Precise Measurement of the Pion Axial Form Factor in theπ+→e+νγDecay

Abstract

We have studied radiative pion decays π + → e + νγ in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 42,209 events we have evaluated absolute π → eνγ branching ratios in the three regions. Minimum χ 2 fits to the integral and differential (E e + , Eγ ) distributions result in the axial-to-vector weak form factor ratio of γ ≡ FA/FV = 0.443(15), or FA = 0.0115(4) with FV = 0.0259. However, deviations from Standard Model predictions in the high-Eγ/low-E e + kinematic region indicate the need for further theoretical and experimental work.PACS numbers: 11.30. Rd, 13.20.Cz, 14.40.Aq In the Standard Model description of radiative pion decay π + → e + νγ, where γ is a real or virtual photon (e + e − pair), the decay amplitude M depends on the vector V and axial vector A weak hadronic currents [1]. Both currents contribute to the structure-dependent terms SD V and SD A associated with virtual hadronic states, while only the axial-vector current contributes to the inner bremsstrahlung process IB. Thus, it is convenient to write the decay amplitude as a sum:where p, k and q are the pion, electron and photon fourmomenta, respectively, e and m e are the electron charge and mass, G F is the Fermi coupling constant, V ud is the CKM quark mixing matrix element, while f π is the pion decay constant. The structure-dependent amplitude is parameterized by the vector and axial vector form factors, F V and F A :The conserved vector current (CVC) hypothesis [4,5] relates F V to the π 0 lifetime yielding F V = 0.0259(5) [6], which agrees with the relativistic quark model and chiral perturbation theory [7]. Chiral symmetry calculations [7,8,9] yield F A in the range 0.010-0.014.The combined π → eνγ event count of all previously published experiments is less than 1,200 events, while the overall uncertainties of the parameter γ ≡ F A /F V extracted from data range from 12 % to 56 % [10,11,12,13,14,15].In this Letter we present a first analysis of the π + → e + νγ (πe2γ) events recorded with the PIBETA detector in the course of a new measurement of the π + → π 0 e + ν (πβ) branching ratio [16,17] from 1999 to 2001.The measurements were performed in the πE1 channel at the Paul Scherrer Institute (PSI), Villigen, Switzerland. The π + beam with p ≃ 113 MeV/c, ∆p/p ≤ 1.3 %, and 24 mr horizontal and vertical divergence, had an average intensity of 6.8 · 10 5 π + /s, and produced a σ x,y = 9 mm beam spot on target. A total of 2.2 · 10 13 π + stops were recorded during the running period.The beam particles were first registered in a 3 mm thick beam counter (BC) placed ∼395 cm upstream of the detector center. The pions were slowed in a 30 mm thick active degrader (AD) and stopped in a 50 mm long segmented active target detector (AT) positioned in the center of the PIBETA detector. The BC, AD and AT detectors are all made of plastic scintillation material.The e + and µ + beam contaminations determined by the time-of-flight method were small, 0.4 % and 0.2 %, respectively. The bac...

Experiments on the Nuclear Interactions of Pions and Electrons

“…The first reaction is an important background to the pion beta decay. As reported in another paper published in 2004 [33], our analysis of the radiative pion decay shows a significant departure from Standard Model (SM) predictions in the (high E γ -low E e ) kinematical region. Detailed cross checks showed that an experimental inefficiency was an improbable explanation.…”

Tensor interaction constraints fromβ-decay recoil spin asymmetry of trapped atoms

“…The PIBETA Collaboration reported a statistically significant deviation from the standard model [14] in π → νeγ decay that could be explained by a finite tensor interaction. The same group has made further dedicated experiments and now eliminated the possibility of a tensor interaction [15].…”

“…Allowed regions are inside:6 He β-ν[5,6], red concentric circles (hashed in between); β + polarization in14 O,10 C [7] (assuming no scalar interaction), blue solid vertical lines (hashed in between); present work, letting b M , d, and both A T and b T float, black dashed hyperbolae. For b T set to 0 (consistent with Ref.…”

The automatic gain-matching in the PIBETA CsI calorimeter

“…During the production runs in 1999-2001 and 2004, we collected a large data sample of pion and muon decay events corresponding to a total of 2.37 × 10 13 pion stops, with stopping rates ranging between 5 × 10 4 π + /s and 1 × 10 6 π + /s [4][5][6].…”