The OPERA experiment, aiming for a better understanding of neutrino physics, has released its final results a couple of weeks ago. Five years of data taking, the observation of 10 events and an analysis that took 6 years have yielded an unambiguous proof that muon neutrinos can be converted into tau neutrinos when they propagate.
This experiment is also a first of its kind in terms of open science.
For this first time in particle physics, a non-LHC experiment has indeed publicly released its data through the CERN Open Data portal.
In this post, I will briefly discuss neutrino physics (that I have already covered at large here), before focus on OPERA and its new results recently published (open access) in Physical Review Letters.
An unrelated item (but not a song) is as usual hidden in this article.
NEUTRINOS IN A NUTSHELL
At the very beginning, there was not only light, but also the Standard Model of particle physics. This theoretical framework has been developed in the last century and allows for predicting how the elementary particles live, eat, drink and dance.
More seriously, the Standard model describes the dynamics of the elementary building blocks of matter. These building blocks consist in a handful of particles underlying the structure of all (visible) matter in the universe. We have 12 of these building blocks, and we focus in this post on three of them, the neutrinos.
Neutrinos date back from the early studies of radioactivity, that are today 100 years old.
In those studies, physicists were studying specific decays of given atomic nuclei into other atomic nuclei together with the emission of an extra electron.
With the tools and knowledge available at that time, it was calculated that the energy of the electron should have a unique value Q. However, data was showing that the electron energy could be anything between 0 and this Q value.
Some energy, i.e. the difference between Q and the observed electron energy, was thus missing. A couple of years later, Pauli postulated that the missing energy was carried away by an invisible particle, a neutrino. Further developments then lead to the unraveling of the weak force and to the fact that the masses of the neutrinos were all zero.
The story does mot end here, in particular because nature is funny and like to be slightly less simple than expected. Instead of a single neutrino, we have three copies, or flavors, of neutrinos: the electron, muon and tau neutrinos.
And things got calm until the discovery that neutrinos could oscillate into each other. An electron neutrino could hence turn into a muon or a tau neutrino when it travels. This has profound consequences, the most important one being that the neutrino masses are non-zero and that the Standard Model must be extended.
In the meantime, many experiments have been built to measure the properties of these oscillations and investigate them in their greatest details. OPERA is one of them.
THE OPERA EXPERIMENT AND ITS GOAL
There are many parameters connected to neutrino oscillations, and the understanding of this phenomenon requires an accurate measurement of all of them. The OPERA experiment was designed to measure, for the first time, the parameters connected to the appearance of a tau neutrino from the oscillation of a muon neutrino.
For this, we need to observe events where a muon neutrino transforms into a tau neutrino. Spoiler: OPERA observed actually 10 of those.
The only issue is that the probability a given neutrino switches to another species is small. Therefore, one needs to make them traveling over long distances to increase the chances.
In the OPERA experiment, one uses a muon neutrino beam prepared at CERN, in Switzerland. The neutrinos are then sent directly 730 kilometers away to Gran Sasso in Italy. At the speed of light, this consists of a trip of about 0.003 second.
During such a ‘long’ trip, the probability that some muon neutrinos turn into tau neutrinos is not negligible.
One can thus expect the presence of some tau neutrinos at Gran Sasso, that one still needs to detect in one way or another. Since neutrino are weakly interacting beasts, this is not trivial.
To this aim, OPERA uses 150.000 bricks made of lead and photographic films arranged in parallel walls. In addition, the apparatus includes a tracking system to identify the bricks in which something happened (so that they could be removed for film development), muon detectors (to detect the muons arising from neutrino decays) and some electronics.
The information collected by the bricks is extracted by developing the photo stored into them. Recalling that tau neutrino decays are standard and easily noticeable, one has thus everything to clearly identify the presence of a real tau neutrino.
The final results of the OPERA experiment are available in this open access paper.
The first important point is that OPERA observed a number of muon neutrinos smaller than the number of muon neutrinos sent by CERN. Some muon neutrinos therefore turned into other neutrino species.
This is not new, as this has already been observed by other experiments in the past.
After the analysis of all data recorded between 2008 and 2012, OPERA drew several conclusions.
First, muon neutrinos have been confirmed to primarily oscillate into tau neutrinos. This stemmed from the unambiguous observation of 10 muon-to-tau neutrino conversions.
By unambiguous, I quantitatively mean that the probability we have no signal and some background fluctuation instead is 0.0000000004. We need certainty to claim anything in physics ;)
And even if 10 events do not seem much, we can do things with that. For instance, the mass difference between the involved neutrinos has been measured (confirming older results by an independent mean), together with the tau neutrino interaction rate and other properties.
The OPERA experiment has measured unambiguously, for the first time, the oscillations of muons neutrinos produced at CERN into tau neutrinos detected at Gran Sasso in Italy.
Those measurements are very important for particle physics. Shedding light on neutrino physics is indeed crucial for physics beyond the Standard Model. One indeed needs to explain neutrino masses somehow, such a thing not manageable in the Standard Model.
On different grounds, the technology developed for OPERA has already numerous applications that range from dark matter searches (i.e. at the SHIP experiment) to applications to other STEM fields. Among those, we can quote glacier and volcano studies, cancer treatment or the mapping of the pyramids.
One should never underestimate the by product of the particle physics experiments ^^
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