies to identify charm quarks in CDF and D$0\llap/$ are briefly described.

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arXiv1993

QCD Corrections to Hadronic $Wγ$ Production with Non-standard $WWγ$ Couplings

The process $p \pbar \rightarrow W^{\pm}γ+ X \rightarrow \ell^\pm νγ+ X$ is calculated to ${\cal O}(α_s)$ for general $CP$ conserving $WWγ$ couplings. At the Tevatron center of mass energy, the QCD corrections to $Wγ$ production are modest, and the Born and inclusive ${\cal O}(α_s)$ cross sections have similar sensitivities to the effects of anomalous couplings. At supercollider energies, the inclusive QCD corrections are large at high photon transverse momenta, reducing the sensitivity to non-standard $WWγ$ couplings by up to a factor~2. The size of the QCD corrections can be reduced significantly, and a large fraction of the sensitivity lost can be regained, if a jet veto is imposed.

hep-ph↗

arXiv1993

Ratios of $W^\pmγ$ and $Zγ$ Cross Sections: New Tools in Probing the Weak Boson Sector at the Tevatron

The ratios ${\cal R}_{γ,\ell}=B(Z\to\ell^+\ell^-)\cdotσ(Zγ) /\allowbreak B(W\to\ellν)\cdotσ(W^\pmγ)$, ${\cal R}_{γ, ν}= B(Z\to\barνν)\cdotσ(Zγ)/\allowbreak B(W\to\ellν)\cdotσ(W^ \pmγ)$, ${\cal R}_{Wγ}=σ(W^\pmγ)/\allowbreakσ(W^\pm)$, and ${\cal R}_{Zγ}=σ(Zγ)/\allowbreakσ(Z)$ are studied as tools to probe the electroweak boson self-interactions. As a function of the minimum photon transverse momentum, ${\cal R}_{γ,\ell}$ and ${\cal R}_{ γ,ν}$ are found to directly reflect the radiation zero present in $W^ \pmγ$ production in the Standard Model. All four ratios are sensitive to anomalous $WWγ$ and/or $ZZγ/Zγγ$ couplings. The sensitivity of the cross section ratios to the cuts imposed on the final state particles, as well as the systematic uncertainties resulting from different parametrizations of parton distribution functions, the choice of the factorization scale $Q^2$, and from higher order QCD corrections are explored. Taking into account these uncertainties, sensitivity limits for anomalous three gauge boson couplings, based on a measurement of the cross section ratios with an integrated luminosity of 25~pb$^{-1}$ at the Tevatron, are estimated.

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arXiv1992

Probing the $WWγ$ Vertex in $e^\pm p\toν\gammaX$

We study the prospects of testing the $WWγ$ vertex in $e^- p\toνγX$ and $e^+ p\toνγX$ at HERA and LEP/LHC. Destructive interference effects between the Standard Model and the anomalous contributions to the amplitude severely limit the sensitivity of both processes to non-standard $WWγ$ couplings. Sensitivity limits for the anomalous $WWγ$ couplings $κ$ and $λ$ at HERA and LEP/LHC are derived, taking into account experimental cuts and uncertainties, and the form factor behaviour of nonstandard couplings. These limits are found to be significantly weaker than those which can be expected from other collider processes within the next few years. At HERA, they are comparable to bounds obtained from $S$-matrix unitarity.

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