Post on 19-Dec-2015
An Updated High Precision Measurement of the Neutral Pion Lifetime via the
Primakoff Effect
A. GasparianNC A&T State University, Greensboro, NC
Outline
Physics Motivation Different methods of lifetime measurements The PrimEx experiment and our first results Control of systematic errors Summary
A. Gasparian PAC33, January 15, 2008 2
0 decay width
eVF
mNc 725.7576 23
3220
0→ decay proceeds primarily via chiral anomaly in QCD. The prediction of chiral anomaly is exact for massless quarks:
Corrections to chiral anomaly prediction: (u-d quark masses and mass differences)
Calculations in NLO ChPT:(J. Goity, at al. Phys. Rev. D66:076014, 2002)Γ(0) = 8.10eV ± 1.0%
~4% higher than LO, uncertainty less than 1%
Precision measurements of (0→) at percent level will provide a stringent test of fundamental predictions of QCD.
0→
Recent calculations in QCD sum rule: (B.L. Ioffe, at al. Phys. Lett. B647, p. 389, 2007)
Γ() is only input parameter0- mixing includedΓ(0) = 7.93eV ± 1.5%
A. Gasparian PAC33, January 15, 2008 3
Decay Length Measurements (Direct Method)
1x10-16 sec too small to measuresolution: Create energetic 0 ‘s,
L = vE/m
But, for E= 1000 GeV, Lmean 100 μm very challenging experiment
Measure 0 decay length
An experiment had been done at CERN, in 1984, P=450 GeV proton beam2 variable (5-250m) foilsResult:(0) = 7.34eV3.1%(total) Dominant systematic error:Uncertainty in P0 (1.5%)
Limitations of method unknown P0 spectrum foil position dependent exptl. bgnd.
A. Gasparian PAC33, January 15, 2008 4
e+e- Collider Experiment e+e-e+e-**e+e-0e+e- e+, e- scattered at small angles (not detected)
only detected
experiment: DORIS II @ DESY
Results: Γ(0) = (7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)
dominant systematic errors: luminosity (~6%)
beam-residual gas interaction
Not included in PDG average
Limitations of method luminosity unknown q2 for **
A. Gasparian PAC33, January 15, 2008 5
Primakoff Method
22
..4
43
3
2Pr
3
sin)(8
QFQ
E
m
Z
d
dme
ρ,ω
Challenge: Extract the Primakoff amplitude
12C target
Primakoff
Nucl. Coherent
Interference Nucl. Incoh.
A. Gasparian PAC33, January 15, 2008 6
Previous Primakoff Experiments DESY (1970)
bremsstrahlung beam, E=1.5 and 2.5 GeV
Targets C, Zn, Al, Pb Result: (0)=(11.71.2) eV
10.%
Cornell (1974) bresstrahlung beam
E=4 and 6 GeV targets: Be, Al, Cu, Ag, UResult: (0)=(7.920.42) eV
5.3% dominant systematic errors: N (4%) and quantameter (2%)
All previous experiments used: Bremsstrahlung (untagged) beam Conventional Pb-glass calorimetry
A. Gasparian PAC33, January 15, 2008 7
PrimEx Experiment
JLab Hall B high resolution high intensity photon tagging facility
New pair spectrometer for photon flux control at high intensities New high resolution hybrid multi-channel calorimeter (HYCAL)
Requirements to Setup: high angular resolution (~0.5 mrad)
high resolutions in calorimeter small beam spot size (‹1mm)
Background: tagging system needed
Particle ID for (-charged part.) veto detectors needed
A. Gasparian PAC33, January 15, 2008 8
PrimEx Milestones Proposal approved in 1999 by PAC15, re-approved by PAC22 (E02-103) in 2002 with A rating.
Full support of JLab (Engineering group, machine-shop, installation, etc.).
In 2000 NSF awarded a collaborative MRI grant of $1 M to develop the experimental setup.
In 4 years the experimental setup, including procurement of all hardware, was designed, constructed and tested.
Commissioning and data taking was performed in August-November 2004 run.
First publication is expected in spring, 2008.
Preliminary results had been released at APS April, 2007 meeting with AIP press conference.
A. Gasparian PAC33, January 15, 2008 9
Luminosity Control: Pair Spectrometer
Dipole
Precision cross section measurement:
photon flux at 1% level required
e-
e+
HYCAL
Photon beam
Scint. Det.
absolute tagging ratios: TAC measurements at low intensities
Checked by cross sections of known EM processes at the 1% level:
Compton scattering e+e- pair production
relative tagging ratios: pair spectrometer at low and high intensities
A. Gasparian PAC33, January 15, 2008 10
Electromagnetic Calorimeter: HYCAL
1152 PbWO4 crystal detectors 576 Pb-glass Cherenkov detectors
Energy resolution Position resolution Good photon detection efficiency @ 0.1 – 5 GeV; Large geometrical acceptance
PbWO4 crystals resolutionPb-glass budget
Design concept hybrid calorimeter
A. Gasparian PAC33, January 15, 2008 11
- Invariant Mass Resolution
A. Gasparian PAC33, January 15, 2008 12
0 Event selection
We measure:
initial photon energy: E and time energies of decayed photons: E1, E2 and time X,Y positions of decayed photons
Kinematical constrains:
Conservation of energy; Conservation of momentum; m invariant mass
Three groups analyzed the data independently
A. Gasparian PAC33, January 15, 2008 13
Differential Cross section
Experimental Yield per
GEANT: acceptances; efficiencies; resolutions;
Diff. cross section
A. Gasparian PAC33, January 15, 2008 14
0 Forward Photoproduction off Complex Nuclei(theoretical models)
Coherent Production A→0A
Primakoff Nuclear coherent
0 rescattering Photon shadowing
Leading order processes:
Next-to-leading order:
A. Gasparian PAC33, January 15, 2008 15
0 Forward Photoproduction off Complex Nuclei(theoretical models)
Incoherent Production A→0A´
Two independent approaches: Glauber theory Cascade Model (Monte Carlo)
Deviation in Γ(0) Extraction:
less than <0.2%
A. Gasparian PAC33, January 15, 2008 16
Fit to Extract 0 Decay Width
Combined average from three groups:
Γ(0) 7.93 eV 2.10%(stat.) 2.0% (syst)
Theoretical angular
distributions smeared with
experimental resolutions
are fit to the data
A. Gasparian PAC33, January 15, 2008 17
PrimEx Current Result
() = 7.93eV2.1%2.0%
0
D
eca
y w
idth
(eV
)
±1.%
A. Gasparian PAC33, January 15, 2008 18
Estimated Systematic Errors Type of Systematic
ErrorsEstimated
contributions in first run
Estimated contributions for current proposal
Photon flux 1.0% 1.0%
Target number <0.1% <0.1%
Background subtraction 1.0% 0.4%
Event selection 0.5% 0.35%
HYCAL response function 0.5% 0.2%
Beam parameters 0.4% 0.4%
Acceptance 0.3% 0.3%
Model errors (theory) 1.0% 0.25%
Physics background 0.25% 0.25%
Branching ratio 0.03% 0.03%
Total 2.0% 1.3%
A. Gasparian PAC33, January 15, 2008 19
Compton Cross section: Theory
Pure QED process: Should be calculable on percent level
Leading Order: Klein-Nishina
Corrections to LO:
Rad. correction (virtual/soft)
Double Compton (hard emiss.)
Klein-Nishina + full rad. Corr. (Monte Carlo Method)
Klein-Nishina + full rad. Corr. (Numerical Integration Method)
A. Gasparian PAC33, January 15, 2008 20
Compton Cross section: Experiment
4.9 5.0 5.1 5.2 5.3 5.4 5.5
Energy (GeV)
0.055
0.060
0.065
0.070
0.075
0.080
0.085
Systematic Uncertainty
P R E L I M I N A R Y
Uncertainties:StatisticalSystematic
Compton Forward Cross Section
Klein-NishinaPrimex Compton Data
4.9 5.0 5.1 5.2 5.3 5.4 5.5
Energy (GeV)
-10
-5
0
5
10
Devi
atio
n (%
)
P R E L I M I N A R Y
Uncertainties:Statistical
Experiment To Theory Comparison
No DeviationExperiment / Theory
Average stat. error: 0.6% Average syst. error: 1.2%
Total: 1.3% Δ
σ/Δ
Ω (
mb
/6.9
msra
d)
A. Gasparian PAC33, January 15, 2008 21
Summary
A state-of-the-art high resolution experimental setup including a high precision EM calorimeter and pair spectrometer has been designed, developed, constructed and commissioned with first physics run in fall, 2004.
Our first result: Γ(0) 7.93 eV 2.10% (stat.) 2.0% (syst.)
The 0 lifetime is one of the few parameter-free predictions in QCD reflecting effects of fundamental symmetry and axial anomaly.
Percent level measurement is a stringent test of QCD at these energies.
Compton and pair-production cross section measurements demonstrate that the systematic errors are controlled at 1.3% level.
The experimental setup is capable for a percent level cross section measurement.
Availability of high resolution and high intensity tagging facility together with recent developments in calorimetry made the Primakoff method the viable way to reach the projected percent level in 0 decay width.
Control of model error in 0 lifetime at 0.25% level has been reached.
Requesting 28 days of beam time to reach the goal of 1.4% on 0 life time.
A. Gasparian PAC33, January 15, 2008 22
The End
A. Gasparian PAC33, January 15, 2008 23
Stability of relative tagging ratios
Monitored by PS during production data taking.
PS+tagger
Tagger
A. Gasparian PAC33, January 15, 2008 24
0 Event selection (cont.)
Three groups analyzed the data independently
A. Gasparian PAC33, January 15, 2008 25
Theoretical Study of 0 Forward Photoproduction off Complex Nuclei
Coherent Production A→0A:
PrimakoffNuclear coherent
0 rescattering Photon shadowing
Absorption of 0
A. Gasparian PAC33, January 15, 2008 26
Model dependence of Γ(0) Extraction
Model error in Γ(0) Extraction can be controlled at < 0.25%
A. Gasparian PAC33, January 15, 2008 27
Some results on Coherent Production A→0A
• Electromagnetic form factors
• Strong form factors
12C
E=5.2 GeV
208Pb
208Pb E=5.2 GeV
Without shadowing
With shadowing
A. Gasparian PAC33, January 15, 2008 28
Incoherent Production A→0A´
Two independent approaches: • Glauber theory• Cascade Model
Deviation in Γ(0) Extraction is <0.2%
A. Gasparian PAC33, January 15, 2008 29
Differential Cross section
Experimental Yield per
GEANT: acceptances; efficiencies; resolutions;
Diff. cross section
A. Gasparian PAC33, January 15, 2008 30
New from Ilya, 011208(animation)
Combined average from three groups:
Γ(0) 7.93 eV 2.10%(stat.)
Theoretical angular
distributions smeared with
experimental resolutions
are fit to the data
A. Gasparian PAC33, January 15, 2008 31
Control of Systematic Errors: Compton
e e
Events Selection Energy conservation
3-momentume conservation
(including co-planarity)
We measure:
Initial photon energy: E and time Energies of scattered particles: E, Ee and time X,Y positions on HYCAL
A. Gasparian PAC33, January 15, 2008 32
Estimated Systematic Errors on Compton (preliminary)
Photon flux 1.0%
Target thickness (+impurity)
0.05%
Coincidence timing 0.03%
Coplanarity 0.065%
Radiative tail cut 0.098%
Geometric cuts stability 0.65%
Background subtraction 0.40%
Yield fit stability 0.063%
Total 1.27%
A. Gasparian PAC33, January 15, 2008 33
PrimEx Collaboration
North Carolina A&T State University University of Massachusetts Idaho State University University of North Carolina WilmingtonJefferson Lab MITCatholic University of America Arizona State University CIAE Beijing, China Norfolk State UniversityBeijing University, China Lanzhou University, ChinaITEP Moscow, Russia IHEP Protvino, Russia Duke University Kharkov Inst. of Physics and Tech. UkraineNorthwestern University IHEP, China
University of Sao Paulo, Brazil Yerevan Physics Institute, ArmeniaRIKEN, Japan JINR Dubna, Russia USTC, China Hampton University George Washington University
A. Gasparian PAC33, January 15, 2008 34
Compton as Stability Control(maybe to question section)
NOV 02
NOV 09
NOV 14
NOV 19
0.24
0.25
0.26
0.27
0.28
0.29
0.30
0.31
0.32Compton Cross Section Time Stability
P R E L I M I N A R Y
Uncertainties:Statistical
E = 5.220 GeV
2% Band
T Counter: 3
Run Number
TheoryPrimEx Compton Data
σ
(mb
)
A. Gasparian PAC33, January 15, 2008 35
Primakoff Method
22
..4
43
3
2Pr
3
sin)(8
QFQ
E
m
Z
d
dme
ρ, ω
Challenge: Extract the Primakoff amplitude
A. Gasparian PAC33, January 15, 2008 36
Compton Cross section
4.9 5.0 5.1 5.2 5.3 5.4 5.5
Energy (GeV)
0.23
0.24
0.25
0.26
0.27
0.28
0.29
0.30
0.31
0.32
P R E L I M I N A R Y
Uncertainties:StatisticalSystematic
Compton Total Cross Section
TheoryPrimEx Compton Data
A. Gasparian PAC33, January 15, 2008 37
Trigger Improvement
A. Gasparian PAC33, January 15, 2008 38
e+e- Pair Production in PrimEx
Agreement with theory at the level of 2.5%
Work in progress to reduce the systematic errors to 1-2% level
A. Gasparian PAC33, January 15, 2008 39
An Example: Precision Measurement of → decay width
All decay widths are calculated from decay width and experimental Branching Ratios (B.R.):
ΓΓ((η→η→ decay) = decay) = ΓΓ((→→) × B.R.) × B.R.
Any improvement in ΓΓ((→→))
will change the whole will change the whole - sector in PDB- sector in PDB
A. Gasparian PAC33, January 15, 2008 40
Compton Cross section
4.9 5.0 5.1 5.2 5.3 5.4 5.5
Energy (GeV)
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
P R E L I M I N A R Y
Forward Compton Scattering Cross Section
Klein-NishinaPrimex Compton Data
A. Gasparian PAC33, January 15, 2008 41
PbWO4 Development:Optical Properties
Optical transparency
A. Gasparian PAC33, January 15, 2008 42
PbWO4 Development
Specified
Size: 20.5x20.5x180 mm3
Tolerances: +0.0-0.1 in trans. +0.3-0.0 in long.
Collaboration managed to double the number of crystals: to from 650 to 1250 Critical for the experiment
A. Gasparian PAC33, January 15, 2008 43
0 decay width: recent theoretical advances
QCD sum rule approach: f0 - f+ caused by strong interaction
shown to be small 0 - mixing included
Γ(0) = 7.93eV ± 1.5%
error is dominated by Γ() decay width
Precision measurements of (0→) at percent level will provide ultimate test of fundamental predictions of QCD.
0→
A. Gasparian PAC33, January 15, 2008 44
(0→) World Data (do not need)
0 is lightest quark-antiquark hadron
The lifetime:
= B.R.( 0 →γγ)/(0 →γγ) 0.8 x 10-16 second
Branching ratio: B.R. ( 0→γγ)= (98.8±0.032)% 0
→
±1%
A. Gasparian PAC33, January 15, 2008 45
Impact of Giant Excitation of Nucleus on 0 Primakoff production
• With nuclear collective excitation, the longitudinal momentum transfer in 0 photo-production is Δin= Δ+Eav, where the average excitation energy Eav for 12C is ~20-25 MeV.
• The ratio of the cross section of the 0 photo-production in the Coulomb field with nuclear excitation to “elastic” electromagnetic production can be estimated as:
Cq
q
ZAEm
N
d
dd
d
inavpel
in
12722
222
for )(qpeak Primakoffat 10)(
)(
2
4.1
• Nuclear Giant Excitation effect for lead is small as well.
Outline
Physics Motivation Different methods of lifetime measurements The PrimEx experiment and our first results Control of systematic errors Summary