An investigation of seasonal variations in the atmospheric neutrino rate

1
An investigation of seasonal variations in the
atmospheric neutrino rate with the AMANDA-II
neutrino telescope
Markus Ackermanna, Elisa Bernardinia for the
IceCube Collaboration (a) DESY Zeuthen,
Platanenallee 6 D-15738 Zeuthen, Germany
LePossible scenarios of particles acceleration
in Blazars. Neutrino are produced in the decay
of mesons.
Physics Motivations One of the major goals of the
large-scale neutrino detectors, AMANDA and
IceCube, is to identify cosmic sources of
high-energy neutrinos. Neutrinos produced in the
interaction of cosmic rays with the atmosphere
form the dominant background for this search (the
much more abundant muons from cosmic ray
interaction can be removed by angular and quality
selection of events). The atmospheric neutrinos
however reach the detectors from all directions
and remain as a residual background in the data
sample. It is essential to study the properties
of this background well to quantify correctly
possible deficits and excesses that would be
interpreted as neutrino sources. Seasonal
temperature variations in the atmosphere cause
fluctuations in the target density and therefore
in the cosmic ray induced muon and neutrino
fluxes. The expected amplitude of these seasonal
variations is calculated here and compared to
AMANDA-II data.
INTRODUCTION
The AMANDA-II telescope The Antarctic Muon and
Neutrino Detector Array (AMANDA) is optimized to
detect neutrino induced muon tracks above a
threshold energy of approx. 50 GeV 1. AMANDA-II
comprises 677 8 photo-multipliers, frozen in the
South Pole glacial ice, at depths between about
1.5 and 2 km. When a muon neutrino undergoes a
charged current interaction in the surrounding
ice (or in the rock below) a muon track emerges,
which can be reconstructed from the arrival time
at the photomultipliers of the Cherenkov photons.

Left Schematic view of AMANDA-II Right
Cherenkov light front emitted from a neutrino
induced muon

• Atmospheric model for temperature variations
• NRLMSISE-00 atmospheric model used to account
  for temperature variations 8,9
• Empirical model for temperatures and densities
  from ground level to several hundred km of
  altitude

Calculation of expected seasonal variations for
atmospheric neutrinos Seasonal variations in the
cosmic ray muon flux due to temperature
fluctuations was calculated and measured for
several experiments (for example MACRO 2 and
AMANDA-B10 4). Even though neutrinos and muons
are produced in same interactions, neutrinos have
to be treated differently due to the differences
shown below
Examples of temperature profiles at different
altitudes for January and July
Muon flux variations in AMANDA-B10

• Relevant differences between muons and neutrinos
• Energy threshold (for AMANDA-II)
• muons 400GeV
• atm. neutrinos 50 GeV
• Location of production
• muons local atmosphere
• atm. neutrinos global atmosphere
• Dominant generation mode 6
• muons Pion decay
• atm. neutrinos Kaon decay

TEMPERATURE MODEL
Left Seasonal variations of atmospheric muon
rates measured in AMANDA-B10 4. Right
Effective temperature above south pole.
Neutrino flux variations vs. latitude
SEASONAL VARIATIONS

• Consequences
• Analytic high energy approximations (valid for
  muons) have to be replaced by a full numerical
  calculation
• Contributions from different geographical
  latitudes and longitudes have to be integrated

• Comparison to AMANDA-II data
• Atmospheric neutrino statistics too low in
  AMANDA-II to resolve seasonal fluctuation
  expected by the calculations
• c2-Test of AMANDA-II point source sample (
  atmospheric neutrino content gt 90 ) shows no
  incompatibility with calculated variations
• Data divided in 3 bins corresponding to
  geographical latitudes of neutrino origin

Amplitude of the seasonal variations of the
atmospheric neutrino rate for neutrinos
originating from a given declination. The
integrated neutrino rate variations are shown
above thresholds of 50 GeV and 1TeV and weighted
for the AMANDA-II effective area.

• Calculation Method
• Monte Carlo integration of neutrino flux 5
  over depth and energy
• Temperature model used to describe atmospheric
  temperature modulations
• (E,q) - parametrization of AMANDA-II effective
  area to calculate event rates

Neutrino flux variations vs. time
AMANDA-II DATA
AMANDA-II atmospheric neutrino rates
Expected seasonal variations

• Results
• Expected seasonal variation of atmospheric
  neutrino event rate for AMANDA-II 0.5 - 3
• Considerably smaller than the seasonal
  variation of the muon rate

Time development of the seasonal variations for
atmospheric neutrinos originating from 65 N.
Besides the combined curve p-decay and K-decay
component variations are shown separately.
Rates of atmospheric neutrinos in AMANDA-II in
bins of 30 days. The years 2000-2003 are
superimposed. The lines show the rates expected
from the calculation normalized to the mean of
the measured rate .
Expected seasonal variations of atmospheric
neutrinos in AMANDA-II for the two latitude bins
above.

• Summary
• Expected seasonal variations of atmospheric
  neutrino rates were calculated for the first time
  for a current high energy neutrino detector
• AMANDA-II atmospheric neutrino statistics are
  too low to resolve seasonal variations of the
  expected magnitude but they are in reach of
  IceCube
• The data in the AMANDA-II point source sample
  is compatible with the calculated variations as
  well as with a flat distribution.

1 Ackermann, M. et al. 2005, These proceedings,
OG.2.5 2 Ambrosio, M. et al. 1997, The MACRO
collaboration, Astrop. Phys. 7, 109-124 3
Barrett, K. et al. 1952, Rev. Mod. Phys. 24,
133 4 Bouchta, A. Proc. 25th ICRC (Salt Lake
City, 1999), HE 3.2.11 5 Gaisser, T. 1990,
Cosmic rays and particle physics, Cambridge
University Press, Cambridge 6 Gaisser, T. 2002,
Atmospheric neutrino fluxes, Talk given at
Neutrino 2002, Munich 7 Gonzalez-Garcia, M.C.
et al. 2005, Phys.Rev. D71, 093010
(hep-ph/0502223) 8 J.M. Picone, et al. 2002, J.
Geophys. Res., 107(A12), 1468 9 Space physics
models on http//nssdc.gsfc.nasa.gov/space/model
SUMMARY