RevModPhys.84.1307

Experiment b neutrino capture on radioactive nuclei

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Unformatted text preview: ent. B. Neutrino capture on radioactive nuclei Neutrino capture on radioactive nuclei, sometimes referred to as enhanced or stimulated beta decay emission, constitutes another thresholdless mechanism in our library of possible neutrino interactions. The process is similar to that of ordinary beta decay " A Z ! A Zþ 1 þ e À þ  e ; N N À1 Rev. Mod. Phys., Vol. 84, No. 3, July–September 2012 (30) 1313 except the neutrino is interacting with the target nucleus e þ AZ ! eÀ þ AZþ1 : N N À1 (31) This reaction has the same observable final states as its beta decay counterpart. What sets this reaction apart from other neutrino interactions is that the process is exothermic and hence no energy is required to initiate the reaction.3 The cross section amplitude is directly related to that of beta decay. Using the formalism of Beacom and Vogel (1999), the cross section can be written as  d G2 jV j2 FðZf ;Ee Þ 2 ¼ F ud Ee pe fV ð0Þ ð1 þ e  cosÞ d cos 2    1 (32) þ 32 1 À e  cos ; 3 where e and  are the electron and neutrino velocities, respectively, Ee , pe , and cos are the electron energy, momentum, and scattering angle, 2 is the axial-to-vector coupling ratio, and jVud j2 is the Cabbibo angle. The Fermi function FðZf ; EÞ encapsulates the effects of the Coulomb interaction for a given lepton energy Ee and final-state proton number Zf . We discuss the coupling strengths fV ð0Þ and 2 later. In Eq. (32), we no longer assume that  ! c. If the neutrino flux is proportional to the neutrino velocity, then the product of the cross section and the flux results in a finite number of observable events. If the neutrino and the nucleus each possess negligible energy and momentum, the final-state electron is ejected as a monoenergetic particle whose energy is above the end-point energy of the reaction. The interaction cross section of very low-energy neutrinos was first suggested by Weinberg (1962). Recently, this process has attracted particular interest thanks to the work by Cocco, Mangano, and Messina (2007), where they considered the process as a means to detect cosmological neutrinos. The reaction has received attention partially due to the advancement of beta decay experiments in extending the reach on neutrino mass scales. The mechanism, like its coherent counterpart, remains to be observed. IV. LOW-ENERGY NUCLEAR PROCESSES: E $ 1–100 MeV As the energy of the neutrino increases, it is possible to probe the target nucleus at smaller and smaller length scales. Whereas coherent scattering only allows one to ‘‘see’’ the nucleus as a single coherent structure, higher energies allow one to access nucleons individually. These low-energy interactions have the same fundamental characteristics as those of lepton scattering, though the manner in which they are gauged and calibrated is very different. And, unlike the thresholdless scattering mechanisms discussed previously, these low-energy nuclear processes have b...
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