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Register a new userNew Precise Measurement of the Pion Weak Form Factors in the Pi+ -> e+ nu gamma Decay
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We have measured the $pi^+to {rm e}^+ ugamma$ branching ratio over a wide region of phase space, based on a total of 65,460 events acquired using the PIBETA detector. Minimum-$chi^2$ 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 $q^2$, the ${rm e}^{+} u$ pair invariant mass squared, parametrized as $F_V(q^2)=F_V(0)(1+acdot q^2)$. The branching ratio for the kinematic region $E_gamma > 10 $MeV and $theta_{{rm e^+}gamma} > 40^circ $ is measured to be $B^{rm exp}=73.86(54) times 10^{-8}$. Earlier deviations we reported in the high-$E_gamma$/low-$E_{{rm e}^+}$ kinematic region are resolved, and we find full compatibility with CVC and standard $V$$-$$A$ calculations without a tensor term. We also derive new values for the pion polarizability, $alpha_E = rm 2.78(10) times 10^{-4} fm^3$, and neutral pion lifetime, $tau_{pi 0} = (8.5 pm 1.1) times 10^{-17} $s.
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We have used the PIBETA large acceptance detector for a precise measurement of the $pi^+ to e^+ ugamma$ radiative decay at rest, with broad phase space coverage. Using the CVC value for the pion vector form factor, $F_V = 0.0259(5)$, we have obtained a new value of the pion axial-vector form factor $gamma = F_A/F_V = 0.429(14)$. However, significant deviations from the SM predictions are evident in our data. The discrepancy can be accounted for by introducing a pion tensor form factor of $F_T = -0.0016(3)$. The question of the true nature of the observed deviations remains open pending further theoretical and experimental study.
The decay K- -> pi0 e- nu is studied using in-flight decays detected with the ISTRA+ spectrometer. About 920K events are collected for the analysis. The lambda+ slope parameter of the decay form-factor f+(t) in the linear approximation (average slope) is measured: lambda+(lin)= 0.02774 +- 0.00047(stat) +- 0.00032(syst). The quadratic contribution to the form-factor was estimated to be lambda+ = 0.00084 +- 0.00027(stat) +- 0.00031(syst). The linear slope, which has a meaning of df+(t)/dt|_{t=0} for this fit, is lambda+ = 0.02324 +- 0.00152(stat) +- 0.00032(syst). The limits on possible tensor and scalar couplings are derived: f_{T}/f_{+}(0)=-0.012 +- 0.021(stat) +- 0.011$(syst), f_{S}/f_{+}(0)=-0.0037^{+0.0066}_{-0.0056}(stat) +- 0.0041(syst).
A new measurement of $B_{pi e2}$, the $pi^+ to e^+ u(gamma)$ decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on $B_{pi e2}$ constitutes the most accurate test of lepton universality available. The accuracy, however, still lags behind the theoretical precision by over an order of magnitude. Because of the large helicity suppression of the $pi_{e2}$ decay, its branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.
The PEN Collaboration is conducting a new measurement of the pi+ -> e+ nu branching ratio at the Paul Scherrer Institute, with the goal uncertainty of delta B/B(pie2)=5E-4 or lower. At present, the combined accuracy of all published pie2 decay measurements lags behind the theoretical calculation by a factor of 40. In this contribution we report on the PEN detector configuration and its performance during two development runs done in 2007 and 2008.
The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay from the pp-->pppi^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent experimental limits significantly reduces the M_U vs. epsilon^2 parameter space preferred by the measured value of the muon anomalous magnetic moment.
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