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Atmospheric Neutrinos Barry Barish Bari, Bologna, Boston, Caltech, Drexel, Indiana, Frascati, Gran Sasso, L’Aquila, Lecce, Michigan, Napoli, Pisa, Roma I, Texas, Torino MACRO

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Neutrino event topologies in MACRO Detector mass ~ 5.3 kton Event Rate: (1) up throughgoing (ToF) ~160 /y (2) internal upgoing (ToF) ~ 50/y (3) internal downgoing (no ToF) ~ 35/y (4) upgoing stopping (no ToF) ~ 35/y Absorber Streamer Scintillator (1)(2)(3) (4)

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Energy spectra of events detected in MACRO ~ 100 GeV for throughgoing ~ 5 GeV for internal upgoing ~ 4 GeV for internal downgoing and for upgoing stopping Low energy events allow us to investigate the oscillation parameter space independently of the throughgoing muon data

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Neutrino induced upward-going muons identified by the time-of-flight method scintillator streamer track T 2 -T 1 ) * c / l

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Upward throughgoing muons External -interactions768 events upward vs downward muons

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Background source for events: pion production by downward ’s (MACRO Collab., Astroparticle Phys. 9(‘98)105) rate of downgoing ’s ~10 5 x rate of upgoing ’s background mainly pions produced at large angles by interactions in the rock around the detector ( + N + ± + X) Estimated background in MACRO: ~ 5% for the stopping muon sample (MACRO Coll., PL B478 (‘00) 5) ~ 1% for the up-throughgoing m (>200 g/cm 2 cut) MACRO Coll., PL B434 (‘98) 451)

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Pion production at large angle upgoing charged produced by interactions in the rock under the detector a background source for upward stopping and throughgoing muons studied 243 events with downgoing plus upgoing particles Estimated background - upward throughgoing ’s: ~1% - low energy events:~5%

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Total observed flux and comparison with Monte Carlo Data number of events: 768 events background (wrong )18 background ( ’s from muons)12.5 Internal neutrino interactions14.6 Total 723 events Prediction (Monte Carlo) 989 ± 17% Bartol neutrino Flux (±14%) GRV-LO-94 cross section (± 9%) Lohmann muon energy loss (± 5%) data/prediction R = 0.731 ± 0.028 stat ± 0.044 syst ± 0.124 theor

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Effects of oscillations on MACRO events Flux reduction depending on zenith angle for the upward throughgoing events Earth underground detector L ~ 10 - 10 4 km distortion of the angular distribution

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Zenith angle distribution E > 1 GeV

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Probabilities of oscillations (for maximal mixing) Probability The peak probability from the angular distribution agrees with the peak probability from the total number of events Probability for no-oscillation: ~ 0.4 %

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Probabilities for sterile neutrino oscillations with maximum mixing Peak probabilities lower than that for tau neutrinos: from the angular distribution: 4.1 % from combination: 14.5 % Probability

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Probabilities for sterile neutrino oscillations

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Test of oscillations with the ratio vertical / horizontal Ratio (Lipari- Lusignoli, Phys Rev D57 1998) can be statistically more powerful than a 2 test: 1) the ratio is sensitive to the sign of the deviation 2) there is gain in statistical significance Disadvantage: the structure in the angular distribution of data can be lost. Measured value ~ 2 (and one sided) from the expected value for sterile neutrino

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Internal upward going events From MC simulation: E ~ 5 GeV ~ 87% due to -CC interactions Results: Phys. Lett. B478 (2000) 5 (141 events) Data - Background = 135 events distribution after all analysis cuts: Upgoing internal muons

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Internal Downgoing ’s and Upward Going Stopping ’s from MC simulation : E ~ 4 GeV mixture upward going stopping ~ 50% internal downgoing ~ 87% from –charged current events DATA - Background = 229 events Upward going stopping Internal downgoing

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Data vs Monte Carlo Predictions : Bartol flux with geomagnetic cutoffs (error ~ 20%) = Q.E. + 1 (Lipari et al., PRL74 (1995) 4384) + DIS (GRV-LO-94 PDF) (error ~ 15%) (E , zenith ): detector response and acceptance (systematic error ~ 10 %) MC: 247 25 sys 62 theo DATA:135 12 stat MC:329 33 sys 82 theo DATA:229 15 stat Internal down + Upgoing stop Internal upward going ’s Low Energy Neutrino Events

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Internal upward events maximum mixing m 2 = 2.5 x 10 -3 eV 2 expected 247 ± 62 thor real data 135 ± 12 stat - - - 146 events Data consistent with a constant deficit in all zenith angle bins ( 2 /d.o.f. = 3.1/4 on shape) Probability for NO oscillations: ~ 4.3% (one side)

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Internal downward + Upward stopping events expected 329 ± 82 th real data 229 ± 15 stat - - - 252 events maximum mixing m 2 = 2.5 x 10 -3 eV 2 Data consistent with a constant deficit in all zenith angle bins Probability for NO oscillations: ~ 12% (one side)

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(Internal upward) / (Internal downward + Upward stopping) Most of the theor. uncertainties canceled (<5%) Systematic errors reduced (~6%) Data: R = 0.59 ± 0.07 stat Expected (No oscillations): R = 0.75 ± 0.04 sys ± 0.04 th compatibility ~ 2.7% Expected oscillations R = 0.58 ± 0.03 sys ± 0.03 th ( maximal mixing and m 2 = 2.5 x 10 -3 eV 2 ) Ratio of event types at low energy

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+ upgoing internal upgoing stop internal downgoing Most of the theor. uncertainties canceled < 5% Systematic errors reduced ~ 6% Data: R = 0.59 ± 0.07 stat Expected (No oscillations): R = 0.75 ± 0.04 sys ± 0.04 th Expected oscillations R = 0.58 ± 0.03 sys ± 0.03 th maximal mixing m 2 = 2.5 x 10 -3 eV 2

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MACRO events: confidence level regions for oscillation

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Summary and Conclusions High energy events: oscillation favoured with large mixing angle and m 2 ~ 2.5x10 -3 eV 2 sterile disfavoured at ~ 2 level Low energy events: event deficit with respect to expectation different for Internal upward, and internal downward + Uuward going stopping no zenith angle distortion compatibility between result and expectation (no oscillations) with a probability < 3 % agreement with hypothesis of oscillation with large mixing angle and 10 -3 < m 2 < 2x10 -2 eV 2 A consistent scenario arises from both high and low energy MACRO events

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