Beam Energy Overview

This topic is principally concerned with setting the absolute energy scale of the colliding beams. Precise knowledge of the collision energy is needed to make mass measurements in both threshold scans and direct reconstruction using kinematic fits. The beam energy scale may also be useful to monitor detector calibrations at high energy using lepton pair production. The top mass measurement (at threshold) and the Higgs mass measurement (via direct reconstruction) both point to a target precision of roughly 100 PPM.

The following is a list of obvious topics and questions. Please feel free to suggest additional questions, provide answers, or express an interest in thinking about any of these issues.

Physics Questions

• Is 100 PPM enough for high energy running?
• Are there other threshold scans (sLeptons for example) which could use higher precision?
• Is this precision needed all the way up to 1 TeV?

Potential Beam Instrumentation

• WISRD-style spectrometer
• BPM-style spectrometer (RF or conventional)
• Moller/Bhabha scattering
• Other ideas (Spin precession, compton scattering, resonant photon production, ...)

Potential Detector Measurements

• Radiative returns
• Mu-pair spectrometry (using tracking)

Operational Questions

• What is the possible variance of beam energy on short time scales?
• How precise do we need real-time measurements (pulse to pulse or train to train)?
• What are the likely correlations with other parameters (lumi, polarization, etc.)?
• Possible to measure anything downstream during collisions?

General Questions (all techniques)

• How much time is required to reach target precision?
• How sensitive is this technique to backgrounds?
• How sensitive is this technique to radiation?
• How often does this technique need calibrating at Z-pole, and for how long?
• Estimated cost in $$$ and in terms of operational difficulty?

WISRD questions

• Possible detector technologies (fiber target, visible synchrotron, ...)
• Optics issues
• Possible to measure energy spectrum as well?

BPM questions

• Radiation sensitivity
• BPM stability
• Sensitivity to disrupted beam
• Possible upstream?

Moller scattering

• Necessary detector precision/stability
• Impact on operations
• Acceptable event rate, time needed to reach target precision
• Backgrounds

Documentation and References