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Register a new userA New Measurement of the $pi^0$ Radiative Decay Width
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High precision measurements of the differential cross sections for $pi^0$ photoproduction at forward angles for two nuclei, $^{12}$C and $^{208}$Pb, have been performed for incident photon energies of 4.9 - 5.5 GeV to extract the ${pi^0 to gammagamma}$ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The ${pi^0 to gammagamma}$ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is $Gamma{(pi^0 to gammagamma)} = 7.82 pm 0.14 ~({rm stat.}) pm 0.17 ~({rm syst.}) ~{rm eV}$. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current PDG average of this fundamental quantity and it is consistent with current theoretical predictions.
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We present a new result on the K^+ --> pi^+ pi^0 gamma decay measurement using stopped kaons. The best fit to the decay spectrum comprised of 10k events gives a branching ratio for the direct photon emission of [3.8+-0.8(stat)+-0.7(syst)] times 10^{-6} in the pi^+ kinetic energy region of 55 to 90 MeV. This result has been obtained with the assumption that there is no component due to interference with the inner bremsstrahlung.
An exclusive measurement of the decay eta --> pi+ pi- gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd --> 3He X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13340pm140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta --> pi+ pi- gamma. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi+ pi- system.
A technique is presented for precision measurements of the area densities, density * T, of approximately 5% radiation length carbon and 208Pb targets used in an experiment at Jefferson Laboratory to measure the neutral pion radiative width. The precision obtained in the area density for the carbon target is +/- 0.050%, and that obtained for the lead target through an x-ray attenuation technique is +/- 0.43%.
Using the production reactions $pdto {}^3mbox{He},omega$ and $ppto ppomega$, the Dalitz plot distribution for the $omega to pi^+ pi^- pi^0$ decay is studied with the WASA detector at COSY, based on a combined data sample of $ (4.408pm 0.042) times 10^4$ events. The Dalitz plot density is parametrised by a product of the $P$-wave phase space and a polynomial expansion in the normalised polar Dalitz plot variables $Z$ and $phi$. For the first time, a deviation from pure $P$-wave phase space is observed with a significance of $4.1sigma$. The deviation is parametrised by a linear term $1+2alpha Z$, with $alpha$ determined to be $+0.147pm0.036$, consistent with the expectations of $rho$-meson-type final-state interactions of the $P$-wave pion pairs.
We present a measurement of $B(pi^0 rightarrow e^+e^- gamma)/B(pi^0 rightarrow gammagamma)$, the Dalitz branching ratio, using data taken in 1999 by the E832 KTeV experiment at Fermi National Accelerator Laboratory. We use neutral pions from fully reconstructed $K_L$ decays in flight; the measurement is based on about 60 thousand $K_L rightarrow pi^0pi^0pi^0 rightarrow gammagamma~gammagamma~e^+e^-gamma$ decays. We normalize to $K_L rightarrow pi^0pi^0pi^0 rightarrow 6gamma$ decays. We find $B(pi^0 rightarrow e^+e^- gamma)/B(pi^0 rightarrow gammagamma)$ $(m_{e^+e^-}$ > 15 MeV/$c^2)$ = $[3.920 pm 0.016(stat) pm 0.036 (syst)] times 10^{-3}$. Using the Mikaelian and Smith prediction for the $e^+e^-$ mass spectrum, we correct the result to the full $e^+e^-$ mass range. The corrected result is $B(pi^0 rightarrow e^+e^- gamma)/B(pi^0 rightarrow gammagamma) = [1.1559 pm 0.0047(stat) pm 0.0106 (syst)]$%. This result is consistent with previous measurements and the uncertainty is a factor of three smaller than any previous measurement.
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