Sarov Tritium Neutrino Experiment (SATURNE)
Neutrino Electromagnetic Properties
In the Standard Model (SM), neutrinos are electrically neutral and do not interact directly (i.e., at the tree level) with electromagnetic fields.
Therefore, the only electromagnetic properties of neutrinos in the SM are the tiny charge radii generated by radiative corrections.
In the models beyond SM (BSM), neutrinos can have also magnetic and electric moments (which are often proportional to the neutrino masses) and tiny electric charges.
The neutrino magnetic moment μν was introduced by Wolfgang Pauli in his famous letter addressed to the "dear radioactive ladies and gentlemen" of the Tübingen meeting
of the German Physical Society in December 1930. The impact of neutrinos with nonzero magnetic moment propagating in matter was studied for the first time in 1932 by
Carlson & Oppenheimer. The cross section of magnetic moment interaction of a neutrino with an electron was calculated by Bethe in 1935.
Before the first observation of electron antineutrinos by Cowan and colleagues in 1956 through the inverse decay process ν̅e + p → n + e+,
whose cross section was estimated to be extremely small by Bethe & Peierls, the magnetic moment interaction was considered as an alternative process for detecting neutrinos
if the neutrino magnetic moment is large enough. The first attempt to detect neutrinos through magnetic moment interaction was done in 1935 by Nahmias
with two Geiger-Müller counters placed near a radioactive source. The neutrinos were not detected, and Nahmias established the upper bound
μν < 2x10-4 μB.
Before their first observation of neutrinos in 1956, in 1954 Cowan and colleagues attempted to measure the neutrino magnetic moment with a liquid scintillation detector
exposed to the electron antineutrino flux of the Savannah River reactor. They obtained the upper limit μν < 10-7 μB. Later, in 1957,
Cowan & Reines obtained μν < 10-9 μB with a larger and better-shielded detector. Since then, the electromagnetic properties
and interactions of neutrinos have been studied in many theoretical and experimental works (see the review papers C. Giunti and A. Studenikin,
Neutrino electromagnetic interactions: A window to new physics, Rev. Mod. Phys. 87 (2015) 531,
arXiv:1403.6344 [hep-ph]
and C. Giunti, K. Kouzakov, Y.-F. Li, and A. Studenikin, Neutrino electromagnetic properties, Annu. Rev. Nucl. Part. Sci. 75, 1 (2025),
arXiv:2411.03122 [hep-ph] and references therein).
Coherent Elastic Neutrino-Atom Scattering (CEνAS)
The process of coherent elastic neutrino-atom scattering (CEνAS) was predicted many years ago by Gaponov and Tikhonov
[Y. V. Gaponov and V. N. Tikhonov, Sov. J. Nucl. Phys. 26, 314 (1977)], using the framework of the V-A theory of weak interaction.
They showed that at neutrino energies Eν ≲ 10 keV a region of coherent optical neutrino phenomena exists where
the neutrino elastic scattering by an atom as a whole dominates. Although it is almost half a century since CEνAS was predicted, it is still to be observed in experiment.
The difficulty of experimental investigation of CEνAS is largely related to the necessity of measuring very small values of atomic recoil energy, which appear not
to exceed several hundreds of meV. In addition, an intense neutrino source with Eν ~ 10 keV is needed.
An experimental setup to observe CEνAS using electron antineutrinos from tritium decay and a liquid helium target was proposed in
Cadeddu M., Dordei F., Giunti C., Kouzakov K.A., Picciau E., Studenikin A.I. Potentialities of a low-energy detector based on 4He evaporation to observe
atomic effects in coherent neutrino scattering and physics perspectives, Phys. Rev. D 100, 073014 (2019), arXiv:1907.03302 [hep-ph].
It was shown to have a potential to set an upper limit on the neutrino magnetic moment of about 4.1x10-13 μB at 90% C.L. using 500 g of tritium.
Elastic Neutrino-Electron Scattering (EνES)
The most sensitive and widely used method for the experimental investigation of the neutrino magnetic moment is currently provided by direct laboratory measurements of
low-energy elastic scattering of neutrinos and antineutrinos with electrons in reactor, accelerator and solar experiments. Since in such measurements
the initial electrons are typically bound in target atoms, the neutrino-electron collision results in atomic ionization.
The SM predicts the electron recoil spectrum to be approximately constant at small recoil energies, whereas the neutrino magnetic moment contribution to the spectrum is
inversely proportional to the recoil energy. Therefore, the smaller the electron recoil energy the larger the neutrino magnetic moment effect.
|