A precision measurement of the neutral pion life time: new results from the PrimEx experiment

1
A precision measurement of the neutral pion life
time new results from the PrimEx experiment
International Conference New Trends in High
Energy Physics Yalta, Crimea, Ukraine September
15 - 22, 2007 I. Larin, ITEP, Moscow on behalf
of the PrimEx collaboration
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Participating Institutions

• a Arizona State University, Tempe, AZ b
  Catholic University of America, Washington, DC
  c Chinese Institute of Atomic Energy, Beijing,
  China d Eastern Kentucky University, Richmond,
  KY e George Washington University, Washington,
  DC f Hampton University, Hampton, VA g
  Institute for High Energy Physics, Chinese
  Academy of Sciences, Beijing, China h Institute
  for High Energy Physics, Protvino, Moscow region,
  Russia i Institute for Theoretical and
  Experimental Physics, Moscow, Russia j Kharkiv
  Institute of Physics and Technology, Kharkiv,
  Ukraine k Massachusetts Institute of
  Technology, Cambridge, MA l Norfolk State
  University, Norfolk, VA m North Carolina AT
  State University, Greensboro, NC n North
  Carolina Central University, Durham, NC o
  Southern University at New Orleans, New Orleans,
  LA p Thomas Jefferson National Accelerator
  Facility, Newport News, VA q Tomsk
  Polytechnical University, Tomsk, Russia r
  University of Illinois, Urbana, IL s
  University of Kentucky, Lexington, KY t
  University of Massachusetts, Amherst, MA u
  University of North Carolina at Wilmington,
  Wilmington, NC v University of Texas at El
  Paso, El Paso, TX w University of Virginia,
  Charlottesville, VA x Yerevan Physics
  Institute, Yerevan, Armenia

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Physics Motivation

• p0 radiation width is one of the most precise
  fundamental predictions of the low energy QCD.
  Number of quark color states is a parameter of
  the calculations
• For massless quarks, width is given by the
  formula
• G(p0 gg) a3Nc2mp3 / 576p3Fp2 7.725 eV (Nc
  3)
• More precise calculations are giving
• 7.93eV 1.5 (QCD sum rules, B.L.Ioffe and
  A.G.Oganesian hep-ph/0701077 )
• 8.10eV 1.0 (Chiral theory, J.L.Goity et al
  Phys Rev D66 076014 / 2002)
• Which precision can be achieved in experimental
  measurements?

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Existing Measurements of p0 Meson Decay Width
Experimental measurements of the radiative width
give 7.84eV 0.56eV. It is 7 error, could it
be reduced, we can check QCD predictions with the
precision close to the theory uncertainties. It
is the challenge for the PrimEx experiment!
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The Primakoff Effect
The PrimEx experiment has utilized the Primakoff
effect to measure p0 radiative width

• p0 photoproduction in coulomb field of nucleus
• Equivalence of photoproduction (gg p0) and
  radiative decay (p0 gg) gives raise to linear
  dependence between cross section of primakoff
  production radiative width
• p0 production for this mechanism goes at very
  small (few MeV) momentum transferred
  (corresponding production angles are fractions of
  milliradian for the PrimEx conditions)

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Thomas Jefferson National Accelerator Facility
(TJNAF)

• PrimEx physics data have been collected during
  the 2004 fall run in TJNAF Hall B
• Narrow monochromatic electron beam, injected from
  FEL
• Energy of electron beam for PrimEx run was 5.75
  GeV
• The PrimEx experiment has utilized tagged photon
  beam with well known energy (in the range of
  4.95.5 GeV) formed from the electron beam

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PrimEx Setup

• For high precision
• JLab Hall B high resolution high intensity
  photon tagging facility (tight photon energy and
  flux control)
• New state of art high resolution hybrid
  multi-channel calorimeter HYCAL (measure po
  angular distribution well to separate various
  amplitudes )
• New pair spectrometer for photon flux control
  at high intensities
• Variety of targets to test validity of
  background extraction (sPrimakoff a Z 2)
• Ability to measure QED processes (Compton and
  pair production) to validate setup and analysis
  techniques

Distance from target to calorimeter 7.3?
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Hall-B Photon Tagger

• Serves as a high precision photon flux meter
• Photon beam energy resolution - 0.1
• Tagging signal time resolution - 0.10.2 nsec

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PrimEx Targets Used For p0 Run
Carbon target pyrolitic graphite with thickness
of 5 X0 or 9.6mm and density of 2.2 g/cm3
Thickness tolerance - 0.1
Lead target monoisotopical lead-208 spinless
nucleus, foil with the thickness of 5 X0 or
0.3mm Thickness tolerance - 0.3
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ee- pair spectrometer
plastic scintillators
Pair spectrometer serves as an online relative
flux monitor detecting ee- pairs from the
target. Needs to ensure high precision of the
beam flux measurements
dipole magnet
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PrimEx Hybrid Calorimeter HyCal

• Transverse size 1.2m x 1.2m
• thermostated at 14?
• Central part
• 1152 PbWO4 modules (scintillators)
• Peripheral part
• 576 lead glass modules (Cherenkov counters)

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PbWO4 Crystals

• Scintillators
• Size 20.5 x 20.5 x 180.0 mm3
• Transverse size and its tolerance
• Specified 20.5 0.0 -0.1mm
• Measured 20.47 0.02mm
• Density 8.3 ?/cm3
• Radiation length 0.89 cm
• Light emission time (fast component) 15 5 ns
• Refraction index 2.2
• Light yield 80 Photons/MeV

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HyCal Calibration

• HyCal calibration has been performed during
  snake scans with HyCal placed on transporter
  and exposed to tagged beam
• Additionally physical calibration with p0s has
  been performed using all collected p0s

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HyCal Resolution

• PWO part of HyCal (5GeV ?)
• 1ns time signal resolution (used for triggering)
• 2.3MeV ? mass resolution
• 0.02 ? production angle resolution
• 1.7 and 0.9 ? energy resolution (w/o and with
  ? mass constraint)
• Lead glass Cherenkov modules
• approximately twice worse resolution values

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PrimEx Production Run

• PrimEx data have been collected during
  October-November 2004
• p0 production runs (with full magnetic field in
  PS-magnet). Working energy range 4.95.5GeV
  (first 11 T-counters).
• ee- production runs (intermediate magnetic field
  in PS allowed pairs to hit calorimeter)
• Compton runs (zero field in PS-magnet allows
  compton pairs to reach HyCal)

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Typical p0 event (real data) on the HyCal event
display
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p0 Reconstruction2? invariant mass spectrum
Crystal part of HyCal
Elastic p0s, Beam energy (well known from Tagger)
has been used to correct cluster energies
(Resolution dominated by coordinate
reconstruction).
Elastic and inelastic p0s, using energies
reconstructed by HyCal
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?0 elasticity spectrum(for PWO part of HyCal)
p0 elasticity E(?) E(p0)
Elastic yield was extracted using fitting
procedure and used in further analysis
s _at_ 0.9
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?0 production terms
Angular distribution enables separation of
amplitudes
Primakoff term
Strong production Incoherent Coherent
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Fit to Extract ?0??? Decay Width p0
distribution on production angle (PWO part of
HyCal)

• p0 production terms
• Sum of all
• Coulomb
• Strong
• Interference
• Incoherent

Theoretical distributions of these processes were
smeared with the experimental resolution
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w and r background for Carbon
p0 distribution on production angle
contribution from r and w was varied with
its cross-section uncertainty (which is
20) Estimated systematic error contribution by
this variation is 0.24
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PrimEx Preliminary Result
?(????) 7.93eV?2.1?2.0
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Systematics check Compton process
Radiation corrections were taken into account in
the theoretical calculations since high precision
is needed
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Systematics check Compton process
Measured Compton cross section at ? beam energy
range 4.95.5GeV is in agreement with the theory
within stat. and syst. errors
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Systematics check ee- Pair Production

• Bethe-Heitler
• Virtual Compton scattering
• Radiative effects
• Atomic screening
• Electron field pair production

Corrections were taken into account in the
theoretical calculations since high precision is
needed
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Systematics check ee- Pair Production

Measured pair production cross section at ? beam
energy range 4.95.5GeV is in agreement with the
theory within stat. and syst. errors
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?0 Rad. Width Error Budget (preliminary)
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PrimEx Future Plans

• publish 2004 run physics results in the nearest
  future
• prepare for the new run 2008-2009 to collect an
  extra statistics with the optimized experimental
  conditions (taking into account existing
  experience and therefore with reduced
  systematical error)
• go to 12GeV beam measurements after upgrade of
  JLab accelerator. Precision measurement of h
  meson radiation width

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Conclusion

• Precision measurement of p0 meson rad. width
  gives the possibility to check QCD prediction,
  based on the number of quark color states.
  Required precision of the measurements has pushed
  the serious requirements to the experimental
  setup
• In 2000-2004 the unique hybrid electromagnetic
  calorimeter (HyCal, which is one of the main
  parts of the PrimEx setup) was assembled at TJNAF
  Hall-B
• Obtained resolution for p0 mass is 2.3MeV for the
  crystal part of HyCal. Usage of the Photon Tagger
  information about beam energy gives further
  improvement of this value up to 1.3MeV
• High quality data (12C and 208Pb nuclei at photon
  beam energy 4.95.5GeV) have been collected
  during fall 2004 run
• Quality of the data is defined by the excellent
  energy and spatial resolutions of the HyCal and
  the precise measurement of the photon beam energy
  and flux with the unique Photon Tagger
• Measured p0 radiation width (preliminary result)
• 7.93eV ? 2.1 (stat.) ? 2.0 (syst.)

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Spare Slides
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Decay Length Measurements (Direct Method)
An experiment had been done at CERN, in 1984,
P450 GeV proton beam 2 variable (5-250?m)
foils Result ?(?0???) 7.34eV?3.1(total)
Dominant systematic error Uncertainty in P?
(?1.5)
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Formfactors

• Distorted formfactors have been calculated with
  most up-to-date charge density distribution
• (E.Offermann, L.Cardman et al, Phys Rev C 91
  Vol.44)

12C charge density
12C strong formfactor
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Elastic p0 yield extraction
Method 1 conventional Elasticity and Invariant
mass distributions fitting
Example of elasticity distribution
Example of invariant mass distribution
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Elastic p0 yield extraction
Method 2 rotation of Elasticity - Invariant mass
2D distribution, hybrid mass analysis
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Elastic p0 yield extraction
Method 3 Invariant mass with elasticity
constraint analysis
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w and r decays background
?0 elasticity spectrum E(?0) E(?)
Data, Carbon target
Simulated contribution from w and r
decays, scaled according to known cross-section
values to Photon beam flux from the data