Post on 31-Dec-2015
description
π0 Lifetime from the PrimEx Experiments
Liping GanUniversity of North Carolina Wilmington
(for the PrimEx Collaboration)
Outline
0 and QCD symmetries PrimEx-I result PrimEx-II status Summary
Properties of 0
0 is the lightest hadron:
m = 135 MeV
0 is unstable.
0 → γγ B.R.(0 →γγ)=(98.8±0.032)%
Lifetime and Radiative Decay width:
= B.R.( 0 →γγ)/(0 →γγ) 0.8 x 10-16 second
2
0 ( ) / 2uu dd
Spontaneous Chiral Symmetry Breaking Gives Spontaneous Chiral Symmetry Breaking Gives
Rise to Rise to ππ00 In massless quark limit
Corrections to theory:Corrections to theory: Non-zero quark masses generate meson massesNon-zero quark masses generate meson massesQuark mass differences cause mixing among the Quark mass differences cause mixing among the mesonsmesons
Since Since ππ00 is the lightest quark-antiquark system in nature, the corrections are small.
0 0 08, , , , , , ,K K K K
(3) (3) (3)L RSU SU SU
Massless Goldstone Bosons
3
Axial Anomaly Determines π0 Lifetime
eVF
mNc 725.7576 23
3220
0→ decay proceeds primarily via the chiral anomaly in QCD. The chiral anomaly prediction is exact for massless quarks:
Corrections to the chiral anomaly prediction:
Calculations in NLO ChPT:Γ(0) = 8.10eV ± 1.0% (J. Goity, et al. Phys. Rev. D66:076014, 2002)Γ(0) = 8.06eV ± 1.0% (B. Ananthanarayan et al. JHEP 05:052, 2002)Calculations in NNLO SU(2) ChPT:Γ(0) = 8.09eV ± 1.3% (K. Kampf et al. Phys. Rev. D79:076005, 2009)
Precision measurements of (0→) at the percent level will provide a stringent test of a fundamental prediction of QCD.
0→
Calculations in QCD sum rule: Γ(0) = 7.93eV ± 1.5% (B.L. Ioffe, et al. Phys. Lett. B647, p. 389,
2007)
Γ(0) is one of the few quantities in confinement region that QCD canis one of the few quantities in confinement region that QCD can
calculate precisely to higher orders!calculate precisely to higher orders!
4
p
k1
k2
5
Primakoff Method
2 3 42 2Pr
. .3 4
8( ) sine m
d Z EF Q
d m Q
ρ,ω
Challenge: Extract the Primakoff amplitude
12C target
Primakoff Nucl. Coherent
Interference Nucl. Incoh.
2
Pr 2
4Pr
2Pr
2
log( )
peak
peak
m
Ed
Ed
d Z E
Requirement:
Photon flux
Beam energy
0 production Angular resolution
Pri PrimEx-I (2004)
JLab Hall B high resolution, high intensity photon tagging facility
New pair spectrometer for photon flux control at high beam intensities 1% accuracy has been achieved
New high resolution hybrid multi-channel calorimeter (HyCal)
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PrimEx Hybrid Calorimeter - HyCal 1152 PbWO4 crystal detectors 576 Pb-glass Cherenkov detectors
HYCALonly
Kinematicalconstraint
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x = 1.3 mm
0 Event selection
We measure:
incident photon energy: E and time energies of decay photons: E1, E2 and time X,Y positions of decay photons
Kinematical constraints:
Conservation of energy; Conservation of momentum; m invariant mass
PrimEx Online Event Display
8
0 Event Selection (contd.)
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Fit Differential Cross Sections to Extract Γ(0)
Theoretical angular distributions smeared with experimental resolutions are fit to the data on two nuclear targets:
10
Verification of Overall Systematical Uncertainties
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
+ e +e Compton cross section measurement
e+e- pair-production cross section measurement
Systematic uncertainties on cross sections are controlled at 1.3% level.
PrimEx-I Result
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(0) = 7.820.14(stat)0.17(syst) eV
2.8% total uncertainty13
PrimEx-I Result (contd.)
Goal for PrimEx-II
PrimEx-I has achieved 2.8% precision (total):
(0) = 7.82 eV 1.8% (stat) 2.2% (syst.)
Task for PrimEx-II is to obtain 1.4% precision
Projected uncertainties:
0.5% (stat.) 1.3% (syst.)
PrimEx-I 7.82eV2.8%
PrimEx-II projected 1.4%
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Estimated Systematic Uncertainties
Type PrimEx-I PrimEx-II
Photon flux 1.0% 1.0%
Target number <0.1% <0.1%
Veto efficiency 0.4% 0.2%
HYCAL efficiency 0.5% 0.3%
Event selection 1.7% 0.4%
Beam parameters 0.4% 0.4%
Acceptance 0.3% 0.3%
Model dependence 0.3% 0.3%
Physics background 0.25% 0.25%
Branching ratio 0.03% 0.03%
Total syst. uncertainties
2.2% 1.3%
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Improvements for PrimEx-II
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1.4 % Total
0.5 % Stat.1.3 % Syst.
Double target thickness (factor of 2 gain)Hall B DAQ with 5 kHz rate, (factor of 5 gain) Double photon beam energy
interval in the trigger
Better control of Background:
Add timing information in HyCal (~500 chan.) Improve photon beam line Improve PID in HyCal (add horizontal veto counters to have both x and y detectors) More empty target data
Measure HyCal detection efficiency
Improvements in PrimEx-II Photon Beam Line
1. Make the primary collimator “tapered”.
2. Triple the Permanent Magnet 3. Reduce the size of the central
hole in Pb-shielding wall
17
Total relative gain: PrimEx-I config. 100 % suggested PrimEx-II config. 19 %
~5 times less background events
Monte Carlo Simulations
Add Timing in HyCal
~500 channels of TDC’s in HYCAL
18
Improvement in PID
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Additional horizontal veto
PrimEx-II Run Status
Experiment was performed from Sep. 27 to Nov. 10 in 2010.
Physics data collected:
π0 production run on two nuclear targets: 28Si (0.6% statistics) and 12C (1.1% statistics).
Good statistics for two well-known QED processes to verify the systematic uncertainties: Compton scattering and e+e- pair production.
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21
22
~ 1.5ns ~ 0.6ns
Before calibration After calibration
HyCal TDC spectrum:HyCal TDC groups scheme:
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Empty target
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~8K Primakoff events25
~20K Primakoff events
( E = 4.4-5.3 GeV)
PrimakoffPrimakoff
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→ 0 +
= 15MeV
→ 30
= 6MeV
ʹ→ (→2) + 20
= 10MeVa0 → (→2) + 0
100MeV
Summary
PrimEx-I result (2.8% total uncertainty): Γ(0) 7.82 0.14(stat.) 0.17(syst.) eV Phys. Rev. Lett., 106, 162302 (2011)
The 0 lifetime is one of the few precision predictions of low energy QCD Percent level measurement is a stringent test of QCD.
Systematic uncertainties on cross sections are controlled at the 1.3% level, verified by two well-known QED processes: Compton and pair-production.
PrimEx-II analysis is in progress. The 0 lifetime at level of 1.4% precision is expected.
New generation of Primakoff experiments have been developed in Hall B to provide high precision measurement on Γ(0)
PrimEx-II (fall 2010): high statistical data set has been collected on two nuclear targets, 12C and 28Si.
27
This project is supported by:
NSF MRI (PHY-0079840) Jlab under DOE contract (DE-AC05-84ER40150)
Thank You!
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2929
Measurement of Γ(→) in Hall D at 12 GeV Use GlueX standard setup for this measurement:
75 m
Counting House
Photon beam line -incoherent tagged photons Pair spectrometer Solenoid detectors (for background rejection) 30 cm LH2 and LHe4 targets (~3.6% r.l.) Forward tracking detectors (for background rejection) Forward Calorimeter (FCAL) for → decay photons Additional CompCal detector for overall control of systematic uncertainties.
CompCal
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0 Forward Photoproduction off Complex Nuclei (theoretical models)
Coherent Production A→0A
Primakoff Nuclear coherent
0 rescattering Photon shadowing
Leading order processes:
(with absorption)
Next-to-leading order:
(with absorption)
31
Theoretical Calculation (cont.) Incoherent Production A→0A´ Two independent approaches:
Glauber theory Cascade Model (Monte Carlo)
Deviation in Γ(0) is less than 0.2%
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Γ(0) Model Sensitivity
Overall model error in Γ(0) extraction is controlled at 0.25%
Variations in absorptionparameter ΔΓ <0.06%
Variations in shadowingparameter ΔΓ <0.06%
Variations in energy dependenceParameter n ΔΓ <0.04%
Primakoff Experiments before PrimEx
DESY (1970) bremsstrahlung beam,
E=1.5 and 2.5 GeVTargets C, Zn, Al, Pb Result: (0)=(11.71.2) eV
10.%
Cornell (1974) bremsstrahlung beam
E=4 and 6 GeV targets: Be, Al, Cu, Ag, UResult: (0)=(7.920.42) eV
5.3%
All previous experiments used: Untagged bremsstrahlung beam Conventional Pb-glass calorimetry
33
Decay Length Measurements (Direct Method)
1x10-16 sec too small to measure
solution: Create energetic 0 ‘s,
L = vE/m
But, for E= 1000 GeV, Lmean 100 μm very challenging experiment
Measure 0 decay length
1984 CERN experiment: P=450 GeV proton beamTwo variable separation (5-250m) foilsResult:(0) = 7.34eV3.1% (total)
Major limitations of method unknown P0 spectrum needs higher energies for improvement
0→
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e+e- Collider Experiment
e+e-e+e-**e+e-0e+e-
e+, e- scattered at small angles (not detected)
only detected
DORIS II @ DESY
Results: Γ(0) = 7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)
Not included in PDG average
Major limitations of method knowledge of luminosity unknown q2 for **
0→
35
36
He
Luminosity Control: Pair Spectrometer
Measured in experiment: absolute tagging ratios:
TAC measurements at low intensities
Uncertainty in photon flux at the level of 1% has been reached Verified by known cross sections of QED processes
Compton scattering e+e- pair production
relative tagging ratios: pair spectrometer at low and high intensities
Combination of: 16 KGxM dipole magnet 2 telescopes of 2x8 scintillating detectors
L. Gan 37User's meeting, 6/7/2011