Borexino, a large volume detector for low energy neutrino spectroscopy, is currently running underground at the Laboratori Nazionali del Gran Sasso, Italy. The main goal of the experiment is the real-time measurement of sub MeV solar neutrinos, and particularly of the mono energetic (862 keV) Be7 electron capture neutrinos, via neutrino-electron scattering in an ultra-pure liquid scintillator. This paper is mostly devoted to the description of the detector structure, the photomultipliers, the electronics, and the trigger and calibration systems. The real performance of the detector, which always meets, and sometimes exceeds, design expectations, is also shown. Some important aspects of the Borexino project, i.e. the fluid handling plants, the purification techniques and the filling procedures, are not covered in this paper and are, or will be, published elsewhere (see Introduction and Bibliography)

We report the direct measurement of the 7 Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV 7 Be neutrinos is 49±3stat±4syst counts/(day·100 ton). The hypothesis of no oscillation for 7 Be solar neutrinos is inconsistent with our measurement at the 4σ C.L.. Our result is the first direct measurement of the survival probability for solar νe in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of 7 Be, pp, and CNO solar νe, and the limit on the magnetic moment of neutrinos.PACS numbers: 13.35. Hb, 14.60.St, 26.65.+t, 95.55.Vj, 29.40.Mc Neutrino oscillations [1] are the established mechanism to explain the solar neutrino problem, which originated from observations in radiochemical experiments with a sub-MeV threshold [2,3] and from real time observation of high energy neutrinos [4,5]. Neutrino oscillations were also observed in atmospheric neutrinos [4] and have been confirmed with observation of reactorν e [6]. Borexino is the first experiment to report a real-time observation arXiv:0805.3843v2 [astro-ph]

This Letter reports a direct measurement of the 7Be solar neutrino signal rate performed with the Borexino low background liquid scintillator detector at the Laboratori Nazionali del Gran Sasso. This is the first real-time spectral measurement of sub-MeV solar neutrinos. The result for 0.862 MeV 7Be neutrinos is 47 ± 7 (stat) ± 12 (sys) counts/(day · 100 ton), consistent with predictions of Standard Solar Models and neutrino oscillations with LMA-MSW parameters

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...

We report a new measurement of the parity-violating asymmetry in elastic electron scattering from the proton at backward scattering angles. This asymmetry is sensitive to the strange magnetic form factor of the proton as well as electroweak axial radiative corrections. The new measurement of A 24.92 6 0.61 6 0.73 ppm provides a significant constraint on these quantities. The implications for the strange magnetic form factor are discussed in the context of theoretical estimates for the axial corrections.