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Physics

At the whim of neutrinos

Unicamp team uses fleeting particle to test an idea of Galileo

SUPER-KAMIOKANDESuper-Kamiokande, a neutrino detector in Hida, JapanSUPER-KAMIOKANDE

One of the most famous experiments attributed to Galileo Galilei, though possibly apocryphal, was carried out from the top of the Tower of Pisa. Two cannon balls of different weights were released at the same time to check whether they would have different acceleration as they moved toward the ground. The story per se may be no more than a folk tale, but the fact remains that the Italian scientist was the first to determine that gravity affects all objects equally, regardless of their mass. End of story?

Not at all. Four centuries later, Brazilian researchers decided to use all the most modern resources of quantum physics – the study of the fleeting particles known as neutrinos – to test the same phenomenon, but they are yet to reach a verdict.

First, what does this old finding of Galileo mean for present day physics? Summed up in what is conventionally referred to as the equivalence principle, it implies that inertial mass (the resistance of an object to changing its state of movement, rest or constant speed) and gravitational mass (which measures the intensity of the force of gravity over a body) are exactly the same. It seems obvious. However, not for the scientists who pore over this question. “Actually, it didn?t have to be like this,” says Marcelo Guzzo, a researcher at the State University of Campinas (Unicamp). “It is surprising it is.”

To explain to what extent this seems to be a mere coincidence, the scientist explains how electromagnetic force works. With a magnetic field, one can induce a particle with an electrical charge to move. If this particle is replaced by another with the same charge, but greater inertial mass, the acceleration caused by the magnetic field drops. This is how almost all forces of nature operate: inertial mass does not make the intensity of the force vary. The sole exception is gravity. Galileo’s experiment shows this is a primitive manner. However, would more rigorous and precise testing uphold his conclusion?

Guzzo and his collaborators decided to use the results of experiments with neutrinos – one of the hardest particles to detect, which produced within the Sun, distant celestial bodies and nuclear reactors – to test the equivalence principle. Neutrinos have only a minuscule amount of mass and no electrical charge. They only interact with the rest of the Universe by means of the weak nuclear force and of gravity, the weakest of the four forces of nature. Given their very small energy, it is a very subtle interaction.

To observe neutrinos, scientist build huge detectors in deep mines and fill them with very pure water and other materials, in the hope that a neutrino might crash into a particle within this system and produce a detectable reaction. In 2009, the Japanese Kamland experiment achieved an important finding: it incontestably confirmed the transformation of a type of neutrino into another, a phenomenon that physicists call flavor oscillation.

This is tied to a peculiar property of quantum mechanics: a particle has no defined specific state until it is measured by means of an interaction process. In practice, a neutrino can have three different flavors (electron, muon and tau) and oscillates all the time among them until it is detected. The Kamland results showed that depending on the distance between the detector and the source that releases the neutrinos, the proportion among the three flavors might vary.

Guzzo and his colleagues compared the measurements taken in the Japanese experiment and in others around the round with the theoretical estimates, in order to analyze the effect of gravity on neutrino oscillation. They found that there might indeed occur a violation of the equivalence principle. The probability of this, however, is ridiculously small. “Something lower than 1 for every 1,015 parts, a figure that only materializes after the fifteenth decimal,” states Guzzo.

The results, submitted to Physical Review D, suggest that up to the precision limit observed, the equivalence between inertial mass and gravitational mass holds true and that Galileo continues to be just as right as he was in the seventeenth century. However, one cannot state that this correspondence will be maintained up to the theoretical limits of measurability.

Mini black holes
The Unicamp group also used neutrino oscillation to test other elements of the cornerstones of physics. One of the studies involved the so-called quantum decoherence, a mechanism whereby a particle loses the characteristic of having all states at the same time and defines itself by one of them. Analyzing this process in the light of neutrino oscillation, one can identify whether something new or different influences the behavior of these particles.  The most interesting hypothesis is that the interaction with mini black holes in space causes this decoherence. A mini black hole is a quantum scale version of a large one. Whereas the latter is created when a star collapses, the former, it is believed, is generated in a particle-sized region and lasts for only fractions of a second before disappearing.

Scientists accept the idea that these odd cosmic phenomena might exist seriously, even though no concrete evidence of their existence has been found to date. Upon analyzing decoherence, Guzzo and his colleagues concluded that yes, these mini black holes might exist and influence neutrino behavior. However, if this is actually happening, “they can’t be very abundant,” says the Unicamp physicist. Moreover, the probability that they exist does not eliminate the possibility that the cause of neutrino decoherence may be something else, according to the study, published online in September in European Physical Journal C. “It might be a mini black hole,” he says, “or another unknown phenomenon.”

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