A Short History of Nearly Everything

1
A Short History of Nearly Everything

• Michele Viti

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Outline

• Myself
• About my work in Zeuthen
• ILC overview
• Beam energy measurement
• An brief overview of my work and results
• Magnetic measurements
• Relative beam energy resolution
• Laser Compton energy spectrometer

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Myself

• I was born 31 years ago somewhere in Italy
• I studied physics at the Perugia university

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Myself

• Master degree in 2004.
• Title of the thesis Evaluation of a Tracking
  Algorithm for the Trigger of KOPIO
  Experiment on the Decay
  
• .
  
• I continued working on this topic until the
  project was canceled by the DOE.

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Myself

• I moved then to Germany and started in February
  2006 my PhD.
• I joined the Linear Collider working under the
  supervision of H.J. Schreiber.
• Title of the thesis Precise and Fast Beam Energy
  Measurements at ILC.

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ILC

• 30 Km electrons/positrons linear accellerator
• Total energy in the cms 500 Gev (upgradeable 1
  Tev)
• High luminosity (21034 /cm2s)
• A machine for precise measurements

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ILC Precise Top Mass Measurements

• Many Standard Model depends strongly on the value
  of the Top Mass.
• Well understood background, clean experimental
  environment
• Best direct measurement of the top mass will be
  at ttbar threshold
• Vary the beam energy (Precise Beam Energy
  Measurements)
• Count number top-antitop events.

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Basic Requirements for Beam Energy Measurements

• In order to make a precise measurement of the top
  quark mass we need to know some input
  parameters very well such as the mean energy of
  the bunch
• We need to have a fast (bunch-by-bunch), precise
  and non-destructive monitor for beam energy
• Direct measurement of energy at the IP is very
  difficult. We want to measure the beam energy
  upstream, downstream the IP plus a slow
  monitoring at the IP
• Relative Energy precision required for upstream
  measurements

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Magnetic Chicane Energy Spectrometer

• Electrons are deflected in this chicane and the
  offset in the mid-chicane is
  anti-proportional to the energy.
• Measuring this position with some special devices
  (Beam Position Monitor, BPM) together with
  B-field integral we have access to the beam
  energy
• Method well tested used at LEP with a precision
  of

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Experiment T474/491

• At the End Station A (ESA) a 4-magnet chicane
  energy spectrometer was commissioned in 2006/2007
  (experiment T474/491).
• The goal is to demonstrate the feasibility of the
  system.

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End Station A

• Characteristic
• Parasitic with PEP II operation
• 10 Hz and 28.5 GeV
• Bunch charge, bunch length energy spread similar
  to ILC
• Prototype components of the Beam delivery System
  and interaction Region.

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End Station A
Beam Parameters at SLAC ESA and ILC
Parameter SLAC ESA ILC-500
Repetition Rate 10 Hz 5 Hz
Energy 28.5 GeV 250 GeV
Bunch Charge 2.0 x 1010 2.0 x 1010
Bunch Length 300-500 mm 300 mm
Energy Spread 0.2 0.1
Bunches per train 1 (2) 2820
Microbunch spacing (20-400 ns) 337 ns
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Experiment T474/491

• Institutes involved SLAC, U.C. Berkeley, Notre
  Dame, Dubna, DESY, RHUL, UCL, Cambridge
• 2006
• January (4 days) commissioning steering BPMs
• April(2 weeks) commissioning cavity BPMS,
  optimization digitization and processing
• July(2 weeks) commissioning interferometer and
  stabilty data taken with frequent calibrations
• 2007
• March(3 weeks) Commissioning and installation
  magnets first chicane data!!!
• July(2 weeks) Additional new BPM in the centre
  of the chicane.

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Magnetic measurements
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Magnetic measurements

• B-field Integral, essential parameter for beam
  energy measurement.
• Need to be measured with an accuracy of 50 ppm.

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Magnetic measurements

• Between November 2006 February 2007
  measurements on these magnets were performed in
  the SLAC laboratories (DESY, Dubna, SLAC).
• Purpose of the measurements
• General understanding and characterization of the
  magnets
• Stability of the B-field and B-field integral
  with fixed current and switching the polarity.
• Monitoring of the residual B-field.
• B-field map.
• Measurement of the temperature coefficient for
  B-field and B-field integral .
• Development and test a procedure to monitor the
  B-field integral.

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Magnetic measurements

• Monitor of the B-field integral in ESA no device
  was available to measure directly this quantity.
• Solution measure the B-field in one point and
  from that determine the integral.
• Basic assumption

When the field is changing in one point, changes
everywhere by the same amount. The field shape
stay constant
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Magnetic measurements

• To measure the B-field in one point an NMR probe
  was used.
• Flip coil technique to measure for B-field
  integral.
• Calibration of the NMR determination of the
  slope and intercept for the relation.
• Comparison of the prediction with the measurement.

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Magnetic measurements

• The total error on the estimation of the B-field
  integral using the one-point B-field measurement
  was
• Main contributions are alignment errors of the
  devices.
• Several suggestions were given to improve the
  results.

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Relative beam energy resolution
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Relative Beam Energy Resolution

• At the End Station A several problems occurred
  for 4-magnet chicane prototype
• A complementary method to cross-check the
  absolute energy measurement was not implemented
• Only relative energy measurement possible at ESA

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Relative Beam Energy Resolution
Beam direction

• The offset d in the mid-chicane point is
  determined by two points, namely Xb and X0
• Xb is measured by the BPMs the mid-chicane and X0
  is extrapolated using BPMs upstream and
  downstream of the chicane.

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Relative Beam Energy Resolution

• BPMs, Beam Position Monitors. They measure the
  transverse position (X and Y) and angle (tilt) in
  the X-Z and Y-Z plane (X and Y).
• Accuracy on position measurement lt 500 nm.

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Relative Beam Energy Resolution

• X0 can be written as
• For zero current magnet XbX0, the BPM measures
  directly X0.
• The coefficients in Eq. above can be determined
  with a minimization.

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Relative Beam Energy Resolution
Beam direction

• One fundamental condition the magnetic chicane
  must work symmetrically
• The upstream path must be restored downstream

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Relative Beam Energy Resolution

• Unfortunately this was not the case of the
  4-magnet chicane in ESA
• For a given current the magnet fields were
  different up to 3
• BPMs downstream could not be used to determine
  X0. This resulted in a worse resolution for d.

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Relative Beam Energy Resolution

• A resolution of 24 MeV was found (Resolution --
  the smallest amount of energy change that the
  instrument can detect reliably)
• For a beam energy of 28.5 GeV this corresponds to
  a relative resolution of
• The largest contribution to this number comes
  from the resolution on d (gt2 microns).

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Laser Compton Energy Spectrometer
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Laser Compton Energy Spectrometer

• At LEP it was possible to have redundant beam
  energy measurement devices ? cross check!!!
• At ILC so far, complementary methods for upstream
  beam energy measurements not implemented.
• Studying the feasibility of an upstream energy
  spectrometer based on Compton backscattering
  (CBS) events.

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Laser Compton Energy Spectrometer

• Compton process with initial electron not at
  rest.
• Energy spectrum for electrons (photons) present a
  sharp cut-off (Compton edge).
• Scattered particles collimated in forward region.

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Laser Compton Energy Spectrometer
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Energy measurement

• , is the center of gravity of the scattered
  photons, or, equivalently, the end point of the
  SR fan.
• , position of beam, possible to measure
  with BPMs
• , position of the electrons with minimum
  energy.

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Laser Compton Energy Spectrometer

• Beam parameters
• Beam energies 50-500 GeV
• Beam size in x (y) 20-50 (2-5) microns
• Geometrical parameters
• Drift distance 25-50 m
• B field 0.28 T, magnet length 3 m
• Laser parameters
• Smaller wavelength preferable (e.g. green laser)
• Pulsed laser with 3 MHz frequency
• Laser spot size 50-100 microns
• Laser pulse energy must ensure 106 scatters
  (e.g. 30 mJ per pulse)
• Crossing angle 8 mrad
• Accuracy required to achieved
• lt 1-2 microns
• lt 1-2 microns
• lt 20 microns

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Laser Compton Energy Spectrometer

• Beam position can measured with a normal BPM
  (very well know and precise technique).
• Edge position Diamond strip detector or quartz
  fiber detector. Basic simulation shows that this
  feasible.
• Photon detection Basically 2 possibilities,
  using the backscattered photons or the
  synchrotron radiation photons.

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Laser Compton Energy Spectrometer

• Number of backscattered photons (BP) 4 order of
  magnitude less than SR photons
• ltEnergygt BP 100 GeV, ltenergygt SR photons 3 MeV.
• 2 possibilities
• Place a thick absorber in front of detector and
  measure the profile of shower (signal from BP
  dominant), quartz fiber detector suitable.
• No absorber, detector in front of the photon beam
  (signal from SR photons dominant). Novel detector
  under development in DUBNA (Xenon gas detector).
• Main problem for both configurations very high
  radiation dose (10-100 GGy per year).

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Conclusions

• A prototype of 4-magnet chicane was built in ESA.
  
• The absolute value of the B-field integral can be
  monitored with an accuracy 184 ppm (ESASLAC note
  and PAC poster)
• The resolution of the chicane was found to be 24
  MeV, where the main contribution is the
  resolution on the beam offset in the mid-chicane
  (to be published)
• A novel method based on Laser Compton events was
  studied(NIM publication). An experiment is under
  study to proof the feasibility (proposal in
  preparation).