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Benoit,
The absolute energy of the beam line depends on our geometry and absolute knowledge of the current in the power supplies. It was measured long time ago (NA8 experiment if I remember well) and found to be about 0.3%. Last year NA61 did a measurement looking the tacks of the beam particles in their spectrometer and found the same value.
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Ilias Efthymiopoulos
CERN/AB Department
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