b. lee roberts, college of william and mary, 24 march 2006 - p. 1/61 the muon: a laboratory for...
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B. Lee Roberts, College of William and Mary, 24 March 2006 - p. 1/61
The Muon: A Laboratory for Particle Physics
Everything you always wanted to know about the
muon but were afraid to ask.
B. Lee RobertsDepartment of Physics
Boston University
[email protected] http://physics.bu.edu/roberts.html
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Outline
• Introduction to the muon• Selected weak interaction
parameters• Muonium• Lepton Flavor Violation• Magnetic and electric dipole
moments• Summary and conclusions.
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The Muon: Discovered in 1936
Discovered in cosmic rays by Seth Neddermeyer and Carl Anderson
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Confirmed by Street and Stevenson
It interacted too weakly with matter to be the “Yukawa” particle which was postulated to carry the nuclear force
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The Muon’s Discovery was a big surprise
• Lifetime ~2.2 s, practically forever• 2nd generation lepton
• mme = 206.768 277(24)
I. I. Rabi
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The Standard Model (Our Periodic Table)
Interact weakly through the
Leptons e e
Interact strongly through the gluons g
Electroweak gage bosons Z0 W±
Quarksu s t
d c b
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Production of The Muon
• produced polarized from the death of a pion
For decay in flight, “forward” and “backward” muons are highly polarized.
It can be produced copiously in pion decaye.g. Paul Scherrer Institut has 108 /s in a beam
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Death of the Muon
• Decay is self analyzing
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What can we learn from the ’s death?
• The strength of the weak interaction– i.e. the Fermi constant GF
• The fundamental nature of the weak interaction– i.e. is it scalar, vector, tensor,
pseudo-scalar, pseudo-vector or pseudo-tensor?
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from radiative corrections
A precise measurement of + leads to a precise determination of the Fermi constantGF
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helped predict the mass of the top quark
Predictive power in weak sector of SM. Difference between the charged and neutral current propagator:
Top quark mass prediction: mt = 177 20 GeV Input: GF (17 ppm), (4 ppb at q2=0), MZ (23 ppm),
2004 Update from D0 mt = 178 4.3 GeV
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Experiment at a glance
1. Collect handful of muons in a few s2. Turn off beam3. Watch them decay4. Repeat
e+
Time in target
Accum.Period
MeasurementPeriod
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Lan @ Paul Scherrer Institut aims for a factor of 20 improvement on
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The Weak Lagrangian (Leptonic Currents)
• Lepton current is (vector – axial vector) “(V – A)”
• It might have been: V±A or S±V±A or most general form:
Scalar ± Vector ± Weak-Magnitism ± PseudoScalar ± Axial-Vector ± Tensor
There have been extensive studies at PSI by
Gerber, Fetscher, et al. to look for other couplings in muon decay.
None were found
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If the Strong Interaction is Present
• Then we have a more general current, which in principle can have all 6 of these components to the current.
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Leptonic and hadronic currents• For nuclear capture (and also in -
decay) there are induced form-factors and the hadronic V-A current contains 6 terms.– the induced pseudoscaler term is important
2nd classvector weak magnitism scalar
axial vector pseudoscalar tensor
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An Aside:
but stop the press! new measurement of the atomic “ortho to para transition rate” seems to remove much of this problem, Clark, Armstrong, et al., PRL 96, 073401 (2006)
The induced pseudoscalar coupling in -capture
further enhanced in radiative muon capture
A new experiment at PSI MuCap hopes to resolve the present 3 discrepancy with PCAC
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Muonium + e- (not +-)Hydrogen (without the proton)
Named by Val Telegdi
discovered by Vernon Hughes
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Muonium
Zeeman splitting
p = 3.183 345 24(37) (120 ppb)
where p comes from proton NMR in the same B field
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muonium and hydrogen hfs → proton structure
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Lepton Flavor
• Remember the puzzle with -decay?– it appeared that energy conservation
did not hold in the decay n → p + e- which should have a mono-energetic e+ in the final state.
e-
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Lepton Flavor
• It took Pauli to propose that energy was conserved, but there was a new neutral particle emitted in the decay (named neutrino by Fermi), so the decay was a 3-body decay with a continuous spectrum.
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Lepton Flavor
• We have found empirically that lepton family number is conserved in muon decay.– e.g.
• What about or
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Lepton Flavor in Muon Decay
me = 0.511 MeV
mm = 104.7 MeV
Why don’t we see → e+ ? Neutrinos oscillate – however, the predicted
Standard Model Charged Lepton Flavor Violation unmeasureably small (from loops).
The standard model gauge bosons (interactions) do not permit lepton flavor-changing interactions, i.e. there is conservation of each lepton flavor separately.
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SM charged leptons do not mix
• Expect charged lepton flavor to be enhanced if there is new dynamics at the TeV scale, in particular if there is Supersymmetry
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In Standard Model we have:
antiparticles
particles
supersymmetric partners(spartners)
SUSY:
(with thanks to Bruce Winstein)
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Supersymmetry Permits Charged Lepton Mixing
• In supersymmetry there is mixing between the charged sleptons
• Many people believe that SUSY is the new physics which will be found at LHC
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Beyond the SM: The Muon Trio:• Lepton Flavor Violation
• Muon MDM (g-2) chiral changing
• Muon EDM
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The First -N e-N Experiment Steinberger and Wolf
• After the discovery of the muon, it was realized it could decay into an electron and a photon or convert to an electron in the field of a nucleus.
• Without any flavor conservation, the expected branching fraction for +e+ is about 10-5.
• Steinberger and Wolf
looked for -N e-N for the first time, publishing a null result in 1955, with a limit Re < 2 10-4
Absorbs e- from - decay
Conversion e- reach this
counter
9”
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The MECO Experiment
Muon Beam Stop
Superconducting Production Solenoid
(5.0 T – 2.5 T)
Superconducting Transport Solenoid
(2.5 T – 2.1 T)
Straw Tracker
Crystal Calorimeter
Muon Stopping Target
Superconducting Detector Solenoid
(2.0 T – 1.0 T)Collimators
10-17 BR single event sensitivity
p beam
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Past and Future of LFV Limits
+e-→-e+
MEG → e – 10-13 BR
sensitivity• under
construction at PSI, first data in 2006
MECO ++A→e+
+A– 10-17 BR
sensitivity• Was approved
at Brookhaven, not funded
Bra
nchi
ng R
atio
Lim
it
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Electric and Magnetic Dipole Moments
In 1950, Purcell and Ramsey propose to search for a neutron EDM to check parity violation
In 1957, Landau and Ramsey independently point out that an EDM violates both P and T
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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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Present EDM Limits
Particle Present EDM limit(e-cm)
SM value(e-cm)
n
future exp
10-24 to 10-25
*projected
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Magnetic Dipole Moments
The field was started by Otto Stern
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Z. Phys. 7, 249 (1921)
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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
Schwinger
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The CERN Muon (g-2) Experiments
The muon was shown to be a point particle obeying QED (Quantum Electrodynamics)
The final CERN precision was 7.3 ppm
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Standard Model Value for (g-2)
relative contribution of heavier things
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Lowest Order Hadronic from e+e- annihilation
Cauchy’s theorem and the optical theorem
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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 a , Dμ , μ → e
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Spin Precession Frequencies: in B field
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If we use an electric quadrupole field for vertical focusing we get
0
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Inflector
Kicker Modules
Storagering
Central orbitInjection orbit
Pions
Target
Protons
π
(from AGS) p=3.1GeV/c
Experimental Technique
π
μνS
Spin
Momentum
B
• Muon polarization• Muon storage ring• injection & kicking• focus by Electric Quadrupoles• 24 electron calorimeters
R=711.2cm
d=9cm
(1.45T)
Electric Quadrupoles
polarized
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muon (g-2) storage ring
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Detectors and vacuum chamber
Detector acceptance depends on radial position of the when it decays.
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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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Can we do better and confront theory more strongly?
• With a 2.7 discrepancy, you’ve got to go further.
• A new upgraded experiment to go from ±0.5 ppm to ± 0.2 ppm was approved by the BNL PAC in September 2004
• It will be considered by the Particle Physics Project Prioritization Panel (P5) next Monday.
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Courtesy K.Olivebased on Ellis, Olive, Santoso, Spanos
In CMSSM, a can be combined with b → s, cosmological relic density h2, and LEP Higgs searches to constrain mass
Allowedband a(exp) – a(e+e- theory)
Excluded by direct searches
Excluded for neutral dark matter
Preferred
same discrepancy no discrepancy
With expected improvements in ahad + E969 the error on the difference
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aμ implications for the muon EDM
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An EDM can also cause spin precession
The EDM causes the spin to precess out of plane.
The motional E - field, β X B, is much stronger than laboratory electric fields (MV/m)
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Muon EDM• use radial E field to “turn off” g-2 precession
so the spin follows the momentum.• look for an up-down asymmetry which builds
up with time • Needs 1018 muons
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Summary and Outlook
• The muon has provided us with much knowledge on how nature works. V-A, GF, induced weak couplings, lepton flavor
conservation, a a precision test of the SM
• New experiments on the horizon may continue this tradition.
• Muon (g-2), with a precision of 0.5 ppm, has a 2.7 discrepancy with the standard model.
• This new physics, if confirmed, could also show up as a muon EDM, as well as in Lepton flavor violation in decay.
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Like most science, this is a work in progress
Stay tuned !
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Two Hadronic Issues:
• Lowest order hadronic contribution• Hadronic light-by-light
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The error budget for E969 represents a continuation of improvements already made
during E821
• Field improvements: 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 0.31 0.21 0.1
Statistical uncertainty (ppm)
4.9 1.3 0.62 0.66 0.14
Total Uncertainty (ppm) 5.0 1.3 0.73 0.72 0.20
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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.
+/- Signs are Important!
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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
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Tests of CVC (A. Höcker – ICHEP04)
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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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MEG @ PSI (10-13 BR sensitivity)
MEG will start running in 2006
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Experimental Experimental boundbound
Largely favouredLargely favoured and confirmed by and confirmed by KamlandKamland
Additional contributionAdditional contribution toto slepton mixingslepton mixing fromfrom VV2121, matrix element , matrix element responsible responsible forfor solar neutrino deficit solar neutrino deficit. (. (J. Hisano & N. Nomura, Phys. Rev. J. Hisano & N. Nomura, Phys. Rev. D59D59 (1999) (1999) 116005)116005)..
All All solar solar experimentsexperiments combinedcombined
tan(tan() = ) = 3030
tan(tan() = 0) = 0
MEG MEG goalgoal
AfterAfterKamlandKamland
Connection with oscillations
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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 time-
varying stray fields.
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Systematic errors on ωa (ppm)
σsystematic 1999 2000
2001
E969
Pile-up 0.13 0.13 0.08 0.07
AGS Background 0.10 0.10 *
Lost Muons 0.10 0.10 0.09 0.04
Timing Shifts 0.10 0.02 0.02
E-Field, Pitch 0.08 0.03 * 0.05
Fitting/Binning 0.07 0.06 *
CBO 0.05 0.21 0.07 0.04
Beam Debunching 0.04 0.04 *
Gain Change 0.02 0.13 0.13 0.03
total 0.3 0.31 0.21 0.11Σ* = 0.11
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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-.
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The Storage Ring Magnet
r = 7112 mmB0 = 1.45 T
cyc = 149 ns
(g-2) = 4.37 s
= 64.4 s
p = 3.094 GeV/c
B Field Measurement
2001
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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
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Improved transmission into the ring
InflectorInflector aperture
Storage ring aperture
E821 Closed End E821 Prototype Open End
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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
Pedestal vs. Time
Near side Far side
E821
E821: Pions @ 3.115 GeV/c
momentum
collimator
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μ EDM may be enhancedabove mμ/me × e EDM
Magnitude increases withmagnitude of ν Yukawa couplings
and tan β
μ EDM greatly enhanced when heavy neutrinos non-degenerate
Model Calculations of EDM
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Beam Needs: NP2
• the figure of merit is Nμ times the polarization.
• we needto 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)/(
Up+
Dow
n)