1. Nearly vertical muons from the lower hemisphere in the Baikal neutrino experiment Zh. Dzhilkibaev - INR (Moscow) for the Baikal Collaboration ( Uppsala, 2006)

2. 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 Baikal Collaboration

4. The Site ● 4 cables x 4km to shore. ● 1100m depth 3600 m 1366 m NT200+

5. NT200+ = NT200 + 3 long outer strings - Height = 210m - = 200m -  = 200m - Volume ~ 5 Mton

6. The NT-200 Telescope -8 strings: 72m height - 192 optical modules = 96 pairs (coincidence) - measure T, Charge - σ T ~ 1 ns - dyn. range ~ 1000 p.e. Effective area: 1 TeV ~2000 m² Eff. shower volume: 10TeV ~0.2Mt Quasar PM: d=37cm Height x  = 70m x 40m,V=10 5 m 3,S v =1200m 2

7. Strategy: Application of series of cuts which are tailored to the response of the telescope to nearly vertically upward going muons. 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,

8. Filtering levels and Cuts on variables Inv. velocity of relativistic muon: c -1 = 3.33 ns/m Inv. velocity of light in water: v -1 water = 4.57 ns/m Time difference per 1 m pass: (v -1 water – c -1 ) ~ 1 ns/m -> (  v) -1 = 1 ns/m Cut 1: | v ij -1 – c -1 | < (  v) -1 + 2  /z ij, where v ij = z ij / (t i – t j ),  = 5 ns, z i > z j  i j z ij Level 1: Time differences of hit channels along strings have to be compatible to vertical upward going muon dt ij = z ij / c Level 0: N hit > 3 along at least one (basic string)

9. Variable: T str = max(| t i – t ik |), t ik = t b – (t t – t b )(z i -z i )/ (z t -z b ) Level 2: Time differences of hit channels on different strings have to correspond to vertical upward going muons Cut 2: basic Str. Str.(N hit <3) t b,z b t t,z t t i,z i t il T str < 80 ns 

10. Level 3: Event length should be large enough Variable: L eff = i b - i t + 1 Cut 3: L eff > 8 ( track length > 50 m) ibib itit Level 4: The center of gravity of hit channels should not be close to the detector bottom Variable: Z amp =  (A i z i )/  A i Cut 4:Z amp > 20 m Level 5: Number of hit channels should be large enough Cut 5: N hit > 4 L eff = 5 Level 6: Reconstructed muon direction should be close to vertical Cut 6: cos(  ) < - 0.75 (  ~ 1.5 o – 2 o )

11. 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)   Phys. Rev., 1998, D58, 093009) CC Muon propagation – MUM (Phys. Rev., 2001, D64, 074015) Simulation of array response – (MC-code, Baikal collaboration) Muons induced by atmospheric neutrinos (MC)     – oscillations,  m 2 =2.5x10 -3 eV 2   sin 2 2  m =1

12. Neutrino induced muons E thr = 10 GeV Atmospheric neutrinos (Bartol-96 flux, oscillations - SK, K2K) (25-30)% muon event suppression due to neutrino oscillations

13. Detection area (NT200) All cuts E  > 10 GeV SK MACRO Baksan

14. 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) 6x10 8 generated events – 4 times larger corresponding to experiment

15. Data analysis Livetime – 502 days (April 1998 – February 2000) Trigger: N hit > 3 --- 1.67x10 8 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).

16. Applied cuts efficiency ( 12 events, 268 days livetime (1999)) - experiment - atm. muons (expectation)  - neutrinos (with oscillations)  - neutrinos (without oscillations)

17. L ef = |i bot -i top +1| (Filtering level 3) Cut 1All cuts

18. Z amp =  A i z i /  A i Z Cut 1 All cuts (Filtering level 4)

19. 24 events - experiment 36.6 events - expected without oscillations 29.7 events - expected with oscillations no osc. osc. Angular distribution of 24 selected muons compatible with expected distribution of muons induced by atmospheric neutrinos

20. 90% C.L. upper limit on the excess muon flux Using Baksan estimations for MSSM(P=0.5; m a =52.5GeV; tg  =8))

21. Ultimate goal of Baikal Neutrino Project: Gigaton (km 3 ) 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 expected sensitivity to excess flux of nearly vertically upward going muons (5 year operation) ~(3-5)x10 -17 cm -2 sec -1 624 m 280m 70m 120m 208m

22. 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.