Online Reconstruction used in
the Antares-Tarot alert system
•The online reconstruction
•concept
•Performance
•Neutrino doublets and high energy events
•DAQ data flow and implementation of alert system
Jürgen Brunner
Reconstruction concept
1.
Select “hot spots” at each detector string
should correspond to point of closest
approach of muon track and detector line
2. Add hits from neighboring floors in narrow time
window
3. Perform minimization of time/distance/amplitude related
function with all selected hit using a simplified geometry
 obtain track parameters
Purity of neutrino selection
• Plot 2 for all upward
•
•
going tracks  clear
double structure visible
Simple 2 cut allows to
choose high purity
sample
Good coverage of full
zenith angular range
Angular resolution
Atmospheric Neutrinos / Muons
Minimum 3-lines in reco
• Mirror solution in
•
•
azimuth do not
permit good pointing
accuracy if fit is done
on only two lines
Minimum 3-line fit
required
Brightest (HE) events
can be as good as
0.5º
1 degree
Median Angular resolution
Avoid explosion of alert rates
• Example:
– sparking OM
• Handful cases seen
• Monitored via trigger rate
•
•
of pure trigger
Typical rate 5Hz
Alert sending veto:
– ~ 20 Hz
• Online reco distinguishes
between tracks and
“Bright points”
Doublet search
Select 2 events coming from the same direction in a time windows of 15 min.
t0 + δt
t0
t0
At the end, we want to
send the alert position
with an error box of ~1°
→ angular resolution
of the 2 events is
around 1.5°
→ maximum angular
distance between the 2
events is around 3°
Doublet search
Estimation of background rate.
2 events coincidence rate:
R2
atm

   atm
 2
t  R1
 2 

2
1
R atm  300 yr Reconstruction on art least 3 lines
1
ΔΩ = 3° x 3°
Δt = 15 min
R
2 atm
 0.007 yr
1
More than 3-sigma effect in 1 year !
Not sufficient to obtain acceptable
number of fake alerts
Selection of High Energy Neutrinos
• Events from multi-line reconstruction are shown here
• Use only basic parameters no sophisticated energy fit
– Number of storeys used in fit
– Total amplitude of hits used in fit
• Exponential fall-off
• Allows smooth selection of cut values to adjust to a
desired fake-alert rate of 1-2 alerts per months
Number of storeys used in fit
Total amplitude of hits used in fit
Candidates for ToO
Candidates for ToO
Selection of HE events
• MC sample of
selected events
• Median neutrino
energy: 4 TeV
• Angular resolution :
0.75 degrees
• Several alerts
already sent
• Some images taken
Implementation
BBalert runs in shore station
- connects into data stream after data filter but before writing
to disk
- Turns in infinite loop (start-stop controlled by AlertControl)
- Needs less than 10msec to process one event
Data writer
Data
Filter
All data
ligier
BBalert
Data flow to onshore farm
ARS
ARS
ARS
ARS
ARS
ARS
time
off-shore
on shore
Ethernet switch
Trigger
Trigger
Trigger
Data flow to onshore farm
ARS
ARS
ARS
ARS
ARS
ARS
time
off-shore
on shore
Ethernet switch
Trigger
Trigger
Trigger
Data flow to onshore farm
ARS
ARS
ARS
ARS
ARS
ARS
time
off-shore
on shore
Ethernet switch
Trigger
Trigger
Trigger
Delay of alert sending
• Time slices of 100msec are treated by each
•
•
•
•
trigger node (unreducable delay)
Online reconstructions gets data from shared
memory of data filter farm output stream
10msec CPU to process one event
Up to 100Hz output rate data processed in real
time on a single computer
~1000 TimeSlices are buffered in each trigger
node
– Useful for excternal alerts (GRBs)
– Induces for ToO an avoidable delay of ~100sec
– Redesign under study
Implementation
BBpaper
Launch the
on-line
reconstruction
ANTARES data
Interactive
Webpage
Internet
Test messages
Monitoring
“I am alive”
GCN
format
Monitoring
Server
TAROT Chile
BBalert
Trigger
Reconstruction
ligier
Pointing telescope
+ data acquisition