Measurement of the spin of the Ξ(1530) resonance

Abstract

The properties of the Ξ ( 1530 ) resonance are investigated in the Λ + c → Ξ − π + K + decay process. The data sample was collected with the BABAR detector at the SLAC PEP-II asymmetric-energy e + e − collider operating at center-of-mass energies 10.58 and 10.54 GeV. The corresponding integrated luminosity is approximately 230 fb − 1 . The spin of the Ξ ( 1530 ) is established to be 3 / 2 . The existence of an S -wave amplitude in the Ξ − π + system is inferred, and its interference with the Ξ ( 1530 ) 0 amplitude provides the first clear demonstration of the Breit-Wigner phase motion expected for the Ξ ( 1530 ) . The P 1 ( cos  θ Ξ − ) Legendre polynomial moment indicates the presence of a significant S -wave amplitude for Ξ − π + mass values above 1.6 GeV / c 2 , and a dip in the mass distribution at approximately 1.7 GeV / c 2 is interpreted as due to the coherent addition of a Ξ ( 1690 ) 0 contribution to this amplitude. This would imply J P = 1 / 2 − for the Ξ ( 1690 ) . Attempts at fitting the Ξ ( 1530 ) 0 line shape yield unsatisfactory results, and this failure is attributed to interference effects associated with the amplitudes describing the K + π + and/or Ξ − K + systems.