π 0 Lifetime from the PrimEx Experiments

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π 0 Lifetime from the PrimEx Experiments Liping Gan University of North Carolina Wilmington (for the PrimEx Collaboration) Outline 0 and QCD symmetries PrimEx-I result PrimEx-II status Summary

description

π 0 Lifetime from the PrimEx Experiments. Outline  0 and QCD symmetries PrimEx-I result PrimEx-II status Summary. Liping Gan University of North Carolina Wilmington (for the PrimEx Collaboration). Properties of  0.  0 is the lightest hadron: m  = 135 MeV. - PowerPoint PPT Presentation

Transcript of π 0 Lifetime from the PrimEx Experiments

Page 1: π 0  Lifetime from the PrimEx Experiments

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

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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

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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

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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!

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p

k1

k2

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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

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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

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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

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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:

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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.

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PrimEx-I Result

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(0) = 7.820.14(stat)0.17(syst) eV

2.8% total uncertainty13

PrimEx-I Result (contd.)

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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

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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

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Total relative gain: PrimEx-I config. 100 % suggested PrimEx-II config. 19 %

~5 times less background events

Monte Carlo Simulations

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Add Timing in HyCal

~500 channels of TDC’s in HYCAL

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Improvement in PID

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Additional horizontal veto

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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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~ 1.5ns ~ 0.6ns

Before calibration After calibration

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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

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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.

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This project is supported by:

NSF MRI (PHY-0079840) Jlab under DOE contract (DE-AC05-84ER40150)

Thank You!

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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)

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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%

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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

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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→

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He

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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