Narrow Results

In the usual holographic approach to QCD, the meson spectrum is generated due to a nontrivial five-dimensional background. We propose an alternative five-dimensional scenario in which the spectrum emerges due to coupling to a scalar field whose condensation is supposed to be dual to the formation of gluon condensate and mimics the scale anomaly in QCD. The spectrum of model has finite number of discrete states plus continuum and reveals a Regge-like behavior in the strong coupling regime.

We present results from a Partial-Wave Analysis (PWA) of diffractive dissociation of 190 GeV/c π^- into π^-π⁺π^- final states on nuclear targets. A PWA of the data sample taken during a COMPASS pilot run in 2004 on a Pb target showed a significant spin-exotic J^PC = 1^-+ resonance consistent with the controversial π₁(1600), which is considered to be a candidate for a non- mesonic state. In 2008 COMPASS collected a large diffractive π^-π⁺π^- data sample using a hydrogen target. A first comparison with the 2004 data shows a strong target dependence of the production strength of states with spin projections M = 0 and 1.

This paper presents an analysis of π^- Pb → X^- Pb → π^-π^-π⁺ Pb events at 190 GeV/c beam momentum and low four-momentum transfer t′ < 0.01 (GeV/c)². Coherent scattering off the lead nucleus as a whole dominates with contributions from Reggeon, Pomeron and photon exchange. Photoproduction becomes apparent at lowest t′ and can be extracted statistically as well as by a partial-wave analysis, indicating also the overlap of diffractive and photo-production of the 3π events.