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Muon (g-2). Past, Present and Future. B. Lee Roberts Department of Physics Boston University. roberts@bu.edu http://physics.bu.edu/roberts.html. 673 (1924). (in modern language). Dirac + Pauli moment. Schwinger term. Lepton Flavor Violation - PowerPoint PPT Presentation
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B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 1/36
Muon (g-2)
Past, Present and Future
B. Lee RobertsDepartment of Physics
Boston University
roberts@bu.edu http://physics.bu.edu/roberts.html
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 2/36
(in modern language)
673 (1924)
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 3/36
Dirac + Pauli moment
Schwinger term
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 4/36
The Muon Trio:• Lepton Flavor Violation
• Muon MDM (g-2) chiral changing
• Muon EDM
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 5/36
Muon (g-2) : Four Past Experiments
• CERN 1 - 1950s – SC precessed in a gradient field
• CERN 2 - 1960s– Dedicated Storage Ring, p = 1.28 GeV/c
• protons from PS injected into the storage ring
• CERN 3 - 1970s– Dedicated Storage Ring
• used injection + → decay to give the kick, The “magic” p3.09 GeV/c,
• BNL E821– Superconducting “superferric” storage ring
• magic , direct muon injection, fast non-ferric kicker
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 6/36
Spin Precession Frequencies: in B field
spin difference frequency = s - c
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 7/36
Use an E field for vertical focusing
spin difference frequency = s - c
0
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 8/36
Spin Precession Frequencies: in B field with both an MDM and EDM
The EDM causes the spin to precess out of plane.
The motional E - field, β X B, is much stronger than laboratory electric fields.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 9/36
Muon (g-2): Store ± in a storage ring
magnetic field averaged over azumuth in the storage ring
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 10/36
Muon (g-2) Present precision: ± 0.5 ppm
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 11/36
Theory and Experiment
• Using these hadronic contributions
K. Hagiwara, et al., Phys. Rev. D69, 093003 (2004)
M. Davier et al., Eur. Phys. J. C 31, 503 (2003), A Höcker, hep-ph/0410081
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 12/36
a with standard model ~2.7
With this discrepancy, a compelling case can be made to do better, and resolve whether this “discrepancy” is significant or not.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 13/36
Can we do a more precise measurement?
• Yes– E969 at BNL has scientific approval to reach 0.2ppm– At a more intense muon facility we could do better.
Will Theory Improve?
• Yes• First, let’s look at the pieces which might
contribute to a potential discrepancy.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 14/36
Why might this be interesting?
• what sources of new physics are there?
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 15/36
aμ is sensitive to a wide range of new physics
• muon substructure
• anomalous couplings• SUSY (with large tanβ )
• many other things (extra dimensions, etc.)
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 16/36
SUSY connection between a , Dμ , μ → e
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 17/36
SUSY, dark matter, (g-2)
CMSSM
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 18/36
E969 = now
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 19/36
E969
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 20/36
SM value dominated by hadronic issues:
• Lowest order hadronic contribution ( ~ 60 ppm)
• Hadronic light-by-light contribution ( ~ 1 ppm)
The error on these two contributions will ultimately limit the interpretation of a more precise muon (g-2) measurement.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 21/36
Lowest Order Hadronic contribution from e+e-
annihilation
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 22/36
Magnitude of the errors
• present hadronic uncertainty ~0.6 ppm• present experimental uncertainty 0.5 ppm
• theory: better R measurements– KLOE– BaBar– SND and CMD2 at Novosibirsk– More work on the strong interaction
• experiment: E969 @ BNL or elsewhere
How could we do better?
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 23/36
Recent News from Novosibirsk
• SND has just released their results for the cross section e+e- → + - over the . – Error on dispersion integral 50% higher
than CMD2– Good agreement with CMD2– Completely independent from CMD2
• Preprint should be on the web soon
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 24/36
How much could the theory improve?
• In their “Annual Reviews” articleDavier and Marciano guess a factor of 2 or so for argument let’s assume theory uncertainty will get to– 0.3 to 0.1 ppm
• Experiment– E969 at BNL (if it runs) could achieve a factor
of 2.5 for a total error of 0.2 ppm– future experiment could reach 0.06 ppm
How much could experiment improve?
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 25/36
E969 at BNL
• Scientific approval in September 2004– at present: no funds for construction or running
• Goal: total error = 0.2 ppm– lower systematic errors– more beam
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 26/36
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– New, open-end inflector – Upgrade detectors, electronics, DAQ
• Improve knowledge of magnetic field B– Improve calibration, field monitoring and
measurement• Reduce systematic errors on ωa
– Improve the electronics and detectors – New parallel “integration” method of analysis
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 27/36
Improved transmission into the ring
InflectorInflector aperture
Storage ring aperture
E821 Closed End P969 Proposed Open End
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 28/36Pedestal vs. Time
Near side Far side
E821: forward decay beam
Pions @ 3.115 GeV/c
Decay muons @ 3.094 GeV/c
This baseline limits how early we can fit data
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 29/36
E969: backward decay beam
Pions @ 5.32 GeV/c
Decay muons @ 3.094 GeV/c
No hadron-induced prompt flash
Approximately the same muon flux is realized
x 1 more
muons
Expect for both sides
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 30/36
E969: Systematic Error Goal
• Field improvements will involve better trolley calibrations, better tracking of the field with time, temperature stability of room, improvements in the hardware
• Precession improvements will involve new scraping scheme, lower thresholds, more complete digitization periods, better energy calibration
Systematic uncertainty (ppm)
1998 1999
2000 2001
E969
Goal
Magnetic field – p 0.5 0.4 0.24 0.17 0.1
Anomalous precession – a
0.8 0.3 0.3 0.21 0.1
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 31/36
Beyond E969?
• It’s not clear how far we can push the present technique.
• To get to 0.06 ppm presents many challenges.
• Perhaps a new storage ring design, and a smaller aperture. – detectors for another factor of 4 will be
very challenging.• At a neutrino factory we certainly we
can get more muons
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 32/36
A new idea (F.J.M. Farley)• Sector focused storage ring, which uses
polarized protons to measure ∫B. dℓ
No need to know / p
Need to know ∫B.dℓ to 20 ppb!!!!! (while E821 already achieved:
Can run well above the magic , so that there are more (g-2) cycles per lifetime.
Many details to be worked out.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 33/36
As always, there are questions …
• Will E969 be funded and reach 0.2 ppm?
• How far can theory be improved?
• a observation from history . . . .
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 34/36
Where we came from:
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 35/36
Today with e+e- based theory:
All E821 results were obtained with a “blind” analysis.
world average
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 36/36
Summary
• (g-2) provides a precise check of the standard model, and accesses new physics in a way complementary to other probes.
• (g-2) is dependent on a standard model value, part of which must be taken from data (e+ e- → hadrons )
• The hadronic contribution will eventually set the limit on useful precision, but substantial improvement can be made beyond the present situation.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 37/36
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 38/36
Fourier Transform: residuals to 5-parameter fit
beam motion across a
scintillating fiber – ~15 turn period
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 39/36
Effects of the CBO on e- spectrum
• CBO causes modulation of N, amplitude ~0.01
• CBO causes modulation of observed energy distribution
• which in turn causes oscillation in A(E), (E), with amplitudes ~0.001, ~1 mrad.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 40/36
Functional form of the time spectrum
• A1 and A2 → artificial shifts in a up to 4 ppm in individual detectors when not accounted for.
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 41/36
Other Systematic Effects: a
• muon losses • gain changes and pedistal shifts• pulse pileup
B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 42/36
E821: Systematic Errors
Systematic uncertainty (ppm)
1998
1999
2000
2001
Spin precession – a 0.8 0.3 0.3 0.21
Systematic uncertainty (ppm)
1998
1999
2000
2001
Magnetic field – p 0.5 0.4 0.24
0.17
Muon spin precession
Magnetic field
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