Mean values <bar_a_x> and <bar_a_y> of the scaled Courant-Snyder forms for the measured shots that are close to the resonance. The quantities bar_a_x and bar_a_y are the scaled CourantSnyder forms of the particles. The statistical error bar (precision) of the measured quantities <bar_a_x> and <bar_a_y> when the beam is on the fixed line is sigma_<bar_a_x> < 0.5 mm^2 and sigma_<bar_a_y> < 1.0 mm^2. In case the beam is not centred on a fixed line, the error bars remain nevertheless within the marker size. The quantity D ( Methods and equation ( 9 )), shown by the colour of the markers, indicates the degree of shift in the bar_a_x and bar_a_y values and, thus, how stable the machine conditions are during the measurement or how close the beam is to the centre of the resonance structure. Small values of D (yellow band) signify that the beam is close to a fixed line and is stationary. If D is large (blue band), then either the beam is far from the fixed line or there has been a drift of the machine parameters ( Methods ).
