Pion beta decay,v ud and π lνγ Emil Frlež for the PIBETA Collaboration University of Virginia 4th International Workshop on CHIRAL DYNAMICS 2003 Theory and Experiment Bonn, GERMANY 8 13 September, 2003
The PIBETA Collaboration: M. Bychkov, E. Frlež, W. Li, R.C. Minehart, D. Počanić, L.C. Smith, W.A. Stephens, B.A. Vandevender, Y. Wang, K.O.H. Ziock University of Virginia, Charlottesville, VA 22904, USA W. Bertl, J. Crawford, Ch. Brönnimann, M. Daum, R. Horisberger, S. Ritt, R. Schnyder, H.-P. Wirtz Paul Scherrer Institut, CH-5232 Villigen, Switzerland T. Kozlowski Institute for Nuclear Studies, PL-05-400 Swierk, Poland B. G. Ritchie Arizona State University, Tempe, AZ 85287, USA V.V. Karpukhin, N.V. Khomutov, A.S. Korenchenko, S.M. Korenchenko, N.P. Kravchuk, N.A. Kuchinsky Joint Institute for Nuclear Research, Dubna, Russia D. Mzhavia, Z. Tsamalaidze Instit. for High Energy Physics, Tbilisi St. Univ., Georgia I. Supek Rudjer Bošković Institute, Zagreb, Croatia
The PIBETA Experiment: MOTIVATION Provide precision tests of Standard Model and QCD predictions: π + π 0 e + ν e main goal SM tests from CKM unitarity π + e + ν e γ(ee) F A /F V, π polarizability (χpt prediction) tensor coupling besides V A µ + e + ν e ν µ γ(ee) departures from V A in L weak π + e + ν e 2nd phase e-µ universality pseudoscalar coupling besides V A massive neutrino, Majoran,...
Pion Beta (πβ) Decay: Theory
Quark-Lepton Universality The basic weak-int. V -A form (e.g., µ decay): e l α ν e ū e γ α (1 γ 5 )u ν is preserved in hadronic weak decays p h α n ū p γ α (G V G A γ 5 )u n with G i 1. Departure from G V = 1 (plain CVC) comes from weak quark mixing (Cabibbo 1963): G V = G µ cos θ C (= G µ V ud ) cos θ C 0.97 which with 3 q generations is generalized in the Cabibbo-Kobayashi-Masakawa matrix (1973): V ud V us V ub V cd V cs V cb V td V ts V tb CKM unitarity cond.: V ud 2 + V us 2 + V ub 2? = 1, provides a means of testing the limits of the SM.
Status of CKM Unitarity V us 0.2196 ± 0.0026 from K e3 decays. V ub 0.0036 ± 0.0007 from B decays. (a) Superallowed Fermi nuclear β decays 1990 Hardy reconciled discrepancies between Ormand & Brown and Towner et al. ft values: V ud 2 + V us 2 + V ub 2 = 0.9962 ± 0.0016, (b) Neutron β decay or 1 2.4σ. 3 V ui 2 = 1.0096 ± 0.0044, or 1 + 2.3σ, i=1 [after Erozolimskiĭ et al. (1990)] 3 V ui 2 = 0.9917 ± 0.0028, or 1 3.0σ. i=1 (c) Pion β decay this work [Abele et al, PRL, May 2002]
Pion Beta Decay: π ± π 0 e ± ν BR 1 10 8 Pure vector transition: 0 0 Theoretical decay rate at tree level: 1 = G2 F V ud 2 ( τ 0 30π 3 1 ) 3 5 f(ɛ, ) 2M + = 0.40692 (22) V ud 2 (s 1 ). With radiative and loop corrections we get: 1 τ = 1 τ 0 (1 + δ), so that the branching ratio is BR(πβ) = τ + τ 0 (1 + δ) = 1.0593 (6) 10 8 (1 + δ) V ud 2.
Recent calculations of pion beta decay radiative corrections (1) In the light-front quark model W. Jaus, Phys. Rev. D 63 (2001) 053009. total RC for pion beta decay: δ = (3.230 ± 0.002) 10 2. (2) In chiral perturbation theory V. Cirigliano, M. Knecht, H. Neufeld and H. Pichl, hep-ph/0209226 χpt with e-m terms up to Oe 2 p 2 theoretical uncertainty of 5 10 4 in extracting V ud from BR ( π e3(γ) ).
Pion Beta (πβ) Decay: Experimental Method
PIBETA Experiment: stopped π + beam segmented active tgt. 240-elem. CsI(p) calo. central tracking digitized PMT readout cosmic µ antihouse stable temp./humidity pure CsI PV π + beam AC1 AD AT MWPC1 BC AC2 MWPC2 10 cm
PIBETA Detector Assembly (1998)
Experimental Method: Summary Detect π + decays at rest (during a delayed 180 ns gate). -30 0 150 ns π stop π gate beam veto Use π + e + ν prescaled for normalization. Accept every πβ trigger unbiased (γγ coincidences above Michel endpoint) 1 τ πβ = 1 τ π + BR eνf presc BR π0 γγ Aeν A πβ Nπβ N eν A πβ, A eν are acceptances for the decay modes, respectively.
Online πβ Energy Spectrum: True πβ events buried deep under overwhelming background!
πβ Signal-to-Background Ratio
πβ Timing and Angular Distributions
πβ: Normalization to π eν Yield
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Radiative Pion Decay (π eνγ): Theory
Anything beside V A in RPD?
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REVIEW OF PARTICLE PROPERTIES Particle Data Group π ± l ± νγ AND K ± l ± νγ FORM FACTORS Written by H. S. Pruys (Zürich University) The electroweak decays of the pseudoscalar mesons are investigated to learn something about the unknown hadronic structure of these mesons, assuming a standard V A structure of the weak leptonic current. The experiments are quite difficult, and it is not meaningful to analyse the results using parameters for both the hadronic structure (decay constants, form factors) and the leptonic weak current (e.g., to add pseudoscalar or tensor couplings to the V A coupling).
LAMPF: L. E. Piilonen et al., Phys. Rev. Lett. 57, 1402-1405 (1986) π eνγ Signal
LAMPF: L. E. Piilonen et al., Phys. Rev. Lett. 57, 1402-1405 (1986) π eνγ Analysis
π eνγ Signal-to-Background Ratios
π eνγ Invariant Masses
π eνγ Timing Distributions
π eνγ Axial-to-Vector Form Factor Ratio
π + e + ν e γ Exp vs Theoretical Branching Ratios: Glimpse at Pion Weak Form Factors Fit Fixed Fitted χ 2 Parameters Parameters F V = 0.0170(80) PGP F A = 0.0116(16) None 2751 F T = 0 SM/ F V = 0.0259(5) F A = 0.0123(4) 275.0 CVC F T = 0 γ = 0.475(18) CVC F V = 0.0259(5) F A = 0.0141(5) 1.0 F T = 0.0022(4)
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π + e + ν e γ: λ Distribution
PIBETA EXPERIMENT: PROGRESS SUMMARY π + π 0 e + ν Branching Ratio Current PIBETA Statistical Uncertainty 0.4 %, Systematic Uncertainty 0.7 % Extracted Branching Ratio in Agreement with CVC Hypothesis and Radiative Corrections π + e + νγ Decay Extracted Axial-to-Vector Form Factor Ratio in Agreement with Chiral Prediction, γ EXP = 0.475 ± 0.018 vs γ CHIRAL = 0.463 (B. R. Holstein 1986) Limit on Phenomenological Tensor Term F T 3 10 3 at 95 % CL