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Observation of seasonal variation of atmospheric multiple-muon events in the MINOS Near and Far Detectors

P. Adamson et al. (MINOS Collaboration)
Phys. Rev. D 91, 112006 – Published 9 June 2015
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  • INTRODUCTION
  • THE MINOS DETECTORS AND MUON DATA
  • MODULATION ANALYSIS
  • DISCUSSION OF RESULTS AND POSSIBLE…
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    We report the first observation of seasonal modulations in the rates of cosmic ray multiple-muon events at two underground sites, the MINOS Near Detector with an overburden of 225 mwe, and the MINOS Far Detector site at 2100 mwe. At the deeper site, multiple-muon events with muons separated by more than 8 m exhibit a seasonal rate that peaks during the summer, similar to that of single-muon events. In contrast and unexpectedly, the rate of multiple-muon events with muons separated by less than 5–8 m, and the rate of multiple-muon events in the smaller, shallower Near Detector, exhibit a seasonal rate modulation that peaks in the winter.

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    • Received 31 March 2015

    DOI:https://doi.org/10.1103/PhysRevD.91.112006

    © 2015 American Physical Society

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    Vol. 91, Iss. 11 — 1 June 2015

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    Images

    • Figure 1
      Figure 1

      Time between neighboring atmospheric multiple-muon events in the MINOS detectors. The data are well described by an exponential over 6 orders of magnitude in instantaneous rate.

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    • Figure 2
      Figure 2

      The reconstructed muon multiplicity data, for events containing more than one reconstructed track, in the Far Detector.

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    • Figure 3
      Figure 3

      The minimum track separation ΔS between any two tracks in multiple-muon events recorded in the FD. The gray (black) histogram is the distribution before (after) the selection to remove misreconstructed single-muon events. Regions of track separation ΔS are defined as A: 0.6–4.5 m, B: 4.5–8.0 m and C: >8  m.

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    • Figure 4
      Figure 4

      The multiple-muon rate in the FD as a function of time for different track separations. Each data point corresponds to one calendar month of data. The solid red lines are the best fit to Eq. (2). The top graph is for the smallest track separation, the middle graph for midrange and the bottom graph for the largest. The vertical lines are year boundaries and the solid horizontal line represents the fit without the cosine term.

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    • Figure 5
      Figure 5

      The multiple-muon rate in the FD for events with ΔS range A from 0.6 to 4.5 m (top graph) and for events with ΔS range C larger than 8 m (bottom) binned according to calendar month. The top figure shows a winter maximum. The bottom figure shows a summer maximum.

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    • Figure 6
      Figure 6

      The multiple-muon rate in the ND as a function of time. Each data point corresponds to one calendar month. A clear modulation in the data is observed with the maximum occurring towards the start of the year. The vertical lines are year boundaries.

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    • Figure 7
      Figure 7

      The top figure is the multiple-muon rate in the ND, binned according to calendar month, which each point showing the average rate for all years of data taking. The figure also shows a cosine fit to the data. The single-muon rate is shown in the bottom figure, showing a clearly different seasonal modulation.

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    • Figure 8
      Figure 8

      The minimum track separation ΔS between any two tracks in multiple-muon events recorded in the ND. The gray (black) histogram is the distribution before (after) the selection to remove misreconstructed single-muon events. Regions of track separation ΔS are defined as A: 0.6–1.8 m, B: 1.8–3.0 m and C: >3  m.

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    • Figure 9
      Figure 9

      The (top) modulation phase relative to 1 Jan. and (bottom) amplitude in the ECMWF temperature data based on a cosine fit are shown as a function of altitude and detector site. These distributions were used to study both the geometry effect (B) and the temperature effect (C). The five points on the left in the top figure show a portion of the atmosphere with a winter maximum temperature, albeit with a small amplitude as indicated in the lower figure.

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    • Figure 10
      Figure 10

      To study a possible temperature effect with altitude, (Sec. 4c in the text), the altitude distribution from corsika for MINOS FD multiple muons are shown for each of the three regions of track separation in Fig. 3.

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