Neutrino scattering on polarized electron target and neutrino magnetic moment

The completed and proposed experiments for the measurement of the neutrino magnetic moment are discussed. To improve the sensitivity of the search for the neutrino magnetic moment we suggest to use a polarized electron target in the processes of neutrino (antineutrino) – electron scattering. It is shown that in this case the weak interaction term in the total cross section is few times smaller comparing with unpolarized case, but the electromagnetic term does not depend on electron polarization. The search of new bounds on the neutrino magnetic moment μν in laboratory experiments would be very important for checking of new Physics beyond the Standard Model (SM) and for different astrophysical implications. The Resonant Spin-Flavor Precession (RSFP) scenario (Akhmedov, 1988; Lim and Marciano, 1988) in the case of a non-vanishing neutrino magnetic moment μν 6= 0 is still considered as a possible solution to the Solar Neutrino Problem (SNP) (Berezinsky, 1998). The observations of neutron stars gave an information about the existence of strong magnetic fields (even more than the critical value Bc = m 2 e/e = 4.41 · 10 G) which can interact with neutrino through its magnetic moment. The nature of a neutrino (Dirac or Majorana) determines the properties of neutrino magnetic moments, so a Dirac neutrino can have diagonal and transition (off-diagonal) moments, while Majorana neutrinos can have transition moments only (Schechter and Valle, 1981). All these hints claim direct laboratory measurements of the neutrino magnetic moment. There is the laboratory bound on the electron neutrino magnetic moment from the experiments with reactor’s neutrino combined by Derbin (1994), μνe < 1.8 · 10 μB. There are also different astrophysical constraints that are even more stringent then previous one. For the detailed discussion and references see Raffelt (1996). To probe neutrino magnetic moment at a more lower level μνe ∼ 10 μB some artificial neutrino sources of well-known activity and spectra can be used. There are some experiments planned with different possible artificial sources, e.g. Cr (Ferrari et al., 1996), Sr (Mikaelyan et al., 1998; Ianni and Montanino, 1999), Pm (Kornoukhov, 1997; Barabanov et al., 1997), Fe (Golubchikov et al., 1996). The recently proposed measurement of neutrino magnetic moment at a level better than 10 μB ( ∼ 3 · 10 μB) with the use of the tritium neutrino emitter (antineutrino source with the energy spectrum endpoint 18.6keV) and a semiconductor cryodetector is planned to reach extra low threshold about 1.1 eV (Trofimov et al., 1998; Bogdanova, 2000) (see Table 1). Reference ν(ν̄) source Detector μν/μB > Ferrari et al., 1996 Cr BOREXINO 5 · 10 Ianni et al., 1999 Cr, Sr BOREXINO 6 · 10 Miranda et al., 1999 Cr, Sr HELLAZ 6 · 10 Beda et al., 1998 Reactor Ge-NaI 3 · 10 TEXONO, Wong, 1999 Reactor, Taiwan CsI 3 · 10 Kurchatov-PNPI, Kozlov, 1999 Krasnoyarsk reactor Si 2 · 10 MUNU, Broggini, 1999 Bugey reactor CF4 2 · 10 Mikaelyan et al., 1998 Sr BOREXINO 1.5 · 10 Golubchikov et al., 1996 Fe Si 5 · 10 Kornoukhov, 1997 Pm Ge, Si or NaI 5 · 10 Barabanov et al., 1997 Pm NaI 3 · 10 Trofimov et al., 1998 TiT2 Si 3 · 10 Table 1. Proposed and planning experiments and neutrino magnetic moments which can be measured. We suggest to use a polarized electron target for more precise measurements of a neutrino magnetic moment in the processes of neutrino (antineutrino) – electron scattering (Rashba and Semikoz, 2000). Kinematics of such scattering for which the recoil electron energy Te = E2 −me should be measured is shown in Fig.1. We have fixed z0x-plane based on the initial neutrino momentum ~k1 directed along z-axis and on the 3-vector part ~ ξe of the four-spin aμ = (0, ~ ξe) at the rest frame of the initial electron (~ p1 = 0) entering the Mishel–Wightman density matrix, ρ(p1) = (p̂1 +me)(1 + γ5â). The cross section of the electron neutrino (antineutrino) scattering off electrons consists of two terms: weak and electromagnetic ones without interference term that can be neglected in the massless limit mν → 0,