CEA DSM Irfu
Reconstruction and analysis of ANTARES 5 line data
Niccolò Cottini
on behalf of the ANTARES Collaboration
XXth Rencontres de Blois
21 / 05 / 2008
21 / 05 / 2008 Niccolò Cottini 2CEA DSM Irfu
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
21 / 05 / 2008 Niccolò Cottini 3CEA DSM Irfu
The ANTARES detector
12 lines, 900 PMTs-2500 m
450 m
~60 m
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The detected signal
• Muon (Cherenkov)– 2 µs crossing time
• 40K decay– Continuous background
30 kHz *
• Bioluminescence– Continuous background
30 kHz *– Occasional bursts ~MHz
Simulation
2 min
* PMT 10’’, threshold 0,3 p.e.
21 / 05 / 2008 Niccolò Cottini 5CEA DSM Irfu
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
21 / 05 / 2008 Niccolò Cottini 6CEA DSM Irfu
The physics signal
Keep all the muon tracks;
then filter to extract the neutrino signal
Muon flux at the detector
Atmosphere
Earth
Sea
107 atmospheric per year3000 atmospheric
per year
Cosmic
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5 line data analyzed
169 days
29 Jan 2007 4 Dec 2007310 days
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43°
interaction
Sea floor
Cherenkov light from
3D PMTarray
Event reconstruction
Reconstruction of trajectoryfrom time, charge and position of PMT hits
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Event reconstruction
z-t event display
z
y
x
21 / 05 / 2008 Niccolò Cottini 10CEA DSM Irfu
Event reconstruction
Complicated pattern; a bundle can be misreconstructed as an upward going track
Atmospheric muon bundle
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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 2 minimization (charge weights)
3) Time residual pdf (charge weights)
Direct Cherenkov emission
Diffused light
Electronics effect
Local minima possible: reiteration with 8 different starting points (prefit rotations)
21 / 05 / 2008 Niccolò Cottini 12CEA DSM Irfu
Reconstruction quality (MC simulation with 12 lines detector)
21 / 05 / 2008 Niccolò Cottini 13CEA DSM Irfu
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: cut
107 neutrino events
90% purity, 0.6 / day
169 days live time
0.17
(~7 reconstructed / day)
Preliminary
21 / 05 / 2008 Niccolò Cottini 15CEA DSM Irfu
An alternative approach under study
Likelihood ratio method
4 discriminative variables:
• cos()
Data, MC, MC
21 / 05 / 2008 Niccolò Cottini 16CEA DSM Irfu
An alternative approach under study
Likelihood ratio method
4 discriminative variables:
• cos()
• of the prefit
Data, MC, MC
21 / 05 / 2008 Niccolò Cottini 17CEA DSM Irfu
An alternative approach under study
Likelihood ratio method
4 discriminative variables:
• cos()
• of the prefit
• Fraction of direct hits (time res<3 ns)
Data, MC, MC
21 / 05 / 2008 Niccolò Cottini 18CEA DSM Irfu
An alternative approach under study
Likelihood ratio method
4 discriminative variables:
• cos()
• of the prefit
• Fraction of direct hits (time res<3 ns)
• Distance covered by the last in time photon
Data, MC, MC
21 / 05 / 2008 Niccolò Cottini 19CEA DSM Irfu
s(x) = pdf of MC atmospheric neutrinos
b(x) = pdf of MC atmospheric muons
y = s(x) / [ s(x) + b(x) ]
An alternative approach under study
Likelihood ratio method
4 discriminative variables:
• cos()
• of the prefit
• Fraction of direct hits (time res<3 ns)
• Distance covered by the last in time photon
Data, MC, MC (x100)
y
21 / 05 / 2008 Niccolò Cottini 20CEA DSM Irfu
yyData, MC, MC (x100)
y
An alternative approach under study – y correlation
Purity = 90%; Efficiency = 27%
y = s(x) / [ s(x) + b(x) ]
21 / 05 / 2008 Niccolò Cottini 21CEA DSM Irfu
An alternative approach under study: neutrino events
Only + y
Under study PRELIMINARY
Data MC (Bartol flux)
MC (arXiv:0802.0562)
62 ± 8 (stat) 95 ± 3 (stat) ± 18 (theor)*
* assumed 20% uncertainty on the absolute neutrino flux
Preliminary result:
1.1 / day in data; from MC: 90% purity, 1° angular resolution
Real data: 56.4 days live time
21 / 05 / 2008 Niccolò Cottini 22CEA DSM Irfu
Conclusion
Analysis of ANTARES 5 line data in progress– Present results
• Good agreement in MC and data down going flux • 107 in 169 days of 5 line data
– Still room for improvement• an alternative study leads to 1.1 / 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
21 / 05 / 2008 Niccolò Cottini 24CEA DSM Irfu
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)
21 / 05 / 2008 Niccolò Cottini 25CEA DSM Irfu
• Step 3: maximum likelihood fit with simple time residual PDF
P( event | track ) = prodi P( ti | tith )
BACKUP: reconstruction algorithm details (2)
21 / 05 / 2008 Niccolò Cottini 26CEA DSM Irfu
)NN).t(P(N
1)t(P bkgsigsig
tot
• Step 4: maximum likelihood fit with full PDF (charge bins)
Distributions obtained from MC muons 100 GeV < E < 100 TeV
BACKUP: reconstruction algorithm details (3)
21 / 05 / 2008 Niccolò Cottini 27CEA DSM Irfu
BACKUP: direct hits
Data, MC, MC
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BACKUP: last in time photon
MC