Pippa Wells21/10/2010 1

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