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W. J. Stirling

Publications and source records attributed to W. J. Stirling.

At least 163 records · Page 9Linked to original sources
arXiv1995

The Measurement of M(W) from the W+W- Threshold Cross Section at LEP2

The rapid increase of the e+e- --> W+W- cross section in the threshold region provides a method for measuring M(W) at LEP2. The dependence of the theoretical cross section -- including the effects of the finite W width, Coulomb interactions and initial state radiation -- on the collider energy and on the mass and width of the W is investigated in order to deduce the optimal collider energy which maximizes the sensitivity to M(W). Statistical and systematic uncertainties are studied in detail. The background contribution from QCD four-jet production is estimated. It is shown that by running LEP2 at a collision energy of approximately 161 GeV and obtaining a luminosity of order 100 inverse picobarns per experiment, an uncertainty on the measured value of M(W) of +-70 MeV should be achievable.

hep-ph↗
arXiv1995

Pinning down the Glue in the Proton

The latest measurements of $F_2$ at HERA allow for a {\it combination} of gluon and sea quark distributions at small $x$ that is significantly different from those of existing parton sets. We perform a new global fit to deep-inelastic and related data. We find a gluon distribution which is larger for $x \lapproxeq 0.01$, and smaller for $x \sim 0.1$, and a flatter input sea quark distribution than those obtained in our most recent global analysis. The new fit also gives $α_s(M_Z^2) = 0.114$. We study other experimental information available for the gluon including, in particular, the constraints coming from fixed-target and collider prompt $γ$ production data.

hep-ph↗
arXiv1994

Gluon Radiation in t\bar{t} Production at the Tevatron p\bar{p} Collider

We present a complete calculation of the matrix elements for the processes $q \bar q, gg \to b W^+ \bar b W^- g$ and $qg \to b W^+ \bar b W^- q$ which are relevant for the study of events with an additional jet in $t \bar t$ production at the Tevatron $p \bar p $ collider. Our calculation includes (i) the contributions from gluons emitted during the top production and decay stages and the interference between these, and (ii) the complete set of Feynman diagrams corresponding to both resonant and non-resonant top production. We study the distribution in phase space of the additional parton jet and make comparisons with previous studies based on the soft-gluon approximation and with results from parton-shower Monte Carlo simulations. The implications for top mass measurements are briefly discussed.

hep-ph↗
arXiv1994

Contributions of below--threshold decays to ${\cal {MSSM}}$ Higgs branching ratios

We calculate all the experimentally relevant branching ratios of the Higgs bosons of the Minimal Supersymmetric Standard Model, paying particular attention to the contributions from below--threshold decays. We show that in some cases these can significantly change the pattern of branching ratios calculated without taking off--shell effects into account

hep-ph↗
arXiv1994

Soft Jets and Top Mass Measurement at the Tevatron

Extra soft jets in top events in $p \bar p$ collisions may arise not only from gluons radiated off initial state partons or final state $b$ quarks, but may also be radiated from the $t$ quarks themselves. We discuss predictions for distributions of soft gluons in $t \bar t$ production at the Tevatron and the implications for attempts to measure the top mass by reconstructing the invariant mass of its decay products.

hep-ph↗
arXiv1994

Spin-dependent Parton Distributions

We perform a global leading-order QCD fit to recent polarized structure function data in order to extract a consistent set of spin-dependent parton distributions. Assuming that there is no significant intrinsic polarization of the quark sea, the data are consistent with a modest amount of the proton's spin carried by the gluon, although the shape of the gluon distribution is not well constrained. We show how inelastic $J/ψ$ production in polarized photon-hadron scattering can, in principle, provide definitive information on the shape of the gluon distribution. (Talk presented by W.J.Stirling at the 27th International Conference on High Energy Physics, Glasgow, July 1994)

hep-ph↗
arXiv1994

Top Momentum and Extra Soft Jets

Top pairs produced at the Tevatron may be accompanied by extra jets due to radiation of soft gluons. Whether these jets should be included in top momentum reconstruction depends on the source of the gluons, which can be radiated off the initial partons, the final state $b$ quarks, or the intermediate top quarks. We compute soft gluon distributions and discuss their implications for top mass measurements.

hep-ph↗
arXiv1994

MRS(1994): parton distributions of the proton

To obtain improved parton densities of the proton, we present a new global analysis of deep-inelastic and related hard scattering data including, in particular, the recent measurements of F2 at HERA, of the asymmetry of the rapidity distributions of W+- production at the FNAL pbar p collider and of the asymmetry in Drell-Yan production in pp and pn collisions. We discuss the evolution of the new partons to low values of Q2.

hep-ph↗
arXiv1994

Additional Soft Jets in ${\rm t\bar{t}}$ Production at the Tevatron $\pp$ Collider

A large fraction of top quark events in $\pp$ collisions at $1.8\ \TeV$ will contain additional soft hadronic jets from gluon bremsstrahlung off the quarks and gluons in the hard processes $\qq, gg \to \tt \to \bb \ww$. These extra jets can cause complications when attempting to reconstruct $m_t$ from the invariant mass of combinations of final-state quarks and leptons. We show how such soft radiation cannot be unambiguously associated with either initial-state radiation or or with final-state radiation off the $b$ quarks. The top quarks can radiate too, and in fact the pattern of radiation has a very rich structure, which depends on the orientation of the final-state particles with respect to each other and with respect to the beam. We calculate the full radiation pattern of soft jets in the soft gluon approximation and compare with several approximate forms which are characteristic of parton shower Monte Carlos. The implications for top mass measurements are discussed.

hep-ph↗
arXiv1994

Angular Correlations and Light Gluinos in Multi-jet Photoproduction at HERA

A study of $3+1$ jet event photoproduction at HERA is presented. We define an angular variable which is sensitive to the topology of the final state jets and is therefore able to discriminate between the different contributing subprocesses, and between QCD and an abelian gluon model. We also investigate the contribution from the direct production of light gluinos to the $3+1$ cross section.

hep-ph↗
arXiv1994

Anomalous quartic couplings in $ W^+W^- γ$ production at $e^+e^- $ colliders

We study the process $e^+e^- \rightarrow W^+W^- γ$ at high-energy $e^+ e^-$ colliders to investigate the effect of genuine quartic $W^+W^-γγ$ and $W^+W^- Zγ$ anomalous couplings on the cross section. Deviations from the Standard Model predictions are quantified. We show how bounds on the anomalous couplings can be improved by choosing specific initial state helicity combinations. The dependence of the anomalous contributions on the collider energy is studied.

hep-ph↗
arXiv1994

Parton Distributions of the Proton

To obtain improved parton densities of the proton, we present a new global analysis of deep inelastic and related data including, in particular, the recent measurements of $F_2$ at HERA, of the asymmetry of the rapidity distributions of $W^\pm$ production at the FNAL $pp$ collider and of the asymmetry in Drell-Yan production in $pp$ and $pn$ collisions. We also incorporate data to determine the flavour dependence of the quark sea distributions. We find that the behaviour of the partons at small $x$ is consistent with the precocious onset of BFKL leading $\log(1/x)$ dynamics. We discuss the ambiguities remaining in the gluon distribution. We present improved predictions for $W$ boson (and $t$ quark) production at the FNAL $\pp$ collider.

hep-ph↗
arXiv1994

Spin-dependent Parton Distributions from Polarized Structure Function Data

In the past year, polarized deep inelastic scattering experiments at CERN and SLAC have obtained structure function measurements off proton, neutron and deuteron targets at a level of precision never before achieved. The measurements can be used to test the Bjorken and Ellis-Jaffe sum rules, and also to obtain information on the parton distributions in polarized nucleons. We perform a global leading-order QCD fit to the proton deep inelastic data in order to extract the spin-dependent parton distributions. By using parametric forms which are consistent with theoretical expectations at large and small $x$, we find that the quark distributions are now rather well constrained. We assume that there is no significant intrinsic polarization of the strange quark sea. The data are then consistent with a modest amount of the proton's spin carried by the gluon, although the shape of the gluon distribution is not well constrained, and several qualitatively different shapes are suggested. The spin-dependent distributions we obtain can be used as input to phenomenological studies for future polarized hadron-hadron and lepton-hadron colliders.

hep-ph↗
arXiv1994

Three-jet final states and measuring the $γγ$ width of the Higgs at a photon linear collider

The identification of the intermediate-mass Higgs process $γγ\to H \to b \bar b$ will be one of the most important goals of a future photon linear collider. Potentially important backgrounds from the continuum $γγ\to c \bar c, b \bar b$ leading-order processes can be suppressed by a factor $m_q^2/s$ by using polarized photon beams in the $J_z=0$ initial-state configuration. We show that the same $m_q^2/s$ suppressions do not necessarily apply to the radiative processes $γγ\to c \bar c g, b \bar b g$. These processes can mimic the two-jet topology of the Higgs signal when two of the three partons are collinear, or when one of the partons is soft or directed down the beam pipe. We calculate the contribution of these processes to the two-jet background in the $J_z=0$ channel. We investigate the effect of imposing additional event shape, jet width and secondary vertex cuts on both signal and background, and show that with reasonable detector capabilities it should be possible to reduce the background to a manageable level.

hep-ph↗
arXiv1994

Gluon Radiation and Energy Losses in Top Quark Production

The emission of energetic gluons in $\tt$ production in $\ee$ annihilation can have important experimental consequences, in particular on top quark mass measurements. We present compact, analytical expressions for the gluon energy distribution and its average value at first order in QCD perturbation theory. Our results are valid for arbitrary masses, collision energies and production currents. We pay particular attention to top quark production near threshold, and show that in certain cases the soft gluon approximation is insufficient to describe the radiation spectrum.

hep-ph↗
arXiv1994

Production of Jet Pairs at Large Relative Rapidity in Hadron-Hadron Collisions as a Probe of the Perturbative Pomeron

The production of jet pairs with small transverse momentum and large relative rapidity in high energy hadron-hadron collisions is studied. The rise of the parton-level cross section with increasing rapidity gap is a fundamental prediction of the BFKL `perturbative pomeron' equation of Quantum Chromodynamics. However, at fixed collider energy it is difficult to disentangle this effect from variations in the cross section due to the parton distributions. It is proposed to study instead the distribution in the azimuthal angle difference of the jets as a function of the rapidity gap. The flattening of this distribution with increasing dijet rapidity gap is shown to be a characteristic feature of the BFKL behaviour. Predictions for the Fermilab proton-antiproton collider are presented.

hep-ph↗
arXiv1993

Light Gluinos in Four-Jet Events at LEP

The light gluino hypothesis can explain the apparent incompatibility between the measurements of $\as$ at low- and high-energy. Such gluinos are produced directlyin four-jet events, for which we perform a detailed analysis. Because the jet energies are not large, the effect of the non-zero gluino mass is important. We take the gluino mass into account in the computation of the cross sections and shape variables. As expected, we find that mass effects tend to reduce the impact of the gluinos in the cross section, weakening the bounds from obtaining assuming massless gluinos.

hep-ph↗