Fermilab Today

Several current and future neutrino experiments will use liquid-argon detectors, such as the MicroBooNE detector pictured here, to discover new physics. The LArSoft software package was created specifically for these experiments, including MicroBooNE and the future DUNE and SBND. Photo: Reidar Hahn

Liquid-argon time projection chamber (TPC) detectors play a critical role in measuring the properties of neutrinos and their interactions with matter. To maximize physics output and minimize cost, liquid-argon TPC experiments in the United States all share and contribute to a common physics software package, LArSoft.

These experiments include MicroBooNE and the Short Baseline Neutrino Detector (SBND) at Fermilab and the future international Deep Underground Neutrino Experiment (DUNE) in South Dakota.

Before the days of automation, when bubble chambers were used to identify and measure subatomic particles, researchers would take photographs of these chambers and scan for particle interactions by hand. This was less than ideal; events are complicated and difficult to reconstruct, so the process was long and vulnerable to human error.

Now, scientists and engineers have the technology to automate this process. Modern physics software has made it possible to analyze data from more complex experiments such as those in liquid-argon TPCs, where researchers expect to see millions of particle interactions.

LArSoft was created specifically for liquid-argon TPCs and is a tool for event simulation, reconstruction and analysis, making it valuable both before and after experiments take data. It allows rapid processing of each event, and the computations can be divided among the thousands of computers at Fermilab and elsewhere, enormously speeding the process of producing physics results.

Fermilab scientist Brian Rebel initiated LArSoft, and Erica Snider of the Scientific Computing Division is LArSoft manager.

"We will see many more events in our current generation of experiments, making it impractical to scan each one by hand," said David Schmitz, a professor at the University of Chicago and co-spokesperson for SBND.

Scientists across multiple collaborations use the common physics algorithms included in LArSoft. In this respect, it is unique in the particle physics world.

"This is an experiment in software organization since it is unusual for people to share detector simulation or reconstruction codes between collaborations," said Tom Junk, co-coordinator of the reconstruction task force and computing liaison for DUNE.

Sharing software has allowed reduced development time and easy transfer of knowledge between collaborations while encouraging communication across experiments, where interaction may not occur on a regular basis.

"LArSoft provides a platform to graduate students and postdocs that allows them to move across the different experiments," said Sam Zeller, MicroBooNE co-spokesperson. "It really helps with information transfer and allows multiple collaborations to benefit from each other's work."

—Diana Kwon

Photo of the Day

Up the stairs to the landing

These stairs, familiar to Fermilab staff, connect the hall of oration with the hall of gustation. Photo: Valery Stanley, WDRS

In the News

Large Hadron Collider finds long-sought signs of rare particle decays

From Los Angeles Times, May 13, 2015

Smashing protons together in search of strange particles, scientists at the Large Hadron Collider near Geneva say they've discovered signs of particle decays that have long been predicted, but have never before been seen.

The decay pattern of the two B mesons, described in the journal Nature, could help researchers test the limits of the standard model of particle physics and probe unexplained cosmic phenomena, including the existence of dark matter and the dearth of antimatter in the universe.

Three years after the dramatic discovery of the Higgs boson — a find that earned the theorists who predicted its existence a Nobel Prize — CERN's Large Hadron Collider has been retrofitted and upgraded to search for particles at even higher energies than before.

"From the scientific standpoint, this is big, heady stuff. All the puzzles of physics could fall into place or they could just remain mysteries based on what we learn from these decays," said study co-author Joel Butler, a member of the collider's Compact Muon Solenoid (CMS) experiment and an experimental particle physicist at Fermilab in Illinois. "This is kind of a fantastic time in physics, where many mysteries might get resolved."

Frontier Science Result: MINOS

Summer, winter and muons

These tracks show a six-muon event in the MINOS far detector.

The MINOS detectors at Fermilab and in Soudan, Minnesota, were built to study neutrino oscillations over a vast distance. But it turns out that they are also powerful cosmic ray muon detectors.

When a cosmic ray strikes an atom in the Earth's atmosphere, it sets off a cascade of particle decay, creating kaons or pions, which in turn decay into muons.

MINOS previously made the first deep measurement of the ratio of positive to negative muons arising from cosmic ray showers, and that number is related to the ratio of positive to negative cosmic shower kaons. That, in turn, has implications for the predicted rates of neutrino detection in neutrino telescopes such as IceCube.

MINOS also measured how the cosmic ray muon rate changed with the seasons of the year. It is well known that this rate fluctuates a few percent, being higher in summer when the higher temperatures lower the atmospheric density, which allows for more pion and kaon decay. MINOS was able to correlate this with temperature and demonstrate sensitivity to the ratio of pions to kaons. This ratio happens to be important for calculations of neutrino rates from targets in beams, such as for MINOS itself.

Now MINOS has made a new measurement of the seasonal variations of underground multiple-muon events. These events come from cosmic ray showers in which two or more muons penetrate the Earth and appear as parallel tracks in the detector.

The answer was unexpected. Instead of being higher in the summer, the seasonal variation of multiple muons differed. In the near detector, about 300 feet below the surface, the rate was at a maximum in the winter. See the figure below showing the rate of multiple muons throughout the year (top) and single muons (bottom). (Day zero is Jan. 1.)

In the far detector, about a half mile below the surface, the multiple muons that were within about 13 feet of each other had a maximum rate in the winter, while the events in which muons were separated by 20 or more feet had a summer maximum.

The difference in depth between the near and far detectors affects the minimum muon energy needed to penetrate the rock and reach the detector. Sophisticated simulations of cosmic ray air showers exist but do not currently include seasonal effects.

The understanding of this unexpected result will require new simulations or new data. It would be a wonderful coincidence if, once again, the reason turned out to be useful for the neutrino community.

—Maury Goodman, Argonne National Laboratory

Multiple-muon events — events in which two or more muons simultaneously penetrate the Earth — seen by the MINOS near detector take a dip in the summer (top). By contrast, single-muon events detected by the MINOS near detector rise in the summer (bottom).

This work was done by, from left, Ricardo Gomes from the Federal University of Goias, Jeff de Jong from Oxford University, Phil Schreiner from Benedictine University and Maury Goodman from Argonne National Laboratory.

In the News

IceCube neutrinos do come in three flavours after all

From Physics World, May 11, 2015

High-energy neutrinos detected by the IceCube experiment in Antarctica are equally distributed among the three possible neutrino flavours, according to two independent teams of physicists. Their analyses overturn a preliminary study of data, which suggested that the majority of the particles detected were electron neutrinos. The latest result is in line with our current understanding of neutrinos, and appears to dash hopes that early IceCube data point to "exotic physics" beyond the Standard Model.

In the News

Hans Kobrak, 1927-2015

From La Jolla Light, Feb. 12, 2015

Editor's note: Hans Kobrak pursued particle physics at Fermilab, including the KTeV project, from 1975 to 2002.

Dr. Hans Kobrak, Adjunct Professor Emeritus in Physics at the University of California, San Diego, died of natural causes on February 4, 2015. Hans was born on January 27, 1927, in Danzig, Germany.