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.