Gratta Group History

In 1995-2000 we built and operated an neutrino oscillation experiment at the Palo Verde Nuclear Generating station near Phoenix, AZ. This experiment, done with a collaboration of about 15 colleagues from University of Alabama, Arizona State, Caltech and Stanford, did not see neutrino oscillations but set what remained for 13 years the second best limit on the neutrino mixing angle theta_13. Only a dozen years later did new experiments at reactors and accelerators improve our measurements.

In 1998 we contributed to the design of the KamLAND detector in Japan and initiated the US component of the KamLAND collaboration. The KamLAND detector started taking data in 2002 and made the first observation of anti-neutrino oscillations with a terrestrial source, providing the best measurement of the 1-2 mass splitting. The first data on this subject made the cover of Phys Rev Lett. Eventually KamLAND reported data where more than one full cycle of oscillation is clearly visible. The progression of oscillation experiment using reactors can be appreciated in Reviews of Modern Physics.

With larger statistics KamLAND also made the first observation of antineutrinos from the Earth's interior. This is the first case of "applied neutrino physics" and was the subject of the PhD thesis of one of our students, winning the APS nuclear physics dissertation prize.

Between 2002 and 2010 we have also developed a new technique for detecting ultra-high energy cosmic-ray neutrinos from the acoustic pulse they produce when showering in water.

Since 2000 we have been planning a very large double-beta decay experiment using 136Xe as a source and detection medium. Our early R&D on this subject discovered that energy resolution in liquid Xe can be substantially improved by using at the same time ionization signals and scintillation. This technique is now used by our EXO-200 detector and several other groups using liquid Xe for Dark Matter searches. The EXO-200 detector is a 200kg enriched Xe double beta detector that has been taking data since June 1, 2011. At the time of turn on EXO-200 was the largest double-beta decay detector in the world and it immediately discovered the 2-neutrino double-beta decay in 136Xe.

With more data and a better analysis EXO-200 has measured the half life of the two neutrino decay with the smallest uncertainty among all two neutrino decays. At 2.165x1021 years this is the slowest process ever directly measured in our universe (Phys Rev C 89, 015502 (2014)). EXO-200 has not (yet) detected the much more important neutrino-less double beta decay, but it has performed one of the most sensitive searches to date (Nature 510 (2014) 229), achieving a sensitivity of 1.9x1025 years; this is about 1015 times the age of the Universe!

In 2014 our new program to measure gravity at micron scale started producing results (Phys Rev Lett 113 (2014) 251801) -- for the time being on the absence of particles with tiny fractional charges. The bound obtained is several times better compared to previous searches.

Site maintenance: Jacopo Dalmasson.
Design: Giulio Gratta