Neutrinos play a unique role in our topic, not only as messenger particles from the non-thermal Universe, but also with respect to their intrinsic properties. Due to their weak interaction with matter, many fundamental parameters of neutrinos such as their mass scale and CP properties are unknown at present.
Of particular interest is the uncharted absolute mass scale of neutrinos. In cosmology, this parameter defines the specific role of relic neutrinos left over from the Big Bang as Hot Dark Matter (HDM) in the evolution of large-scale structures of the Universe. In particle physics, this parameter provides unique access to (novel) mass generation mechanisms for elementary particles. Accordingly, a model-independent measurement of the small neutrino rest mass would be of major importance not only for our topic but the entire Helmholtz programme. From the experimental point of view, this is extremely challenging in view of the exceedingly small neutrino mass values, being at least five orders of magnitude smaller than the electron mass. Consequently a large effort in our topic is devoted to perform this crucial task with the KATRIN experiment at KIT.
The measurement of neutrino oscillation properties with the IceCube neutrino observatory at the South Pole documents the possibility to measure neutrino properties with experiments in the South Pole ice. With the proposed PINGU extension of the IceCube neutrino observatory the measurement of the neutrino mass hierarchy comes into reach, which is the second, although smaller activity in this sub-topic.
Neutrino mixing and masses
The first evidence for a non-zero neutrino rest mass by observing flavour oscillations of atmospheric (and solar) neutrinos has recently been labelled as one of the top five discoveries in physics over the past 25 years by the Institute of Physics. Today, after more than two decades of neutrino oscillation experiments we have obtained detailed information on the ν-mixing angles θij and mass splittings Δmij2. However, we do not know the absolute values of the mass eigenstates m1,m2 and m3, and the resulting pattern: either a hierarchical case with m1<<m2<<m3, an inverted hierarchy with m1,m2>>m3 or, finally, a quasi-degenerate case with m1≈m2≈m3. At present, no single experimental method exists to measure the absolute values of all three mass eigenstates mi with the required precision. Therefore, a two-fold strategy is followed: first, to improve the experimental sensitivity on the absolute neutrino mass scale by one order of magnitude to cover the entire parameter range of quasi-degenerate masses with the KATRIN experiment, and, secondly, to determine the ordering of mass hierarchy with the proposed PINGU in-fill array for the IceCube neutrino observatory, see figure below.
Left: KATRIN sensitivity at 90% c.l. for the absolute mν mass scale as function of exposure.
Right: Estimated significance for determining the neutrino mass hierarchy with PINGU.
Experimental activities at KIT
The core experimental activity in neutrino physics is focused on the 'Karlsruhe Tritium Neutrino Experiment' (KATRIN), currently under assembly at Tritium Laboratory Karlsruhe (TLK) at KIT, which will improve the present sensitivity of direct neutrino mass experiments by one order of magnitude from 2 eV=c2 down to 200 meV/c2 (90% CL) after 3 full years of data taking. The large-scale experiment will scan the endpoint region of tritium β-decay with an unprecedented precision by combining an ultra-luminous gaseous molecular tritium source with a high-resolution electrostatic retarding spectrometer. This will also allow searching for light sterile neutrinos on the eV-mass scale as suggested by the reactor oscillation anomaly and other experimental hints. It also allows to look for novel types of weak interactions such as right handed currents and specific violations of Lorentz symmetry in β-decay. Finally, the unique tritium source luminosity and stability can be exploited in explorative studies to search for sterile neutrinos on the keV-mass scale as a potential candidate for Warm Dark Matter.
Neutrino physics at the South Pole
The second part of our activities is the 'Precision IceCube Next Generation Upgrade' (PINGU), a proposed in-fill array for the IceCube neutrino observatory at the South Pole. The primary physics goal for PINGU is the determination of whether the neutrino mass hierarchy is normal or inverted, which is one of the last unmeasured fundamental properties of the neutrino sector. The additional strings will lower the energy threshold of the IceCube neutrino observatory down to a few GeV, so that a measurement of the survival probability of GeV-scale atmospheric muon neutrinos and anti-neutrinos as a function of their energy and their zenith angle can be used to distinguish between normal and inverted mass hierarchies. Besides measuring the mass hierarchy, PINGU will have enhanced sensitivity to neutrino flavour mixing parameters via a high-precision measurement of muon-neutrino disappearance and tau-neutrino appearance in the flux of atmospheric neutrinos, and may also enable neutrino-based tomography of the Earth and contribute to indirect searches for Dark Matter. The achievable resolution on the energy and direction of the observed neutrinos is critical for the success of the mass hierarchy. Therefore, detailed studies have been performed to achieve a realistic estimate of the performance of the proposed instrumentation. Preliminary estimates based on algorithms developed for DeepCore/IceCube analysis and applied to simulations of the PINGU array indicate that the mass hierarchy can be established at 3σ confidence with 1-2 years of data, and at 5σ confidence with 2-5 years of data, see figure above. The stated time ranges reflect the different deployment options as well as different assumptions about the efficiency in reconstructing GeV-scale atmospheric neutrinos.
Further information on the CP properties of neutrinos, as well as on neutrino masses, albeit in a model-dependent way, will be provided by on-going and future searches for 0νββ processes. This will shed light on the Majorana nature of neutrinos and on lepton number violation. These laboratory experiments are supplemented by cosmological studies on large-scale structure (LSS) formation and evolution, combining information from the early epoch of CMB (Planck) with galaxy surveys and weak lensing studies to obtain information on the sum of neutrino masses ∑mi. Inferring neutrino mass results from cosmological observations is notoriously difficult due to partly strong degeneracies amongst the parameters in specific underlying models and tensions between different data sets of observations. Therefore model-independent information on the fundamental mass scale of neutrinos is indispensable.
Theory and phenomenology
The wealth of new information which can be expected in the PoF 3 period (and beyond) implies a large potential for surprising discoveries if dedicated experiments (KATRIN, PINGU) and cosmological studies are combined, and if information on the CP properties of neutrinos (0νββ searches) is included as well. These issues will be addressed and studied in theoretical astroparticle physics group at DESY, and, in parts, by a newly established Helmholtz YIG group at KIT. Discrepancies could hint at non-standard cosmological scenarios and/or non-standard neutrino properties. Of particular interest will be the existence of sterile neutrinos as suggested by the reactor oscillation anomaly, the search for non-standard neutrino properties and non-standard interactions of neutrinos. In the case of neutrino masses it is important to note that mass models depend on the mechanism of electroweak symmetry breaking, and may even originate from physics beyond the Standard Model at the TeV scale. Therefore cross-topic activities with the topic Fundamental Particles and Forces are essential and arise naturally. Furthermore, the origin of CP violation, large enough to describe the matter-antimatter asymmetry of the Universe, may reside in the lepton sector. Finally, non-standard neutrino properties and interactions, such as quantum decoherence, electric or magnetic moments, or neutrino decays, may only be visible at very high energies, very long (cosmological) distances, or in extreme matter conditions. Information on these issues may therefore come from high-energy neutrinos from astrophysical objects, such as studied with the IceCube laboratory.
Measuring the absolute mass scale of neutrinos with a sensitivity of 200 meV/c2 imposes major scientific and technological challenges. Most importantly, it implies that the experimental precision of KATRIN in β-decay spectroscopy has to be improved by two orders of magnitude as compared to the previous experiments. This in turn requires significant improvements of key experimental parameters such as source activity, energy resolution and background rate. In addition, it requires a much better (by one order of magnitude) control of systematic effects. As these are dominated by parameters related to the tritium source, a sub-eV sensitivity can only be achieved by a gaseous molecular tritium source of highest intensity (1011 Bq), highest isotopic purity (> 95%) and highest stability (10-3). To detect the minute spectral modifications close to the endpoint at 18.6 keV, a very large electrostatic spectrometer acting as integrating filter with a narrow width ΔE = 0.93 eV is required.
A key element for the successful operation of KATRIN is the unique expertise provided by Tritium Laboratory Karlsruhe, which offers more than 20 years of tritium handling expertise and holds the licence to store and process large amounts of tritium. This is of particular importance to maintain the required tritium throughput of up to 10 kg per year by cycling in a closed loop, a benchmark that has never been reached before and that can only be compared to the full ITER operation in the late 2020s. Over the next two years, the extensive tritium processing infrastructure will have to be adjusted to the continuous 24/7 operation of KATRIN so that more than 200 days of data taking can be collected per year, while at the same time maintaining fusion-related duties and activities.
A particular challenge arises from the required stability of the source intensity at the 10-3 level. This calls for a tritium loop design with comparable stabilities of the injection pressure and of the source beam tube temperature at 30 K. Also, the high-luminosity source has to be decoupled from the electrostatic spectrometer, which has to be operated at a very low intrinsic background rate of 0.01 counts per second (cps). This necessitates a tritium retention factor in open source geometry in excess of 1014, as a less efficient retention would dramatically increase the background rate. So failsafe and robust tritium retention techniques have to be at hand. Likewise, the exceedingly large spectrometer dimensions pose many challenges, most notably with regard to maintaining excellent vacuum conditions, making the KATRIN spectrometer the largest UHV recipient worldwide. Finally, a fully adiabatic transport of electrons over the entire 70 m long beam line has to be guaranteed, necessitating the development of an advanced code for fast calculation of electromagnetic fields and particle tracking with machine precision.