PANIC05

1
The Radiative Pion Decay Anomalie Revisited

• (by the PIBETA Collaboration)

2
Motivation to measure p? eng

• Determine weak form factors of the Pion
• Testing the Standard Model
• CVC hypothesis
• Search for scalar and tensor currents

3
The Decay Rate Amplitude
Inner Bremsstrahlung Amplitude
Structure-dependent Amplitude

Radiation from strong interaction
states Parametrize by form factors FV and FA
Axial vector current ? pure QED
FV (q) from CVC
FA (q) model dependent calculation
q0 for regions A and B but not for C
4
Collaboration
University of Virginia USA K.A.Assamagan, M.
Bychkov, E.Frlež, J.E.Koglin, W.Li, R.C.Minehart,
D.Pocanic, P.L.Slocum, L.C.Smith, W.A.Stephens,
B.Vandevender, Y.Wang, K.O.H.Ziock, Paul-Scherre
r Institut CH W.Bertl, Ch.Brönnimann,
J.F.Crawford, M.Daum, Th.Flügel, R.Frosch,
Z.Hochman, R.Horisberger, B.Krause, H.Obermeier,
D.Renker, S.Ritt, R.Schnyder, H.P.Wirtz, and
Hallendienst, Elektronic divisions, workshops and
and and Inst. for Nuclear Studies, Swierk,
PL T.Kozlowski, Arizona State University,
Tempe, USA D.W.Lawrence, B.G.Ritchie, Joint
Inst. of Nuclear Research, Dubna,
RU V.A.Baranov,V.A.Kalinnikov, V.V. Karpukhin,
N.V.Khomutov, A.S.Korenchenko, S.M.Korenchenko, N.
P.Kravchuk, N.A.Kuchinsky, A.M.Rozhdestvensky,
V.V.Sidorkin, E.P.Velicheva, Inst. for High
Energy Physics, Tbilisi, GUS D.Mzhavia,
Z.Tsamalaidze, Rudjer Boškovic Institute,
Zagreb, HR I.Supek
University of Virginia USA O.A.
Rondon-Aramayo Joint Inst. of Nuclear Research,
Dubna, RU Y. Bystritsky, A. Moisenko, E.P.
Velicheva, V. Volnykh Rudjer Boškovic
Institute, Zagreb, HR M. Korolija, D.
Mekterovic University of Zurich, CH P. Robmann,
T. Sakhelashvili, S. Scheu, U. Straumann, A. van
der Schaaf, University of Sofia, BL M.V.
Chizhov (theoretical support)
up to 2001
up to 2004
spokesperson graduate student technician
5
The Decay Rate

• Kinematics

Range of x 1-y lt x lt 1 Range of y
0 lt y lt 1
6
Kinematics
Qeg 180º
Qeg 180º
x
7
The PIBETA Detector
E.Frlez et al., NIM A526, 300 (2004)
8
The PIBETA Detector
9
Trigger
240 CsI Crystals grouped to 60 clusters Out of
60 clusters 10 Superclusters were formed (SC)
Those 10 Supercluster provide the Triggerinput
10
Normalization p? en Decay
Energy subtraction and background calibration by
2 indep. Methods (a)ADC, b) TDC) Agreement better
than 1 Corrected for radiative corrections
11
Analysis p? eng Decay
Only events from 1-arm-high trigger used
Basic procedures same as for p?en events
Eetotal EeCsI Eetarget EePV
Egtotal EgCsI
12
p? eng Background
m-decays rejected by kinematics, lifetime
13
p? eng Energy Calibration
a) From p? en decay
b) DL ? l1-l2
(Is sensitive to asymmetries in energy
calibration for e and g )
14
p? eng Some Distributions
Region B
15
p? eng Results
Global Fit
3 different studies
Fit variables

1. classical 3 regions, Egt20MeV
2. Extended regions, Egt15MeV
3. More (8) regions

Form factor ratio
a q dependence
(
)
FV
16
p? eng Resulting Bran. Ratios
3 Regions, Threshold 20 MeV
PRELIMINARY !
3 Regions, Threshold 15 MeV
8 Regions, Threshold 15 MeV
17
Results Form Factor Ratio g
THEORY
Scheck 73
Holstein 86
Bijnens 97
Geng 04
0.468 0.0118 This experiment
PRELIMINARY !
18
Main Result

• FA 0.0118 0.0003
• Fv 0.0262 0.0015
• a 0.241 0.093

PRELIMINARY !
First time measured
From theory
0.0263 0.0009 CVC Vaks
58 0.0270 0.0273 cPT Geng 04
19
Radiative Muon Decay
Goal - Used as control measurement -
determination of (SM 0)
400 000 m measured in region Eg gt 10 MeV qeg gt
30o
BR (4.40 0.09)x10-3 BRtheor 4.30x10-3
or
This is about as good as the result of
Eichenberger et al. 84
20
Conclusion

• First analysis of 2004 data finished
• Second analysis in good progress
• FA from 1999-2001 data confirmed
• FV precision 6 fold increased
• q-dependence first measured
• Tentativ tensor form factor consistent with zero
  (SM prediction)

21
Outlook

• We investigate data of 1999-2001 to understand
  the cause of the discrepancy in region B.
• Once discrepancy is understood those data will be
  combined with 2004 data tripling the statistics
  to 60000 events in a broad kinematic region.
• (pre-PIBETA aera 1200 events)

22
Results (p?eng) (2001-3)
The resulting g
(assuming CVC determines FV)
Region A sensitive to SD ? measure (1g)2 ?
g only
g 0.480 ? FA 0.0124(4)
Regions B, C also dependent on SD- ? measure
(1-g)2 ? sign!
g 0.443 ? FA 0.0115(4)
published E. Frlez et al., Phys. Rev. Lett. 93
(2004) 181804
23
A Tensor Form Factor ?

• Fact An FT ? 0 is incompatible with the Standard
  Model
• We could not find an exp. Explanation (e.g.
  X-checks with )
• Radiative corrections recalculated (Kuraev,
  Bystritsky, Velicheva hep-ph 0310275)
• A similar effect seen by the ISTRA group

V.N. Bolotov et al., Phys.Lett. B243, 308 (1990)
A.A. Poblaguev proposed FT -0.0056(17)
Phys.Lett. B286, 169 (1992) and
revised it to FT -0.0115(33) Phys.Rev. D68,
054020 (2003) (based on all available data sets)
?

• Fiction
• P. Herczeg could not rule out FT on basis of all
  existent b-decay data (Leptoquarks)
• M.V. Chizhov proposed a new intermediate chiral
  boson with anomalous
  interaction with matter
• M.D.Scadron et al. propose a non-CVC FV (linear
  sigma model)

Phys. Rev. D49, 247 (1994)
Mod.Phys.Lett. A8, 2753 (1993) hep-ph 0307100 and
hep-ph 0310203
Nucl. Phys. A724, 391 (2003)
How