Transcript Slide 1

Muon (g-2)
Past, Present and Future
B. Lee Roberts
Department of Physics
Boston University
[email protected]
http://physics.bu.edu/roberts.html
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673 (1924)
(in modern language)
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Dirac + Pauli moment
Schwinger term
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The Muon Trio:
• Lepton Flavor Violation
• Muon MDM (g-2)
chiral changing
• Muon EDM
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Muon (g-2) : Four Past Experiments
• CERN 1 - 1950s
– SC m precessed in a gradient field
• CERN 2 - 1960s
– Dedicated Storage Ring, pm = 1.28 GeV/c
• protons from PS injected into the storage ring
• CERN 3 - 1970s
– Dedicated Storage Ring
• used p injection + p→m decay to give the kick,
The “magic” g = 29.7; pm = 3.09 GeV/c,
• BNL E821
– Superconducting “superferric” storage ring
• magic g, direct muon injection, fast non-ferric kicker
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Spin Precession Frequencies: m in B field
spin difference frequency = ws - wc
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Use an E field for vertical focusing
0
spin difference frequency = ws - wc
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Spin Precession Frequencies: m in B field
with both an MDM and EDM
The motional E - field,
β X B, is much stronger
than laboratory electric
fields.
The EDM causes the
spin to precess out
of plane.
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Muon (g-2): Store m ± in a storage ring
magnetic field averaged
over azumuth in the
storage ring
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Muon (g-2) Present precision: ± 0.5 ppm
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Theory and Experiment
M. Davier et al., Eur. Phys. J. C 31, 503 (2003), A Höcker, hep-ph/0410081
K. Hagiwara, et al., Phys. Rev. D69, 093003 (2004)
• Using these hadronic contributions
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D am with standard model ~2.7 s
With this discrepancy, a
compelling case can be
made to do better, and
resolve whether this
“discrepancy” is
significant or not.
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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.
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Why might this be interesting?
• what sources of new physics are there?
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aμ is sensitive to a wide range of new
physics
• muon substructure
• anomalous
couplings
• SUSY (with large tanβ )
• many other things (extra dimensions, etc.)
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SUSY connection between am , Dμ , μ → e
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SUSY, dark matter, (g-2) DE821
CMSSM
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D E969 = Dnow
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D E969 = 0
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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.
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Lowest Order Hadronic contribution from e+e- annihilation
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Magnitude of the errors
• present hadronic uncertainty ~0.6 ppm
• present experimental uncertainty 0.5 ppm
How could we do better?
• theory: better R measurements
–
–
–
–
KLOE
BaBar
SND and CMD2 at Novosibirsk
More work on the strong interaction
• experiment: E969 @ BNL or elsewhere
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Recent News from Novosibirsk
• SND has just released their results for
the cross section e+e- → p + p - over the r.
– Error on dispersion integral 50% higher than
CMD2
– Good agreement with CMD2
– Completely independent from CMD2
• Preprint should be on the web soon
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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
How much could experiment improve?
• 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
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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
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Strategy of the improved experiment
• More muons – E821 was statistics limited
sstat = 0.46 ppm, ssyst = 0.3 ppm
–
–
–
–
Backward-decay, higher-transmission beamline
Double the quadrupoles in the p 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
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Improved transmission into the ring
Inflector aperture
Inflector
Storage ring aperture
E821 Closed End
P969 Proposed Open End
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E821: forward decay beam
Pions @ 3.115 GeV/c
Decay muons @ 3.094 GeV/c
Near side
Far side
This baseline
limits how early
we can fit data
Pedestal
vs. Time
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Roberts, NuFact
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E969: backward decay beam
Pions @ 5.32 GeV/c
Decay muons @ 3.094 GeV/c
Expect for
both sides
No hadron-induced
prompt flash
Approximately the same
muon flux is realized
x1
more
muons
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E969: Systematic Error Goal
Systematic uncertainty (ppm) 1998 1999 2000 2001 E969
Goal
Magnetic field – wp
0.5
0.4
0.24
0.17
0.1
Anomalous precession – wa
0.8
0.3
0.3
0.21
0.1
• 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
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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
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A new idea (F.J.M. Farley)
• Sector focused storage ring, which uses
polarized protons to measure ∫B. dℓ
No need to know
mm / mp
Need to know ∫B.dℓ to
20 ppb!!!!! (while E821
already achieved:
Can run well above the
magic g , so that there
are more (g-2) cycles per
lifetime.
Many details to be
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2005
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As always, there are questions …
• Will E969 be funded and reach 0.2 ppm?
• How far can theory be improved?
• a observation from history . . . .
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Where we came from:
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Today with e+e- based theory:
All E821
results were
obtained
with a “blind”
analysis.
world average
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Summary
• (g-2)m provides a precise check of the
standard model, and accesses new
physics in a way complementary to other
probes.
• (g-2)m 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.
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Fourier Transform: residuals to 5-parameter fit
beam motion
across a
scintillating
fiber – ~15
turn period
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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),
f(E), with amplitudes ~0.001, ~1 mrad.
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Functional form of the time spectrum
• A1 and A2 → artificial shifts in wa up to
4 ppm in individual detectors when not
accounted for.
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Other Systematic Effects: wa
• muon losses
• gain changes and pedistal shifts
• pulse pileup
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E821: Systematic Errors
Muon spin precession
Systematic uncertainty
(ppm)
Spin precession – wa
1998 199 2000 200
9
1
0.8 0.3 0.3 0.21
Magnetic field
Systematic uncertainty 1998 1999 200
(ppm)
0
Magnetic field – wp
0.5
200
1
0.4 0.24 0.17
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