pxzz.m
function pxzz = pxzz(P0,k,r,tx,ty,pos)
    x = pos(1);
    y = pos(2);
    z = pos(3);
    xr = x - tx;
    yr = y - ty;
    d = sqrt(xr.^2 + yr.^2 + z.^2);
    epsilon = 1e-6; % small error
    sinTheta = sqrt(xr.^2 + yr.^2)./d + epsilon;
    
    pxzz = (1./(d.^7.*k.*r.*sinTheta.^3)).*(2.*exp(d.*i.*k).*P0.*xr.*((-k).*r.*sinTheta.*(d.^2.*(1 + 2.*sinTheta.^2 + d.*i.*k.*(1 - 2.*sinTheta.^2)) + (-1 + d.^2.*i.^2.*k.^2.*(1 - 3.*sinTheta.^2) + d.*i.*k.*(1 + 9.*sinTheta.^2) + sinTheta.^2.*(-9 + k.^2.*r.^2.*(-1 + sinTheta.^2))).*...
       z.^2).*besselj(0, k.*r.*sinTheta) + (d.^2.*(2 + sinTheta.^2 + k.*(2.*d.*i - (d.*i.*(1 + d.*i.*k) + k.*r.^2).*sinTheta.^2 + k.*r.^2.*sinTheta.^4)) + ...
      (2.*(-1 + d.*i.*k.*(1 + d.*i.*k)) - (3 - 3.*d.*i.*k + d.^3.*i.^3.*k.^3 + 2.*k.^2.*(-1 + d.*i.*k).*r.^2).*sinTheta.^2 + k.^2.*(-7 + 3.*d.*i.*k).*r.^2.*sinTheta.^4).*z.^2).*besselj(1, k.*r.*sinTheta)));
end