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Stephen Godfrey

Publications and source records attributed to Stephen Godfrey.

At least 91 records · Page 5Linked to original sources

What Can We Learn About Leptoquarks At LEP200?

We investigate the discovery potential for first generation leptoquarks at the LEP200 $e^+e^-$ collider. We consider direct leptoquark searches using single leptoquark production via resolved photon contributions which offers a much higher kinematic limit than the more commonly considered leptoquark pair production process. Depending on the coupling strength of the leptoquark, search limits can be obtained to within a few GeV of $\sqrt{s}$. We also consider LQ limits that can be obtained from t-channel interferences effects in $e^+e^- \to hadrons$.

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Search Limits for Extra Neutral Gauge Bosons at High Energy Lepton Colliders

We study and compare the discovery potential for heavy neutral gauge bosons (Z') at the various $e^+e^-$ and $μ^+μ^-$ colliders that have been proposed. Typical search limits for the e^+e^- colliders are 2-10 times sqrt{s} with the large variation reflecting the model dependence of the limits. The search limits for the mu^+ mu^- colliders are slightly lower. Polarization and flavour tagging are important in realizing the highest discovery limits possible. Because the search limits are based on indirect inferences of deviations from standard model predictions, they are sensitive to systematic errors.

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Leptoquark Production and Identification in High Energy mu^+ mu^-, e^+ e^- and e gamma Collisions

We study single leptoquark production in $e^+e^-$, $eγ$, and $μ^+μ^-$ collisions due to the quark content of the photon. Leptoquarks can be produced in substantial numbers for masses very close to the centre of mass energy of the collider which results in equivalently high discovery limits. Using polarization asymmetries in an $eγ$ collider the ten different types of leptoquarks listed by Buchmüller, Rückl and Wyler can be distinquished from one another for leptoquark masses essentially up to the kinematic limit. Thus, if a leptoquark were discovered an $eγ$ collider could play a crucial role in determining its origins.

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t tbar Production via Vector Boson Fusion at High Energy e^+ e^- Colliders

We examine t tbar production via vector boson fusion at high energy e^+ e^- colliders using the effective vector-boson approximation. We show cross sections as functions of CM energy for various Higgs masses ranging from 100 GeV up to 1 TeV, and also for M_H = infinity which corresponds to the LET. We give expressions for sigma(V_i V_j -> t tbar) in the 2M_(W,Z)/sqrt(s) = 0 approximation and show how this approximation effects the results.

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Measurement of the W W gamma and W W Z Couplings in the Process $e^+e^- \to l νq\bar{q}'$

We studied the sensitivity of the process $e^+e^-\to \ell νq\bar{q}'$ to anomalous trilinear gauge boson couplings of the $WWγ$ and WWZ vertices at the center of mass energies $\sqrt{s}=500$ GeV and 1 TeV. The bounds for the couplings we obtained result from an analysis of a five dimensional angular differential cross section. In our calculations we included all tree level Feynman diagrams contributing to the final state as well as the finite widths of the vector bosons. Both unpolarized and polarized beams were considered. We found that the 500 GeV measurements will be at the level of loop contributions to the couplings and may show hints of new physics while the 1 TeV should be sensitive to new physics at the loop level. We also explored $\ellνq\bar{q}$ final states off the W resonance and found that useful information could be extracted from this region of phase space.

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Probing the Heavy Quark Content of the Photon Using b Tagging at High Energy $eγ$ and $e^+e^-$ Colliders

We suggest a method for probing the quark content of the photon using $b$ tagging at high energy $e^+e^-$ and $eγ$ colliders. We find that heavy quark tagging provides a sensitive and effective probe of the quark content of the photon especially in the low $x$-region where the various models differ the most. This process is complementary to others that have been studied in the literature and can contribute to more precise determinations of quark and gluon distributions inside the photon.

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Leptoquark Mass Limits from Single Leptoquark Production at LEP and LEP200

We investigate the discovery potential for first generation leptoquarks at the LEP and LEP200 $e^+e^-$ colliders. We consider single leptoquark production via resolved photon contributions which offers a much higher kinematic limit than the more commonly considered leptoquark pair production process. This process also has the advantage that it is independent of the leptoquark chirality, is almost insensitive to whether the leptoquark is scalar or vector, and only depends on the leptoquark charge. We estimate that from the nonobservation of energetic $e-jet$ events, the limits at LEP for all types of leptoquarks are $M_{LQ} > 90$~GeV. For the just completed $\sqrt{s}=161$~GeV run at LEP200 we estimate potential scalar leptoquark mass limits of $M_{LQ} > 130$~GeV for $Q_{LQ}=-2/3, \; -4/3$ and $M_{LQ} > 148$~GeV for $Q_{LQ}=-1/3, \; -5/3$ and for vector LQ's the limits are 140 and 154~GeV respectively. Ultimately, LEP200 should be able to achieve discovery limits for leptoquarks of $\sim 188$~GeV for the $\sqrt{s}=190$~GeV runs. In all cases we assume electromagnetic strength coupling.

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The $ξ(2220)$ Revistited

We calculated the decay widths of the $1^3F_2$ and $1^3F_4$ $s\bar{s}$ mesons and compared them to the measured properties of the $ξ(2220)$ (now known as the $f_4(2220)$). Including previously neglected decay modes we found that the width of the $^3F_2$ state $s\bar{s}$ meson is much larger than previously believed making this explanation unlikely. On the other hand the predicted width of the $^3F_4$ state, although broader than the observed width, is consistent within the uncertainties of the model. This interpretation predicts large partial widths to $K K^*(892)$ and $K^*(892) K^*(892)$ final states which should be looked for. A second possibility that would account for the different properties of the $ξ(2220)$ seen in different experiments is that two hadronic states exist at this mass. The first would be a broader $^3F_4$ $s\bar{s}$ state which is seen in hadron beam experiments while the second would be a narrow state with high glue content seen in the gluon rich $J/ψ$ radiative decay. Further experimental results are needed to sort this out.

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Properties of the Strange Axial Mesons in the Relativized Quark Model

We studied properties of the strange axial mesons in the relativized quark model. We calculated the $K_1$ decay constant in the quark model and showed how it can be used to extract the $K_1 (^3P_1) - K_1 (^1P_1)$ mixing angle ($θ_K$) from the weak decay $τ\to K_1 ν_τ$. The ratio $BR(τ\to ν_τK_1 (1270))/BR(τ\to ν_τK_1(1400))$ is the most sensitive measurement and also the most reliable since the largest of the theoretical uncertainties factor out. However the current bounds extracted from the TPC/Two-Gamma collaboration measurements are rather weak: we typically obtain $-30^o \lesssim θ_K \lesssim 50^o$ at 68\% C.L. We also calculated the strong OZI-allowed decays in the pseudoscalar emission model and the flux-tube breaking model and extracted a $^3P_1 - ^1P_1$ mixing angle of $θ_K \simeq 45^o$. Our analysis also indicates that the heavy quark limit does not give a good description of the strange mesons.

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W-Pair Production in the Process $e^+e^- \to \ell νq\bar{q}'$ and Measurement of the $WWγ$ and $WWZ$ Couplings

We performed a detailed analysis of the process $e^+e^-\to \ell νq\bar{q}'$ where we included all tree level Feynman diagrams that contribute to this final state. We studied the sensitivity of this process to anomalous trilinear gauge boson couplings of the $WWγ$ and $WWZ$ vertices using two popular parametrizations. We used a maximum likelihood analysis of a five dimensional differential cross-section based on the $W$ and $W$ decay product angular distributions. We concentrated on LEP-200 energies, taking $\sqrt{s}=175$ GeV, and energies appropriate to the proposed Next Linear Collider (NLC), a high energy $e^+e^-$ collider with center of mass energies $\sqrt{s}=500$ and 1~TeV. At 175 GeV, $g_1^Z$ can be measured to about $\pm 0.2$, $κ_Z$ to $\pm 0.2$ and $κ_γ$ to $\pm 0.3$, $λ_Z$ to $\pm 0.2$ and $λ_γ$ to $\pm 0.3$. at 95\% C.L. assuming 500~pb$^{-1}$ integrated luminosity. Although these will be improvements of existing measurements they are not sufficiently precise to test the standard model at the loop level and are unlikely to see deviations from SM expectations. At 500~GeV with 50~fb$^{-1}$ integrated luminosity, $g_1^Z$ can be measured to about $\pm 0.01$, $κ_Z$ and $κ_γ$ to $\pm 0.005$ and $λ_Z$ and $λ_γ$ to $\pm 0.003$ at 95\% C.L. while at 1 TeV with 200~fb$^{-1}$ integrated luminosity, $κ_V$ and $λ_V$ can be measured to about $\pm 0.005$ and $\pm 10^{-3}$ respectively. The 500~GeV measurements will be at the level of loop contributions to the couplings and may show hints of new physics while the 1~TeV should be sensitive to new physics at the loop level.

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Measurement of the $WWγ$ and $WWZ$ Couplings at LEP200: The Benefits of Higher Energy?

We performed a detailed analysis of the process $e^+e^-\to \ell νq\bar{q}'$ to determine its sensitivity to anomalous trilinear gauge boson couplings of the $WWγ$ and $WWZ$ vertices and how the sensitivity varies with energy and integrated luminosity. We included all tree level Feynman diagrams that contribute to this final state and used a maximum likelihood analysis of a five dimensional differential cross-section based on the $W$ and $W$ decay product angular distributions. For constant luminosity, increasing $\sqrt{s}$ from 175~GeV to 192~GeV (220~GeV) improves the measurement sensitivity by a factor of 1.5 to 2 (2 to 3) depending on the parameter measured. However, the lower luminosity expected at higher $\sqrt{s}$ will reduce these improvements. In any case, the sensitivities for $\sqrt{s}$=175~GeV and L=500~pb$^{-1}$ of $δg_1^Z = \pm 0.22$, $δκ_Z = \pm 0.20$, $δκ_γ=\pm 0.27$, $δL_{9L}= \pm 55$, and $δL_{9R}= ^{+330}_{-230}$ are likely to be at least an order of magnitude too big to see the effects of new physics.

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Quartic Gauge Boson Couplings

Quartic vertices provide a window into one of the most important problems in particle physics; the understanding of electroweak symmetry breaking. I survey the various processes that have been proposed to study quartic gauge boson couplings at future $e^+e^-$, $eγ$, $γγ$, $e^-e^-$, and $pp$ colliders. For the lowest dimension operators that do not include photons, it appears that the LHC will provide the most constraining measurements. However, precision measurements at high energy $e^+e^-$ colliders involving $W^+W^-$ rescattering are also quite sensitive to the effects of a strongly interacting weak interaction. For quartic couplings involving photons, $γγ$ collisions appear to be the best place to measure these couplings. Measurements using gauge boson production in $eγ$ collisions are almost as precise as the $γγ$ processes with $e^+e^-\to VVV$ about an order or magnitude less sensitive.

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Comparison of Discovery Limits for Extra Z Bosons at Future Colliders

We study and compare the discovery potential for heavy neutral gauge bosons ($Z'$) at various $e^+e^-$ and $p\stackrel{(-)}{p}$ colliders that are planned or have been proposed. Typical discovery limits are for the TEVATRON $\sim 1$ TeV, DI-TEVATRON $\sim 2$ TeV, LHC $\sim 4$ TeV, LSGNA (a 60 TeV $pp$ collider) $\sim 13$ TeV while the $e^+e^-$ discovery limits are $2-10\times \sqrt{s}$ with the large variation reflecting the model dependence of the limits. While both types of colliders have comparable discovery limits the hadron colliders are generally less dependent on the specific $Z'$ model and provide more robust limits since the signal has little background. In contrast, discovery limits for $e^+e^-$ limits are more model dependent and because they are based on indirect inferences of deviations from standard model predictions, they are more sensitive to systematic errors.

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Probing the Heavy Quark Content of the Photon Using b Tagging at Electron Photon Colliders

We study the prospects for probing the quark content of the photon using b tagging at high energy electron-photon colliders. We find that heavy quark tagging provides a sensitive and effective probe of the quark content of the photon. Using a 500 GeV $e^+e^-$\ NLC in electron-photon mode, the cross section for $eb_γ\rightarrow eb$\ can be measured to 10\% with b tagging. This is sufficient to differentiate between the various photon structure functions.

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Issues in Light Meson Spectroscopy: The Case for Meson Spectroscopy at Cebaf

I review some outstanding issues in meson spectroscopy. The most important qualitative issue is whether hadrons with explicit gluonic degrees of freedom exist. To answer this question requires a much better understanding of conventional $q\bar{q}$ mesons. I therefore begin by examining the status of conventional meson spectroscopy and how the situation can be improved. The expected properties of gluonic excitations are discussed with particular emphasis on hybrids to give guidance to experimental searches. Multiquark systems are commented upon as they are likely to be important in the mass region under study and will have to be understood better. In the final section I discuss the opportunities that CEBAF can offer for the study of meson spectroscopy.

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Resolved Photon Contributions to Leptoquark Production in $e^+ e^-$ and $eγ$ Collision

We calculate the resolved photon contribution to leptoquark production at $eγ$ colliders for the center of mass energies $\sqrt s=500$~GeV and 1~TeV. We also calculate the resolved photon contribution to leptoquark production at $e^+ e^-$ colliders for the center of mass energies $\sqrt{s} = 1$~and~2~TeV. In both cases we find that these contributions are considerably larger than the standard contributions considered in the literature.

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Summary of the Extensions of the Standard Model Working Group

We summarize the results of the extended gauge group working group of the Madison-Argonne Workshop on Present and Future Colliders. Contributions are described on the previously unexamined two photon fusion production of heavy leptons, new studies of $Z'$ couplings to $ν\barν$ and $q\bar{q}$, and previously unexplored vector leptoquark production. More detailed accounts of these studies can be found in individual contributions.

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Single Top Production in $eγ$ Collisions

We study single top production in high energy $eγ$ collisions. The process $eγ\toνb\bar t$ can serve as a unique tool for measuring the $Wtb$ coupling. We show that allowing for realistic luminosity and backgrounds, the size of this coupling can be determined to within 10 (5)\% in an $eγ$ collider built around a 500 (1000) GeV $e^+e^-$ collider.

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