The e+e- energy spread at collision is far wider at a high luminosity linear collider than in any earlier machine. In order to make precise measurements of physics processes it will be necessary to measure three distinct sets of data for every physics run:
- The absolute value of the mean energy from the linacs;
- The shape of the energy spectra from the linacs;
- The luminosity-weighted energy spectrum for physics events, including the effects of beamstrahlung, of linac energy spread and of initial state radiation.
UCL worked with collaborators to develop an upstream calorimeter to measure absolute beam energy. It uses cavity beam position monitors (BPMs) in a dedicated spectrometry chicane which has been added to the Beam Delivery System design (see Mark Woodley’s talk at BDIR meeting).
There was a similar spectrometer at the CERN LEP machine which almost achieved the desired precision but had instabilities which have never been fully understood.
We participated in two test beam programmes which aimed at reducing the instability and establishing the spectrometry technique. One of these programmes is the Nanobpm collaboration, which takes data at the ATF test facility at the KEK Lab. in Japan – specializing in BPM performance. The second programme was at SLAC End Station A where we will developed a prototype chicane to study the stability of the whole spectrometer system.
This programme formed part of the LC-ABD programme, workpackage 4.2, with additional funding from EuroTeV. Other colleagues working at End Station A set up tests of a prototype downstream chicane to measure the beam spectrum.
Neither of these techniques produces a luminosity-weighted spectrum. The UCL group proposed in 1990 that the best way to complete the measurement of the luminosity spectrum with the accuracy required will be to measure the acollinearity of Bhabha scattering (e+e- elastic) events in the endcap region of the detector (~100 to 300 milliradians).
Although this acollinearity reflects the momentum difference of the colliding particles, rather than on their sum, it is sensitive to both the energy spread and the beamstrahlung and automatically gives luminosity-weighted results. UCL performed physics studies of this technique, with special attention to the precise measurement of the top quark mass.