Early heavy target data 9 rev mod phys vol 84 no 3

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Unformatted text preview: nging from 0.65 to 1.09 GeV were obtained in the period from the late 1960s to early 1990s resulting from fits to both the total rate of observed events and the shape of their measured Q2 dependence [for a recent review, see Lyubushkin et al. (2009)]. In addition to providing the first measurements of MA and the QE cross section, many of these experiments also performed checks of CVC, fit for the presence of second-class currents, and experimented with different forms for the axialvector form factor. By the end of this period, the neutrino QE cross section could be accurately and consistently described by V-A theory assuming a dipole axial-vector form factor with MA ¼ 1:026 Æ 0:021 GeV (Bernard et al., 2002). These conclusions were largely driven by experimental measurements on deuterium, but less-precise data on other heavier targets also contributed. More recently, some attention has been given to reanalyzing these same data using modern vector form factors as input. The use of updated vector form factors slightly shifts the best-fit axial mass values obtained from these data; however, the conclusion is still that MA $ 1:0 GeV.9 Modern day neutrino experiments no longer include deuterium but use complex nuclei as their neutrino targets. As a result, nuclear effects become much more important and produce sizable modifications to the QE differential cross section from Eq. (57). With QE events forming the largest contribution to signal samples in many neutrino oscillation experiments, there has been renewed interest in the measurement and modeling of QE scattering on nuclear targets. In such situations, the nucleus is typically described in terms of individual quasifree nucleons that participate in the scattering process (the so-called ‘‘impulse approximation’’ approach) (Frullani and Mougey, 1984). Most neutrino experiments use a relativistic Fermi-gas model (Smith and Moniz, 1972) when simulating their QE scattering events, although many other independent particle approaches have been developed in recent years that incorporate more sophisticated treatments. These include spectral function (Nakamura and Seki, 2002; Benhar et al., 2005; Ankowski and Sobczyk, 2006; Benhar and Meloni, 2007; Juszczak, Sobczyk, and Zmuda, 2010), superscaling (Amaro et al., 2005), RPA (Nieves, Amaro, and Valverde, 2004; Nieves, Valverde, and Vicente Vacas, 2006; Leitner and Mosel, 2009; Sajjad Athar, Chauhan, and Singh, 2010), and plane-wave impulse approximation-based calculations (Butkevich, 2010). In concert, the added nuclear effects from these improved calculations tend to reduce the predicted neutrino QE cross section beyond the Fermi-gas model-based predictions. These reductions are typically on the order of 10%–20% (Alvarez-Ruso, 2010). Using Fermi-gas model-based simulations and analyzing higher statistics QE data on a variety of nuclear targets, new experiments have begun to repeat the axial-vector measurements that fueled much of the early investi...
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