Abstract
We present first results of a partial wave analysis of the diffractive reaction $\pi- Pb \to \pi- \pi+ \pi- Pb$ based on data from the COMPASS experiment taken during a pilot run in 2004 using a 190 GeV/c $\pi-$ beam on a lead target. The analysis was performed in the region of squared four-momentum transfer $t'$ between 0.1 and 1.0 (GeV/c)^2. The $\pi- \pi+ \pi-$ final state shows a rich spectrum of well-known resonances. In addition a spin-exotic $J^{PC} = 1^{-+}$ state with significant intensity was observed at 1.66 GeV/c^2 in the $\rho(770) \pi$ decay channel in natural parity exchange. The resonant nature of this state is manifest in the mass dependence of its phase difference to $J^{PC} = 1^{++}$ and $2^{-+}$ waves. The measured resonance parameters are consistent with the disputed $\pi_1(1600)$. An outlook on the analyses of the much larger data set taken during 2008 and 2009 is given.
Highlights
The naıve Constituent Quark Model (CQM) describes light mesons as bound color-singlet states of quarks and antiquarks with flavors u, d, and s grouped into SU(3)flavor multiplets
We present first results of a partial wave analysis of the diffractive reaction π− Pb → π−π+π− Pb based on data from the COMPASS experiment taken during a pilot run in 2004 using a 190 GeV/c π− beam on a lead target
In order to reproduce the high-mass tail of the wave and its interference with the 1++0+[ρπ]S and the 2−+0+[ f2π]S waves, a second BreitWigner for the a2(1700) with its parameters fixed to the PDG values [32] was added to the fit
Summary
The naıve Constituent Quark Model (CQM) describes light mesons as bound color-singlet states of quarks and antiquarks with flavors u, d, and s grouped into SU(3)flavor multiplets. In the CQM the spin J, parity P, and charge conjugation C of a meson are given by (1) J = |L − S |, . In addition a meson is characterized by its isospin I and G-parity which is defined as (2) G = (−1)I+L+S. Both quantum numbers are conserved in strong interactions. In lattice QCD [2] simulations the lightest glueball is predicted to have ordinary scalar quantum numbers JPC = 0++ and a mass of about 1.7 GeV/c2. The interpretation of the data is, complicated by the mixing of the f0(1500) with other states
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