10.10.2000 J.Wenninger - LEP fest 1

Energy Calibration at LEP

Spins, Tides

Vagabond Currents

J. WenningerLEP fest 10.10.2000

and

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Polarization at LEPUnder the influence of synchrotron radiation, the LEP beams polarize spontaneously (align their spins) in the transverse (vertical) direction.

Polarization is a slow and delicate process which requires a lot of care and special machine conditions !

record PT = 57%

routine PT = 5 -10%

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At LEP :

Ideal machine :

PTmax = 92.4%

Up to 60.6 GeV

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Resonant Depolarization

The number of precessions/turn is proportional to the energy :

Resonant depolarizationResonant depolarization

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To determine the energy Measure !Measure !

Principle : Get a fast magnet (“kicker”). Sweep the B-field and observe PT.

If kicker frequency and match, PT

is rotated away from the vertical axis.

The interest of PT : magnetic moments precess in B-fields.

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In the control room : Intrinsic accuracy :

E < 0.4 MeVE/E < 10-5

E < 0.4 MeVE/E < 10-5

Resonant Depolarization II

Sweep the magnet frequency over a selected interval (~ 22 Hz). Observe the effect on PT.

This is more than one orderof magnitude better than any other method !

But it requires an largeamount of DEDICATEDbeam time !

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Z Resonance ScansGood regions for PT are ~ 50 MeV wide and spaced by 441 MeV.

Convenient for Z mass and width measurements !

Calibrations cannot beperformed during “physics”

(no PT with colliding beams)

Beam energy modelBeam energy model

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Extrapolation in time

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Stressed Rings

Sensitivity of the energy tocircumference changes :

At LEP resonant depolarization is sensitiveto circumference changes of C/C ~ 10-9 !

At LEP resonant depolarization is sensitiveto circumference changes of C/C ~ 10-9 !

1991 : the first calibrations revealed unexplained fluctuations of the beam energy. A SLAC ground motion expert suggested… tides !

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The beam samplesdifferent fields.

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Earth Tides

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Tide bulge of a celestial body of mass M at a distance d :

= angle(vertical,the celestial body)

Earth tides :

The Moon contributes 2/3,

the Sun 1/3. NO 12 hour symmetry

(direction of Earth rotation axis). Not resonance-driven

(unlike Sea tides !). Accurate predictions.

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Moonrise over LEP

Fall of 1992 : The historic tide experiment !

The total strain is 4 x 10-8 (C = 1 mm)

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Success in the Press !

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Underground Water

Underground water Rainfall Lake levels Other ?

1993 : Unexpected energy “drifts” over a few weeks were traced to cyclic circumference changes of ~ 2 mm/year.

Circumference changemeasured with the radialbeam position.

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The Crack in the ModelSpring of 1994 : the beam energy model seemed to explain all observed

sources of energy fluctuations...

An unexplained energy increase of 5 MeV wasobserved in ONE experiment.

It will remain unexplained for two years…

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The Field Ghost

Short term fluctuations

• Long term increase (hysteresis)

Energy increase of ~ 5 MeV

over a LEP fill !

• Quiet periods in the night !

The data showed (unexpected) :

Human activity !Human activity !

But which one ??

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PipebustersThe explanation was given by the Swiss electricity company EOS...

Vagabond currents from

trains and subways

Vagabond currents from

trains and subways

Source of electrical noiseand corrosion

(first discussed in …1898 !)

I blast your pipes !

~20%

~80%

Vagabond (Earth) current

DC railway

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Vagabonding Currents

Injection lines (Point 1) Point 6 (Versoix river)

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LEP is affected by the French DC railway line Geneva-Bellegarde

A DC current of 1 A is flowing on the LEP vacuum chamber.

Entrance/exit points :

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TGV for Paris

The current on the railway tracks The current on the vacuum chamber The dipole field in a magnet

correlate perfectly !

Because energy calibrations were usually performed :

• At the end of fills (saturation) During nights (no trains !)

we “missed” the trains for many years !

we “missed” the trains for many years !

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November 1995 : Measurements of

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Epilogue

• More than 50 24-hour days of machine time were devoted to energy calibration between 1993 and 2000...

• The mass and width of the Z boson were measured with a remarkable accuracy (see forthcoming talks). The beam energy contributes ~ 1.5 MeV to the total errors. Work is in progress on for the W mass...

• The LEP Energy Calibration Working Group was a very successful collaboration between physicist from the machine and the experiments, building ties between the two communities.

• Many other effects besides tides and trains are included in the LEP energy model. There is not enough time to give details ...

• 5 years (1991-1995) were needed to unravel most of the beam energy “mysteries”.

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LEP Laser Polarimeter