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

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

2. The Baikal Collaboration 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.

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

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

6. -8 strings: 72m height - 192 optical modules = 96 pairs (coincidence) - measure T, Charge - σT ~ 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=105m3,Sv=1200m2 The NT-200 Telescope

7. 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.

8. 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 m zij j dtij = zij / c

9. 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 til m Tstr < 80 ns Cut 2: tb,zb

10. Level 3: Event length should be large enough Leff = 5 Variable: Leff = ib - it + 1 it Cut 3: Leff > 8 ( track length > 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 > 20 m Level 5: Number of hit channels should be large enough Cut 5: Nhit > 4 ib Level 6: Reconstructed muon direction should be close to vertical Cut 6: cos(q) < - 0.75 (dq ~ 1.5o – 2o)

11. 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, dm2=2.5x10-3 eV2, sin22qm=1 n m - (Phys. Rev.,1998, D58, 093009) CC Muon propagation – MUM(Phys. Rev., 2001, D64, 074015) Simulation of array response – (MC-code, Baikal collaboration)

12. Neutrino induced muons Ethr = 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 Em > 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) 6x108 generated events – 4 times larger corresponding to experiment

15. 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 eventsselected 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. Lef = |ibot-itop+1| (Filtering level 3) Cut 1 All cuts

18. (Filtering level 4) Zamp = SAi zi/SAi Cut 1 Z All cuts

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

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

21. 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 fluxof nearly vertically upward going muons (5 year operation) ~(3-5)x10-17 cm-2sec-1 Ultimate goal of Baikal Neutrino Project: Gigaton (km3) Volume Detector in Lake Baikal 208m 624m 70m 70m 120m 280m

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.