Non-vanishing and sign changes of Hecke eigenvalues for Siegel cusp   forms of genus two (with an Appendix by E. Kowalski and A. Saha)
  In this paper, we show that half of non-zero coefficients of the spinor zeta function of a Siegel cusp form of genus 2 are positive and half are negative. 
Emmanuel Royer, Jyoti Sengupta (TIFR), Jie Wu (IECL)
math.NT