Filed in: Main.Osc SNS Home Page · Modified on : Wed, 05 Jun 13
There exists a need to address and resolve the growing evidence for short-baseline neutrino oscillations and the possible existence of sterile neutrinos. Such non-standard particles, ﬁrst invoked to explain the LSND νµ → νe appearance signal, would require a mass of ∼ 1 eV/c2 , far above the mass scale associated with active neutrinos. More recently, the MiniBooNE experiment has reported a 2.8σ excess of events in antineutrino mode that is consistent with neutrino oscillations and with the LSND antineutrino appearance signal. MiniBooNE has also observed a 3.4σ excess of events in their neutrino mode data. In addition, lower than expected neutrino-induced event rates from calibrated radioactive sources and nuclear reactors can be explained by the existence of sterile neutrinos. Fits to the world’s neutrino and antineutrino data are consistent with sterile neutrinos at this ∼ 1 eV/c2 mass scale, although there is some tension between measurements from disappearance and appearance experiments. The existence of these sterile neutrinos will impact design and planning for next generation neutrino experiments. It should be conclusively established whether such totally unexpected light sterile neutrinos exist. The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, built to usher in a new era in neutron research, provides a unique opportunity for US science to perform a deﬁnitive search for sterile neutrinos.
The 1.4 MW beam power of the SNS is a prodigious source of neutrinos from the decay of π + and µ+ at rest. These decays produce a well speciﬁed ﬂux of neutrinos via π + → µ+ νµ , τπ = 2.7×10−8 s, and µ+ → e+ νe νµ , τµ = 2.2 × 10−6 s. The low duty factor of the SNS (∼ 695 ns beam pulses at 60 Hz, DF = 4.2 × 10−5) is more than 1000 times less than that found at LAMPF. This smaller duty factor provides a reduction in backgrounds due to cosmic rays, and allows the νµ induced events from π + decay to be separated from the νe and νµ induced events from µ+ decay.
The OscSNS experiment will make use of this prodigious source of neutrinos. The OscSNS detector will be centered at a location 60 meters from the SNS target, in the backward direction. The cylindrical detector design is based upon the LSND and MiniBooNE detectors and will consist of an 800-ton tank of mineral oil with a small concentration of b-PBD scintillator dissolved in the oil, that is covered by approximately 3500 8-inch phototubes for a photocathode coverage of 25%. The cylindrical design will allow us to map the event rates as a function of L/E, to look for any sinusoidal dependence indicative of oscillations.
This experiment will use the monoenergetic 29.8 MeV νµ to investigate the existence of light sterile neutrinos via the neutral-current reaction νµ C → νµ C ∗ (15.11 MeV ). This reaction has the same cross section for all active neutrinos, but is zero for sterile neutrinos. An observed oscillation in this reaction is direct evidence for sterile neutrinos. OscSNS can also carry out an unique and decisive test of the LSND νµ → νe¯ appearance signal. In addition, OscSNS can make a sensitive search for νe disappearance by searching for oscillations in the reaction νe C → e− Ngs , where the Ngs is identiﬁed by its beta decay. It is important to note that all of the cross sections involved are known to two percent or better.
The SNS represents a unique opportunity to pursue a strong neutrino physics program in a cost-eﬀective manner, as an intense ﬂux of neutrinos from stopped π + and µ+ decay are produced during normal SNS operations. The existence of light sterile neutrinos would be the ﬁrst major extension of the Standard Model. Sterile neutrino properties are central to dark matter, cosmology, astrophysics, and future neutrino research. The OscSNS experiment would be able to prove whether sterile neutrinos can explain these existing short-baseline anomalies.