arXiv subjects

T. Gehrmann

Publications and source records attributed to T. Gehrmann.

161 records · Page 9Linked to original sources

arXiv1997

QCD Corrections to Double and Single Spin Asymmetries in Vector Boson Production at Polarized Hadron Colliders

The production of $W^{\pm}$ and $Z^0$ bosons in collisions of longitudinally polarized protons allows to access various combinations of the polarized quark distributions from the study of double and single spin asymmetries. We compute the ${\cal O}(α_s)$ corrections to these asymmetries as function of the vector boson rapidity $y$ in the hadron-hadron centre-of-mass frame. Detailed studies of the numerical impact of the next-to-leading order corrections are presented. The theoretical uncertainties of an extraction of polarized quark distributions from future vector boson production data are investigated in detail.

hep-ph↗

arXiv1997

Radiative Corrections to the Photon + 1 Jet Rate at LEP

We present the results of a calculation of the rate for the annihilation of an $e^+e^-$ pair into a jet containing an energetic photon and a single further jet at order $αα_{s}$. By comparing these results with the existing data on the photon + 1 jet rate from the ALEPH Collaboration, we make a first determination of the process independent quark-to-photon fragmentation function $D_{q \to γ}(z,μ_{F})$ at order $αα_{s}$. We also compare our prediction for the inclusive photon energy spectrum with the recent OPAL measurement.

hep-ph↗

arXiv1997

The Bethe-Heitler Process in Polarized Photon-Nucleon Interactions

We calculate the Bethe-Heitler cross section for the production of lepton pairs in the field of a longitudinally polarized nucleon, taking into account the lepton masses and the target mass. This process is a dominant background to the detection of open charm from semi-leptonic decay modes, which is a potential probe of the polarized gluon distribution in the nucleon.

hep-ph↗

arXiv1997

Spin Physics

A summary of new experimental results and recent theoretical developments presented in the ``Spin Physics'' working group is given.

hep-ph↗

arXiv1997

The polarized Drell-Yan process at O(α_s)

The results of a recent calculation of the QCD corrections to the polarized Drell-Yan process are summarized. Some implications for the production of vector bosons at RHIC are discussed.

hep-ph↗

arXiv1997

QCD corrections to the longitudinally polarized Drell-Yan process

In this paper we calculate the O($α_s$) corrections to the $x_F$- and $y$-distributions of lepton pairs produced in collisions of longitudinally polarized hadrons. The numerical importance of these corrections is studied and consequences for the extraction of the polarized sea quark distributions from a measurement of the longitudinally polarized Drell-Yan cross section are discussed.

hep-ph↗

arXiv1996

Drell-Yan asymmetries at HERA-$\vec{\rm N}$

The production of Drell-Yan pairs in the double polarized mode of HERA-$\vec{\rm N}$ is studied at next-to-leading order in QCD. It is found that a measurement of this observable could yield vital information on the polarization of the light quark sea in the nucleon.

hep-ph↗

arXiv1996

Time-reversal-odd asymmetries at HERA

Longitudinal polarization of the HERA electron beam allows the study of spatial asymmetries in the final state of single inclusive hadron production in $\vec{e}p$-collisions. We estimate the size of a particular production asymmetry which is related to the time reversal properties of the underlying parton level subprocess.

hep-ph↗

arXiv1996

Working Group Report on the Structure of the Proton

We summarize the developments on the structure of the proton that were studied at the Workshop on "HERA Physics" that was held in Durham in September 1995. We survey the latest structure function data; we overview the QCD interpretations of the measurements of the structure functions and of final state processes; we discuss charm production and the spin properties of the proton.

hep-ph↗

arXiv1996

The Effects of a New Vector Boson on the Top Quark Cross Section at the Tevatron

It has recently been shown that a new neutral vector boson $Z'$, with a mass of order 1~TeV and weak couplings to quarks only, could explain both the anomalous values of $R_b$ and $R_c$ and the apparent excess of large $E_T$ jet events measured by the CDF collaboration. We calculate the effects of $Z'$ exchange on the $t\bar{t}$ production cross section at Tevatron $p \bar p$ collider energies, including next-to-leading-order QCD corrections. We find a significant enhancement of the cross section and study the resulting $t \bar t$ invariant mass distribution, which could provide a decisive test of the $Z'$ model.

hep-ph↗

arXiv1995

Polarized Parton Distributions in the Nucleon

The distribution of the spin of the nucleon among its constituents can be parametrized in the form of polarized parton distribution functions for quarks and gluons. Using all available data on the polarized structure function $g_1(x,Q^2)$, we determine these distributions both at leading and next-to-leading order in perturbation theory. We suggest three different, equally possible scenarios for the polarized gluon distribution, which is found to be only loosely constrained by current experimental data. We examine various possibilities of measuring polarized parton distributions at future experiments.

hep-ph↗

arXiv1995

Production of $J/ψ$-pairs at HERA-$\vec{\rm N}$

The production of $J/ψ$-pairs as a possible measure of the polarized gluon distribution $ΔG(x)$ is studied for proton--nucleon collisions at $\sqrt{s} =40\;\mbox{GeV}^2$ (HERA-$\vec{\rm N}$). Possibilities of reconstructing the helicity state of at least one of the $J/ψ$'s are critically reviewed. The observation of production asymmetries in the single polarized mode of HERA-$\vec{\rm N}$ is found to be not feasible.

hep-ph↗

arXiv1995

Present Status of Polarized Parton Distributions

A review of the present knowledge on polarized parton distributions is given. The effects of perturbative evolution on these distributions are discussed qualitatively and a comparison of various recent parametrizations is made.

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arXiv1995

Analytic Approaches to the Evolution of Polarised Parton Distributions at Small $x$

The $Q^2$ evolution of polarised parton distributions at small $x$ is studied. Various analytic approximations are critically discussed. We compare the full evolution with that obtained from the leading-pole approximation to the splitting functions, and show that the validity of this approximation depends critically on the $x \to 0$ behaviour of the starting distributions. A new analytic solution which is valid at small $x$ is obtained, and its domain of applicability is discussed.

hep-ph↗

arXiv1995

Deep Inelastic Electron-Pomeron Scattering at HERA

We have substantially extended our analysis of the partonic content of the pomeron. Ambiguities in the normalization of the pomeron flux and the possiblity of a direct coupling of the pomeron to single quarks are discussed, yielding two equally possible scenarios for the parton distributions in the pomeron. We discuss various experimental tests of these scenarios.

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

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.

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