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Molecule for low diffusion TPCs for rare event searches

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Catching a cosmic Houdini in the act will shed new light on the enigmatic neutrino

Theoretically, every matter particle has a corresponding antimatter particle, yet we observe much more matter than antimatter. The hypothesis that the mysterious neutrino may be its own antineutrino addresses this asymmetry and may soon be supported by data thanks to enhanced detection capability.

Fundamental Research

Theories and experimental discoveries over the last century have led to remarkable insight into the (very few) fundamental building blocks of our Universe, the stuff of which everything is made. The Standard Model of particle physics, developed in the 1970s, is our most complete description of the particle world. However, the Standard Model has some significant and acknowledged gaps. With the support of the Marie Skłodowska-Curie Actions (MSCA), the MELODIC project paved the way to filling in some of those gaps with unique information about neutrinos, matter and antimatter, and the physics beyond the Standard Model.

Now you see it, now you don’t

Particles and their antiparticles have the same masses and equal but opposite charges, and annihilate each other when in proximity. This has interesting implications for the chargeless neutrino, and many now believe these enigmatic particles may act as both matter and antimatter. One of the best ways to study antineutrinos is to detect the occurrence of neutrinoless double beta decay (0νββ), until now only hypothesised to occur. Double-beta decay (2νββ), a rare event that has been observed in several isotopes, results in the emission of two electrons and two neutrinos. MSCA fellow Neus López March notes: “0νββ is a hypothetical process in which an atomic nucleus undergoes a radioactive decay with the emission of two electrons and no neutrinos. This can only happen if the neutrino is its own antineutrino and the two particles annihilate each other. The observation of this process could explain why there is more matter than antimatter in the Universe, one of the most important open questions in particle physics.”

The fog clears

López March set out to enhance the ability to detect 0νββ in a high-pressure xenon-gas time projection chamber (TPC) by reducing electron diffusion, thus enhancing resolution and improving background rejection. 0νββ can be detected by measuring the energy of the electrons released in the decay of the isotope xenon-136 to see if the total adds up to exactly 2.458 MeV, meaning that no energy was carried away by undetectable neutrinos. “We exploited xenon doped with helium to diminish diffusion, exploiting the fact that the electrons are better cooled via elastic collisions with the helium than with xenon. This prevents the point-ionisation trace from becoming a cloud, enhancing detection. Then, the use of an array of silicon photomultipliers provided topological information distinguishing between two-electron and single-electron events,” López March explains. Employing these principles, López March’s team of physicists and mechanical and electronic engineers designed, built and operated the NEXT-DEMO++ detector, a demonstrator for the Neutrino Experiment with a Xenon TPC at IFIC, all within the 2-year MSCA fellowship.

Ready for what comes NEXT

Preliminary results from the demonstrator confirmed that transverse diffusion is reduced by a factor of 3 when doping the xenon with 15 % helium. A publication is in progress. This enhancement could be just what we need to finally observe a double-neutrino vanishing act providing proof that neutrinos are also antineutrinos, forever altering the description of our particle Universe.

Keywords

MELODIC, neutrino, xenon, matter, antimatter, double beta decay, neutrinoless, electron, Standard Model, antineutrino, Neutrino Experiment with a Xenon TPC (NEXT)

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