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Giovanni De Lellis: Opera Results

March 12, 2013

The “missing tile” in a landscape of oscillation measurements at the start of the XXIth century was the tau neutrino appearance in a muon neutrino beam. This could be investigated by a CERN beam directed to the Gran Sasso mine, 730 km away. The CNGS neutrino beam originates from a 400 GeV proton beam with neutrinos from pion decay having a mean energy of 17 GeV. The beam is directed to the Gran Sasso underground laboratory, but contrary to common belief (Italian readers will understand this pun better than foreigners – it was Mariastella Gelmini, Italian Minister from the Berlusconi government, who in 2011 claimed that we had built such a thing to have neutrinos arrive at us from CERN) it does not travel through a tunnel, but instead through rock of the Earth crust.

ImageOpera has been taking data since 2008, with a total of 1.8*10^20 protons on target. This is 20% less than the value expected at the time of experiment proposal. Data taking with beam has been just concluded at the end of 2012 with the shutdown of the CERN machine.

The principle of detection is a massive target of 1.25 kilotons of bricks of lead with emulsions interleaved. There is a tracker that allows the extrapolation of tracks to the primary vertex, identifying the brick which should contain the fine-grained information on the primary (and secondary, if present) vertex. Tau leptons generated by charged-current interactions in the lead bricks are in fact identified by their secondary vertex and its characteristics.

There are as usual two topologies: the short topology has a primary interaction and a tau decay point both within the same lead brick. A long topology has the two vertices in different bricks.

The detector is made of two equal parts, supermodules. Two targets alternated with muon spectrometers. The target is made by 150,000 bricks. One brick is 10 radiation lengths thick. In each brick there are 60 emulsion sheets, for a total of 9 million: the largest emulsion production that was ever done in HEP.

Reconstruction of vertices is three-dimensional and one can very clearly see decays from charm, for instance, with micrometric accuracy. Charm production is a benchmark, since charm hadrons and tau have similar lifetime. They have collected 50 charmed hadron candidates in the 2008-2010 data taking period, and with them they can study agreement with simulations and validate reconstruction efficiencies and kinematics. A total of 6000 reconstructed interactions have been recorded overall.

They have 19 electron neutrino candidates, whose energy distribution is perfectly consistent with the expected beam contamination; a very small signal of oscillated ones might be present at low energy. This allows to place limits on the non-standard oscillations at large dm^2 values. I found it strange that Opera chooses a bayesian analysis and places limits in a two-dimensional plot overlaid with results of other experiments that have been derived using frequentist methodologies: the bayesian technique should at the very least be shown to have robust coverage properties, but I fear that is not the case… Later I will paste here the figure.

Giovanni then showed very nice and animated event displays of the first tau neutrino candidate events. The first is a muonless event taken on August 22nd 2009. A second tau neutrino candidate was taken on April 23rd 2011. This one shows two tracks pointing to the primary vertex, one of which, the tau, decays to three prongs, one of which produces a hadronic interaction downstream. It is crucial, in order to make backgrounds negligible, to assess the muon-less nature of the event. The decay happens in the plastic base, so there is no nuclear fragment. It is a low-density material, so it suppresses backgrounds a lot. The flight length is of 1.54 millimeters. The primary track has 2.8 GeV of momentum and is incompatible with being a muon, since the track is absorbed downstream. Two of the decay tracks are also found to be hadrons as expected.

Since it is very important to identify the muon-less property of primary tracks, and residual backgrounds are muons from charm, emitted at large angles where they cannot be identified, an intensive study was performed to extend the angular acceptance of the detector. The procedure is now well established. They have thus improved their results and they will publish them very soon.

To conclude, Giovanni mentioned that Opera collected data for five years, getting 1.8E20 protons-on-target worth of neutrino flux. The data analysis for 2010-2012 is ongoing. They found two tau lepton candidates; they also found 19 electron neutrino candidates, and exclude a non-standard oscillation with 6 events observed with energy below 30 GeV where 9.4+-1.3 are expected, placing an upper limit of 7.2E-3 at 90% confidence level on sin^2(2 theta_new).

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