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Progress Report on the π0 LifetimeExperiment (PrimEx) at JLab
Dustin E. McNultyMIT/JLab
PrimEx Collaborationmcnulty@jlab.org
November 1, 2006
PrimEx Collaboration Jefferson Lab Hall B
Progress Report on the π0 LifetimeExperiment (PrimEx) at JLab
Outline
Physics Motivation
π0 Photoproduction Cross Section
Experimental Overview
Preliminary Compton Results
π0 Analysis Status
Summary and Outlook
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 1
PrimEx Collaboration Jefferson Lab Hall B
PrimEx Collaboration
Institutions
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 2
PrimEx Collaboration Jefferson Lab Hall B
Physics Motivation
π0 decay rate is a fundamental prediction of confinement scale QCD.
Presence of closed loop triangle diagramresults in nonconserved axial vector current,
Chiral Anomaly
even in the limit of vanishing quark masses.
In the leading order (chiral limit), the anomaly leads to the decay amplitude:
Aπ0 γγ
αem
4πFπεµνρσkµk
νε
ρε
σ (1)
or the reduced amplitude,
Aγγ
αem
4πFπ
0 02513 GeV
1 (2)
where Fπ
92 42
0 25 MeV is the pion decay constant.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 3
PrimEx Collaboration Jefferson Lab Hall B
Physics Motivation
The π0 γγ decay width predicted by this amplitude is
Γπ0 γγ
m3π
Aγγ
2
64π
7 725
0 044 eV (3)
Current Particle Data Book value is 7 84
0 56 eV
The above result for the decay amplitude is exact in the chiral limit,however for non-vanishing quark masses there are corrections:
Due to isospin sym-breaking (mu md), π0, η and η mixing induced.
Further corrections induced by terms in the Chiral Lagrangian.
Next to Leading Order prediction for the decay width is 8 10 eV
1%
Calc. using Chiral Perturbation Theory and 1/Nc expansion.J.L.Goity et al, Phys. Rev. D66, 076014 (2002); B.Moussallam, Phys. Rev. D51, 4939 (1995)
This is 4% higher than current experimental value!
A precision measurement of the π0 decay width is needed.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 4
PrimEx Collaboration Jefferson Lab Hall B
Physics Goal
Use the Primakoff effect to measure Γπ0 γγ to within 1 5% uncertainty
Experiments
π0 →γγ
Dec
ay W
idth
(eV
)
CERN(Direct)
Cornell(Primakoff)
DESY(Primakoff)
Tomsk(Primakoff)
PrimExExperiment
Leading OrderChiral Anomaly
Next to LeadingOrder, ±1%
7
8
9
10
11
12
0 1 2 3 4 5 6 7
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 5
PrimEx Collaboration Jefferson Lab Hall B
The Primakoff Effect
π0 photoproduction fromCoulomb field of nucleus.
Equivalent production (γγ
π0)and decay (π0 γγ) mechanismimplies Primakoff cross sectionproportional to π0 lifetime.
Primakoff π0 produced at veryforward angles.
dσP
dΩ Γ
π0 γγ
8αemZ2
m3β3E4
Q4
Fem
Q
2sin2θπ (4)
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 6
PrimEx Collaboration Jefferson Lab Hall B
Full Cross Section Componentsdσπ0
dΩ
dσP
dΩ
dσC
dΩ
dσI
dΩ
2 dσP
dΩ
dσC
dΩcos
φ
(5)
Primakoff Nucl.Coherent Incoherent Interference
Primakoff:Proportional to Z2,peaked at θπ0
m2π0
2E2γ
Nuclear Coherent:
dσC
dΩ
C A2
FN
Q
2sin2θπ (6)
Nuclear Incoherent:
dσI
dΩ
ξA
1 G
Q
dσH
dΩ(7)
Interference:
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 7
PrimEx Collaboration Jefferson Lab Hall B
Experiment Overview
Conducted at Jefferson Lab, Fall 2004
Used 5 75 GeV continuous e ! beamand Hall B γ-tagging facility
Tagged photons incident on5%X0 targets: 12C and 208Pb
New PrimEx/Hall B calorimeter(HyCal), upstream of CLAS,designed to detect π0 decay γ’s
Measured 3 physical processes (absolute cross sections): Primary - π0
production, Secondary - Compton and e
"
e ! pair production
Improvements over previous experiments: Precision tagged γ flux andincident γ energy info, enhanced π0 angular and mass resolution, andidentification and subtraction of background event contamination
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 8
PrimEx Collaboration Jefferson Lab Hall B
Experiment Overview
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 9
PrimEx Collaboration Jefferson Lab Hall B
Hall B Photon Tagger
e-beam Photon-beam
PrimEx energy Range0.95 0.85
Single dipole magnet combined with a hodoscope containing twoplanar arrays of plastic scintillators to detect energy-degraded electronsfrom a thin bremsstrahlung radiator.
Tagger has 0 1% energy resolution and is capable of 50 MHz rates.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 10
PrimEx Collaboration Jefferson Lab Hall B
Photon Flux Control
PrimEx goal: Total uncertainty in photon flux
#
1 0%.
Number of tagged photons on target (Nγ) calibrated periodically usinga Total Absorption Counter (TAC).
Any drifts in the tagging ratio, occuring between calibration points, aremonitored online with the e
"
e ! pair spectrometer.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 11
PrimEx Collaboration Jefferson Lab Hall B
Hybrid Calorimeter – “HyCal”.
Optimal performance/costdesign
1 2 m $ 1 2 m, 1728 channels
576 Lead-glass (outer layers)
1152 Lead-Tungstenatecrystal (inner layers)
Lead-glass PbWO4
Energy Res. (∆E/E) 3 5 % 1 2 %
Position Res. (∆x,y) % 5 mm % 1 5 mm
Angular Res. (∆θπ0 ) % 675 µrad % 300 µrad
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 12
PrimEx Collaboration Jefferson Lab Hall B
HyCal
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 13
PrimEx Collaboration Jefferson Lab Hall B
HyCal Calibration
Full x,y motion allowed each ch. to be scanned through tagged γ beam.
Performed at both the beginning and end of the experiment.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 14
PrimEx Collaboration Jefferson Lab Hall B
Compton Scattering:
PbWO4
:(x,y,E’)γ
θγscattered photon
incident photon scattered atomic electron
Lead-Glass
Lead-Glass
e:(x,y,E)
beamline
HyCal
MeasuredMeasured Eo
Measured
A well known process, useful in several ways:
Detector/beam alignment
HyCal gain monitoring
Overall check of PrimEx setup to measure absolute cross sections
& Dedicated ”Double-Arm” Compton Runs:
Performed on a weekly basis, BPS
0, Ibeam
% 5 10 nA
Both e and scattered photon detected in HyCal
Compton Cross Section Measured: 12C and 0 5%X04Be
& “Single-Arm” Compton Data:
Dominant Source of Events in π0 production dataruns
BPS
% 2 T, Ibeam
% 100 nA, only scattered photon detected
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 15
PrimEx Collaboration Jefferson Lab Hall B
Beam Alignment Monitoring using Single-Arm Compton(Y
-
Y) m
ean
(X
- X
) mea
nC
ompt
Com
pt
.
Only scattered γ measured
X ' reported HyCal coord
XCompt
' calc. (x,y) fromHycal E and Compton kin.
If beam alignmentperfect: (XCompt-X) = 0
Technique tracksalignment at 0 1 mm level
Jump in X correlated withbeamline BPM
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 16
PrimEx Collaboration Jefferson Lab Hall B
Very Preliminary Compton Cross Section
0.2
0.21
0.22
0.23
0.24
0.25
0.26
0.27
0.28
0.29
0.3
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6
Ebeam (GeV)
tota
l cro
ss se
ctio
n (m
b) p
er e
lect
ron
Klein-Nishina
PrimEx data, C12 target
0.2
0.21
0.22
0.23
0.24
0.25
0.26
0.27
0.28
0.29
0.3
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6
Ebeam (GeV)
tota
l cro
ss se
ctio
n (m
b) p
er e
lect
ron
Klein-Nishina
PrimEx data, Be target
.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 17
PrimEx Collaboration Jefferson Lab Hall B
π0 Analysis Status: γγ Invariant Mass
110 120 130 140 150 160 170 180 190
Eve
nts/
0.5M
eV
0
2
4
6
8
10
12
14
310× Invariant Massγ Two
(MeV)γγm
C target12
detector4PbWO
) = 2.52 MeVγγ(mσ
110 120 130 140 150 160 170 180 190
Eve
nts/
0.5M
eV2
4
6
8
10
12
14
16
18
310× Invariant Massγ Two
(MeV)γγm
C target12
detector4PbWO+ Tagger Energy Constrain
) = 1.31 MeVγγ(mσ
.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 18
PrimEx Collaboration Jefferson Lab Hall B
π0 Analysis: Event Timing
trigPhoton.tdiff (ns)-40 -20 0 20 40
trigPhoton.tdiff (ns)-40 -20 0 20 40
Yie
ld /
ns0 π
10
210
310
410
pk_int 292± 8.214e+04
mean 0.004± -3.447
sigma 0.0027± 0.9732
slope 2.0e-05± 1e-05
intercept 2.6± 192.5
time (ns)Σtdiff = TAGM[0] time - HyCal total
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 19
PrimEx Collaboration Jefferson Lab Hall B
π0 Analysis: Event Elasticity
tagger/EclusterPairE0.8 0.9 1 1.1 1.2
tagger/EclusterPairE0.8 0.9 1 1.1 1.2
Yie
ld/0
.010
0 π
0
2
4
6
8
10
12
14
16
310×
/ ndf 2χ 60.57 / 31
const1+2 301± 6.522e+04
mean1 0.000± 1.002
sigma1 0.00018± 0.01536
const2 1787± 1.369e+04
mean2 0.0031± 0.9744
sigma2 0.00118± 0.02461
p3 40± 1.142e+05
p2 50± -3.079e+05
p1 67± 2.611e+05
p0 48± -6.658e+04
301.2± = 65224 ctsPeak
tagger/EclusterPairElasticity = E
, Radiator BbeamAll I
tdiff cut: [-7.4,0.6] nsC, crystal only12
:[0.9,1.1] = 21120=>24.5%ctsBkgd
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 20
PrimEx Collaboration Jefferson Lab Hall B
π0 Experimental Yield: 12C
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
’sγ/2
.403
e+12
inci
dent
o
Yie
ld/0
.02
0 π
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1310× C, crystal only)12 Photoproduction Yield (0π
yield0πElastic s = 649590πTotal
]= 5498os:[0,0.30π
Preliminary
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 21
PrimEx Collaboration Jefferson Lab Hall B
π0 Experimental Yield Comparison: 12C
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
o Y
ield
/0.0
20 π
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1310× C)12-Yield (0πCalculated
(degrees)θ Production Angle, 0π
------ Coulomb ------ Nuclear Coherent------ Interference ------ Incoherent
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
o Y
ield
/0.0
20 π
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1310× C)12-Yield Comparison (0π
(degrees)θ Production Angle, 0π
---- Calculated Experiment•
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 22
PrimEx Collaboration Jefferson Lab Hall B
π0 Lifetime Extraction
Convert Yield to Cross Section.
dσexp
dθπ0
Nyieldπ0
θπ0
Nγ
$ Nt
$ επ0
θπ0
$ ∆θπ0(8)
where Nγ
( # of γ’s on target (preliminary uncertainty
)
1%).
where Nt
( target atoms/cm2 (thickness mapped to % 03% uncertainty).
where επ0
( experimental acceptance (uncertainty still being evaluated)
Fit experimental cross section with parameterization:
dσexp
dθπ0
bpT 2p
* bcT 2c
* biT 2i
* 2cosφ bpbcTpTc (9)
where the parameter bp
Γγγ
Vary the four parameters (bp, bc, bi, and φ) and minimize χ2.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 23
PrimEx Collaboration Jefferson Lab Hall B
π0 Experimental Cross Section Comparison: 12C
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
ob/
rad)
/0.0
2µ (θ
/dσC
ross
Sec
tion,
d
0
5
10
15
20
25
30
35C)12Calculated Total Cross Section (
(degrees)θ Production Angle, 0π
------ Coulomb ------ Nuclear Coherent------ Interference ------ Incoherent
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
ob/
rad)
/0.0
2µ (θ
/dσC
ross
Sec
tion,
d0
5
10
15
20
25
30
35C)12Total Cross Section Comparison (
(degrees)θ Production Angle, 0π
---- Calculated Experiment•
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 24
PrimEx Collaboration Jefferson Lab Hall B
Comparison of +Independent π0 Analyses: 12C
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5
b/ra
d)µ (θ
/dσE
xper
imen
tal C
ross
Sec
tion,
d
5
10
15
20
25
30
35C, Geom. Acc. Corrected Only)12 Photoprod. Cross Section (0π
PreliminaryI. Larin
D. McNulty
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 25
PrimEx Collaboration Jefferson Lab Hall B
Summary and Outlook
High Quality precision π0 photoproduction data on 12C and 208Pb targetsusing 4 9
)
Etaggedγ
)
5 5 GeV has been collected and analyzed by thePrimEx Collaboration.
State of the art performance by the Hall B tagger and PrimEx calorimeter —delivering precision photon flux statistics combined with stellar energy andcoordinate resolutions.
Three % independent π0 analysis groups; two groups have achieved niceagreement, third group coming with comparison soon.
Preliminary Compton cross section results in good agreement with theory;final radiative corrections still pending.
Preliminary π0 results should be expected early next year–including crosssections and lifetime.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 26
PrimEx Collaboration Jefferson Lab Hall B
Extra Slide: Inelastic Bkgd Correction
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3 3.5
)o B
ackg
roun
d Y
ield
(per
0.1
00 π
0
0.2
0.4
0.6
0.8
1
1.2
1.4
310×C, crystal only)12 (0πθ Inelastic Bkgd vs. 0π
hr6Entries 0
/ ndf 2χ 15.51 / 23
p0 9.819± 265
p1 45.35± -317.9
p2 40.56± 2330
p3 13.67± -1188
p4 4.186± -104.5
p5 1.199± 129.1
p6 0.3064± -15.23
/ ndf 2χ 15.51 / 23
p0 9.819± 265
p1 45.35± -317.9
p2 40.56± 2330
p3 13.67± -1188
p4 4.186± -104.5
p5 1.199± 129.1
p6 0.3064± -15.23
/ ndf 2χ 15.51 / 23
p0 9.819± 265
p1 45.35± -317.9
p2 40.56± 2330
p3 13.67± -1188
p4 4.186± -104.5
p5 1.199± 129.1
p6 0.3064± -15.23
/ ndf 2χ 15.51 / 23
p0 9.819± 265
p1 45.35± -317.9
p2 40.56± 2330
p3 13.67± -1188
p4 4.186± -104.5
p5 1.199± 129.1
p6 0.3064± -15.23
C, crystal only)12 (0πθ Inelastic Bkgd vs. 0π
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 27
PrimEx Collaboration Jefferson Lab Hall B
Extra Slide: Inelastic Bkgd Correction
(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3
)o Y
ield
(per
0.0
20 π
0
0.2
0.4
0.6
0.8
1
1.2
310× C, crystal only)12 Photoproduction Yield (0π
yield + bkgd0πElastic
bkgd0πInelastic s = 213580πTotal
]= 833os:[0,0.30π
Preliminary
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 28
PrimEx Collaboration Jefferson Lab Hall B
Extra Slide: π0 Experimental Yield: 208Pb
Production Angle (degrees)0π0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
’sγ Y
ield
/0.0
4 de
g/1.
377e
+12
inci
dent
0 π
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
310× Pb, all channels)208 Photoproduction yield (0π
Fri Nov 18 13:51:23 2005
yield0πElastic Preliminary
.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 29
PrimEx Collaboration Jefferson Lab Hall B
Extra Slide: π0 Experimental Yield: 12C
Production Angle (degrees)0π0 0.5 1 1.5 2 2.5 3 3.5 4
’sγ Y
ield
/0.0
4 de
g/3.
439e
+12
inci
dent
0 π
0
0.5
1
1.5
2
2.5
3
310× C, all channels)12 Photoproduction yield (0π
Mon Oct 31 13:29:41 2005
yield0πElastic Preliminary
.
Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 30
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