Nearly vertical muons from the lower hemisphere in the Baikal neutrino experiment

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Nearly vertical muons from the lower hemisphere
in the Baikal neutrino experiment
Zh. Dzhilkibaev - INR (Moscow) for the Baikal
Collaboration ( Uppsala, 2006)
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• Institute for Nuclear Research, Moscow, Russia.
• Irkutsk State University, Irkutsk, Russia.
• Skobeltsyn Institute of Nuclear Physics MSU,
  Moscow, Russia.
• DESY-Zeuthen, Zeuthen, Germany.
• Joint Institute for Nuclear Research, Dubna,
  Russia.
• Nizhny Novgorod State Technical University,
  Nizhny Novgorod,
• Russia.
• 7. St.Petersburg State Marine University,
  St.Petersburg, Russia.
• 8. Kurchatov Institute, Moscow, Russia.

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The Site
3600 m
1366 m
NT200

• 4 cables x 4km to shore.
• 1100m depth

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NT200NT200 3 long outer strings

• - Height 210m
• ? 200m
• Volume 5 Mton

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The NT-200 Telescope
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Neutrinos from the Center of the Earth
Basic Idea Search for a statistically
significant excess of neutrino
induced nearly vertically upward going
muons with respect to the expectation for
atmospheric neutrinos.
Background Downward going atmospheric muons,
pair and bremsstrahlung
cascades below the array,
bare atm. muons close to horizon below the
array,
Strategy Application of series of cuts which
are tailored to the response of
the telescope to nearly vertically upward
going muons.
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Filtering levels and Cuts on variables
Level 0 Nhit gt 3 along at least one (basic
string)
Level 1 Time differences of hit channels along
strings have to be compatible to vertical upward
going muon
Inv. velocity of relativistic muon c-1 3.33
ns/m Inv. velocity of light in water
v-1water 4.57 ns/m Time difference per 1 m
pass (v-1water c-1) 1 ns/m -gt (dv)-1 1
ns/m Cut 1 vij-1 c-1 lt (dv)-1 2d/zij ,
where vij zij / (ti tj), d 5 ns, zi gt zj
i
m
zij
j
dtij zij / c
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Level 2 Time differences of hit channels on
different strings have to correspond to vertical
upward going muons
Str.(N hitlt3)
basic Str.
tt,zt
Variable Tstr max( ti tik ),
tik tb (tt tb)(zi-zi)/ (zt-zb)
ti,zi
til
m
Tstr lt 80 ns
Cut 2
tb,zb
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Level 3 Event length should be large enough
Leff 5
Variable Leff ib - it 1
it
Cut 3 Leff gt 8 ( track length gt
50 m)
Level 4 The center of gravity of hit
channels should not be close to the detector
bottom
Variable Zamp S(Ai zi)/ SAi
Cut 4
Zamp gt 20 m
Level 5 Number of hit channels should be large
enough
Cut 5 Nhit gt 4
ib
Level 6 Reconstructed muon direction should
be close to vertical
Cut 6 cos(q) lt - 0.75 (dq 1.5o
2o)
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Muons induced by atmospheric neutrinos (MC)
Atm. neutrino flux - BARTOL 96 (Phys.ReV.,
1995, D53, 1314)
Neutrino propagation the Earth profile
(Astropart.Phys. 1996, 5, 81)
cross-sections
(Phys. Rev., 1998, D58, 093009)
nm nt oscillations, dm22.5x10-3 eV2,

sin22qm1
n
m - (Phys. Rev., 1998, D58, 093009)
CC
Muon propagation MUM (Phys. Rev., 2001, D64,
074015)
Simulation of array response (MC-code, Baikal
collaboration)
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(25-30) muon event suppression due to neutrino
oscillations
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Detection area (NT200)
All cuts Em gt 10 GeV
SK
MACRO
Baksan
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Atmospheric muons (MC)
Primary cosmic ray spectrum and composition
(Cosmic Rays, 1999, 6, 37)
Air shower generation - CORSIKA (Rep. 6019,
Forschungszentrum Karlsruhe (1998))
QGSJET
(Phys. At. Nucl., 1993, 56, 346)
Muon propagation - MUM (Phys.
Rev., 2001, D64, 074015)
Simulation of array response (MC-code,
Baikal collaboration)
6x108 generated events 4 times larger
corresponding to experiment
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Data analysis
Livetime 502 days (April
1998 February 2000) Trigger
Nhit gt 3 --- 1.67x108 events detected
after Cut 1 --- 54534
events selected after all Cuts
--- 24 events selected
Atm. neutrinos --- 36.6 events
without oscillations (expectation)
--- 29.7 events with oscillations
Atm. muons --- 1.9
events expected (background)
Systematic uncertainties 27 Within stat. and
syst. uncertainties 24 detected events are
compatible with the expected background
induced by atmospheric neutrinos (with or without
oscillations).
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Lef ibot-itop1
(Filtering level 3)
Cut 1
All cuts
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(Filtering level 4)
Zamp SAi zi/SAi

Cut 1
Z
All cuts
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Angular distribution of 24 selected muons
compatible with expected
distribution of muons induced by
atmospheric
neutrinos


no osc.
osc.
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90 C.L. upper limit on the excess muon flux
Using Baksan estimations for MSSM(P0.5
ma 52.5GeV tgb8))
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Ultimate goal of Baikal
Neutrino Project Gigaton (km3)
Volume Detector in Lake Baikal
208m
624m
70m
70m
120m
280m
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Conclusion
Neutrino telescope NT200 in Lake Baikal is taking
data since April 1998. With NT200 data from
1998-99 (502 days) 24 events were selected as
nearly vertically upward going muons. Number of
events, as well as their angular distribution, is
compatible with expectation for muons induced by
atmospheric neutrinos. Limits on the excess of
muon flux due to WIMP annihilation in the center
of the Earth have been derived. These limits
belong to the most stringent limits obtained by
Baksan, MACRO, SK and AMANDA experiments.
Analysis of data from 2000-2002 years is in
progress.