Progress Report for the BPM Energy Spectrometer Test Experiment at ESA

The main physics programme of the International Linear Collider (ILC) requires a mea- surement of the beam energy with a relative precision of 10 4 or better. To achieve this goal a spectrometer using high resolution beam position monitors (BPM) and ac- curately monitored bending magnets has been proposed. A prototype spectrometer chicane using 4 dipoles is now commissioned in End Station A (ESA) at SLAC, in- tending to demonstrate the required stability of this method and investigate possible systematic effects and operational issues. In this contribution we will describe the ex- perimental setup for this ESA test experiment (T-474/491), which has been finalised during two runs in 2007, and present results from the BPM commissioning runs in 2006. The design of the International Linear Collider is driven by the broad precision physics pro- gramme of electroweak, Higgs, QCD and possible SUSY measurements. The uncertainty on the energy of the colliding electron and positron bunches contributes directly to the system- atic error on e.g. top quark W and Higgs masses (2), making a precise energy measurement of the beam of crucial importance. At LEP2, an energy spectrometer was successfully com- missioned, achieving an accuracy of 1.9 × 10 4 (3). Dipoles in the bending sections of the storage ring were used to induce a deflection of the lepton beam. With accurate knowledge of the total integrated field of these bending magnets together with a measurement of the deflection itself, one can derive the energy of the beam. The ILC energy spectrometer has similar design requirements in terms of accuracy, however to limit the emittance growth due to synchrotron radiation in the beam delivery system, the introduced dispersion in the spectrometer chicane has been restricted to 5 mm. Also, at the ILC the measurement has to be done in a single shot. High resolution RF cavity BPM systems are therefore preferred to strip-line or button BPMs as these can achieve resolutions well below a micron (4), needed for a precision energy measurement. As a proof of principle, a test beam experiment (T-474/491) was proposed (5) at ESA at the Stanford Linear Accelerator Center (SLAC), focusing on studying the achievability and