Paschal Coyle: Antares
The Antares collaboration searches for astrophysical sources of neutrinos, as well as for new physics such as dark matter or magnetic monopoles. Besides fundamental physics and astrophysics, there are however a few additional things that are peculiar of the unique environment in which the detector operates. The site allows studies of oceanography, sea biology, seismology, environmental monitoring.
The collaboration is composed by 150 scientists and engineers from 31 institutes in 7 European countries. The detector is located 40 km south of the Mediterranean coast of southern France, off Toulon. At that location they have an eccellent view of the center of our galaxy by looking at upward-going neutrinos.
In the sea, the angular resolution is better than in ice, and can reach 0.3 to 0.1 degrees for the direction of a shower. The detector consists of 12 lines of photomultiplier tubes, 885 in each. The last line of PMT detectors was installed in May 2008. Recently a new junction box was connected to the infrastructure: this is dedicated to sea science projects.
At this point Coyle made a sort of Freudian slip when he showed a graph with green and black or red dots, each representing a different PMT. He said “90% of them are STILL… 90% of them are giving data”. It begged the question, what is lifetime of PMTs in Antares ? However unfortunately I could not ask it, since there was very little time for question after the talk.
The speaker then explained that a difficulty for the experiment with respect to ice arrays is that of course the detector is not stable: it moves around in sea currents; since however they need to know where it is with high accuracy to avoid spoiling the angular resolution, they have a positioning system which every 2 minutes measures inclination and rotation of hte arrays.
The device can measure attenuation length precisely with multi-wavelength beacons that fire signals which the arrays record.
Track reconstruction is performed with a maximum likelihood fit to the muon hypothesis. Neutrinos and muons are distinguished by a “quality parameter” of the fit. One isolates the upward going neutrinos by reconstructing the cos(theta) of the track arrival.
Coyle then showed a nice histogram of counts acquired by looking in the direction of the Moon. Our satellite of course shadows downward-going muons, since cosmics are stopped by it. The graph showed a nice dip, underlying that their angular resolution is indeed what they claim. Unfortunately his talk is not online yet, so I cannot attach the relevant plot… Similarly, the angular resolution for upward going neutrinos is of about 0.5 degrees.
He then showed a nice sky map from a live time of 295 days. Events plotted were constituted for 60% by atmospheric neutrinos, for 40% by downgoing muons reconstructed as upgoing.
They did a candidate list search, with a list of 24 likely places in the sky from where one might observe neutrinos. They calculated the probability of the observed events from those sources. The most significant candidate is a galactic microquasar, whose probability of being generated by background sources alone is 6.8%, so not significant. Still interesting to watch.
Antares does not see any significant sources, so they set a limit on the flux at 10^-7 GeV cm^-2 s^-1, integrated in solid angle. However, they have not used an energy estimator in the analysis yet. Significant improvements in limit will be obtained once that is used.
They are also doing GRB-triggered searches: alerts from SWIFT satellite and from Fermi. >1300 have been recorded to Jan 2011. No signal is observed.
They can also do the opposite: if they see two neutrinos within 15 minutes and 3 degrees they send an alert within 10 seconds, to telescopes which look at 1.9×1.9 degrees of sky around the potential source. They use two 25 cm “Tarot” telescopes in France, plus another system called Rotse, four 45cm telescopes.
Coyle concluded his presentation by remarking that Antares is a major step in the direction of constructing a multi-km^3 array in deep sea. Let us hope we will see that built soon!