Thus, the two components of the crosssection (1) exhibit quite different dependencies on the recoil-electron kineticenergy T. The µ ν cross section is givenby dσ ( µ ) dT = 4 παµ ν (cid:18) T − E ν (cid:19), (3)where α is the fine-structure constant. (1)In the case of reactor experiments, where one deals with electron antineutri-nos, the SM term is given by dσ SM dT = G F m e π " ( g V + g A ) + ( g V − g A ) (cid:18) − TE ν (cid:19) + ( g A − g V ) m e TE ν, (2)where E ν is the incident antineutrino energy, g A = − / g V = (4 sin θ W +1) /Ģ, with θ W being the Weinberg angle. In the ultrarelativistic limit m ν →Ġ, itis given by an incoherent sum of the SM contribution, which is due to weakinteraction that conserves the neutrino helicity, and the helicity-flipping con-tribution, which is due to µ ν, dσdT = dσ SM dT + dσ ( µ ) dT. One stud-ies an inclusive cross section for (anti)neutrino-electron scattering which isdifferential in the energy transfer T. Therefore,the major task faced by experiments is to enhance their sensitivity to the µ ν value.The strategy of experiments searching for NMM is as follows. At the same time, there are different theoreticalscenarios beyond SM that predict much higher µ ν values, thus giving hopeto observe NMM experimentally in the not too distant future. E-mail: E-mail: µ ν ≤ × − µ B (cid:16) m ν (cid:17), where m ν is a neutrino mass.
This bound, which is due tothe GEMMA experiment with a HPGe detector at Kalinin nuclear powerstation, is by an order of magnitude larger than the constraint obtained inastrophysics : µ ν ≤ × − µ B. × − µ B, where µ B = e/ (2 m e ) is a Bohr magneton. The current best upper limit on the NMMvalue obtained in such direct laboratory measurements is µ ν ≤. The latter are also being intensively searched in reac-tor, accelerator and solar experiments on low-energy elastic(anti)neutrino-electron scattering. Among these nontypical neutrino featuresthe most studied and well understood theoretically are neutrino magneticmoments (NMM). The stepping approximation is shown to be wellapplicable practically down to the ionization threshold.PACS: 13.15.+g, 14.60.StĮlectromagnetic properties of neutrinos are of particular interest, for theyopen a door to “new physics” beyond the Standard Model (SM) (see, for in-stance, the review articles ). In contrast to the recent theoretical prediction, no considerableenhancement of the electromagnetic contribution with respect to the free-electron case is found. The sensitivity of this process to neutrino magnetic momentsis analyzed. We consider theoretically ionization of a helium atom by impact of an electronantineutrino. Studenikin a,b, a Faculty of Physics, Lomonosov Moscow State University, 119991 Moscow,Russia b Joint Institute for Nuclear Research, 141980 Dubna, Russia F e b On sensitivity of neutrino-helium ionizing collisions to neutrinomagnetic moments
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