Low-Energy Neutrinos

Neutrinos are the most elusive of the elementary particles in the standard model. Nearly massless and without electric or strong nuclear charge, neutrinos pass unimpeded through ordinary matter, traveling close to the speed of light. Only very rarely they undergo weak interactions with electrons or atomic nuclei that can be observed in dedicated detectors, huge in target mass and deep underground to reduce the otherwise overwhelming background rates created by natural radioactivity and cosmic rays.

Since its discovery in 1956, many experiments have tried to unravel the peculiar properties of the neutrino. Today, we know that neutrinos come in three types or flavors similar to the quark families and -- very unlike the quarks -- are able to change their initial flavor in periodic patterns while propagating through space, a process known as neutrino oscillations. While the basic principles of the oscillations are by now well understood, experiments are still struggling to determine the full set of fundamental parameters governing the oscillations. Moreover, there are experimental hints that the 3-flavor picture might not be complete, and additional inactive „sterile" neutrino flavors might be present.

Their elusiveness makes neutrinos to rather ideal probes to investigate the interior of astrophysical objects: For instance, our sun is a bright source of low-energy neutrinos created in the nuclear fusion reactions at the heart of solar energy output. The neutrino flux detected at Earth allows for a direct observation of the conditions in the solar interior which imprint on the rate and energy spectrum.

The activities of our group evolve both around neutrino oscillations and astrophysical observations. Presently, we are involved in both data analysis and hardware development for the Borexino experiment at the Gran Sasso underground laboratory (LNGS). Well-shielded from cosmic rays by the Abruzzo mountains, the international collaboration has created a low-threshold neutrino detector of unprecedented radiopurity, potentially able to detect the entire solar neutrino spectrum. Moreover, the Borexino detector will be used in a short-baseline oscillation experiment (SOX) searching for the transition of active to sterile neutrinos, hoping for a first glimpse of particles beyond the standard model.

On a longer time scale, we are involved in the R&D and design of a future large-scale experiment JUNO for the determination of the neutrino mass ordering or hierarchy. To be constructed close to the city of Jiangmen in southern China, JUNO will not only allow for a precision measurement of oscillation parameters but will also act as a next-generation observatory for astrophysical neutrinos.

Figure 1: View of the Inner Detector of Borexino  Figure 2: The Gran Sasso mountains in Abruzzo