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An Overview of the IsoDAR Experiment

IsoDAR has two primary science goals: sterile neutrino searches using inverse beta decay events and nonstandard interaction searches using electron antineutrino - electron elastic scattering events. With the very large anti-electron event sample, other studies can be considered such as neutrino decay studies, neutrino magnetic moment searches, and precision 8Li beta-decay spectrum measurements.

Sterile Neutrino Search

Searches for light sterile neutrinos with mass ∼1 eV are motivated by observed anomalies in several experiments. These results come from a wide range of experiments covering neutrinos, anti-neutrinos, different flavors, and different energies. This diversity has motivated a number of proposals to address them. Many of these proposals, however, do not have sufficient sensitivity to make a definitive > 5σ statement about the existence of sterile neutrinos. A definitive experiment demands that the neutrino (or antineutrino) source be very high intensity and compact in size so that the production point is well known. This type of source then needs to be coupled with a large detector that has free hydrogen so that a large sample of inverse beta-decay events can be recorded. The IsoDAR concept meets these requirements by using the high-power DAEδALUS injector cyclotron to generate a very high intensity isotope DAR source. Such a complex situated next to a kiloton-scale scintillator detector such as KamLAND would enable a definitive search for sterile neutrinos by observing a deficit of antineutrinos as a function of the distance L and antineutrino energy E across the detector—the definitive signature of neutrino oscillation. This is the concept behind the IsoDAR proposal.

The proposed IsoDAR target is to be placed adjacent to the KamLAND detector. The antineutrinos propagate 9.5 m through a combination of rock, outer muon veto, and buffer liquid to the active scintillator volume of KamLAND. The scintillator is contained in a nylon balloon 6.5 m in radius bringing the total distance from target to detector center to 16.1 m. The antineutrinos are then detected via the Inverse Beta Decay (IBD) interaction. This interaction has a well-known cross section with an uncertainty of 0.2%, and creates a distinctive coincidence signal between a prompt positron signal, Evis = Eνe −0.78 MeV, and a delayed neutron capture giving a 2.2 MeV gamma ray within ∼200 μs.

KamLAND was designed to efficiently detect IBD. A standard analysis has a 92% efficiency for identifying IBD events. In IsoDAR’s nominal 5 year run, 8.2 × 105 IBD events are expected. The sterile neutrino analysis uses an energy threshold of 3 MeV. Due to the very effective background rejection provided by the IBD delayed coincidence tag, this threshold enables use of the full KamLAND fiducial volume, R<6.5 m and 897 metric tons, with negligible backgrounds to the reactor anti-neutrinos signal.

To understand the sensitivity relative to other proposals, the IsoDAR 95% CL is compared to other electron antineutrino disappearance experiments in the two neutrino oscillation parameter space in the above figure. In just five years of running, IsoDAR could rule out the entire global 3+1 allowed region, sin2new = 0.067 and Δm2 = 1 eV2 at 20σ. This is the most definitive measurement among the proposals in the most probable parameter space of Δm2 between 1-10 eV2.

Precision Electroweak Tests of the Standard Model

In addition to the 8.2 × 105 IBD interactions, the IsoDAR neutrino source, when combined with the KamLAND detector, could collect the largest sample of low-energy νe-electron scatters (ES) that has been observed to date. Approximately 2600 ES events would be collected above a 3 MeV visible energy threshold over a 5 year run, and both the total rate and the visible energy can be measured.

In the standard model, the ES differential cross section is given by: where T ∈[ 0, 2E2ν/(me+2Eν) ] is electron recoil energy, Eν is the energy of the incoming νe, and the weak coupling constants gR and gL are given at tree level by gR = sin2 θW and gL = 1/2 + sin2 θW . Eq. 2.1 can also be expressed in terms of the vector and axial weak coupling constants, gV and gA, using the relations gR = 1/2 (gV − gA) and gL = 1/2 (gV + gA).

The ES cross section can therefore be used to measure the weak couplings, gV and gA, as well as sin2 θW , a fundamental parameter of the Standard Model. Although sin2 θW has been determined to high precision, there is a longstanding discrepancy between the value obtained by e+e collider experiments and the value obtained by NuTeV, a precision neutrino-quark scattering experiment. Despite having lower statistics than the NuTeV, IsoDAR would measure sin2 θW using the purely leptonic ES interaction, which does not involve any nuclear dependence. This could therefore shed some light on the value of sin2 θW measured by neutrino scattering experiments.

The ES cross section is also sensitive to new physics in the neutrino sector arising from nonstandard interactions (NSIs), which are included in the theory via dimension six, four-fermion effective operators. NSIs give rise to weak coupling corrections and modify the Standard Model ES cross section given above.

The ES interaction used for these electroweak tests of the Standard Model is very different from the IBD interaction used for the sterile neutrino search. The IBD signal consists of a delayed coincidence of a positron and a 2.2 MeV neutron capture γ, whereas the ES signal consists of a single isolated event in the detector. Another difference is that at IsoDAR energies, the IBD cross section is several orders of magnitude larger than the ES cross section. In fact, if just 1% of IBD events are mis-identified as ES events, they will be the single largest background. A total of 2584 signal events, 705 IBD mis-ID background events and 2870 non-beam background events are expected assuming a nominal 5 year IsoDAR run with a 90% duty factor.

A graphic showing the IsoDAR beam target and shielding, with 7Li sleeve.

Use of the Spectrum End points for Physics and Calibration

In addition to the rich physics provided by the IsoDAR beam, the 8Li neutrino beam can be used as a calibration source for the reactor, geo- and solar- neutrino analyses. Similar to the decays of 12B from muon spallation, the beam provides an isotropic source of positrons and neutrons from the IBD interactions that can be used to calibrate the energy scale. These events will not suffer from the electronics effects that appear in the muon spallation analyses.

The measured electron and positron spectra may be interesting to 8B solar neutrino analyses like those done by Super-K and SNO. The 8Li decay proceeds through the same wide excited state of 4Be as 8B. These measurements would be a good test of the conversion procedure from accelerator based beta and alpha spectrum 8B experiments to the 8B neutrino spectrum.

Studies have also shown that the sterile neutrino oscillation analysis has the statistical power to determine the 8Li endpoint along with identifying the sterile neutrino effects. Preliminary estimates indicate that the endpoint can be determined with an uncertainty of around 0.015 MeV using this type of analysis.


Our Latest Work on IsoDAR

  • Work with Larry Bartoszek, of Bartoszek Engineering, on the target design.
  • Regular meetings betwen MIT, INFN, and BEST cyclotrons to discuss planning execution of an IsoDAR central region test, which was held in Vancouver summer of 2014.
  • Productive meetings at ERICE to discuss the way forward for a Baseline Design Report
  • Writing a paper on the phenomenology of electron antineutrino-electron scatters with IsoDAR.
  • Work on the MIST-1 Ion Source completed Fall, 2017. See our News section for details!

Further Reading

  • Cost-Effective Design Options for IsoDAR. ArXiv
  • Proposal for an Electron Antineutrino Disappearnace Search Using High-Rate 8Li production and Decay. ArXiv and also chosen as a PRL Highlight
  • IsoDAR@KamLAND: A Conceptual Design Report for the Technical Facility. ArXiv
  • IsoDAR@KamLAND: A Conceptual Design Report for the Conventional Facility. ArXiv