Miuon (g-2) experiment at BNL and Precise Measurements of Hadronic Cross-Sections at VEPP-2M

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Seminar Dedicated to 75th
Anniversary of Academician L.M.Barkov
Miuon (g-2) experiment at BNL and Precise
Measurements of Hadronic Cross-Sections
at VEPP-2M
Guennadi Fedotovitch Budker Institute of Nuclear
Physics On behalf of the (g-2) and CMD-2
Collaboration Novosibirsk
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Outline

• Motivation of BNL (g-2) experiment
• Method
• Experiment
• Results of 2000 data analysis
• Calculations of cross sections
• e?e? ? e?e? (?)? ???? (?)? ???? (?)
• Some new results from CMD-2 and SND
• Conclusions

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(No Transcript)
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Standard Model Summary
d (ppm)
? 10-11

• Uncertainty in hadronic VP will continue to
  shrink
• But, hadronic light by light remains somewhat
  difficult

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Why probe the difference of g from 2?
QED WEAK HADRONIC
Standard Model g-2
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Looking Beyond the Standard Model
A variety of possible contributions (at the 0.35
ppm level)

• Muon substructure Dam (mm /L)2 sensitivity
  L ? 5 TeV LHC domain
• W anomalous magnetic moment aW sensitivity
  0.02 LEPII 0.05, LHC 0.2
• W substructure Dam (mW /L)2 sensitivity L ?
  400 GeV LEPII 100-200 GeV
• Supersymmetry (for large tan?)

Dominant Diagrams
i.e. For tan ß 40 MSUSY 1.2 TeV? Dam
0.35ppm MSUSY 700 GeV? Dam 1 ppm MSUSY 350
GeV? Dam 4 ppm
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The g-2 Principle
For a relativistic particle undergoing cyclotron
motion in a magnetic field, the spin rotation
frequency is given by
While the cyclotron frequency is given by
So, in the particle's rest frame, the spin vector
rotates relative to the momentum vector at the
frequency
Proportional to am ... not g!
wa is independent of g !
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Measuring (g-2)µ
Make a pion beam, then select highest energy
muons from parity violating p ?m nm decay
Polarized Muon Source
Precession in Uniform B-field
Ultra-precise dipole storage ring allowing muons
to precess through as many g-2 cycles as possible
In parity violating muon decay, m ? e ne
nm , the positron is preferentially emitted in
the muon spin direction
Polarimeter vs. Time
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The Magic ?
Polarized muons enter storage ring and precess in
uniform B-field
Need vertical focusing to store beam, but want to
avoid perturbing the magnetic field.
Use electrostatic quadrupole focusing
In the presence of a transverse electric field,
the spin rotation frequency gets modified
For Magic p3.09 GeV/c the second term does not
affect the rotation frequency!
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Polarimeter

• In rest frame, positron emitted preferentially
  along direction of muon spin
• In lab frame, positrons receive a boost along
  the direction of motion

Result More positrons above a given energy
threshold when spin is pointing forward, fewer
when spin is pointing backward
A counting experiment vs. time
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The g-2 Time Spectrum
In each positron detector, the time spectrum
follows the following energy-dependent form
Asymmetry is 0.4 above 1.8 GeV threshold
Statistical error of fit
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Getting to High Precision...
Recall ?aaµ(e/mc)B

• Need statistics - billions of muons at the magic
  momentum

• Need precise knowledge of the B-field at all times

• Need to know the stored beam distribution
  averaged over the field region

• Need very stable measurement of positron arrival
  times over a wide range of rates, plus moderate
  energy resolution

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Aspects of BNL E821
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Beamline and Injection Modes
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Storage Ring / Kicker
Radius 7112 mm Aperture 90 mm Field 1.45 T Pm
3.094 GeV/c
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Positron Detector
24 Calorimeters inside the ring

• Lead/Scintillating Fiber
• 10 Radiation Lengths
• Energy resol ?10

Requirements over 600 µs measuring time

• Timing shifts lt 60 ps
• Gain change lt 0.3

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NMR System

• 375 NMR probes placed above and below the beam
  vacuum chamber all around the ring

• 17 probe NMR trolley operates in vacuum to map
  out field in storage region

• Calibration probes reference to "standard"
  spherical probe

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Magnetic Field Measurement
Systematic Uncertainties for the ?p Analysis.
Source of Errors
Size ppm
Absolute Calibration of Standard
Probe Calibration of Trolley Probe Trolley
Measurements of B-field Interpolation with Fixed
Probes Uncertainty from Muon Distribution Others T
otal
0.05 0.15 0.10 0.10 0.03 0.10 0.24
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4.5 Billion e with Egt2GeV
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1999 Analysis Strategy
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E821 Data Runs

• 1997 p engineering run 13 ppm measurement
  published
• 
  
  (R.M.Carey et al., PRL 82 (1999) 1632)
• 1998 m engineering run 5 ppm measurement
  published
• 
  (H.N.Brown
  et al., Phys.Rev. D62 (2000) 091101)
• 1999 m run 1.3 ppm
  measurement published
• 
  (H.N.Brown et al., hep-ex/0102017 v3 27 Feb 2001)
• 2000 m run 0.7 ppm
  measurement published
• 
  (G.V.Benett et al.., PRL 89 (2002) 1804)
• 2001 m- run In progress

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VEPP-2M collider
CMD-2 1992-2000 SND 1995-2000
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How the luminosity are measured?
when e?e? and ???? are not clearly separated

E.MeV
2?E 720 MeV
ee
With a QED fixed ratio
??
(NI)
??, ??
(mips)
E?, MeV
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Dispersion applications
Fine structure constant ?(MZ)
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e?e? ? ???? cross section (PPCMD)
Polarization of vacuum by leptons and hadrons is
included in resonance dressed cross section
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e?e? ? ????
Main sources of
systematic errors Event separation (0.2)
Feducial
volume (0.2)

Radiative correction (0.4)
Correction for pion
losses (0.2) Detection efficiency (0.2)
Beam energy
determination (0.1)
Total (0.6)
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ee-???-?0 (SND)
Fit A(????) A(????)
A(????) A(????) A(????) A(ee-???0
???)
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? ?KLKS
ee????KLKS
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ee-?KLKS, KS???- (CMD-2)
2E1.0-1.04 GeV L2 pb-1, N2.7?105
?0(??KLKS )1413?6?24 nb m?1413?6?24 MeV/c2
??4.280?0.033?0.025 MeV systematic error
in ?(ee- ?KLKS ) 1.7 2E1.05-1.38 GeV,
L5.8 pb-1, N103 systematic error in ?(ee-
?KLKS ) 5-10
solid curve is VDM with ?(770) , ?(783) ,
?(1020) X dash curve is VDM with ?(770) ,
?(783) , ?(1020) only
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ee-?KK-
Separation of kaons CMD-2 - dE/dX in DC SND -
the distribution of the energy deposition in
the calorimeter Different detectors, different
methods, but good agreement! systematic error in
?(ee- ?KK- ) ?6
Cross section can not be described by ?(770),
?(783) and ?(1020) only (solid curve)
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ee-?4?
ee-???0 ,????-?0
ee-?2?2?-

CMD-2 data in 4? channel is lower!
After reanalysis CMD-2 data agrees with SND data
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What is a? build from?
Relative contributions to a?
Relative contributions to uncertainty a?
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Comparison with ? decays
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(g-2)/2 of muon
e?e? ? hadrons a?(exp) - a?(theor)
(22.1?11.3)?10-10 (1.9 ?) ? ? hadrons??
a?(exp) - a?(theor) (7.4 ?10.5) ?10-10 (0.7 ?)
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Measurements of R at low S after VEPP-2M
VEPP-2M All major modes, contributing to R, are
measured. Data analysis is in progress.
Precision lt1 is expected for energy range below
1 GeV, 1 10 for energy range up to 1.4 GeV. Up
to 1.5-fold improvement in precision of HC to
(g-2)/2 is expected ISR experiments (KLOE,
BABAR, BELLE) Measure ?(e?e?? hadrons) through
e?e?? ? hadrons Main question is What
systematic errror will be achieved? VEPP-2000
(first beam spring 2004) Direct measurement
?(e?e?? hadrons) 10-fold increase of luminosity,
wider c.m. energy range Upgrades of CMD-2 SND
Up to 2-fold improvement in precision of HC to
(g-2)/2 is expected
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Conclusions

• Muon anomalous is measured with 0.7 ppm
• Three billion negative muon decays is in progress
  now.
• Accuracy about 0.8 ppm is expected
• Final average result will have error about 0.5
  ppm
• Good agreement between SM calculations for (g-2)?
  based on
• e?e? ? hadrons with experimental value
• Fancy flight plan to improve aµ up to 0.06 ppm.
  Ten times else !!!
• Total systematic error for the main channel
  ???? 0.6
• VEPP-2M has been stopped at 2000. New results
  still arrives
• New data are required to improve accuracy in a?