The fifth shot of a tau neutrino

http://t.co/urnbKwoiSY by @ulaulaman about #neutrino #tau #Opera #particlephysics

From the press release:

The OPERA (Oscillation Project with Emulsion-tRacking Apparatus) international experiment at the National Institute for Nuclear Physics (INFN) Gran Sasso Laboratory (Italy) has detected the fifth occurrence of a tau neutrino. The neutrino started its flight at CERN as muon neutrino and, after traveling 730 km through the Earth, it arrived at Gran Sasso Laboratories showing up as a tau neutrino. This important result was announced yesterday during a seminar held at the Gran Sasso Laboratories. According to the Spokesperson of the international research team, Giovanni De Lellis, from Federico II University and INFN in Naples, "The detection of a fifth tau neutrino is extremely important: the direct observation of the transition from muon to tau neutrinos has now achieved for the first time the 5 sigma statistical precision, the usual particle physics threshold for a discovery. We can thus definitely report the discovery of the appearance of tau neutrinos in a muon neutrino beam." The detection of tau neutrinos from the oscillation of muon neutrinos was the motivation of the OPERA project, designed in the late nineties. "This task is extremely difficult due to two conflicting requirements: a huge, massive detector and a micrometric accuracy. The challenge is to bring to the thousands ton scale a detector based on the nuclear emulsion technology, a photographic technique unique in ensuring the required accuracy", De Lellis says.

The tau neutrino was discovered in july 2000 by DONUT collaboration (arXiv). It is produced in the tau decay, where tau is a lepton, an elementary particle with a negative electric charge and spin 1/2 and with a mass of 1776.82 ± 0.16 MeV: with a great simplification we can say that tau is an electron with a very big mass!
Now, first of all I share the paper about the fourth observation:
Observation of tau neutrino appearance in the CNGS beam with the OPERA experiment

The OPERA experiment is searching for $\nu_\mu \rightarrow \nu_\tau$ oscillations in appearance mode, i.e., via the direct detection of $\tau$ leptons in $\nu_\tau$ charged-current interactions. The evidence of $\nu_\mu \rightarrow \nu_\tau$ appearance has been previously reported with three $\nu_\tau$ candidate events using a sub-sample of data from the 2008–2012 runs. We report here a fourth $\nu_\tau$ candidate event, with the $\tau$ decaying into a hadron, found after adding the 2012 run events without any muon in the final state to the data sample. Given the number of analyzed events and the low background, $\nu_\mu \rightarrow \nu_\tau$ oscillations are established with a significance of $4.2 \sigma$.

and the last paper from OPERA collaboration: Limits on muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam

The OPERA experiment, exposed to the CERN to Gran Sasso $\nu_\mu$ beam, collected data from 2008 to 2012. Four oscillated $\nu_\tau$ Charged Current interaction candidates have been detected in appearance mode, which are consistent with $\nu_\mu \rightarrow \nu_\tau$ oscillations at the atmospheric $\Delta m^2$ within the "standard" three-neutrino framework. In this paper, the OPERA $\nu_\tau$ appearance results are used to derive limits on the mixing parameters of a massive sterile neutrino.

About neutrino oscillations, I remember an old post.

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DONUT Collaboration (2001). Observation of Tau Neutrino Interactions, Physics Letters B, 504 (3) 218-224. DOI: 10.1016/S0370-2693(01)00307-0
OPERA Collaboration (2014). Observation of tau neutrino appearance in the CNGS beam with the OPERA experiment, Progress of Theoretical and Experimental Physics, 101C01 DOI: 10.1093/ptep/ptu132