New Precise Measurement of the Pion Weak Form Factors inπ+→e+νγDecay

Abstract

We have measured the pi+-->e+ nugamma branching ratio over a wide region of phase space, based on a total of 65 460 events acquired using the PIBETA detector. Minimum-chi2 fits to the measured (E(e+), E(gamma) energy distributions result in the weak form factor value of F(A)=0.0119(1) with a fixed value of F(V)=0.0259. An unconstrained fit yields F(V)=0.0258(17) and F(A)=0.0117(17). In addition, we have measured a=0.10(6) for the dependence of F(V) on q2, the e+ nu pair invariant mass squared, parametrized as F(V)(q2)=F(V)(0)(1+aq(2)). The branching ratio for the kinematic region E(gamma)>10 MeV and theta(e(+)gamma)>40 degrees is measured to be B(expt)=73.86(54)x10(-8). Earlier deviations we reported in the high-E(gamma)-low-E(e+) kinematic region are resolved without a tensor term. We also derive new values for the pion polarizability alpha(E)=2.78(10)x10(-4) fm3 and neutral pion lifetime tau(pi0)=(8.5+/-1.1)x10(-17) s.

Global effective-field-theory analysis of new-physics effects in (semi)leptonic kaon decays

“…The interest of P 2γ in the context of NP is that their kinematic distributions are sensitive to the tensor operator [60,61], which enter with new form factors [62] (q = p − k): 9) although G P T is kinematically suppressed in the amplitude and can be neglected in first approximation. In fact, the PIBETA collaboration has obtained a stringent constraint on these contributions in π e2γ [57] that, using the calculation of F π T obtained in ref. [62], leads to the bound [47]:…”

“…In addition to the QED correction (internal-bremsstrahlung) to the P → ν decay, we have the so-called "structure-dependent" terms which can be extracted separately from experiment [56][57][58]; in the SM they depend on P → γ hadronic form factors and some of them are not chirally suppressed [3,59]. The interest of P 2γ in the context of NP is that their kinematic distributions are sensitive to the tensor operator [60,61], which enter with new form factors [62] (q = p − k): 9) although G P T is kinematically suppressed in the amplitude and can be neglected in first approximation.…”

New studies of allowed pion and muon decays

“…Furthermore, thanks to lower and better controlled backgrounds, the PEN data will enable us to improve on the precision of the previous PIBETA results on pion [13] and muon [16,17] radiative decays, both of which are sensitive to non-(V − A) interactions. Besides providing the most stringent limit on tensor interactions to date [18], the PIBETA π → eνγ results [13] also furnish fundamental inputs for chiral perturbation theory related to pion structure.…”

“…Besides providing the most stringent limit on tensor interactions to date [18], the PIBETA π → eνγ results [13] also furnish fundamental inputs for chiral perturbation theory related to pion structure. On the other hand, muon radiative decay, being largely free from hadronic corrections, provides model-independent information on non-(V −A) admixtures in the lagrangian.…”

“…[10], and used in a series of rare pion and muon decay measurements [11,12,13]. The PEN apparatus, shown in Fig.…”