We present the results of a search for extraterrestrial electron antineutrinos (ν e 's) in the energy range 8.3 MeV < E ν e < 31.8 MeV using the KamLAND detector. In an exposure of 4.53 kton-year, we identify 25 candidate events. All of the candidate events can be attributed to background, most importantly neutral current atmospheric neutrino interactions, setting an upper limit on the probability of 8 B solar ν e 's converting into ν e 's at 5.3 × 10 −5 (90% CL), if we assume an undistorted ν e shape. This limit corresponds to a solar ν e flux of 93 cmor an event rate of 1.6 events (kton-year) −1 above the energy threshold (E ν e 8.3 MeV). The present data also allows us to set more stringent limits on the diffuse supernova neutrino flux and on the annihilation rates for light dark matter particles.
The recent progress in establishing the existence of finite neutrino masses and mixing between generations of neutrinos has been remarkable, if not astounding. The combined results from studies of atmospheric neutrinos, solar neutrinos, and reactor antineutrinos paint an intriguing picture for theorists and provide clear motivation for future experimental studies. In this review, we summarize the status of experimental and theoretical work in this field and explore the future opportunities that emerge in light of recent discoveries.
Data corresponding to a KamLAND detector exposure of 0.28 kton yr has been used to search for nu;(e)'s in the energy range 8.3
We report the first measurement of the vector analyzing power in inclusive transversely polarized elastic electron-proton scattering at Q 2 ϭ0.1 (GeV/c) 2 and large scattering angles. This quantity must vanish in the single virtual photon exchange, plane-wave impulse approximation for this reaction, and can therefore provide information on two photon exchange amplitudes for electromagnetic interactions with hadronic systems. The observable we have measured is driven by the imaginary part of the two photon exchange amplitude, the hadronic side of which is simply the Compton amplitude for the proton with two virtual photons. We find a small but nonzero value of AϭϪ15.4Ϯ5.4 ppm. The recent development and refinement of experimental methods for measurements of small ͑few parts per million, or ppm͒ parity violating effects in polarized electron scattering ͓1-3͔ provides a new technique for further studies of the electromagnetic structure of the proton. We have exploited these methods for the first time to measure the small vector analyzing power in the elastic scattering of 200 MeV electrons from the proton at large laboratory scattering angles (130°р р170°), corresponding to a four-momentum transfer squared of Q 2 ϭ0.1 (GeV/c) 2 . This parity conserving quantity is associated with transverse electron polarization, in contrast to the parity violating longitudinal ͑i.e., helicitydependent͒ asymmetry. It has been previously noted ͓4͔ that transverse polarization effects will be suppressed by the relativistic boost factor 1/␥. Nevertheless, as demonstrated here, the development of the technology to measure small parity violating asymmetries, along with the ability to produce transversely polarized electron beams at high energies, now renders these transverse polarization effects amenable to measurement.The vector analyzing power is a time-reversal odd observable that must vanish in first-order perturbation theory, and can only arise in leading order from the interference of twophoton exchange ͑second order͒ and single-photon exchange amplitudes. Our observation of this quantity therefore demonstrates the viability of a new technique to access the physics associated with the absorption of two virtual photons by a hadronic system. Thus, the study of vector analyzing powers provides another method to study processes in which two photons couple to the proton, i.e., the Compton amplitude, that is complementary to virtual Compton scattering ͑VCS͒, in which there is presently a great deal of interest as a means to further probe the structure of the proton ͓5͔. VCS involves the coupling of one virtual and one real photon to a hadronic system, but in practice includes Bethe-Heitler amplitudes, associated with radiation of a real photon from the electron, with which care must be taken for proper treatment to allow a correct interpretation of those measurements. In contrast, the two-photon exchange amplitude involves the coupling of two virtual photons to a hadronic system, and the vector analyzing power in elastic electron-proton...
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