Transcript Slide 1

New Results on Muon (g-2)
Past, Present and Future
Experiments
B. Lee Roberts
Department of Physics
Boston University
[email protected]
http://physics.bu.edu/roberts.html
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Vernon W. Hughes
1921-2003
The g-2 Collaboration
Boston University, Brookhaven National Laboratory, Budker Institute,
Cornell University, University of Heidelberg (* KVI), University of Illinois, KEK,
University of Minnesota,
Tokyo
Institute
of-11Technology,
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B. Lee
Roberts, SPIN2004
–Trieste
September 2004 Yale University
Outline
•
•
•
•
•
Prehistory: Stern to CERN
Theory of Muon (g-2)
E821: from 7.3 ppm to 0.5 ppm
The Future: E969 from 0.5 to 0.20 ppm
Summary and Outlook
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(in modern
language)
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Dirac + Pauli moment
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Dirac Equation Predicts g=2
• radiative corrections change g
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The Lowest Order Radiative Corrections
The vertex correction:
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Electric and Magnetic Dipole Moments
Transformation properties:
An EDM implies both P and T are violated. An EDM at
a measureable level would imply non-standard model
CP. The baryon/antibaryon asymmetry in the universe,
needs new sources of CP.
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Matrix Element for MDM and EDM
• MDM (g-2)
chirality changing
• EDM
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The CERN Muon (g-2) Experiments
The muon was shown to be a point particle obeying
QED
The final CERN precision was 7.3 ppm
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Standard Model Value for (g-2)
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Two Hadronic Issues:
• Lowest order hadronic contribution
• Hadronic light-by-light
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Lowest Order Hadronic from e+e- annihilation
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Evaluating the Dispersion Integral
Agreement between
Data (BES) and pQCD
use data
(within correlated
systematic errors)
use QCD
Better agreement
between exclusive
and inclusive (2)
data than in 19971998 analyses
use QCD
from A. Höcker
ICHEP04
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a(had) from hadronic t decay?
• Must assume CVC, no second-class currents,
make the appropriate isospin breaking
corrections.
decay has no isoscalar piece,
while e+e- does
Let’s look at the branching ratio and Fπ from
the two data sets:
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Tests of CVC
(A. Höcker – ICHEP04)
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Shape of Fp from e+e- and hadronic t decay
Comparison between t data and e+e- data from CDM2
(Novosibirsk)
zoom
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New precision data
from KLOE confirms
CMD2
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KLOE Data on R(s)
Pion Formfactor
45
45
2p contribution to amhadr
CMD-2
KLOE
35
• KLOE has evaluated the Dispersions Integral
for the 2-Pion-Channel
2
in the Energy Range 0.35 <sp<0.95 GeV30
40
25
ampp = (388.7  0.8stat  3.5syst  3.5theo) 10-1020
15
10
• Comparison with CMD-2 in the Energy Range 0.37 <sp<0.93 GeV2
KLOE
CMD2
(375.6  0.8stat  4.9syst+theo)
(378.6  2.7stat  2.3syst+theo) 10-10
10-10
5
0
0.4
1.3% Error
0.50.9% 0.6
Error0.7
0.8
0.9
sp [GeV2]
• At large values of sp (>mr2) KLOE is consistent with CMD and therefore
They confirm the deviation from t-data!
.
Courtesy of G. Venanzone
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A. Höcker at ICHEP04
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SM Theory from ICHEP04 (A. Höcker)
amhad [e+e– ] = (693.4 ± 5.3 ± 3.5)  10 –10
am SM [e+e– ] = (11 659 182.8 ± 6.3had ± 3.5LBL ± 0.3QED+EW)  10 –10
amweak
Weak contribution
= + (15.4 ± 0.3)  10 –10
Hadronic contribution from higher order : amhad [( /p)3] = – (10.0 ± 0.6)  10 –10
Hadronic contribution from LBL scattering: amhad [LBL] = + (12.0 ± 3.5)  10 –10
not yet published
not yet published
BNL E821 (2004):
amexp =(11 659 208.0  5.8) 10 10
Observed Difference with Experiment:
preliminary
am exp – am SM =
(25.2 ± 9.2) 
10 –10
 2.7 ”standard deviations“
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Hadronic light-by-light
• This contribution must be
determined by calculation.
• the knowledge of this
contribution limits knowledge
of theory value.
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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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In CMSSM, am can be combined with b → s, cosmological
relic density Wh2, and LEP Higgs searches to constrain c mass
Excluded by
direct searches
Allowed 2s band
am(exp) – am(e+e- theory)
Excluded for
neutral dark
matter
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Courtesy K.Olive
based on Ellis, Olive,
Santoso, Spanos
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The CMSSM plot with error on Dam of 4.6 x 10-10
(assuming better theory and a new BNL g-2 experiment)
Dam=24(4.6) x 10-10 (discrepancy at 6 s)
Current Discrepancy
Dam = 0 (4.6) x 10-10
Standard Model
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Spin Precession Frequencies:
spin
difference
The
motional
E - field, frequency
β X B, is much stronger
than laboratory electric
fields.
= ws - wc
The EDM causes the
spin to precess out
of plane.
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Experimental Technique
νμ
Protons
(from AGS)
π
π
Pions


S
m

Spin
polarized m
Momentum
m  m
Inflector

B
p=3.1GeV/c
(1.45T)
Target
• Muon polarization
• Muon storage ring
• injection & kicking
• focus by Electric Quadrupoles
• 24 electron calorimeters
Injection orbit
Ideal orbit
Kicker
Storage
Modules
ring
R=711.2cm
d=9cm
Electric Quadrupoles
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Experiment - Field Measurement
The system is calibrated in situ against a
standard* before and after data taking with
beam
(I) calibration uncertainties
The field values along the muon trajectory
are measured several times per week with
17 NMR probes mounted on a trolley.
(II) measurement uncertainties
The field is tracked continually with ~160
out of 375 NMR probes in the top and
bottom walls of the vacuum chamber.
(III) interpolation uncertainties
(IV) apparatus response and
field perturbations (IV)
muon (g-2) storage ring
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Field Shimming
2001
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Magnetic Field Uncertainty
Systematic uncertainty
(ppm)
Magnetic field – wp
1998 1999 2000 2001
0.5
0.4
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Beam Dynamics
beam
storage
region
mismatch between entrance channel and storage volume, +
imperfect kick causes coherent beam oscillations
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Coherent Betatron Oscillations
2pr is one turn around
the ring
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Frequencies in the g-2 Ring
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Detectors and vacuum chamber
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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
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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Muon Frequency Error
Systematic uncertainty
(ppm)
Spin precession – wa
1998 1999 2000 2001
0.8
0.3
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0.3
0.21
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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.
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Life Beyond E821?
• With a 2.7 s discrepancy, you’ve got to go
further.
• A new upgraded experiment was approved
by the BNL PAC in September
E969
• Goal:
total error = 0.2 ppp
– 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
– 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
Time 2004
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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
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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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Summary
• g-2 continues to be at the center of
interest in particle physics.
• E821 reached 0.5 ppm precision with a
2.7 s discrepancy with SM
– using e+e- data for the hadronic piece
• E969 has scientific approval, physics
reach is x 2 to 2.5 over E821. Should
clarify comparison with SM. (still need $)
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Outlook
• Scenario 1
– LHC finds SUSY
– (g-2) helps provide information on important
aspects of this new reality, e.g. tan b
Stay tuned !
• Scenario 2
– LHC finds the Standard Model Higgs at a
reasonable mass, nothing else, (g-2)
discrepancy and m might be the only
indication of new physics
– virtual physics, e.g. (g-2), mEDM, m→e
conversion would be even more important.
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E821 ωp systematic errors (ppm)
E969
(i
(I)
)
(II)
(III)
(iv)
*higher multipoles, trolley voltage and temperature response, kicker eddy currents, and timevarying stray fields.
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Systematic errors on ωa
(ppm)
σsystematic
1999
Pile-up
0.13
0.13
0.08
0.07
AGS Background
Lost Muons
Timing Shifts
0.10
0.10
0.10
0.10
0.10
0.02
*
0.09
0.02
0.04
E-Field, Pitch
Fitting/Binning
CBO
Beam Debunching
Gain Change
total
0.08
0.07
0.05
0.04
0.02
0.3
0.03
0.06
0.21
0.04
0.13
0.31
*
*
0.07
*
0.13
0.21
2000
2001 E969
0.05
0.04
0.03
0.11
Σ* = 0.11
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