Like a halo of sparkles

Imagine: Owl shift at 4:00 a.m. You’re sipping on a cold cup of coffee, sleepily starting and stopping runs and checking all the monitors when a huge bunch of low-energy neutrinos bursts on the scene. It’s all over in ten seconds… it’s a supernova!

This is the dream of the Supernova Burst and Low-Energy (SNB/LE) Neutrino Physics Working Group. The group is developing high-quality reconstruction tools for the anticipated low-energy signals from both supernovae and the sun.

Participants at the summer 2016 Hack Days. Photo: Inés Gil-Bottela, sparkly halos courtesy nus2surf

SNB events will appear as short, perhaps a few tens-of-cm, stub-like tracks from electrons emerging from neutrino interactions on argon.   De-excitation products — gamma rays, protons, or neutrons shaken off by the disturbed nucleus — are likely to accompany these signals, creating little blips surrounding the track like a halo of sparkles.   Other events, for instance, electrons kicked elastically by the neutrinos, will look like simple stubs — halo-less, like the rest of us.

So far, reconstruction algorithms for low-energy neutrinos in DUNE lag behind those for high-energy events, but this is changing. The Far Detector Optimization Task Force preliminary report details their progress so far.

“We have made improvements to the low-energy reconstruction tools this year that are necessary for several SNB/LE projects to move forward,” said AJ Roeth, an undergrad at Duke who is working on identifying the neutral current interaction of neutrinos, “including mine!”

Complicating things, little blips from radioactivity in the argon or detector materials may fake de-excitation gamma rays. Understanding these and other backgrounds may be especially important for the photon detectors, since they correct for the attenuation of charge as it drifts and therefore largely determine the energy resolution of a signal. Backgrounds also impact triggering and data acquisition (DAQ), and the Radiopurity, Photon and DAQ groups are coordinating with SNB/LE and with each other in these areas.

The first “Hack Days” event in summer 2016 set the stage for the significant progress the group has since made in finding standard reconstruction algorithms that work for low-energy events, and learning how to tweak them appropriately. Another event in January 2017 will focus on the work that is underway on new algorithms specialized for these events.

“We hope to have a realistic reconstruction for low-energy events ready before the final Task Force report is due in May,” said Inés Gil-Botella, co-convener of the SBN/LE group. “We would also like to combine the charge and light signals to improve the low-energy resolution and compare the performance of single- and dual-phase LAr technologies by then.”