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LEP Energy Calibration

LEP Energy Calibration. Or the saga of 1001 shifts…. Pippa Wells, CERN. 10 Sep 2008. First circulating protons. WRONG. 11 Dec 1989. First circulating protons. 1989 & 1990 @20GeV Infer speed of protons by comparing RF frequency for e and p on central orbit

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LEP Energy Calibration

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  1. LEP Energy Calibration Or the saga of 1001 shifts…. Pippa Wells, CERN Pippa Wells

  2. 10 Sep 2008 First circulating protons WRONG Pippa Wells

  3. 11 Dec 1989 First circulating protons • 1989 & 1990 @20GeV • Infer speed of protons by comparing RF frequency for e and p on central orbit • Magnetic measurements (ref. magnet & flux loop) to extrapolate to 45GeV • LEP circumference shrank between measurements by 1.6±0.8mm (hint for future?) • 20 MeV uncertainty on MZ Pippa Wells

  4. Beam energy at the interaction points • Beam energy varies around the ring • Synchrotron radiation in the arcs • Energy restored by the RF cavities (originally at IP2 & IP6) • If cavities are precisely positioned, beams gain the same energy on the way into the IP and on the way out. • ECM equal at all IPs • Copper cavities drivenby TWO frequencies • RF power oscillates between storage and main cavity. • Aligned to wrong freq! • ECM shifts at L3 and OPAL of 10~20 MeV Pippa Wells

  5. RF model • Careful logging of the exact conditions of the RF system to calculate ECM at each IP as a function of time • New superconducting cavities at all 4 IPs gradually replaced copper cavities (energy increase for LEP2) Pippa Wells

  6. Resonant depolarisation • Electron spin aligns with vertical B field of dipoles due to synchrotron radiation • Slow (hours) build up of polarisation if the beam orbit is sufficiently smooth. • Spins precess - number of precessions per orbit (spin tune): • Best polarisation buildup for half-integer s • Monitor polarisation and scan frequency of externalB field to measure s Pippa Wells

  7. RDP gives o(100keV) instantaneous precision on average Ebeam (Coloursrefer to different bunches) Choose Z lineshape scan points at non-integer s Measure Ebeam at ends of fills with RDP Resonant depolarisation Pippa Wells

  8. Length of beam orbit fixed by RF freq Earth tides change length of tunnel (1mm in 27km). Magnets move w.r.t. beam Extra contribution from quadrupole fields off central orbit changes Ebeam Amplitude ~10MeV We found the moon Montes Jura Lunar Hadron Collider? Pippa Wells

  9. We found water • Long term changes to LEP circumference, C • But some discrepancies remained until 1995, especially for measurements during long fills Pippa Wells

  10. 1995 - installed two NMR probes in LEP dipoles on opposite sides of the ring Noise related to human activity in daytime; quiet over night General trend - energy increases during fill Measuring Ebeam at the end of fill gives a ~5 MeV bias on average NMR probes Pippa Wells

  11. Vagabond currents from French DC electric trains Measured current on beam pipe and NMR field change We found the trains Pippa Wells

  12. Corrections for pre-1995 data • Model to correct for magnetic behaviour, extrapolating back from end-of-fill resonant depolarisation measurements • Time of day • Time from start of fill • Magnet temperatures • (RF configuration, and other IP effects) • Confirmed with more NMR probes in the tunnel during LEP2 times Final ECM uncertainty on Z mass : 1.7 MeV MZ = 91.1875 ± 0.0021 GeV Pippa Wells

  13. Depolarising effects increase with energy LEP2: Calibrate 16 NMR measurements to resonant depolarisation measurments Validate extrapolation with flux loop measurements and other methods LEP2 - beyond RDP Ebeam constraint for MW Pippa Wells

  14. Detailed mapping of the spectrometer field by “the mole” Require 1micron precision from BPMs Cross-calibrate with RDP, then ramp to physics energy (short term stability). The LEP spectrometer Pippa Wells

  15. Use two types of events: e+e- ff e+e- Z Z  ff √s’, effective mass of the ff system Calculate √s’ from event kinematics Compare with well known value of MZ Statistically limited at LEP2 Could be a useful technique for a future linear collider Cross check with radiative returns Pippa Wells

  16. LEP energy and the future Final ECM uncertainty on Z mass : 1.7 MeV MZ = 91.1875 ± 0.0021 GeV ECM uncertainty on (LEP) W mass : 10 MeV MW = 80.376 ± 0.033 GeV • Very detailed studies and many careful checks went into the LEP energy calibration • The Z (and W) mass measurements will set the energy scale for higher energy studies for the foreseeable future Pippa Wells

  17. Post Script Physics Coordinators’ report - update to 2010 (Previous OPAL plenary Dec 2005) Gabriella Pásztor and Pippa Wells Pippa Wells

  18. 2010 Pippa Wells

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