Spin density matrix of the $$\omega $$ ω in the reaction $${\bar{p}p}\,\rightarrow \,\omega \pi ^0$$ p ¯ p → ω π 0

Abstract

The spin density matrix of the ω has been determined for the reactionp p → ωπ 0 with unpolarized inflight data measured by the Crystal Barrel LEAR experiment at CERN. The two main decay modes of the ω into π 0 γ and π + π − π 0 have been separately analyzed for variousp momenta between 600 and 1940 MeV/c. The results obtained with the usual method by extracting the matrix elements via the ω decay angular distributions and with the more sophisticated method via a full partial wave analysis are in good agreement. A strong spin alignment of the ω is clearly visible in this energy regime and all individual spin density matrix elements exhibit an oscillatory dependence on the production angle. In addition, the largest contributing orbital angular momentum of thep p system

Coupled channel analysis of $${\bar{p}p}\,\rightarrow \,\pi ^0\pi ^0\eta $$, $${\pi ^0\eta \eta }$$ and $${K^+K^-\pi ^0}$$ at 900 MeV/c and of $${\pi \pi }$$-scattering data

“…No details about the sources of all non-interfering background contributions must be known a priory. This method is described in detail in [17] and was successfully applied in earlier analyses by the CLAS collaboration [18,19] as well as in a recent re-analysis of Crystal Barrel data [9]. In principle the method relies on the fact, that background events cannot reproduce the narrow mass shape of resonances as e.g.…”

“…The amplitudes for the individualp p channels are defined in a similar way as explained in [9]. The various contributing initialp p states are expressed as an expansion in terms of I G J PC states.…”

“…The various contributing initialp p states are expressed as an expansion in terms of I G J PC states. It has turned out from semi-classical calculations [26] as well as from data analyses, as described in [9] for example, that only states up to a maximal orbital momentum L max p p of thep p system contribute. The complete reaction chain starting from thep p annihilation system down to the decay into the final state particles is taken into account.…”

“…Several different hypotheses have been tested in order to get a first glance on the potentially contributing resonances. Based on the outcome from [9] the maximal orbital momentum of the initialp p system has been chosen to be L max p p = 4 for all fits. Also the production amplitudes have been limited to L max X s r = 5.…”

“…Based on the fitted amplitudes the spin density matrix (SDM) elements for the two vector mesons φ(1020) and K * (892) ± can be determined which provide the full information on the underlying production process. The comparison with the ω production in the channel p p → ωπ 0 [9] delivers new information about thep p production process of light mesons with strange-quark content. The reactionp p → K + K − π 0 has already been studied in detail for beam momenta between 1.0 and 2.5 GeV/c [10].…”