db1acce038539a246364a93d8cbe34e0.ppt
- Количество слайдов: 22
Nearly vertical muons from the lower hemisphere in the Baika 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, Germany. 5. Joint Institute for Nuclear Research, Dubna, Russia. 6. Nizhny Novgorod State Technical University, Nizhny Novgorod, 7. Russia. 8. 7. St. Petersburg State Marine University, St. Petersburg, Russia. 9. 8. Kurchatov Institute, Moscow, Russia.
The Site 1366 m 3600 m NT 200+ ● ● 4 cables x 4 km to shore. 1100 m depth
NT 200+ = NT 200 + 3 long outer strings - Height = 210 m - = 200 m - Volume ~ 5 Mton
-8 strings: 72 m 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 Te. V ~2000 m² Eff. shower volume: 10 Te. V ~0. 2 Mt Height x = 70 m x 40 m, V=105 m 3, Sv=1200 m 2 Quasar PM: d=37 cm
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-1 water = 4. 57 ns/m Time difference per 1 m pass: (v-1 water – c-1) ~ 1 ns/m -> (dv)-1 = 1 ns/m Cut 1: | vij-1 – c-1 | < (dv)-1 + 2 d/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. 5 o – 2 o)
Muons induced by atmospheric neutrinos (MC) Atm. neutrino flux - BARTOL 96 (Phys. Re. V. , 1995, D 53, 1314) Neutrino propagation – the Earth profile (Astropart. Phys. 1996, 5, 81) n n CC cross-sections (Phys. Rev. , 1998, D 58, 093009) 2 -3 2 m t – oscillations, dm =2. 5 x 10 e. V , sin 22 qm=1 n m - (Phys. Rev. , 1998, D 58, 093009) Muon propagation – MUM (Phys. Rev. , 2001, D 64, 074015) Simulation of array response – (MC-code, Baikal collaboration)
Neutrino induced muons Ethr = 10 Ge. V Atmospheric neutrinos (Bartol-96 flux, oscillations - SK, K 2 K) (25 -30)% muon event suppression due to neutrino oscillations
Detection area (NT 200) All cuts Em > 10 Ge. V 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, D 64, 074015) Simulation of array response – (MC-code, Baikal collaboration) 6 x 108 generated events – 4 times larger corresponding to experiment
Data analysis Livetime – 502 days (April 1998 – February 2000) Trigger: Nhit > 3 --- 1. 67 x 108 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 (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. 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 Using Baksan estimations for MSSM(P=0. 5; ma =52. 5 Ge. V; tgb=8))
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 Te. V cascades ~ 0. 5 -1. 0 km³ 208 m 624 m expected sensitivity to excess flux of nearly vertically upward going muons (5 year operation) ~(3 -5)x 10 -17 cm-2 sec-1 70 m 280 m 120 m
Conclusion Neutrino telescope NT 200 in Lake Baikal is taking data since April 1998. With NT 200 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.