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A. Donnachie

Publications and source records attributed to A. Donnachie.

14 recordsLinked to original sources
arXiv2016

Photoproduction of $a_0(980)$ and $f_0(980)$

There is no generally accepted view of the structure of the light-quark non-strange scalar mesons. A variety of models have been proposed that encompass $qq\bar{q}\bar{q}$, molecular, $q\bar{q}$ and glueball states in various combinations. Previously we considered scalar-meson photoproduction in a simple Regge-pole model and showed that it was experimentally viable. Recent data on the photoproduction of $a_0(980)$ and $f_0(980)$ confirm this. We extend our model to incorporate Regge cuts, based on our knowledge of $π^0$ photoproduction. The theoretical predictions are compared to the $a_0(980)$ and $f_0(980)$ photoproduction data.

hep-ph↗
arXiv2014

Central soft production of hadrons in $pp$ collisions

The high-energy behaviour of soft scattering observables such as total cross sections, elastic scattering at small momentum transfer, diffractive dissociation and central production have been described successfully in the context of Regge theory, with the same basic structure holding as energies have increased. For elastic scattering and diffraction dissociation the defining energies were those of the ISR, the $Sp\bar{p}S$ collider and the Tevatron. The elastic scattering data from the LHC demonstrate the continuing applicability of Regge theory. Preliminary data on diffraction dissociation promise to add to our understanding and now, for the first time, we can expect to test fully these concepts in central production. Although the latter is the principal objective of this discussion, understanding the first two is an essential prerequisite as they define the formalism and establish parameters.

hep-ph↗
arXiv2004

Evidence for Quark Spin-flip in Pomeron Exchange

Spin-parity analyses of the $ωπ$ system in the reaction $γp \to (ωπ) p$ for photon laboratory energies from 20 to 70 GeV have shown that production of the $J^P=1^+$ $b_1(1235)$ meson dominates, with a $J^P=1^-$ background at the level of 20%. Using vector-meson dominance arguments, this background is shown to be consistent with the data on $e^+e^- \to ωπ$. The energy dependence of the data imply that the mechanism is a combination of Reggeon and Pomeron exchange. Assuming that the latter is relevant only for the $J^P = 1^-$ component and extrapolating to W=200 GeV, it is argued that this accounts for most of the preliminary $ωπ$ signal observed by the H1 Collaboration in the same reaction. A residual peak can be ascribed to the $b_1(1235)$, which requires a quark spin-flip from Pomeron exchange. Precisely the same mechanism occurs in the reaction $πp \to a_1(1260) p$.

hep-ph↗
arXiv2004

Does the hard pomeron obey Regge factorisation?

While data for the proton structure function demand the presence of a hard-pomeron contribution even at quite small Q^2, previous fits to the pp and p\bar p total cross sections have found that in these there is little or no room for such a contribution. We re-analyse the data and show that itmay indeed be present and that, further, it probably obeys Regge factorisation: sigma^{gamma p}(s,Q_1^2)sigma^{gamma p}(s,Q_2^2)= sigma^{pp}(s)sigma^{gamma gamma}(s,Q_1^2,Q_2^2) for all values of Q_1^2 and Q_2^2.

hep-ph↗
arXiv2003

Evolution at small x

At present there is no correct theory of evolution of F_2(x,Q^2) at small x. It is a mixture of hard and soft pomeron exchange and perturbative QCD very successfully describes the evolution of the hard-pomeron component. This allows the gluon density to be calculated. It is somewhat different from what is conventionally supposed, but it leads to a clean PQCD description of the data for the charm structure function. Perturbative QCD breaks down for the evolution of the soft-pomeron component of F_2(x,Q^2).

hep-ph↗
arXiv2002

Radiative decays: a new flavour filter

Radiative decays of the $1^3D_1$ orbital excitations of the $ρ$, $ω$ and $ϕ$ to the scalars $f_0(1370)$, $f_0(1500)$ and $f_0(1710)$ are shown to provide a flavour filter, clarifying the extent of glueball mixing in the scalar states. A complementary approach to the latter is provided by the radiative decays of the scalar mesons to the ground-state vectors $ρ$, $ω$ and $ϕ$. Discrimination among different mixing scenarios is strong.

hep-ph↗
arXiv2000

Rho Electroproduction and the Hadronic Contribution to Deeply Virtual Compton Scattering

A two-gluon-exchange model incorporating perturbative and non-perturbative effects is presented for rho electroproduction which provides an excellent description of all current data. This is then used to calculate the contribution from the rho to deeply virtual Compton scattering via the vector-meson-dominance transition rho -> gamma. This is found to be sufficiently large to provide a significant contribution through interference with the perturbative QCD term.

hep-ph↗
arXiv2000

Observing the Odderon: Tensor Meson Photoproduction

We calculate high-energy photoproduction of the tensor meson $f_2(1270)$ by odderon and photon exchange in the reaction $γ+ {\rm{p}} \to f_2(1270) + {\rm{X}}$, where X is either the nucleon or the sum of the N(1520) and N(1535) baryon resonances. Odderon exchange dominates except at very small transverse momentum, and we find a cross section of about 20 nb at a centre-of-mass energy of 20 GeV. This result is compared with what is currently known experimentally about $f_2$ photoproduction. We conclude that odderon exchange is not ruled out by present data. On the contrary, an odderon-induced cross section of the above magnitude may help to explain a puzzling result observed by the E687 experiment.

hep-ph↗
arXiv1999

Gamma*Gamma* Reactions at High Energies

The total hadronic gamma*gamma* cross sections at high energy are calculated as a function of energy and photon virtuality in a model combining Regge exchange, the quark box diagram (a fixed pole in Regge language) and soft and hard pomeron exchanges evaluated in the context of dipole-dipole scattering. Good agreement is obtained with the data for the real gamma-gamma cross section and for the real photon structure function. However the model prediction for the gamma* gamma* cross section is too small. This is attributed to an incorrect extrapolation of the Q^2 dependence of the hard pomeron adopted here. Parametrising it independently shows that the hard part of the cross section can be well represented by a simple Regge pole with intercept ~ 1.3.

hep-ph↗
arXiv1999

Odderon and photon exchange in electroproduction of pseudoscalar mesons

We investigate the reaction $e p \to e PS X$ where PS denotes a pseudoscalar meson $π^0, η, η'$, or $η_c$ and X either a proton or resonance or continuum states into which the proton can go by diffractive excitation. At high energies photon and odderon exchange contribute to the reaction. The photon exchange contribution is evaluated exactly using data for the total virtual photon-proton absorption cross section. The odderon exchange contribution is calculated in nonperturbative QCD, using functional integral techniques and the model of the stochastic vacuum. For the proton we assume a quark-diquark structure as suggested by the small odderon amplitude in $pp$ and $p \bar{p}$ forward scattering. We show that odderon exchange leads to a much larger inelastic than elastic PS production cross section. Observation of our reaction at HERA would establish the soft odderon as an exchange object on an equal footing with the soft pomeron and would give us valuable insight into both the nucleon structure and the mechanism of high energy diffractive scattering.

hep-ph↗
arXiv1999

Two Photon Reactions at High Energies

Cross sections for the reactions gamma^(*) gamma^(*) --> hadrons and gamma^(*) gamma^(*) --> 2 vector mesons are calculated as functions of energy (sqrt(s) > 20 GeV) and photon virtualities. Good agreement with experiment is obtained for the total hadronic cross section and, after allowing for a valence-quark contribution from the hadronic part of the photon, with the photon structure function at small x. The cross section for vector meson production are shown to be experimentally accessible for moderate values of Q^2. This is sufficient to probe the nature of the hard pomeron which has recently been proposed.

hep-ph↗