Why is there more matter than anti-matter in Universe?

This question and several other physics questions, like what is the mass of the neutrino, are the ultimate goals to answer for one of the largest physics experiments in the world [1, 2].

The GERDA detector is composed of a 590 m³ tank that holds highly purified water. Inside the water tank is another tank covered with copper plates that holds 63 m³ of liquid argon at a temperature of -186 °C. Inside the liquid argon there are 7 strings with in total 37 germanium crystals weighing about 1 kg each (Fig.1). The whole detector is covered by 1700 meters of rock inside the mountain under the highest peak in the Apennine mountains, Corno Grande.

A new publication

A new article in Science magazine [link] summarizes the finding obtained in the GERDA experiment that started taking data in November 2011 for Phase I and ended Phase II in 2018. The key results are that:

• The collaboration succeeded in constructing a "background-free experiment" which was described in a recent article in Nature [link]. Due to a combination of selecting radiopure materials and making intelligent data-analysis algorithms, the background in the region of interest around 2039 keV could be reduced to essentially zero.
• All past claims of detecting the so-called neutrinoless double beta decay of ⁷⁶Ge can be refuted.
• The lower limit for the half-life of the neutrinoless double beta decay in ⁷⁶Ge is 10²⁶ years.
• The upper limit for the mass of the elusive neutrino is now 0.16 eV.
• The 2-neutrino double beta decay in ⁷⁶Ge was measured with unprecedented accuracy. Its half-life is 1.84×10²¹ years.

JRC contribution

JRC has played an important role for the conception of the GERDA detector since it joined as partner in 2005. All 40 germanium crystals used in the GERDA detector have been tested and characterised in the underground facility HADES of SCK·CEN, in which JRC-Geel operates a laboratory for ultra low-level gamma-ray spectrometry and detector testing (Fig. 2). In addition, JRC-Geel has been vital for performing so-called radiopurity measurements of a vast amount of materials such as electronics, cables, grease, metals etc. used in the GERDA detector. Natural radioactivity is present everywhere albeit in small amounts. For example, the human body has about 100 Bq per kg of natural radioactivity. But even 1 Bq (one decay per second) is a lot for the GERDA experiment. The present limit to the half-life for the neutrinoless double-beta decay of ⁷⁶Ge corresponds to 53 micro-Bq. This is about one million times less than the natural radioactivity in your hand.

The future

The technical solutions made by the GERDA team consisting of scientists from Europe – mainly Germany, Italy and Russia - have proved to be highly successful. This has triggered all other scientists looking for ⁷⁶Ge neutrinoless double beta decay to stop their work on alternative methods and merge with the GERDA team in a world-wide collaboration. This collaboration, called LEGEND [http://legend-exp.org/], plans in the first phase to expand the existing GERDA detector by adding more enriched germanium detectors and try to reach a mass of 200 kg. Requests have been to JRC made to have the new detectors tested in HADES.