Muon g2 Past and Future

1
Muon (g-2) Past and Future

• Beam Dynamics in the
• Muon (g-2) Storage Ring

B. Lee Roberts Department of Physics Boston
University
roberts_at_bu.edu http//physics.bu.edu/robert
s.html
2
Outline of the Talk

• Brief review of magnetic moments (including the
  theory of a?)
• Spin motion in a magnetic field
• Overview of the experimental technique
• The precision storage ring magnet
• The fast muon kicker
• The electrostatic quadrupoles
• Beam dynamics in the storage ring
• The new experiment E969
• Outstanding challenges for the future
• Summary and conclusions

3
Muon (2nd generation lepton)
Source weak decay
4
Magnetic Moments g-factors, etc
s spin
g gyromagnetic ratio
? magnetic moment

• Dirac Equation predicts g 2
• In nature, radiative corrections make g ? 2

5
Magnetic Moments ctd.
6
Unlike the EDM, there is a large SM value for the
MDM
The Electron to the level of the experimental
error (4ppb),
Contribution of µ, (or anything heavier than the
electron) is 4 ppb.
For the muon, the relative contribution of
heavier particles
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Standard Model Value for (g-2)
8
Lowest Order Hadronic contribution from ee-
annihilation
9
a(had) from hadronic ??decay?

• Assume CVC, no 2nd-class currents, isospin
  breaking corrections.
• n.b. ?? decay has no isoscalar piece, while ee-
  does
• Many inconsistencies in comparison of ee- and
• ??decay
• - Using ee- data and CVC to predict ?
  branching ratio gives 0.7 to 3.6 ?
  discrepancies with reality.
• - F? from ? decay has different shape from ee-.

a??(Had) is very much a work in progress!
10
All E821 results were obtained with a blind
analysis.
with ee- data only for the hadronic
contribution.
An interesting, but not definitive discrepancy
with theory.
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Why people are interested SUSY ( large tanß )
12
Traditionally,

• For many years, muon (g-2) has provided strong
  and serious constraints on models of physics
  beyond the standard model.

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Spin Precession Frequencies ??in B field
spin difference frequency ?s - ?c
14
Spin Precession Frequencies E and B field
0
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Experimental Technique
polarized ?
Pions
Protons
Inflector
(from AGS)
p3.1GeV/c
Target
(1.45T)
Injection orbit

• Muon polarization
• Muon storage ring
• injection kicking
• focus by Electric Quadrupoles
• 24 electron calorimeters

Storage ring
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The Beamline
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The Production Target
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Decay Channel
19
Plan View of the Injection Line
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The Inflector Exit
storage region
beam
mismatch between entrance channel and storage
volume, plus an imperfect kick causes
coherent beam oscillations
21
The Inflector
Length 1.7 m Central field 1.45 T
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Magnetic Circuits
Ohms law
23
Schematic of the Magnet
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Winding the Coils
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The Finished Coils
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Coil Interconnect
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Inserting a Pole Piece
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muon (g-2) storage ring
29
Mapping the Field
NMR B-field mapping trolley
Fixed probes monitor dipole and quadrupole field
components
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Azimuthal Variation
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averaged over azimuth
32
The Kick
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Injection Simulation
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The Kicker Modulator
38
Kicker Plate Geometry
electrodes
39
The Kicker Current Pulse
eddy currents less than 0.1 ppm on Bdl after 20
?s
measured with the faraday effect
40
Vacuum Chamber and Quadrupole
41
The Electrostatic Quadrupoles ?? polarity

-
-

24 kV at full power, 17 kV for beam
scraping after injection
42
The Ring Layout
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Scraping the Beam

• V0 24 kV
• VS 17 kV
• Beam is lifted and moved sideways
• Scraped on collimators to minimize losses

44
Ring ??-Function
for 4-fold symmetry
45
Weak Focusing Betatron

• Detector acceptance depends on the radial
  coordinate x. The beam moves coherently radially
  relative to a detector with the Coherent
  Betatron Frequency (CBO)

46
Coherent Betatron Frequency
CBO amplitude modulates the signal in the
detectors.
47
Tune Plane
48
Muon Decay
?-decay parity violating
49
Electron Detectors
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In the 1999 Data Set A Surprise
Nature gives us 5 parameters
Storage ring plus bunched beam gives us more
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Frequencies in the (g-2) Ring
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Fiber Beam Monitors
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Measuring the Tune
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The Tune During Scraping

• The tune change with scraping is clearly visible
  from the fiber harps

56
CBO in the 2001 Data Set
Residuals from fitting the 5-parameter function
57
Beam Debunching after Injection
Ring momentum acceptance
58
Fourier Transform vs. Debunching Model
Debunching model
modifiedFT
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Exclusion/Limitations on New Physics
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Can we improve the sensitivity of this
confrontation between experiment and theory?

• Yes
• E969 at BNL has scientific approval to go from
  0.5 ppm ? 0.2ppm
• funding decision will be made in spring 2006
• Will Theory Improve beyond 0.6 ppm?
• Yes
• better R measurements from KLOE, BaBar, Belle,
  SND and CMD2 at Novosibirsk
• More work on the strong interaction
• Theory could eventually improve to 0.2 ppm

61
Strategy of the improved experiment

• More muons E821 was statistics limited
  ?stat 0.46 ppm, ?syst 0.3 ppm
• Backward-decay, higher-transmission beamline
• Double the quadrupoles in the ? decay line

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E821 forward decay beam
Decay muons _at_ 3.094 GeV/c
This baseline limits how early we can fit data
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E969 backward decay beam
Decay muons _at_ 3.094 GeV/c
No hadron-induced prompt flash
Approximately the same muon flux is realized
Then we double the number of quadrupoles in the
decay channel
x 2
64
Improved transmission into the ring
Inflector aperture
Storage ring aperture
E821 Closed End
P969 Proposed Open End
x 2
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E969 Systematic Error Goal
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Systematic errors on ?a (ppm)
Beam manipulation
Backward beam
S 0.11
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Timescales in the ring

• Muon lifetime ?? 64.4 ?s
• Cyclotron period ?C 149 ns
• Scraping time (E821) 7 to 15 ?s
• Total counting time 700 ?s
• Total number of turns 4000

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Relative Amplitude of the CBO effect
??a
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Lost Muons and CBO are Major Issues

• Two schemes proposed to eliminate CBO and losses
• Drive CBO with an oscillating dipole to scrape,
  then slip the phase by ? and damp it
• Suggested by Yuri Orlov
• Pulsed Octupole for 30 turns
• Suggested by Yuri Shatunov

70
Oscillating Dipole Solution

• Use Fiber Harps to measure phase of CBO
• Sample Parameters
• L 0.5 m
• N 20 turns
• Ex0 7.4 kV/cm
• f 470 kHz

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2-meter long coil!
Y. Shatunov, SPIN04
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Y. Shatunov, SPIN04
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CBO damping
Muon population
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Challenges with Octupole

• Eddy currents affecting B0?
• We can only tolerate effects on B dl at the 0.05
  ppm level
• Too many muons lost?

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Summary

• E821 at BNL achieved 0.54 ppm relative accuracy
  on a?
• 0.46 ppm statistical
• 0.28 ppm systematic
• This represents a factor of 14 over the CERN
  experiment

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Where we came from
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Today with ee- based theory
All E821 results were obtained with a blind
analysis.
world average
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Summary

• E821 at BNL achieved 0.54 ppm relative accuracy
• 0.46 ppm statistical
• 0.28 ppm systematic
• This represents a factor of 14 over the CERN
  experiment
• E969 Aims to achieve an additional factor of 2.5
• from 0.5 ppm ? 0.2 ppm
• Will more than double the physics reach when
  confronting theory
• Please come join us on E969!

Thank you
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Summary

• E821 at BNL achieved 0.54 ppm relative accuracy
• 0.46 ppm statistical
• 0.28 ppm systematic
• This represents a factor of 14 over the CERN
  experiment
• E969 Aims to achieve an additional factor of 2.5
• from 0.5 ppm ? 0.2 ppm
• Will more than double the physics reach when
  confronting theory
• Please come join us on E969!

Thank you