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Neutrinos are among the elementary particles, which are the fundamental units of matter and do not fragment into smaller units. Along with electrons, they are categorized as leptons and are known as “ghost particles” because of their extremely low mass* and near-light-speed motion, which results in weak interactions and makes it challenging to detect their presence. Since their discovery, they have become a subject of study for scientists since, although being well known to exist and being extremely abundant in the universe (so much so that each of us is passed by millions of them every day), it is quite difficult to ascertain their features. For instance, their lack of electric charge is well recognized, although their mass is not precisely established. Neutrinos can interact with other leptons through the weak force in three different ways, or “flavors,” depending on the interaction. Some neutrinos associate with electrons (electron neutrinos), while others engage with muons (muonic neutrinos), and still others are connected to tau (tauonic neutrinos).

That is not all, though. Oscillation, a characteristic of these particles that allows them to change from one kind to another (or from one flavor to another) while they are moving through space, is a pretty unusual quality. High-energy neutrinos can potentially change into an electrically charged particle, such as an electron, a muon, or a tau, leaving a detectable and quantifiable trail. To be able to identify them and learn a little bit more about them, many people every day engage in their research and work across the globe. However, finding them requires very special experimental conditions due to their extreme elusiveness and transmutation by oscillation property. For example, the construction of subway detectors that avoid cosmic radiation contamination and the availability of large volumes of sensitive material that increases the probability of capturing visible signals despite their weak interaction Giving up in the face of these challenges is not an option because their research is crucial because it enables us to understand the interior structure of nucleons (protons and neutrons) and nuclei as well as supply us with critical information about weak interactions.

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Over the years, a number of techniques have been suggested to find these phantom particles. The most cutting-edge method is “Earth-skimming,” which makes use of detectors positioned on the ground in order to observe how these neutrinos interact with nucleons. Other methods for looking for astrophysical neutrinos involve atmospheric cascade detectors and subway devices (from sources outside the Earth). The High-Altitude Water Cherenkov Observatory, or HAWC for short, is situated in Mexico, more specifically in the state of Puebla, at a height of roughly 4,100 meters above sea level on the slopes of the Sierra Negra Volcano. Here, they are able to keep track of two thirds of the sky in depth to find gamma rays with high energy (GeV and TeV). On a surface of 22,000 square meters, 300 water-based Cherenkov detectors—devices that instantly detect visible light or UV photons and enable particle identification—make up the HAWC.

Each detector looks like a cylindrical steel structure that is at least 5 meters high and 7.3 meters in diameter, with a plastic bladder that is attached to the base and holds four photomultiplier tubes and a plastic bladder with a volume of about 200,000 liters of water. The tubes enable the observation of Cherenkov light, which is created when charged particles move across a water volume. Authors from the Institute of Physics of the UNAM made the suggestion to use the HAWC observatory to find neutrinos that graze the Earth in an article that was published in 2022 and was titled Characterization of the background for a neutrino search with the observatory. Data collected between June and October 2017 and January and May 2018 were used for this. In order to identify and prevent any sources of contamination that could affect their results, they standardized their tests. They limited their simulation and analysis to the region at the foot of the Pico de Orizaba Volcano because it offered the best protection from atmospheric muons (noise signals). This allowed them to measure the overall error rate at around 30% while completely controlling any potential random errors, which was an acceptable result given the straightforward model they used.

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Only two of the three types of neutrinos, muon and tauon neutrinos, could be researched using this technique, according to the authors of the article. Because neutrino-induced electrons quickly start electromagnetic showers after they are created, they are absorbed by the volcano and are consequently hard to find. While tau, despite having a shorter half-life compared to muons, have a survival probability that is correlated with the length of their decay, moons have a long half-life that allows them to avoid being absorbed by the mountain and produce a detectable trace within the HAWC matrix. Therefore, the likelihood of capturing them increases with their time to disintegrate. They hope that their work will be used in the future to develop more effective triggers and analysis techniques that can significantly increase the number of detections with experiments like theirs, despite the fact that the number of signals they can capture is limited (at least one neutrino-induced muon every two years on average). As we can see, this study is not only crucial for this area of physics, but it also marks a major accomplishment for Mexico because it proved for the first time that it is possible to detect neutrinos using a method that was previously thought to be impossible due to background radiation from space. This method was previously thought to be ineffective due to cosmic background noise.

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