Transcript Document

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
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
Standard Model Summary
d (ppm)
aμQED  116 584 706  3
151  4
 0.03
1st
ahad

μ
6 918  67
 0.53
h.o.
ahad

μ
 179  17
 0.14
aμweak
aμSM 

 0.025
116 591 596  67
 0.6
 10-11
Uncertainty in hadronic VP will continue to shrink
But, hadronic “light by light” remains somewhat
difficult
Why probe the difference of g from 2?
a"NEW
"

 aexp t  atheory

QED + WEAK + HADRONIC
Standard Model g-2
e+
e-
QED
Z0
WK
e+
e-
B
µ
e+
µ
µ
µ
W
µ
W
e-
e+
e-
Looking Beyond the
Standard Model
A variety of possible contributions (at the 0.35 ppm leve
 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)
2
a
SUSY

 100 GeV 
 140 10 
 tan 
 M SUSY 
11
i.e. For tan β = 40
MSUSY = 1.2 TeV Da ~0.35ppm
MSUSY = 700 GeV Da ~1 ppm
MSUSY = 350 GeV Da ~4 ppm
Dominant Diagrams
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:
eB
eB
ws  g
 (1   )
2mc
 mc
eB
wc 
 mc
So, in the particle's rest frame, the spin vector rotates relative to the
momentum vector at the frequency
eB
eB
eB
eB
wa  w s  wc  g
 (1   )

 a
2mc
 mc  mc
mc
Proportional to a ... not g!
wa is independent of  !
Measuring (g-2)μ
Polarized Muon
Source
Precession in
Uniform B-field
Polarimeter vs.
Time
Make a pion beam, then select
highest energy muons from parity
violating     decay
Ultra-precise dipole storage ring
allowing muons to precess through
as many g-2 cycles as possible
In parity violating muon decay,
  e  e   , the
positron is preferentially
emitted
in the muon spin direction
The Magic γ
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:


e 
1 
wa 
 a B   a  2
   E
mc 
 1 



2
 1  1/ a
For Magic p=3.09 GeV/c the
second term does not affect
the rotation frequency!
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 pointingAbackward
counting experiment vs. time
The g-2 Time Spectrum
In each positron detector, the time spectrum follows the following energydependent
form: is ~0.4 above 1.8 GeV threshold
Asymmetry
N (t )  N0e
 t /  
Statistical error of fit:
dw a
2

w a w a  A N e
1  A cos(wat   )
Getting to High Precision...
Recall: ωa=aµ(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
Aspects of BNL E821
Beamline and Injection Modes
Storage Ring / Kicker
Radius
7112 mm
Aperture 90 mm
Field
1.45 T
P
3.094 GeV/c
Positron Detector
24 Calorimeters
inside the ring:
•
Lead/Scintillating
Fiber
• 10 Radiation Lengths
• Energy resol 10%
Requirements over
600 μs measuring time:
•
•
Timing shifts < 60 ps
Gain change < 0.3%
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
•
Magnetic Field Measurement
Systematic Uncertainties for the ωp 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
Total
0.05
0.15
0.10
0.10
0.03
0.10
0.24
4.5 Billion e+ with E>2GeV
dN / dt  N 0e

t

1  A cosw at  a 
1999 Analysis Strategy
Magnetic Field
Secret Offsets
Secret Offsets
Data Production
Fitting / Systematics
E821 Data Runs
1997
+ engineering run 13 ppm measurement published
(R.M.Carey et al., PRL 82 (1999) 1632)
1998
+ engineering run 5 ppm measurement published
(H.N.Brown et al., Phys.Rev. D62 (2000) 091101)
1999
+ run
1
2000
+ run
(H.N.Brown et al., hep-ex/0102017 v3 27 Feb 2001)
0.7 ppm measurement published
(G.V.Benett et al.., PRL 89 (2002) 1804)
"
2001
1.3 ppm measurement published
- run
In progress
VEPP-2M collider
CMD-2 1992-2000
SND
1995-2000

Ldt

50pb

1
How the luminosity are measured?
P+
L=
2E = 370 MeV
ee
Nee
ee  ee()


ee

—
when
and
are not clearly separated
E+.MeV
L=
Nee(1 + R)
P
2E = 720 MeV
ee
ee  ee () +  () 
With a QED fixed ratio
N(e+ e  μ +μ  (γ))
Rμ 
N(e+ e  e+ e (γ))

(NI)
, 
(mips)
E, MeV
Dispersion applications
e+
The contribution of the hadronic
vacuum polarization to a
 αmμ 
a μ (Had;1)= 

3π




Η
2 
ds
2 s2 K(s)R(s)
4m


π
Fine structure “constant” (MZ)
α(0)
α(s)=
1-Δαl (s)-Δα(5had) (s)-Δα top (s)
e-
Δα
(5)
hadrons
Η
αs
' R(s )
(s)=- P  2 ds ' '
3π 4mπ s (s-s )

'
ee   cross section (PPCMD)

2
|F |2
+


+

2
2
2  E, MeV
+


Polarization of vacuum by leptons
and hadrons is included in
resonance:
“dressed” cross section
“compensators”
2
σ
vis
theor
(e e  π π )=  ... Fπ (z1,z 2 ,s) test D(z1 )D(z 2 )σ(z1,z 2 ,s)dz1dz 2
+ –
+ –
2
e e     
L= 317.3 nb-1
114000  events
in  meson region
2
|F |
M ρ = (775.65 ± 0.64 ± 0.50) MeV (  0.54σ)
Γρ = (143.85 ± 1.33 ± 0.80) MeV (  0.39σ)
Γ(ρ  e + e – ) = (7.06 ± 0.11± 0.05) keV (  1.7σ)
Br(ω  π + π – ) = (1.30 ± 0.24  0.05) % (nc)
arg d = 13.3o  3.7 o  0.2o (nc)
2  E, MeV
Main sources of systematic errors
Event separation
(0.2%)
Radiative correction (0.4%)
Detection efficiency (0.2%)
Feducial volume
(0.2%)
Correction for pion losses (0.2%)
Beam energy determination (0.1%)
Total (0.6%)
e+e-+-0 (SND)
Fit: A(w) + A() + A() + A(w’)
+ A(w’’) + A(e+e-w0 )
 KLKS
L= 1294 nb-1
2.72 105 KLKS events
e+eKLKS
M φ = (1019.483  0.011 ± 0.025) MeV (nc)
Γφ = (4.280 ± 0.033 ± 0.025) MeV (nc)
σ0 = (1413 ± 6± 24) nb (  1.6σ)
Bree BrKL KS = (1.001 ± 0.004  0.017) 10-4 (  1.6σ)
+
e e K
LKS,
KS
+
  (CMD-2)
• 2E=1.0-1.04 GeV
L=2 pb-1, N=2.7105
0(KLKS )=1413624 nb
m=1413624 MeV/c2
=4.2800.0330.025 MeV
systematic error
in (e+e- KLKS ) 1.7%
• 2E=1.05-1.38 GeV,
L=5.8 pb-1, N=103
systematic error
in (e+e- KLKS ) 5-10%
solid curve is VDM with
(770) , w(783) , (1020) + X
dash curve is VDM with
(770) , w(783) , (1020) only
e+e-K+KSeparation 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
(e+e- K+K- ) ~6%
Cross section can not be described by
(770), w(783) and (1020) only (solid curve)
e+e-4
e+e-w0 ,w+-0
CMD-2 data in 4 channel is lower!
e+e-2+2-
After reanalysis CMD-2 data
agrees with SND data
What is a build from?
Relative
contributions to a
Relative
contributions
to uncertainty a
Comparison with  decays
(g-2)/2 of muon
ντ
W
Η

CVC
τ


Η
e+
•


Η

e-

Η
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 )
M.Davier et al., hep-ph/0308213
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 <1% 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
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