Calibration – where the rubber meets the road

Calibration Task Force leaders Kendall Mahn (far left) and Sowjanya Gollapinni (second from right) enjoy the “selfie booth” at the January collaboration meeting with fellow DUNE collaborators Jen Raaf and Karl Warburton. Credit: M.Hronek

As electrons drift in a LArTPC under the influence of an electric field, they can undergo absorption by impurities and recombination with ions. Effects due to these and other energy loss mechanisms are known to be highly convolved and difficult to disentangle. The collaboration’s Calibration Task Force, led by Professors Sowjanya of the University of Tennessee, Knoxville, and Kendall Mahn of Michigan State University, is working on developing a calibration strategy that will likely include a range of instrumentation and measurements.

The DUNE collaboration meeting in January dedicated one plenary and two parallel sessions to this still understaffed effort that is crucial to determining the physics reach of DUNE.

According to Gollapinni the electric field in a LArTPC is never as uniform as one might expect. The calibration needs to untangle how the energy losses affect the collected signal, and determine the field non-uniformity and the effects it has on the loss mechanisms.

The task force is pipelining lessons from ICARUS, the DUNE 35-ton prototype and MicroBooNE as it recruits more participants.

A UV laser run from MicroBooNE in 2015. Using lasers is an option under consideration by the Calibration Task Force. Credit: MicroBooNE

“Filippo Varnini from INFN shared with us a great deal of information from the ICARUS experience,” said Gollapinni. “Using cosmic-ray muons to understand cathode planarity, ICARUS was able to see the impacts on the field, the drift distance, the electron drift velocity, and hence the momentum of muons from multiple coulomb scattering. Now MicroBooNE, with plenty of data, is developing many new techniques for calibration.”

The task force will also rely on models to understand the impacts. The models are still incomplete, however, and have been generated using different electric field values than DUNE requires and much smaller drift volumes than those planned for the Far Detector. The much longer drift lengths in the Far Detector will greatly magnify effects that smaller LArTPCs are still struggling with, in particular signal dispersion, which occurs in three dimensions around and along a particle track. Bo Yu of BNL has been running simulations to investigate the potential sources of electric field distortions. Fermilab’s Tom Junk is using a standard sample of fully simulated and reconstructed cosmic-ray muons generated by MUSUN (MUon Simulations UNderground) to study detector alignment and related issues for a single-phase module at the 4850-foot level.

Many experiments use cosmic-ray muons to calibrate their detectors. At the DUNE Far Detector’s deep underground location — chosen specifically to minimize interference from cosmics — relatively few will reach it (estimated ~4,000/10kt/day). With an eye towards maximizing the usefulness of the available cosmic rays and muons from other sources such as beam-induced rock muons, Josh Klein of the University of Pennsylvania is investigating the use of a cosmic ray tracker (CRT).

A couple of other efforts are also underway. The group is exploring options for implementing a system of lasers, which provide a known input and rely on ionization tracks that are not subject to effects such as recombination. Juergen Reichenbacher from the South Dakota School of Mines and Technology, and Robert Svoboda from University of California, Davis, are actively investigating the use of sources for low-energy calibration.

Gollapinni and Mahn hold regular task force meetings with their few active members, and are hoping to attract more collaborators to a workshop at Fermilab in March.

“Calibration is where the rubber meets the road,” said Mahn. “We have a giant, cold, detector… how do you peer inside without disturbing it? How do you make sure it has a uniform electric field across 50 meters or more? That’s daunting! But that’s where I look forward to working with people to find creative approaches to solving this problem.”