Muon (g-2)

B. Lee Roberts, PANIC05, Santa Fe, 27 October, 2005 - p. 1/35

Muon (g-2)

Status and Plans for the Future

B. Lee RobertsDepartment of Physics

Boston University

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

mailto:[email protected]


Magnetic moments, g-factors


The Muon Trio:• Lepton Flavor Violation

• Muon MDM (g-2) chiral changing

• Muon EDMDirac, Pauli moment


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.


Present EDM Limits

Particle Present EDM limit(e-cm)

SM value(e-cm)

n

future exp 10-24 to 10-25 *

final limit will be better, * projected.


Unlike the EDM, there is a large SM value for the MDM

The Electron: to the level of the experimental error (4ppb),

Contribution of μ, (or anything heavier than the electron) is ≤4 ppb.

For the muon, the relative contribution of heavier particles


Standard Model Value for (g-2)


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


a(had) from hadronic decay?

• Assume: CVC, no 2nd-class currents, isospin breaking corrections.

• n.b. decay has no isoscalar piece, while e+e- does• Many inconsistencies in comparison of e+e- and decay:

- Using CVC to predict branching ratios gives 0.7 to 3.6 discrepancies with reality.

- F from decay has different shape from e+e-.


New Physics; SUSY (with large tanβ )


SUSY connection between a , dμ , μ → e


Muon (g-2) : E821

• Superconducting “superferric” storage ring– superconducting inflector– fast muon kicker– magic , – direct muon injection with a fast non-

ferric kicker


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 motional E - field, β X B, is much stronger (~GV/m) than laboratory electric fields.



The EDM causes the spin to precess out of plane and increases ||

The motional E - field, β X B, is much stronger (~GV/m) than laboratory electric fields.



The if the EDM were 2.8 x 10-18 e cm, a would increase by 2.9 ppm.


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


Can we improve the sensitivity of this confrontation between experiment and

theory?• Yes

– E969 at BNL has scientific approval to go from 0.5 ppm → 0.2ppm

– At a more intense muon facility we could do better.

Will Theory Improve beyond 0.6 ppm?• Yes

– better R measurements from: KLOE, BaBar, Belle, SND and CMD2 at Novosibirsk

– More work on the strong interaction

• Theory could eventually improve to ~0.2 ppm


Exclusion/Limitations on New Physics


SUSY, dark matter, (g-2)

CMSSM (constrained minimal supersymmetric model)

scal

ar m

ass

gaugino mass


Future Comparison: E969 = now


Future Comparison: E969

Historically (g-2) has played an important role in restricting models of new physics.


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


Strategy of the improved experiment

• New, open-end inflector • Upgrade detectors, electronics, DAQ

x 2 in flux


E969: Systematic Error Goal

Systematic uncertainty (ppm)

1998 1999 2000 2001 E969Goal

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


aμ implications for the muon EDM

This paper, published before our February 2001 announcement predicts a large muon EDM, and a corresponding SUSY contribution to aμ comparable to what we might be observing, with the e- EDM predicted to be 0.1 of the present limit.


aμ implications for the muon EDM


Dedicated EDM measurement:• operate with ≈ 5 << 29.3• use a radial E-field to turn off (g-2)

precession

• Place detectors above and below the vacuum chamber and look for an up/down asymmetry which builds up with time


Beam Needs: NP2

• the figure of merit is Nμ times the polarization. We need

to reach the 10-24 e-cm level.• Since SUSY calculations range from 10-22 to

10-32 e cm, more muons is better.

= 5*10-7

(Up+

Dow

n)

time (s)

(Up-

Dow

n)


Where E821 came from:


Today:

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) provides serious constraints on physics beyond the standard model.

• The hadronic contribution will eventually set the limit on useful precision, but substantial improvement can and will be made, both in theory and experiment beyond the present situation.



Recent News from Novosibirsk

• SND has just released their results (hep-ex/0506076) for the cross section e+e- → + - over the . – Error on dispersion integral 50% higher

than CMD2– Good agreement with CMD2– Completely independent from CMD2


Improved transmission into the ring

InflectorInflector aperture

Storage ring aperture

E821 Closed End P969 Proposed Open End


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 proton driver/neutrino factory

we certainly we can get more muons


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


Better agreement between exclusive and inclusive (2) data than in 1997-1998 analyses

Agreement between Data (BES) and pQCD (within correlated systematic errors)

use QCD

use data

use QCD

Evaluating the Dispersion Integral

from A. Höcker ICHEP04


Tests of CVC (A. Höcker – ICHEP04)


Shape of F from e+e- and hadronic decay

zoom

Comparison between t data and e+e- data from CDM2 (Novosibirsk)

New precision data from KLOE confirms

CMD2

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Muon (g-2)