Transcript BAIKAL Experiment: main results obtained with the neutrino

Nearly vertical muons from
the lower hemisphere in the
Baikal neutrino experiment
Zh. Dzhilkibaev - INR (Moscow)
for the Baikal Collaboration
( Uppsala, 2006)
The Baikal Collaboration
1. Institute for Nuclear Research, Moscow, Russia.
2. Irkutsk State University, Irkutsk, Russia.
3. Skobeltsyn Institute of Nuclear Physics MSU, Moscow, Russia.
4. DESY-Zeuthen, Zeuthen, Germany.
5. Joint Institute for Nuclear Research, Dubna, Russia.
6. Nizhny Novgorod State Technical University, Nizhny Novgorod,
Russia.
7.
St.Petersburg State Marine University, St.Petersburg, Russia.
8.
Kurchatov Institute, Moscow, Russia.
The Site
NT200+
●
●
4 cables x 4km to
shore.
1100m depth
NT200+
=
NT200
+
3 long outer
strings
- Height = 210m
-  = 200m
- Volume ~ 5 Mton
-8 strings: 72m height
- 192 optical modules
= 96 pairs (coincidence)
- measure T, Charge
- σT ~ 1 ns
- dyn. range ~ 1000 p.e.
The NT-200 Telescope
Effective area: 1 TeV ~2000 m²
Eff. shower volume: 10TeV
~0.2Mt
Height x  = 70m x 40m,V=105m3,Sv=1200m2
Quasar PM:
d=37cm
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.
Filtering levels and Cuts on variables
Level 0: Nhit > 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 -> (dv)-1 = 1 ns/m
Cut 1:
| vij-1 – c-1 | < (dv)-1 + 2d/zij ,
where
vij = zij / (ti – tj), d = 5 ns, zi > zj
i
zij
j
m
dtij = zij / c
Level 2: Time differences of hit channels on different strings
have to correspond to vertical upward going muons
Str.(N hit<3)
basic Str.
tt,zt
Variable:
Tstr = max(| ti – tik |),
tik = tb – (tt – tb)(zi-zi)/ (zt-zb)
ti,zi
Cut 2:
til
Tstr < 80 ns
tb,zb
m
Level 3:
Variable:
Event length should be large enough
Leff = ib - it + 1
Cut 3:
Leff > 8
( track length > 50 m)
Leff = 5
it
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 > 20 m
Level 5: Number of hit channels should be large
enough
Cut 5:
Level 6:
Cut 6:
Nhit > 4
ib
Reconstructed muon direction should
be close to vertical
cos(q) < - 0.75
(dq ~ 1.5o – 2o)
Muons induced by atmospheric neutrinos (MC)
Atm. neutrino flux - BARTOL 96
(Phys.ReV., 1995, D53, 1314)
Neutrino propagation – the Earth profile (Astropart.Phys.
n
1996, 5, 81)
cross-sections (Phys. Rev., 1998, D58, 093009)
nm nt – oscillations, dm2=2.5x10-3 eV2,
sin22qm=1
CC
m - (Phys. Rev., 1998, D58, 093009)
Muon propagation – MUM (Phys. Rev., 2001, D64, 074015)
Simulation of array response – (MC-code, Baikal collaboration)
Neutrino induced muons
Ethr = 10 GeV
Atmospheric neutrinos
(Bartol-96 flux, oscillations - SK, K2K)
(25-30)% muon event suppression due to neutrino oscillations
Detection area (NT200)
All cuts
Em > 10 GeV
SK
MACRO
Baksan
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
Data analysis
Livetime – 502 days (April 1998 – February 2000)
Trigger: Nhit > 3 --- 1.67x108 events detected
after Cut 1
--- 54534 events selected
after all Cuts
--24 events selected
Atm. neutrinos --(expectation) --Atm. muons
(background)
---
36.6 events without oscillations
29.7 events with oscillations
1.9 events expected
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).
Applied cuts efficiency
( 12 events, 268 days livetime (1999))
•- experiment
•
- atm. muons
- neutrinos
- neutrinos
(expectation)
(with oscillations)
(without oscillations)
Lef = |ibot-itop+1|
Cut 1
(Filtering level 3)
All cuts
Zamp = SAi zi/SAi
Cut 1 Z
(Filtering level 4)
All cuts
Angular distribution of 24 selected muons
compatible with expected distribution of muons induced by
atmospheric neutrinos
no osc.
osc.
24 events - experiment
36.6 events - expected without
oscillations
29.7 events - expected with
oscillations
90% C.L. upper limit on the excess muon flux
Baikal Amanda SK Baksan MACRO
T, days
502
422
1680
5402
1298
Using Baksan estimations
for MSSM(P=0.5; ma =52.5GeV; tgb=8))
Ultimate goal of Baikal Neutrino Project:
Gigaton (km3) Volume Detector in Lake Baikal
Sparse instrumentation:
90 - 100 strings with 12-16 OMs
= 1300 - 1600 OMs
effective volume for >100 TeV
cascades ~ 0.5 -1.0 km³
208m
624m
expected sensitivity to excess flux of
nearly vertically upward going muons
(5 year operation)
~(3-5)x10-17 cm-2 sec-1
70m
70m
280m
120m
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.