B. Lee Roberts, NuFact WG4: 24 June 2005 - p. 1/36 Muon (g-2) Past, Present and Future B. Lee...

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

[email protected] http://physics.bu.edu/roberts.html


(in modern language)

673 (1924)


Dirac + Pauli moment

Schwinger term


The Muon Trio:• Lepton Flavor Violation

• Muon MDM (g-2) chiral changing

• Muon EDM


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


Spin Precession Frequencies: in B field

spin difference frequency = s - c


Use an E field for vertical focusing

spin difference frequency = s - c

0


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.


Muon (g-2): Store ± in a storage ring

magnetic field averaged over azumuth in the storage ring


Muon (g-2) Present precision: ± 0.5 ppm


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


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.


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.


Why might this be interesting?

• what sources of new physics are there?


aμ is sensitive to a wide range of new physics

• muon substructure

• anomalous couplings• SUSY (with large tanβ )

• many other things (extra dimensions, etc.)


SUSY connection between a , Dμ , μ → e


SUSY, dark matter, (g-2)

CMSSM


E969 = now


E969


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.


Lowest Order Hadronic contribution from e+e-

annihilation


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?


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


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?


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


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


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


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


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


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


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.


As always, there are questions …

• Will E969 be funded and reach 0.2 ppm?

• How far can theory be improved?

• a observation from history . . . .


Where we came from:


Today with e+e- based theory:

All E821 results were obtained with a “blind” analysis.

world average


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.


Fourier Transform: residuals to 5-parameter fit

beam motion across a

scintillating fiber – ~15 turn period


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.


Functional form of the time spectrum

• A1 and A2 → artificial shifts in a up to 4 ppm in individual detectors when not accounted for.


Other Systematic Effects: a

• muon losses • gain changes and pedistal shifts• pulse pileup


E821: Systematic Errors


1998

1999

2000

2001

Spin precession – a 0.8 0.3 0.3 0.21


1998

1999

2000

2001

Magnetic field – p 0.5 0.4 0.24

0.17

Muon spin precession

Magnetic field

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