A these low q2 interactions produce no nuclear recoil

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Unformatted text preview: A 50 0 0 20 40 60 80 100 120 140 160 E (GeV)  A ! À Aþ ;  A ! þ AÀ : " (78) While the cross sections for these processes are predicted to be comparatively small, coherent pion production has been observed across a broad energy range in both NC and CC interactions of neutrinos and antineutrinos. Figure 22 shows a collection of existing measurements of coherent pion production cross sections for a variety of nuclei. A valuable compilation of the same is also available in Vilain et al., 1993. Most of these historical measurements were performed at higher energies (E > 2 GeV). Table XIII provides a listing of more recent measurements of coherent pion production, most in the form of cross section ratios that were measured at low energy (E < $2 GeV). Experiments measuring coherent pion production at these very low neutrino energies have typically observed less coherent pion production than predicted by models which well describe the high-energy data. In addition, the production of CC coherent pion events at low energy has been seemingly absent from much of the experimental data (Hasegawa et al., 2005; Hiraide et al., 2008), although refined searches have indicated some evidence for their existence (Hiraide, 2009). To date, it has been a challenge to develop a single description that can successfully describe existing coherent pion production measurements across all energies. The most common theoretical approach for describing coherent pion production is typically based on Adler’s partially conserved axial current (PCAC) theorem (Adler, 1964) which relates neutrinoinduced coherent pion production to pion-nucleus elastic scattering in the limit Q2 ¼ 0. A nuclear form factor is then invoked to extrapolate to nonzero values of Q2 . Such PCACbased models (Rein and Sehgal, 1983) have existed for many years and have been rather successful in describing coherent pion production at high energy (the prediction shown in Fig. 22 is such a model). With the accumulation of increasingly large amounts of low-energy neutrino data, revised FIG. 22. Measurements of absolute coherent pion production cross sections from a variety of nuclear targets and samples, as indicated in the legend. Both NC and CC data are displayed on the same plot after rescaling the CC data using the prediction that NC ¼ 1 CC (Rein and Sehgal, 1983). In addition, data from 2 various targets have been corrected to carbon cross sections assuming A1=3 scaling (Rein and Sehgal, 1983). Also shown is the prediction from Casper (2002). approaches have been applied to describe the reduced level of coherent pion production observed by low-energy experiments. Two such approaches have been developed. The first class of models are again based on PCAC (Rein and Sehgal, 1983; Belkov and Kopeliovich, 1987; Paschos and Kartavtsev, 2003; Kopeliovich, 2005; Paschos et al., 2006; Berger and Sehgal, 2009; Hernandez et al., 2009; Paschos et al., 2009). The other class is microscopic mo...
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This document was uploaded on 09/28/2013.

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