Barionski model predvidio neočekivano malu iskustvenu vektorsku stranost nukleona

Most of model considerations of the hidden nucleon strangeness, as well as some preliminary experimental evidence, led to the expectations of relatively sizeable strange vector form factors of the proton. For example, it seemed that the contribution of the fluctuating strange quark-antiquark pairs accounts for as much as one tenth of the proton\u27s magnetic moment. By the same token, baryon models which failed to produce the "vector strangeness" of the nucleon seemed disfavoured. Recently, however, more accurate measurements and more sophisticated data analysis, as well as lattice simulations, revealed that the form factors associated with the vector strangeness of the nucleon are much smaller than thought previously; in fact, due to the experimental uncertainties, the measured strange vector-current proton form factors may be consistent with zero. In the light of that, we re-asses the merit of the baryon models leading to little or no vector strangeness of the nucleon. It is done on the concrete example of the baryon model which essentially amounts to the MIT bag enriched by the diluted instanton liquid.Većina teorijskih razmatranja skrivene stranosti nukleona, kao i neki prethodni eksperimentalni podaci, potaknuli su očekivanja relativno velikih stranih vektorskih faktora oblika protona. Npr., činilo se da doprinos kolebanja parova strani kvark– antikvark iznosi čak desetinu magnetskog momenta protona. Zbog tih su stajališta bili zanemareni barionski modeli koji nisu davali “vektorsku stranost” nukleona. Međutim, nedavnija točnija mjerenja i bolje analize mjernih podataka, kao i simulacije na rešetkama, našli su da su faktori oblika povezani s vektorskom stranosti nukleona mnogo manji nego se prije mislilo. U stvari, uzevši u obzir točnost eksperimentalnih podataka, mjereni strani vektorski faktori oblika mogu biti jednaki nuli. U tom svjetlu obnavljamo razmatranje barionskih modela koji daju malu ili nultu vektorsku stranost nukleona. To smo načinili s posebnim primjerom barionskog modela koji je u biti MIT vreća obogaćena razrijeđenom tekućinom instantona

Barionski model predvidio neočekivano malu iskustvenu vektorsku stranost nukleona

Most of model considerations of the hidden nucleon strangeness, as well as some preliminary experimental evidence, led to the expectations of relatively sizeable strange vector form factors of the proton. For example, it seemed that the contribution of the fluctuating strange quark-antiquark pairs accounts for as much as one tenth of the proton\u27s magnetic moment. By the same token, baryon models which failed to produce the "vector strangeness" of the nucleon seemed disfavoured. Recently, however, more accurate measurements and more sophisticated data analysis, as well as lattice simulations, revealed that the form factors associated with the vector strangeness of the nucleon are much smaller than thought previously; in fact, due to the experimental uncertainties, the measured strange vector-current proton form factors may be consistent with zero. In the light of that, we re-asses the merit of the baryon models leading to little or no vector strangeness of the nucleon. It is done on the concrete example of the baryon model which essentially amounts to the MIT bag enriched by the diluted instanton liquid.Većina teorijskih razmatranja skrivene stranosti nukleona, kao i neki prethodni eksperimentalni podaci, potaknuli su očekivanja relativno velikih stranih vektorskih faktora oblika protona. Npr., činilo se da doprinos kolebanja parova strani kvark– antikvark iznosi čak desetinu magnetskog momenta protona. Zbog tih su stajališta bili zanemareni barionski modeli koji nisu davali “vektorsku stranost” nukleona. Međutim, nedavnija točnija mjerenja i bolje analize mjernih podataka, kao i simulacije na rešetkama, našli su da su faktori oblika povezani s vektorskom stranosti nukleona mnogo manji nego se prije mislilo. U stvari, uzevši u obzir točnost eksperimentalnih podataka, mjereni strani vektorski faktori oblika mogu biti jednaki nuli. U tom svjetlu obnavljamo razmatranje barionskih modela koji daju malu ili nultu vektorsku stranost nukleona. To smo načinili s posebnim primjerom barionskog modela koji je u biti MIT vreća obogaćena razrijeđenom tekućinom instantona

Precise Measurement of pi+ -> e+ nu Branching Ratio

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.Comment: pdflatex, 11 pages, 5 figures, to be published in "Progress in High-Energy Physics and Nuclear Safety", NATO Science for Peace Series: B - Physics and Biophysic

New Precise Measurement of the Pion Weak Form Factors in the Pi+ -> e+ nu gamma Decay

We have measured the $\pi^+\to {\rm e}^+\nu\gamma$ 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 ${\rm e}^{+}\nu$ pair invariant mass squared, parametrized as $F_V(q^2)=F_V(0)(1+a\cdot q^2)$. 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.Comment: 4 pages, 2 PDF figure

PEN: a sensitive search for non-(V−A) weak processes

Abstract A new measurement of Bπe2, the π + → e + ν(γ) decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on Bπe2 constitutes the most accurate test of lepton universality available. The accuracy, however, still lags behind the theoretical precision by over an order of magnitude. Thanks to the large helicity suppression of πe2 decay, the branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study. Key words: semileptonic pion decays, muon decays, lepton universality PACS: 13.20. Cz, 13.35.Bv 14.40.Aq Historically, the π→eν decay, also known as π e2 , provided an early strong confirmation of the V − A nature of the electroweak interaction. At present, thanks to exceptionally well controlled theoretical uncertainties, its branching ratio is understood at the level of better than one part in 10 4 . The most recent independent theoretical calculations are in very good agreement and give