Luminosity spectrum measurement in future e+e− linear colliders using large-angle Bhabha events

In future e+e− linear colliders, the colliding energy cannot be monochromatic due to the large effect of beam-beam interactions before e+e− collisions. The luminosity must have a continuous spectrum which distributes around the nominal energy and has a long tail toward the low-energy region. An accurate measurement of the luminosity spectrum is indispensable for a precise experiment at high-luminosity e+e− linear colliders. It has been found that the luminosity spectrum can be determined by measuring the acollinearity angle of Bhabha events by a tracking device with good angular resolution. The beam parameters, i.e. the beam-energy spread, the topological beam size and the number of particles in a beam bunch, can be also extracted from the measured luminosity spectrum by a likelihood fitting.