$\overline pp$ annihilation into four charged pions at rest and in flight

The spin-parity analysis of the data on the annihilation reaction at rest in liquid and in gaseous hydrogen at 3 bar pressure and in flight at momentum of MeV/c, collected by the Obelix spectrometer at the LEAR complex of CERN, is presented. The relative branching ratios and are obtained. It is also shown that the inclusion of the exotic meson , J^PC = 1^{- +} , mass and width , GeV/c², in its decay to , improves the fit and some implications of these results are briefly discussed. The relative S and P-wave annihilation percentages in four charged pions at two target densities are obtained.Received: 2 December 2003, Published online: 23 April 2004     

N\bar N annihilation into two body channels

A simple model is constructed forN¯N annihilations into two non-strange mesons. Determining the ratio of isospin one to isospin zero annihilation by experimental information we predict the branching ratios for these channels. The agreement with experiment is surprisingly good.     

A new model forN \bar N annihilation

A new quark model forN \(\bar N\) annihilation is proposed. It is argued that a linear superposition of the so-called³P₀ and³S₁ models is more consistent with QCD and the inclusion of quark degrees of freedom inNN scattering, and is also suggested by the data. A comparison is made, in Born approximation, with the angular distribution for \(p\bar p \to \Lambda \bar \Lambda \) .     

A nonperturbative description ofN \bar N annihilation

A nonperturbative picture ofN \(\bar N\) annihilation is proposed. In particular, different parts of the nucleon and the antinucleon do not behave independently, but are tied together by their topological charge. We describe theN \(\bar N\) state as a coherent soliton-antisoliton configuration, and argue that vector mesons play an important role in the decay process. SomeS-wave selection rules are derived. Their relevance to the present LEAR experiments is discussed.     

The\bar pp \to e^ + e^ - reaction at rest and the hierarchy of the annihilation ranges

A new procedure is devised to evaluate the proton form-factor at the ¯pp threshold from experimental data on ¯ppe ⁺ e ^– annihilation at rest. Comparison of the ¯ppe ⁺ e ^– and¯pp ^+– processes reveals that annihilation ranges for various channels are ranked in line with quark dynamics.This work has been funded in part by the German Federal Minister for Research and Technology (BMFT) under the contract number 06HD756     

Pion correlations and resonance effects in \bar{p}p annihilation to 4\pi^0 at rest

We study correlations in the exclusive reaction at rest with complete reconstruction of the kinematics for each event. The inclusive distribution is fairly flat at small invariant mass of the pion pair while a small enhancement in the double differential distribution is observed for small invariant masses of both pion pairs. Dynamical models with resonances in the final state are shown to be consistent with the data while the stochastic HBT mechanism is not supported by the present findings. Received: 26 February 2002 / Revised version: 22 July 2002 / Published online: 30 August 2002     

Observation and analysis ofE mesons in\bar p p annihilation at rest in H2 gas

Antiproton-proton annihilation at rest in a gaseous H₂ target at NTP into the final state π⁺ π^? K ^± π^? (K ⁰) with an undetectedK ⁰ or \(\bar K^0 \) has been investigated. We observe theE(1420) resonance in the invariant mass spectrum (K ⁰)_miss K ^± π^? with massM _E =1413±8 MeV/c² and widthГ ^E =62 ± 16MeV/c² and find evidence for the production of thef ₁(1285). The absolute branching ratio of \(\bar p\) p → π⁺ π^? E ⁰,E ⁰ →K ₀ ^L K ^± π ^? at (61±6)%P wave annihilation is (3.0±0.9)·10^?4 of all annihilations. The observed suppression of theE production fromP wave with respect to theS wave together with some simple selection rules suggest that the quantum numbers of theE(1420) areJ ^pc=0^?+ and not I⁺⁺.     

Proton spectrum from\bar pd \to 5\pi p at rest and the elementary\bar pnannihilation

We calculate the shape of the proton spectator distribution in \(\bar pd \to 5\pi p\) at low proton momenta. The sensitivity to atomicS- andP-state orbits and to the size of elementarys- andp-wave annihilation is discussed. Implications for the elementary annihilation amplitude \(\bar pn \to 5\pi \) subthreshold are presented.     

Studies in\bar p p annihilation

Evidence is presented that \(\bar p\) p annihilation proceeds via the formation of two jets, with average transverse momentum of the order of the nucleon mass. The implications of this behaviour is linked to the measured cross sections and multiplicities, and explanations for it have been sought in the underlying gluon and quark constituents of the \(\bar p\) p system. The confrontation of the suggested constituent mechanisms and experimental data lead to the concept that charm meson production may play a significant role in the \(\bar p\) p experiments at the highest available energies.     

The inclusive proton spectrum for\bar pd \to 3\pi p at rest re-examined

We re-analyze the spectator proton distribution for ¯pd→π ⁺ π^? π^? p. For this channel previous calculations underestimated the cross section for proton momenta of 300 to 600 MeV/c by about a factor 7 while the channels with higher pion multiplicities and the ¯pd→K¯ Kmπp reactions were well explained. By using detailed experimental information on ¯pn and ¯pp annihilation, meson rescattering can in fact explain the cross section also for the 3π case. The¯ pp annihilation followed by charge exchange pion rescattering is much larger than in the 5π case. Vector meson exchange is small but not negligible at high spectator momenta.     

Production of colourless scalar resonance inp\bar p annihilation

We analyse under what conditions the production of colourless scalar resonance (Higgs boson) can be enhanced and its subsequent decays into gauge bosons and/or light Higgs bosons can provide a test for them. We discuss the expected experimental signatures and point out difficulties of obtaining large coupling to light quarks in the framework of a multi Higgs doublet model with horizontal symmetry.     

N \bar N Annihilation creates two mesons

N \(\bar N\) annihilation is assumed to occur exclusively via two-meson production, with nearest threshold dominance. A phenomenological expression for the branching ratios, implementing the rule, contains two parameters. The set ofs- andp-wave (q \(\bar q\) ) mesons is used. Reasonable results are obtained for rates measured inp \(\bar p\) annihilation at rest. The numerous channels are projected into the multipion and kaon final states for comparison with in-flight experiments. A good overall agreement is reached up top _lab～3 GeV/c; beyond, more mesons should be included to provide new thresholds.     

Manifestation of the bound state \bar NN (1870) in the annihilation process \bar pd \to p_s 5\pi

It is shown that the existence of an N?N bound state with a mass of approximately 1870 MeV and width 10 MeV follows from analysis of the experimental data on the momentum distribution of recoil protons in the annihilation process p?d→p _s5π.     

Dynamical selection rules from $p \bar p$ annihilation at restin three meson final states

The hadronic annihilation branching-ratios in quasi-two-body final states have been obtained from the observation of the reactions , at rest in hydrogen targets at different densities. The enhancement or suppression of specific hadronic channels connected to dynamical selection rules is observed in the production of different resonances both in protonium S and P-waves, systematically investigated for the first time. Besides the well known and enhancement from ³S₁ and ¹S₀ partial waves, the dominance of one isospin source in production, well established from S-waves, is confirmed also in P-waves (¹S₀, I = 0; ³S₁, I = 1; ¹P₁, I = 0; ³P₁, I = 0; ³P₂, I = 1). In addition, the experimental data clearly show a strong suppression of and final states from P-wave which has a remarkable coincidence with pattern production.Received: 4 February 2004, Published online: 5 May 2004