arXiv subjects
Publications and source records attributed to A Donnachie.
It is fundamental that the phase of an elastic scattering amplitude is related to its energy variation. We repeat a previous fit to proton-proton and proton-antiproton elastic scattering data from 13 to 13000 GeV, taking better account of the very high accuracy of the 13 TeV data. The conclusion remains that there is no evidence for the existence of an odderon in the small-t data
A simple application of Regge theory, with 9 free parameters, provides a good fit to elastic scattering data at small t from 13.76 GeV to 13 TeV. It yields a value for rho, the ratio of the real part of the forward amplitude to its imaginary part, close to 0.14 at 13 TeV. Although the exact value obtained for rho is sensitive to what functional form is chosen for the fit, there is no strong case for the presence of an odderon contribution to forward scattering.
We propose an approach to DGLAP evolution at small x that circumvents the usual problem that a perturbation expansion is not valid there. The data for the charm structure function are important to motivate the method, and it describes them much more successfully than the conventional approach.
The light-quark non-strange scalar mesons a0(980), f0(980), f0(1370), a0(1450), f0(1500) and f0(1710) are of great interest as there is no generally accepted view of their structure which can encompass q-q-qbar-qbar, molecular, q-qbar and glueball states in various combinations. It has been shown previously that the radiative decays of the scalar mesons to rho and omega are a good probe of their structure and provide good discrimination among models. Scalar meson photoproduction is proposed as an alternative to measuring radiative decays and it is shown that it is a feasible proposition.
The scalar mesons f0(1370), f0(1500) and f0(1710) are of interest as there is as yet no consensus of their status, or indeed of the existence of the f0(1370). Radiative decays to rho and omega have been shown to provide effective probes of their structure and to discriminate among models. Scalar-meson photoproduction is proposed as an alternative and it is shown that it represents a feasible approach.
A model is presented based on a dipole picture with a hard and a soft pomeron. It is assumed that large dipoles couple to the soft pomeron and small dipoels couple to the hard pomeron. Most of the parameters of the model are predetermined from proton-proton scattering and the only free parameter is the radius Rc, defining the transition from small to large dipoles. This is fixed by the proton structure function. The model then successfully predicts the proton charm and longitudinal structure functions, J/psi photoproduction, the DVCS cross section, the total gamma-p and gamma-gamma cross sections and the photon structure function.
It has been shown that the energy dependence of the reaction gamma p --> b1(1235) requires a contribution from pomeron exchange. This necessitates spin-flip at the quark level as the transition is from a 3S1 state to a 1P1 state. The same mechanism occurs in the reaction pi p --> a1(1260) p, which is a 1S0 to 3P1 transition.
In contrast to theoretical expectations, experimental results at sqrt(s)=200 GeV for the reaction gamma p --> pi0 X show no evidence for odderon exchange. The upper limit on the cross section is an order of magnitude smaller than the theoretical estimate. It is argued that chiral symmetry leads to a large suppression, taking the thoeretical estimates well below the data. Two additional arguments are presented which may decrease the theoretical estimate further. The calculations are more sensitive to the assumptions made in evaluating the hadronic scattering amplitude than in the processes considered previously and lattice gauge calculations indicate that the odderon intercept may be appreciably lower than usually assumed. These two latter effects are particularly relevant for the reactions gamma p --> f2(1270)X and gamma p --> a2(1320)X for which the data upper limits are also below the theoretical predictions, but not so dramatically as for gamma p --> pi0 X.
We refute past suggestions that the data on soft diffraction dissociation demand modifications to conventional Regge theory.
The gluon distribution is dominated by the hard pomeron at small $x$ and all $Q^2$, with no soft-pomeron contribution. This describes well not only the DGLAP evolution of the hard-pomeron part of $F_2(x,Q^2)$, but also charm photoproduction and electroproduction, and the longitudinal structure function, all calculated in leading-order pQCD.
We explain how Regge theory and perturbative evolution may be made compatible at small x. The result not only gives striking support to the two-pomeron description of small-x behaviour, but gives a rather clean test of perturbative QCD itself. When x is very small, or Q^2 is very large, the proton's gluon distribution function is significantly larger than is commonly believed. Perturbative evolution is invalid below Q^2\approx 5 GeV^2.
Radiative decays of the $1^3S_1$ radial and $1^3D_1$ orbital excitations of the $ρ$, $ω$ and $ϕ$ are calculated in the quark model, using wave functions obtained variationally from the Hamiltonian with standard quark-model parameters. The larger radiative widths should be measurable at new high-intensity facilities being proposed, and in some cases may be measurable in data from existing experiments. The radiative decays are a strong discriminator between the $1^3S_1$ and $1^3D_1$ excitations, and can also be used to provide unique information about the decay products.
The known analytic properties of the Compton amplitude at small $Q^2$ place significant constraints on its behaviour at large $Q^2$. This calls for a re-evaluation of the role of perturbative evolution in past fits to data.
The current situation for vector meson spectroscopy is outlined, and it is shown that the data are inconsistent with the generally-accepted model for meson decay. A possible resolution in terms of exotic mesons is given. Although this resolves some of the issues, fresh theoretical questions are raised.
The current situation for vector meson spectroscopy is outlined, and it is shown that the data are inconsistent with the generally-accepted model for meson decay. A possible resolution in terms of exotic (hybrid) mesons is given. Although this hypothesis resolves some of the issues, fresh theoretical questions are raised. It is argued that high-precision $e^+e^-$ annihilation data provide an excellent laboratory for studying many aspects of nonperturbative QCD.
We present a model based on a dipole picture with a hard and a soft pomeron in which large dipoles couple to the soft pomeron and small dipoles couple to the hard pomeron. The parameters in the model are fixed by proton-proton scattering and the proton structure function. The model is then applied successfully to the proton charm structure function, the proton longitudinal structure function, J/psi photoproduction, deep virtual Compton scattering, the real photon-proton total cross section, the real photon-photon cross section, and the photon structure function. Differences between our predictions and data on charm production in real photon-photon interactions and the virtual gamma-gamma cross section are discussed.
New structure-function data are in excellent agreement with the existence of a hard pomeron, with intercept about 1.4. It gives a very economical description of the data. Having fixed 2 parameters from the data for the real-photon cross section $σ^{γp}$, we need just 5 further parameters to fit the data for $F_2(x,Q^2)$ with $x\leq 0.001$. The available data range from $Q^2=0.045$ to 35 GeV$^2$. With guesses consistent with dimensional counting for the $x$ dependences of our three separate terms, the fit extends well to larger $x$ and to $Q^2=5000$ GeV$^2$. With no additional parameters, it gives a good description of data for the charm structure function $F_2^c(x,Q^2)$ from $Q^2=0$ to 130 GeV$^2$. The two pomerons also give a good description of both the $W$ and the $t$ dependence of $γp\to J/ψp$.
A two-component model is developed for diffractive electroproduction of $ρ^0$, $ϕ$ and $\jpsi$, based on non-perturbative and perturbative two-gluon exchange. This provides a common kinematical structure for non-perturbative and perturbative effects, and allows the role of the vector-meson vertex functions to be explored independently of the production dynamics. A good global description of the vector-meson data is obtained.