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Register a new userA new evaluation of the pion weak form factors
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Dinko Pocanic
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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.
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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.
Using $e^+e^-$ annihilation data taken at the CESR collider with the CLEO-c detector, measurements of hyperon pair production cross sections and elastic and transition electromagnetic form factors have been made at the charmonium resonances: $psi(2S)$, $sqrt{s}=3.69$ GeV, $|Q^2|=13.6$ GeV$^2$, $mathcal{L}=48$~pb$^{-1}$; $psi(3770)$, $sqrt{s}=3.77$ GeV, $|Q^2|=14.2$ GeV$^2$, $mathcal{L}=805$~pb$^{-1}$; and $psi(4170)$, $sqrt{s}=4.17$ GeV, $|Q^2|=17.4$ GeV$^2$, $mathcal{L}=586$~pb$^{-1}$. %High efficiency particle identification has resulted in good statistical precision in the results. Results with good statistical precision are obtained with high efficiency particle identification. Systematics of pair production cross sections, and form factors with respect to the number of strange quarks in the hyperons are studied, and evidence is presented for effects of diquark correlations in comparative results for $Lambda^0$ and $Sigma^0$, both of which have the same $uds$ quark content.
We present lattice QCD calculations of nucleon electromagnetic form factors using pion masses $m_pi$ = 149, 202, and 254 MeV and an action with clover-improved Wilson quarks coupled to smeared gauge fields, as used by the Budapest-Marseille-Wuppertal collaboration. Particular attention is given to removal of the effects of excited state contamination by calculation at three source-sink separations and use of the summation and generalized pencil-of-function methods. The combination of calculation at the nearly physical mass $m_pi$ = 149 MeV in a large spatial volume ($m_pi L_s$ = 4.2) and removal of excited state effects yields agreement with experiment for the electric and magnetic form factors $G_E(Q^2)$ and $G_M(Q^2)$ up to $Q^2$ = 0.5 GeV$^2$.
We consider how to extract the pion form factors in the epsilon regime. Using the correlators with non-zero momenta and taking appropriate ratios of them, we eliminate the dominant finite volume effect from the zero-momentum pion mode. Our preliminary lattice result for the pion charge radius is consistent with the experiment.
The update of the e^+e^-topi^+pi^- process cross section, measured in the energy region sqrt{s}<1 GeV with SND detector at VEPP-2M collider is presented.
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