# Neutrino Physics

The neutrino group at IKP-2 was formed thanks to the recruitment initiative of the Helmholtz Association in November 2015. The group is specialized in the low-energy neutrino physics based on the liquid scintillator detection technique with the major involvement in the the Borexino (Italy) and JUNO (China) experiments. The scientific gaols cover a broad spectrum of arguments, including fundamental questions of neutrinos physics (neutrino oscillations, neutrino mass ordering) as well as approaches that use neutrinos as unique messengers about astrophysical objects, including the Earth and the Sun. The group is specialised in the data analysis and event reconstruction techniques and it is involved in the construction of the OSIRIS detector.

### Borexino

Borexino is a 280-ton liquid scintillator detector placed at the Laboratori Nazionali del Gran Sasso (LNGS), the largest underground laboratory in the world. Coverage by the 1400 m thick rock provides a cosmic-ray-flux reduction by one million times. The experiment is contained in a stainless steel dome of 18 m in diameter and consists of the Outer Detector (OD) and the Inner Detector (ID). The OD serves as a shield against external background and as a Cherenkov veto for cosmogenic muons. It is filled with 2400 tons of ultra pure water and is equipped with 208 PMTs. The ID consists of a stainless steel sphere with 2200 PMTs installed inside and two nested nylon vessels. It is filled with 1040 tons of shielding liquid outside and 280 tons of liquid scintillator inside the inner nylon vessel. The liquid scintillator is the radio-purest liquid in the world.

The primary goal of the Borexino detector is the measurement of solar neutrinos from the pp-fusion cycle that powers the sun and the discovery of neutrinos coming from the CNO-cycle. In addition, Borexino is one of the two detectors in the world that can measure geoneutrinos, which can be used as a tool to study the Earth's mantle. The other physics goals of the detector include the study of: muons, neutrino magnetic moment, diffuse supernova background, non-standard interactions, double beta decay searches, and atmospheric neutrinos.

### JUNO

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose, liquid scintillator-based neutrino experiment, proposed in 2008 to determine the Neutrino Mass Hierarchy (MH) by detecting reactor antineutrinos. The detection mechanism exploits the inverse beta-decay (IBD) reaction, where an antineutrino interacts with a proton of the scintillator and generates a positron and a neutron. The JUNO complex is currently under construction in China, with a rock overburden above the experimental hall of around 700 m, and is located 53 km away from both Yangjiang and Taishan nuclear power plants.

The scintillation light produced in the central detector is collected by more than 40.000 photo-multiplier tubes (PMTs) installed on a spherical structure with 20m radius. The central scintillator detector is surrounded by a cylindrical water pool, designed to detect the Cherenkov light from atmospheric muons and to shield against the environmental radioactivity, acting as a veto detector. On top of the water pool there is another muon detector, made of scintillating strips, which has the role to accurately identify the muon tracks.

Additionally, the reactor antineutrino flux can be exploited for a measurement of the solar oscillation parameters Θ12 and Δm212 with a sub-percent accuracy, which would represent the most precise measurement in the neutrino solar sector. Supernovae neutrinos can also be observed in case of a stellar explosion, inferring important information on the source. The fine energy resolution can also be exploited to observe solar neutrinos by means of elastic scattering on electrons. Other sources potentially accessible to JUNO are geoneutrinos, generated in radioactive decays inside the Earth, and atmospheric neutrinos, produced after cosmic-ray interactions in the atmosphere. Exotic searches include non-standard interactions, sterile neutrinos, proton decay, and dark matter.

### OSIRIS

The purpose of the Online Scintillator Internal Radioactivity Investigation System (OSIRIS) is to monitor whether the LS meets these requirements during the whole process of filling the 20 kton of LS into the JUNO central detector. OSIRIS can contain 20 tonnes of LS and the samples from each purification batch will continuously flow throuh it.

The principle of measuring the levels of 238U and 232Th contamination lies in identification of the fast time-coincident decays of the isotope pairs 214Bi-214Po and 212Bi-212Po, respectively. In addition, OSIRIS will be able to measure the 14C concentration down to a 14C/12C ratio of 10-17 at 90% confidence level.

The OSIRIS detector design, consisting of two optically separated vessels, is shown in the figure above. The inner vessel is an acrylic cylinder with a diameter and height of 3m each. It holds the scintillator and is observed by 64 20''-PMTs. These PMTs are held by a steel frame which is fully contained in the outer vessel. Due to the purpose of OSIRIS to constantly monitor the LS quality, it has an inlet on the top and an outlet in the bottom of the inner vessel.

The outer vessel is a stainless steel cylinder filled with water, which serves as a buffer volume to shield against external radioactivity from the surrounding rock. It is equipped with additional 12 20''-PMTs that will detect the Cherenkov light from cosmogenic muons.

The readout design of the PMTs will use a novel approach. To maximize the quality of the signal, the readout electronics as well as the digitizer is placed directly inside the base of the PMT. This enables better impedance matching of the PMT base and therefore a higher quality of the signal. OSIRIS will run a trigger-less readout scheme. The waveforms from individual PMTs will be sent, together with a synchronized time stamp, to the outside DAQ system. Here, a dedicated software will sort the waveforms and build the "offline trigger". The IKP group is involved in the development of the latter.

# Local contact persons in IKP-2:



Prof. Dr. Livia Ludhova