Because of their weak interaction with other matter, detecting neutrinos is exceptionally challenging. On rare occasions, however, a neutrino will collide with a water molecule, creating a cascade of particles that emits a blue light known as Cerenkov radiation. Instruments such as KM3NeT are designed to detect this faint glow deep underwater.
The KM3NeT (Kilometer Cube Neutrino Telescope) is a vast neutrino observatory situated beneath the Mediterranean Sea. Its detection capabilities rely on spotting neutrino interactions in the water. One component of this array, known as ORCA (Oscillation Research with Cosmics in the Abyss), played a central role in the latest research. Located near Toulon, France, ORCA operates at a depth of approximately 2,450 meters.
However, merely tracking neutrinos is not sufficient to explore the nuances of quantum gravity. Researchers are also on the lookout for a subtle signature known as "decoherence."
Neutrinos are known to undergo flavor oscillations as they travel, a process in which they switch between different types, or "flavors." This behavior is driven by quantum superposition, where a neutrino exists simultaneously in a mix of three mass states. Coherence maintains the stability of this superposition, allowing oscillations to proceed predictably. Theories of quantum gravity suggest that interactions with the environment might disturb this coherence, leading to a phenomenon termed decoherence, which could dampen or halt these oscillations.
"There are several theories of quantum gravity which somehow predict this effect because they say that the neutrino is not an isolated system. It can interact with the environment," explains Nadja Lessing, a physicist at the Instituto de Fisica Corpuscular of the University of Valencia and the corresponding author of the study, conducted by a global consortium of scientists.
"From the experimental point of view, we know the signal of this would be seeing neutrino oscillations suppressed." This suppression would indicate that during their journey through space-and to the KM3NeT sensors in the Mediterranean depths-neutrinos might have interacted with their surroundings in a way that disrupted their oscillations.
In the analysis conducted by Lessing and her team, no evidence of decoherence was observed in the neutrinos detected by KM3NeT/ORCA. Nonetheless, this outcome holds significance for the field.
"This," explains Nadja Lessing, "means that if quantum gravity alters neutrino oscillations, it does so with an intensity below the current sensitivity limits." The study succeeded in establishing tighter upper bounds on potential decoherence effects, surpassing constraints from prior atmospheric neutrino observations. It also points to promising paths for future investigations.
"Finding neutrino decoherence would be a big thing," says Lessing. To date, no direct observation of quantum gravity has been made, prompting heightened focus on neutrino experiments as a potential gateway. "There has been a growing interest in this topic. People researching quantum gravity are just very interested in this because you probably couldn't explain decoherence with something else."
Research Report:Search for quantum decoherence in neutrino oscillations with six detection units of KM3NeT/ORCA
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