Can DUNE realize Einstein’s dream?

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Image: D. Brandonisio, Fermilab

Einstein dreamed of unifying the strong, weak and electromagnetic forces into one, and theorists have been pursuing Grand Unified Theories (GUTs) ever since. Most GUTs predict rates for nucleon decay that cover a range directly accessible with the next generation of large underground detectors. Find it and you’ve got the “smoking gun” – and Einstein’s dream.

The DUNE TPC designs offer excellent tracking and particle-identification capabilities, with sensitivity to key proton decay channels. The job of the nucleon decay (NDK) group in the DUNE Far Detector Optimization Task Force is to figure out what improvements are needed so that DUNE won’t miss this tiny signal, if it occurs.

In the CDR, we estimated a sensitivity of τ/BR (a standard measure of lifetime over branching ratio) of greater than 3.8 x 1034 years for the golden mode, “proton goes to antineutrino K+,” at 90% confidence level assuming the full 40 kiloton DUNE far detector running for 10 years. If this is right, DUNE would provide a big step forward in sensitivity with respect to the best published limit for the same mode, of greater than 5.9 x 1033 years, by the Super-Kamiokande collaboration.

“More than that, it is the exquisite level of information and control of backgrounds that comes from the LArTPC technology that gives credence to a single event being accepted as evidence for an observation of proton decay,” said Bob Wilson, of Colorado State University.

The DUNE sensitivity estimate relied on preliminary estimates for the signal efficiency and background rates. The question now is whether this high sensitivity can be maintained, or perhaps even surpassed, as we inject more realism into our projections for DUNE capabilities.

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The proton decay “golden channel.” Courtesy: J. Raaf

The NDK group, made up of about a dozen scientists, is refining these estimates with more realistic simulations, taking into account event reconstruction effects.

“Over the last year we have successfully integrated GENIE and LArSoft for simulation and reconstruction of NDK events and can now fully reconstruct them,” said Aaron Higuera, of the University of Houston. “Now we are evaluating our reconstruction capabilities and looking for improvements to reach the full potential of DUNE.”

For the first time, the group has evaluated tracking and particle identification efficiencies for golden mode events. These studies help to identify areas where reconstruction improvements are needed.

The performance of the photon detector system is also important for NDK physics.

“Detecting this light enables 3D localization of the signal and is an important step in extracting a lot of the information that makes DUNE sensitive to this decay channel,” said Kevin Wood, of the Stony Brook University. “We have estimated a detection efficiency of roughly 99% across the fiducial volume through a detailed Monte Carlo simulation, which appears to be satisfactory.”

The group is also exploring different TPC wire spacing and wire angle configurations, and extending DUNE NDK studies to other decay modes. Hector Mendez and two colleagues from the University of Puerto Rico, have been simulating and reconstructing decay modes with leptons in the final state, e.g., proton decaying to a charged lepton plus a neutral rho or K meson, to evaluate and demonstrate DUNE’s sensitivity to them. After all, nobody really knows which particles to expect in a NDK final state, so we want to cast as wide a net as possible.