Giorgio Gratta: The EXO Program and Results
The field of neutrinoless double beta decay has been revived in the recent past with new results. EXO is one of the main experiments investigating this process.
The speaker started discussing why Xenon is so much better than other materials for this field of research. One does not need to grow crystals. Xe can be re-purified during the experiment, there are no long-lived isotopes to activate. It can be easily transferred from one detector to another. The enrichment in 136Xe is easy and safe: it is a noble gas (so no chemistry is involved), a centrifuge is cheaper for it.
EXO-200 is a TPC, two halves reading ionization and scintillation, each with anode wires for charge collection and avalanche photodiodes for light collection. Everything is built in low-activity material (a copper vessel), 1.5 mm thin to minimize materials. Avalanche photodiodes are ideal for this application because they are clean and lightweight, and sensitive in the VUV region.
The vessel is surrounded by lead, and a cosmic-ray veto. It is placed in a salt mine, a underground location 655m underground. 1600m water equivalent. Low levels of Radon because this is a salt mine. In Carlsbad, New Mexico.
By doing a linear combination of ionization and scintillation signals one improves the resolution on the signal. They measure the Radon content with the amount of ionization and scintillation of the beta and alpha decay signals. The signal can be fit as a function of time after a fill. This has an exponential slope and then a constant part.
They observe no signal in the region of interest, but this is also good news because the region is very background-clean. One expects 60 counts in two years of operation in 140kg of liquid xenon.
Slide 26 has the fit much above the data, begs a question – there seems to be a deficit of observed events with respect to the fit. The speaker after the talk said that the fit chisquare is good, but it looks suspicious to have a deficit right in the region around the signal box, which is the data most relevant for placing a limit. I will paste the relevant figure here later….
From a profile likelihood they get a limit on the half-life of >1.6*10^25 years, at 90% CL. The fit looks funny in the distribution where they extract the limit, with backgrounds overestimated.
This has been published in PRL already. In the future, they want to analyze a three-times larger dataset already collected. A better analysis and reconstruction is in progress. They want to install a Radon suppression system in the air around the cryostat. An electronics upgrade is under discussion. They are approved to run until the end of 2014, but probably will go on for two more years after that.
Th e present limit (an envelope of limits) is above the band for the inverted hierarchy scenario, but is expected to get into the band in the future, with the nEXO upgrade.
In summary, EXO-200 took data since June 2011. Discovered the double beta decay with neutrinos in 136Xe in the first weeks of operation. Now searching for the neutrinoless mode.