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Chris Tunnel: The NuStorm Project

March 13, 2013

Today’s question: what machine can we build now to elucidate claims of sterile neutrinos using muon decay beams ? A 3.8 GeV muon storage ring with a mINOS-like detector at 2 km.

In 1980 D.Neuffer proposed 1-GeV muon decay ring for neutrino oscillations. In 1998 S.Geer started the modern Neutrino Factory effort. In summer 2011 NuSTORM is born. NuStorm has a smaller facility in terms of size of detector, providing fewer decays at lower energy. Taking 50 GeV protons to a 100 kW target station (Tallium or Carbon are considered for the target). Then extract 5 GeV pions, and derive muons of 3.8 GeV in a decay ring. Want a L/E of the order 1, so the far detector needs to be at about 2 km. A near detector at 50 meters is also needed for normalization purposes. Can also do electron neutrino cross sections there.

The decay ring should have 150 m straight sections and a large-aperture FODO scheme. One may anticipate few % flux uncertainty from this design.

Assuming 5 years of running and a fairly conservative estimate of the components, this yields 1.7×10^18 useful muon decays. At the near detector hall one can do an oscillation analysis, but also an electron neutrino test beam with which to do precision cross section measurements. Specifically, electron neutrinos for future long-baseline experiments. At the far detector, for the sterile nu program, one will have a magnetized iron detector, similar in size to MINOS. 1.3 kton fiducial volume, with 5-6 meters of diameter. Using plates of iron 1-2 cm thick.

The channel to probe from LSND is muon antineutrinos appearing as electron antineutrinos. If you do a CPT operation you get the situation in nuStorm, where you get electron neutrinos going into muon neutrinos. Your flux uncertainties are much smaller. The caveat is due to muon antineutrino backgrounds. So how would one extract the signal ?

The signal of electron neutrinos appearing as muon neutrinos would be sought in charged current interactions, with 332 events appearing in 5 years in a 1.3 kton far detector. Backgrounds are up to hundreds of thousands from the various sources. With simulations they studied the background rejection and they get a neutrino energy spectrum which has a large background with small backgrounds. This would be a “smoking gun” of the effect.

The speaker then discussed the expected sensitivity of the setup. He said S.Parke once told him he wanted 10-sigma expectations, since he’d seen so  many 5-sigma expectations vanish. So he showed plots for 5- and 10-sigma effects.

They so believe they can probe signals at 10-sigma significance in a wide range of values in the plane sin^2 2 theta_emu vs dm^2, extending to (0.01,1 eV^2).

Then Chris spoke about the cross section measurement. At the near site one gets millions of events from NC and CC, and the determined cross sections have high precision.

This experiment could be built either at Fermilab or at CERN. At FNAL the beam from the main ring could give interactions in a far detector located in the dzero site, where it would fit in the assembly hall (although oddly at an angle with respect to the building orientation). A sketch of the layout is shown in the aerial picture below.

nustorm1

In conclusion, the speaker stressed the three legs of the nuStorm project: the sensitivity to sterile neutrinos; the measurement of neutrino cross sections (somebody said this could yield over 60 possible thesis topics); and the accelerator R&D, with a fundamentally different beam.

There is significant european involvement in the project. And people will be studying neutrinos for a long time, so developing new beams today is a good way to invest in the field.

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