Low energy antiproton-proton annihilation and meson spectroscopy

Recent results in antiproton-proton annihilation at rest from the Crystal Barrel at CERN/LEAR are reviewed. Rates for annihilation at rest into various two-body channels have been measured. In spite of the complexity of the annihilation mechanism, simple tests based on the underlying quark constituents can be applied. The annihilation process is also useful to investigate the spectrum of light quark mesons and to search for exotic (non-q¯q) mesons. Three scalar states have been found in the 1500 MeV mass region. One of them is a good candidate for the ground state glueball.     

Single pseudoscalar meson production in diffractive ep scattering

Exclusive pseudoscalar meson production in scattering at high energies is a direct probe for a possible “odderon” exchange in soft hadronic processes. Using a simple phenomenological ansatz for the odderon, we demonstrate how it can be separated from the contribution due to photon-photon fusion, and the relevant parameters be measured. Total cross sections and differential distributions are presented for , and production. Results are given from both a full calculation and one using the equivalent photon approximation. The accuracy of the latter is discussed. Received: 18 December 1997 / Published online: 10 March 1998     

Coherent meson production in antinucleon annihilation on nuclei

A phenomenological optical potential for antinucleon-nucleon (N?N) interaction in an energy range of 1 to 15 GeV is presented in an eikonal model. In addition to the discussion of the strength and energy dependence, the potential is used to describe initial state interactions for coherent meson production on nuclei. To describe nuclear matter effects, Hartree-Fock Bogoliobuv (HFB) densities are used. As a first step we calculate the production of two mesons, namely pions. To cover the large energy range, the final state pion-nucleus ( π A) interaction is improved by adding resonances beyond the Δ resonance to a Kisslinger-type ansatz. Results for N?N and π A cross sections are presented.     

SU(3) analysis of antiproton-proton annihilation into two mesons

We analyse existing data on antiproton-proton annihilation into two mesons using as minimal assumptions charge conjugation invariance, the quark line rule and SU(3). The analysis is greatly facilitated by using a set of invariant SU(3) couplings which is directly related to the contributing quark line diagrams.     

Quark model for the production of pseudoscalar mesons in annihilation

A modification of the quark model for annihilation permits the calculation of the relative probabilities of all observed channels with pseudoscalar mesons. Agreement with the experimental data on proton-antiproton annihilations at rest into three and more mesons is obtained.     

General formalism in collinear regime for pseudoscalar meson production in NN collisions

The spin structure of the matrix element for the pseudoscalar meson production processes in nucleon-nucleon collisions is established in the collinear kinematical regime in terms of 3 independent scalar amplitudes. This result is valid for any reaction mechanism and for any energy of the colliding and the produced particles. The complete experiment for the full reconstruction of all 3 complex amplitudes must contain two different classes of polarization experiments. The polarization transfer coefficients can be used to determine the moduli of all 3 amplitudes, whereas the spin correlation coefficients for + collisions are sensitive to the relative phases of different amplitudes. Received: 13 August 2001 / Accepted: 20 September 2001     

Evidence for the production of heavy vector mesons in antiproton-proton annihilation at rest

Stopping antiprotons in a hydrogen target, 26 electron pairs have been found, which can be attributed to the reaction $\text{p}\text{p}\text{→}\text{V}{}^0\text{(}\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{)+π}{}^0$. The analysis of the data supports the existence of the ρ″ (1600) and the ρ′ (1250) mesons.     

Pion multiplicity distribution in antiproton-proton annihilation at rest

The pion multiplicity distribution is widely believed to reflect the statistical aspects of annihilation at rest. We try to reproduce it in a grand canonical picture with explicit conservation of electric charge, isospin, total angular momentum, and the parity quantum numbersP, C, andG via the projection operator formalism. Bose statistics is found to be non-negligible, particularly in fixing the interaction volume. The calculated pion multiplicity distribution for n =5 turns out to depend strongly on the conservation of the angular momentum and connected quantum numbers, as well as on the spin state occupation inS-wave annihilation. However, the empirical Gaussian pion multiplicity distribution cannot be reproduced. This calls in question either the statistical ansatz or the rather old data themselves.Work supported in part by DFG, BMFT and GSI     

Exchange-component analysis of neutral pseudoscalar meson photoproduction

π⁰ and η photoproduction data in the region of 4 GeV are decomposed into t-channel exchange components of definite isospin and parity. The latter are discussed in terms of Regge exchange models and compared to vector-meson dominance related processes. The isospin-one natural parity-exchange component is smooth and structureless. The dominant unatural parity exchange in π⁰ photoproduction has isospin zero. As in π^± photoproduction the ratio of natural to unnatural parity exchange is found to be larger than the vector-dominance expectation. Complementary information from a recent spin decomposition of γp → π⁰p gives further insight into the SU (3) properties of the exchanges. We predict unmeasured quantities and stress, in particular, the importance of studying γn → π⁰n with polarised photons.     

Hydrodynamic description of D meson production in high-energy heavy-ion collisions

The large values and constituent-quark-number scaling of the elliptic flow of low-pT D mesons imply that charm quarks,initially produced through hard processes,...     

Regge description of nucleon-nucleon polarisation

Recent polarisation data for pp and pn elastic scattering within the momentum range 6 to 45 GeV/c are economically described by a simple Regge model. A crucial feature of the model is the inclusion of a non-factorising pomeron which couples both to helicity flip and to non-flip. This allows an understanding of the observed t-dependent structure and rapid energy dependence of the pp polarisation. Predictions are made for pp and pn polarisation measurements at higher momenta. Fragmentary data on $\text{p}\text{p}$ elastic polarisation are compared to predictions.     

Pure annihilation decay of strange beauty meson into two charm heavy mesons

In this study, the heavy to heavy decay of \begin{document}$ B^0_s\rightarrow D^{*+}D^- $\end{document} is evaluated through the factorization approach by using the final state interaction as an effective correction. Under the factorization approach, this decay mode occurs only through the annihilation process, so a small amount is produced. Feynman's rules state that six meson pairs can be assumed for the intermediate states before the final meson pairs are produced. By taking into account the effects of twelve final state interaction diagrams in the calculations, a significant correction is obtained. These effects correct the value of the branching ratio obtained by the pure factorization approach from \begin{document}$ (2.41\pm1.37)\times10^{-5} $\end{document} to \begin{document}$ (8.27\pm2.23)\times10^{-5} $\end{document}. The value obtained for the branching ratio of the \begin{document}$ B^0_s\rightarrow D^{*+}D^- $\end{document} decay is consistent with the experimental results.     

Single-pion production in antiproton-proton interactions

Single-pion production in $\text{p}\text{p}$ interactions at 9.1 GeV/c is analysed and cross sections are given. The p_lab dependence of the isospin amplitudes is determined by a fit to the world data. The data are also compared with the predictions of a reggeized Deck model. The results and the $\text{N}\text{π}\text{N}$ amplitudes are compared with the results at other energies and with the corresponding NπN amplitudes.     

The annihilation of virtual photons into pseudoscalar mesons

We investigate the possibility to constrain the pion distribution amplitude from the transition. For a surprisingly large range in the two photon virtualities we find that the transition form factor is essentially independent of the distribution amplitude. This in turn entails a parameter-free prediction of QCD. The form factors are also briefly discussed. We estimate that experimental studies might be feasible at the existing experiments BaBar, Belle, and CLEO. Received: 30 August 2001 / Published online: 23 November 2001     

Inclusive production of vector and tensor mesons in antiproton-proton annihilation and non-annihilation interactions at 9.1 GeV/c

Cross sections have been determined for the inclusive production of vector $\text{(?}{}^0\text{, ω,}\text{K}{}^1\text{)}$ and tensor (f, A₂^±) mesons in $\text{p}\text{p}$ reactions at 9.1 GeV/c for both annihilation and non-annihilation processes. Distributions in the Feynman variable x and transverse momentum squared, p_T², have been examined for the ?⁰, ω and f mesons. The slopes for p_T² appear to be exponential and decrease with increasing particle mass for both annihilation and non-annihilation reactions, furthermore the slopes have consistently higher values for non-annihilation reactions. Comparisons with other data indicate that the ratio ?⁰/π^? is independent of antiproton momentum in annihilation processes.     

D and D meson production near 4 GeV in e+e− annihilation

We have studied momentum spectra for D⁰ ($\text{D}$⁰) and D^± mesons produced near 4 GeV in e⁺e^? annihilation and find that production is dominated by two-body reactions involving D and D¹ states where the D¹ (charged and neutral) have masses near 2.01 GeV/c². A detailed study of D momentum spectra near E_cm = 4.028 GeV allows determination of masses and branching ratios of D and D¹ states. Lastly we find that events containing a D⁰ meson conserve strangeness, thus establishing upper limits on D⁰-$\text{D}$⁰ mixing and on the effects of charm changing neutral currents.     

Thermal emission of pions in antiproton-proton annihilation at 12 GeV/c

Using data of \(\bar pp\) annihilation at 12 GeV/c we have found that centrally-emitted pions follow a thermal energy spectrum with an inclusive temperature of 111±2 MeV, the same as has been found for pions produced in πp collisions at 16 GeV/c. An analysis of individual high-multiplicity events to search for fluctuations in the temperature parameter on an event-byevent basis has been made, using two different methods. We obtained the value 0.18±0.02 for the relative dispersion of the temperature fluctuations.     

The graal experiment: Beam polarization asymmetries in pseudoscalar meson photoproduction

The GRAAL tagged and polarized γ beam has been operating with a maximum energy of 1100 MeV. For the first time asymmetry results in ν photoproduction were obtained and will be shown. Interesting results have also been obtained for π⁰ and π⁺ photo-production, that largely confirm the already existing ones, but also extend in not yet explored angular and energetic regions, especially for backward scattering and energies greater than 900 MeV.