Reconstruction and analysis of ANTARES 5 line data

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Reconstruction and analysis of ANTARES 5 line data

• Niccolò Cottini

on behalf of the ANTARES Collaboration
XXth Rencontres de Blois 21 / 05 / 2008
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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

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The ANTARES detector
60 m
450 m
-2500 m
12 lines, 900 PMTs
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The detected signal

• Muon (Cherenkov)
• 2 µs crossing time
• 40K decay
• Continuous background ? 30 kHz

Simulation
2 min

• Bioluminescence
• Continuous background ? 30 kHz
• Occasional bursts MHz

PMT 10, threshold 0,3 p.e.
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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

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The physics signal
Muon flux at the detector
Keep all the muon tracks then filter to extract
the neutrino signal
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5 line data analyzed
169 days
29 Jan 2007
4 Dec 2007
310 days
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Event reconstruction
Reconstruction of m trajectoryfrom time, charge
and position of PMT hits
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Event reconstruction
z-t event display
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Event reconstruction
Atmospheric muon bundle
Complicated pattern a bundle can be
misreconstructed as an upward going track
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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
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Reconstruction quality (MC simulation with 12
lines detector)
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5 line data reconstruction
Dominated by the tail of atmospheric muons badly
reconstructed as upward going
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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)
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An alternative approach under study

• Likelihood ratio method
• 4 discriminative variables
• cos(q)

Data, m MC, n MC
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An alternative approach under study

• Likelihood ratio method
• 4 discriminative variables
• cos(q)
• q of the prefit

Data, m MC, n MC
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An alternative approach under study

• Likelihood ratio method
• 4 discriminative variables
• cos(q)
• q of the prefit
• Fraction of direct hits (time reslt3 ns)

Data, m MC, n MC
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An alternative approach under study

• Likelihood ratio method
• 4 discriminative variables
• cos(q)
• q of the prefit
• Fraction of direct hits (time reslt3 ns)
• Distance covered by the last in time photon

Data, m MC, n MC
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An alternative approach under study

• Likelihood ratio method
• 4 discriminative variables
• cos(q)
• q of the prefit
• Fraction of direct hits (time reslt3 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)
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An alternative approach under study L y
correlation
y s(x) / s(x) b(x)
y
Purity 90 Efficiency 27
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An alternative approach under study neutrino
events
Real data 56.4 days live time
Only L L y
Under study PRELIMINARY
Data n MC (Bartol flux) m MC (arXiv0802.0562)
62 8 (stat) 95 3 (stat) 18 (theor)
Preliminary result 1.1 n / day in data from MC
? 90 purity, ? 1 angular resolution
assumed 20 uncertainty on the absolute
neutrino flux
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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

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BACKUP track equation
Time resolution 2 ns
Position resolution 10 cm
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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, K0.05 (from MC simulation)
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BACKUP reconstruction algorithm details (2)

• Step 3 maximum likelihood fit with simple time
  residual PDF

P( event track ) prodi P( ti tith )
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BACKUP reconstruction algorithm details (3)

• Step 4 maximum likelihood fit with full PDF
  (charge bins)

Distributions obtained from MC muons 100 GeV lt E
lt 100 TeV
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BACKUP direct hits
Data, m MC, n MC
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BACKUP last in time photon
m MC