arXiv subjects

A. D. Martin

Publications and source records attributed to A. D. Martin.

At least 271 records · Page 15Linked to original sources

arXiv1996

Diffractive $J/ψ$ photoproduction as a probe of the gluon density

We use perturbative QCD, beyond the leading $\ln Q^2$ approximation, to show how measurements of diffractive $J/ψ$ production at HERA can provide a sensitive probe of the gluon density of the proton at small values of Bjorken $x$. We estimate both the effect of the relativistic motion of the $c$ and $\overline{c}$ within the $J/ψ$ and of the rescattering of the $c\overline{c}$ quark pair on the proton. We find that the available data for diffractive $J/ψ$ photoproduction can discriminate between the gluon distributions of the most recent sets of partons.

hep-ph↗

arXiv1996

Constraints on gluon evolution at small x

The BFKL and the unified angular-ordered equations are solved to determine the gluon distribution at small $x$. The impact of kinematic constraints is investigated. Predictions are made for observables sensitive to the gluon at small $x$. In particular comparison is made with measurements at the HERA electron-proton collider of the proton structure function $F_2 (x, Q^2)$ as a function of $\ln Q^2$, the charm component, $F_2^c(x,Q^2)$ and diffractive $J/ψ$ photoproduction.

hep-ph↗

arXiv1995

The description of F2 at small x incorporating angular ordering

We study the perturbative QCD description of the HERA measurements of $F_2 (x, Q^2)$ using a gluon distribution that is obtained from an evolution incorporating angular ordering of the gluon emissions, and which embodies both GLAP and BFKL dynamics. We compare the predictions with recent HERA data for $F_2$. We present estimates of the charm component $F_2^c (x, Q^2)$ and of $F_L (x, Q^2)$.

hep-ph↗

arXiv1995

Higher-order Coulomb Corrections to the Threshold e+ e- --> W+ W- Cross Section

The QED Coulomb correction is one of the most important corrections to the e+ e- --> W+ W- total cross section near threshold. We calculate these corrections through second order, and discuss the implications for extracting M_W from a threshold cross section measurement at LEP2. Analytic expressions are derived in various kinematic limits.

hep-ph↗

arXiv1995

Description of F2 and the gluon at small x

We give a brief overview of the perturbative QCD description of the proton deep-inelastic structure function F2(x, Q2) at small x. We discuss GLAP and BFKL approaches, and then we review progress towards a more unified treatment.

hep-ph↗

arXiv1995

The $α_S$ Dependence of Parton Distributions

We perform next-to-leading order global analyses of deep inelastic and related data for different fixed values of $α_S (M_Z^2)$. We present sets of parton distributions for six values of $α_S$ in the range 0.105 to 0.130. We display the $(x, Q^2)$ domains with the largest parton uncertainty and we discuss how forthcoming data may be able to improve the determination of the parton densities.

hep-ph↗

arXiv1995

BFKL predictions at small x from k_T and collinear factorization viewpoints

Hard scattering processes involving hadrons at small $x$ are described by a $k_T$-factorization formula driven by a BFKL gluon. We explore the equivalence of this description to a collinear-factorization approach in which the anomalous dimensions $γ_{gg}$ and $γ_{qg}/α_S$ are expressed as power series in $α_S \log (1/x)$, or to be precise $α_S/ω$ where $ω$ is the moment index. In particular we confront the collinear-factorization expansion with that extracted from the BFKL approach with running coupling included.

hep-ph↗

arXiv1995

THE GLUON DISTRIBUTION AT SMALL x OBTAINED FROM A UNIFIED EVOLUTION EQUATION.

We solve a unified integral equation to obtain the $x, Q_T$ and $Q$ dependence of the gluon distribution of a proton in the small $x$ regime; where $x$ and $Q_T$ are the longitudinal momentum fraction and the transverse momentum of the gluon probed at a scale $Q$. The equation generates a gluon with a steep $x^{- λ}$ behaviour, with $λ\sim 0.5$, and a $Q_T$ distribution which broadens as $x$ decreases. We compare our solutions with, on the one hand, those that we obtain using the double-leading-logarithm approximation to Altarelli-Parisi evolution and, on the other hand, to those that we determine from the BFKL equation.

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↗

arXiv1995

COULOMB EFFECTS IN W+ W- PRODUCTION

We calculate the Coulomb effects on the cross section for e+ e- --> W+ W- taking into account the instability of the W bosons. We carefully explain the consequences of instability throughout the energy range which will be accessible at LEP2. We present a formula which allows these effects to be easily implemented.

hep-ph↗

arXiv1994

Measuring$|V_{ub}|$attheLEP/SLC

We show that the observation of energetic charged leptons from semileptonic $B$-meson decays at the LEP and SLC $Z$--boson factories offers a unique opportunity to measure the quark mixing matrix element $|V_{ub}|$. We present various distributions of the $B$ decay products to show that the $b \to u$ decays can be cleanly separated from the $b \to c$ decays, with a signal of $O(100)$ events per million $Z$ bosons produced.

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

Low x phenomena

We review recent developments in the application of perturbative QCD to phenomena at small x.

hep-ph↗

arXiv1994

Determination of V_ub at the Z-boson resonance

We show that the observation of energetic charged leptons from semileptonic $b$ becays at the LEP and SLC $Z$--boson factories offers a unique opportunity to measure the quark mixing matrix element $V_{ub}$. We present various distributions of the $b$ decay products to show that the $b \to u$ decays can be cleanly separated from the $b \to c$ decays, with a signal of $O(100)$ events per million $Z$ bosons produced.

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

Dijet Production at HERA as a Probe of BFKL Dynamics

We calculate the rate for the deep-inelastic electroproduction of dijets at HERA. We study the weakening of the azimuthal (back-to-back) correlation between the jets, as $x$ decreases, to see whether it can be used to identify BFKL dynamics from conventional fixed-order QCD effects. We show how this may give information on the transverse momentum ($k_T)$ dependence of the gluon distribution of the proton.

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

Transverse Energy Flow at HERA

We calculate the transverse energy flow accompanying small $x$ deep-inelastic events and compare with recent data obtained at HERA. In the central region between the current jet and the remnants of the proton we find that BFKL leading $\ln(1/x)$ dynamics gives a distinctively large transverse energy distribution, in approximate agreement with recent data.

hep-ph↗