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LEP Energy Calibration
Or the saga of 1001 shifts….
Pippa Wells, CERN
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First circulating protons 10 Sep 2008
WRONG
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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
11 Dec 1989
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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
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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)
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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
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Resonant depolarisation
• RDP gives o(100keV) instantaneous precision on average Ebeam
• (Colours refer to different bunches)
• Choose Z lineshape scan points at non-integer s
• Measure Ebeam at ends of fills with RDP
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We found the moon• 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
Montes Jura
Lunar Hadron Collider?
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We found water
• Long term changes to LEP circumference, C• But some discrepancies remained until 1995, especially for
measurements during long fills
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NMR probes
• 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
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We found the trains
• Vagabond currents from French DC electric trains
• Measured current on beam pipe and NMR field change
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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
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LEP2 - beyond RDP• Depolarising effects increase with energy• LEP2: Calibrate 16 NMR measurements
to resonant depolarisation measurments
• Validate extrapolation with flux loop measurements and other methods
Ebeam constraint for MW
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The LEP spectrometer
• 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).
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Cross check with radiative returns• Use two types of
events:e+e- ffe+e- ZZ 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
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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
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Post Script
Physics Coordinators’ report -
update to 2010
(Previous OPAL plenary Dec 2005)
Gabriella Pásztor and Pippa Wells
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2010