Reconstruction and analysis of ANTARES 5 line data

1. Reconstruction and analysis of ANTARES 5 line data Niccolò Cottini on behalf of the ANTARES Collaboration XXth Rencontres de Blois 21 / 05 / 2008

2. Outline of the talk • The ANTARES experiment • Detector layout • Data acquisition system • Data analysis • Event reconstruction • Selection of neutrino events • standard approach • an example of the work being pursued Niccolò Cottini

3. The ANTARES detector ~60 m 450 m -2500 m 12 lines, 900 PMTs Niccolò Cottini

4. Muon (Cherenkov) 2 µs crossing time 40K decay Continuous background  30 kHz * The detected signal Simulation 2 min • Bioluminescence • Continuous background  30 kHz * • Occasional bursts ~MHz * PMT 10’’, threshold 0,3 p.e. Niccolò Cottini

5. m The Trigger • Front end chip digitizes charge and time of a light signal • “ALL DATA TO SHORE” SCHEME: • all data transmitted through multiplexed Gigabit links • computer farm running a software trigger: • the whole data flow can not be written to disk • look in all directions for light signals compatible with a muon track • muon triggered event rate: ~1 Hz Niccolò Cottini

6. 107 atmospheric m per year 3000 atmospheric n per year Atmosphere Sea Earth Cosmic n The physics signal Muon flux at the detector Keep all the muon tracks; then filter to extract the neutrino signal Niccolò Cottini

7. 5 line data analyzed 169 days 29 Jan 2007 4 Dec 2007 310 days Niccolò Cottini

8. 3D PMTarray Cherenkov light from m 43° Sea floor m nm interaction Event reconstruction Reconstruction of m trajectoryfrom time, charge and position of PMT hits Niccolò Cottini

9. z y x Event reconstruction z-t event display Niccolò Cottini

10. Event reconstruction Atmospheric muon bundle Complicated pattern; a bundle can be misreconstructed as an upward going track Niccolò Cottini

11. Reconstruction algorithm • Muon track fit in 3 steps: 1) Prefit: (x,y,z,t) linear fit through the photon hits 2) Improved c2 minimization (charge weights) 3) Time residual pdf (charge weights) Direct Cherenkov emission Electronics effect Local minima possible: reiteration with 8 different starting points (prefit rotations) Diffused light Niccolò Cottini

12. Reconstruction quality (MC simulation with 12 lines detector) Niccolò Cottini

13. 5 line data reconstruction Dominated by the tail of atmospheric muons badly reconstructed as upward going Niccolò Cottini

14. Neutrino events in standard analysis: L cut 169 days live time Preliminary 0.17 107 neutrino events 90% purity, 0.6 n / day (~7 reconstructed n / day) Niccolò Cottini

15. An alternative approach under study • Likelihood ratio method • 4 discriminative variables: • cos(q) Data, m MC, n MC Niccolò Cottini

16. An alternative approach under study • Likelihood ratio method • 4 discriminative variables: • cos(q) • q of the prefit Data, m MC, n MC Niccolò Cottini

17. An alternative approach under study • Likelihood ratio method • 4 discriminative variables: • cos(q) • q of the prefit • Fraction of direct hits (time res<3 ns) Data, m MC, n MC Niccolò Cottini

18. An alternative approach under study • Likelihood ratio method • 4 discriminative variables: • cos(q) • q of the prefit • Fraction of direct hits (time res<3 ns) • Distance covered by the last in time photon Data, m MC, n MC Niccolò Cottini

19. Data, m MC, n MC (x100) y An alternative approach under study • Likelihood ratio method • 4 discriminative variables: • cos(q) • q of the prefit • Fraction of direct hits (time res<3 ns) • Distance covered by the last in time photon s(x) = pdf of MC atmospheric neutrinos b(x) = pdf of MC atmospheric muons y = s(x) / [ s(x) + b(x) ] Niccolò Cottini

20. Data, m MC, n MC (x100) y y An alternative approach under study : L – y correlation y = s(x) / [ s(x) + b(x) ] y Purity = 90%; Efficiency = 27% Niccolò Cottini

21. An alternative approach under study: neutrino events Real data: 56.4 days live time Only L;L + y Under study PRELIMINARY Preliminary result: 1.1 n / day in data; from MC:  90% purity, 1° angular resolution * assumed 20% uncertainty on the absolute neutrino flux Niccolò Cottini

22. Conclusion Analysis of ANTARES 5 line data in progress • Present results • Good agreement in MC and data down going flux • 107 n in 169 days of 5 line data • Still room for improvement • an alternative study leads to 1.1 n / day, with 90% purity • 10 lines data now available • 12 lines soon available! • Search for neutrino cosmic sources started… Niccolò Cottini

23. BACKUP: track equation Time resolution: ~2 ns Position resolution: ~10 cm Niccolò Cottini

24. BACKUP: reconstruction algorithm details (1) • Step 1: linear prefit with coincidences and large amplitude hits • Step 2: minimization of the M-estimator formula • Ai = charge, ri = time residual, fang = angular factor, K=0.05 (from MC simulation) Niccolò Cottini

25. BACKUP: reconstruction algorithm details (2) • Step 3: maximum likelihood fit with simple time residual PDF P( event | track ) = prodi P( ti | tith ) Niccolò Cottini

26. BACKUP: reconstruction algorithm details (3) • Step 4: maximum likelihood fit with full PDF (charge bins) Distributions obtained from MC muons 100 GeV < E < 100 TeV Niccolò Cottini

27. BACKUP: direct hits Data, m MC, n MC Niccolò Cottini

28. BACKUP: last in time photon m MC Niccolò Cottini