Session GB: Mini-Symposium: Neutrino Properties and Interactions: Results, Challenges, and Implications III

Weak Neutral-Current Axial-Vector Form Factor and Neutrino-Nucleon Scattering

We perform a phenomenological analysis, where we combine a calculation of the strange quark electromagnetic form factor from lattice QCD with (anti)neutrino-nucleon scattering differential cross section data from MiniBooNE experiments to determine the weak axial-vector form factor $G_A^Z(Q^2)$. We show that the precise value of $G_A^Z(0)$ obtained from the lattice calculation greatly improves the precision of the form factor extraction. Finally, we show that a consistent determination of the form factor from neutrino and anti-neutrino scattering data requires a non-zero contribution from the strange quark EM form factor in the neutral current scattering process.   

New cold matter effects in neutrino oscillations experiments

Recently (arXiv:180306332) we showed that the electron density in cold matter exhibits large spikes close to the atomic nuclei sites. We showed that these spikes in the electron densities, 3-4 orders of magnitude larger that those inside the Sun`s core, have no effect on the neutrino emission and absorption probabilities or on the neurinioless double beta decay probability. However, it was not clear if the effect of these density spikes is consistent with an average constant electron density in condensed matter. We now investigated these effects by a direct integration of the coupled equation of motion describing the propagation of neutrinos through cold matter, and we found significant differences between the two approaches for a baseline similar to that from Fermilab to Gran Sasso. These results will be reported, including the effects of cold matter electron densities on the evolution of the mixing amplitudes for the vacuum mass eigenstates.   

Improving the Sensitivity of nEXO to Neutrinoless Double Beta Decay

The nEXO Collaboration has conceived a 5000kg liquid-xenon time projection chamber (TPC) that will enable two orders of magnitude greater sensitivity on the neutrinoless double-beta decay ($0\nu\beta\beta$) half-life over present experiments. Such sensitivity arises from the TPC’s capability to simultaneously measure multiple event characteristics. Combined with the use of a large homogenous detector volume, this allows to precisely assess the backgrounds while exploiting the signal from the entire liquid xenon volume. The sensitivity reach is also made possible by a strong radioassay program that carefully screens candidate detectors and by a combination of active and passive shielding in ultra-low-background detector design. In this talk, I will review the elements behind nEXO’s sensitivity, including a background model from new radioassay results, and a more detailed modeling that incorporates reconstruction of time-correlated events and interactions in the liquid xenon outside of the central TPC region. These developments, combined with others, suggest nEXO’s sensitivity will exceed $10^{28}$ years for the $0\nu\beta\beta$ decay half-life of $^{136}$Xe.   

Simulation of charge readout with segmented charge tiles in nEXO

nEXO is a proposed experiment to search for $0\nu\beta\beta$ decays of $^{136}$Xe in a single phase liquid xenon time projection chamber (TPC) with 5 tonnes of liquid xenon. The nEXO TPC is designed to use segmented charge tiles as the anode to read out ionization electrons. A dedicated simulation package is developed to study the performance of this anode design. A multivariate method and a deep neural network are developed to distinguish $0\nu\beta\beta$ decays and background events arising from radioactivity in the detector materials using the simulated charge signals. The nEXO TPC with charge tiles forming the anode shows promising capability to distinguish signal and backgrounds in the study. A half-life sensitivity for $0\nu\beta\beta$ decays is estimated with the discriminators, which suggests the potential for $\sim$20$~(32)\%$ employing the multi-variate~(deep neural network) methods considered here, relative to the sensitivity estimated in the nEXO pre-conceptual design report.   

R&D on Ba-Tagging for nEXO Using Electron Microscopy

nEXO is a proposed 5 ton LXe neutrino-less double beta decay ($0\nu\beta\beta$) experiment to discover whether neutrinos are Majorana particles. nEXO is one of the most sensitive proposed ton-scale $0\nu\beta\beta$ experiments. With a projected half-life sensitivity of about $10^{28}$ years, it can cover the entire $\nu$ inverted mass hierarchy. This limit is partly due to radioactive backgrounds around the $0\nu\beta\beta$ Q-value which mimic a $^{136}$Xe decay. However, $^{136}$Xe decays into $^{136}$Ba++, and identification of the remnant Ba ion (ie, Ba-tagging) would allow nEXO to reject all background events. This would increase the sensitivity by a factor of 4, and allow nEXO to probe well into the normal mass hierarchy. We present results using Scanning Transmission Electron Microscopes (STEM) to image and robustly identify single Ba atoms using Energy-Dispersive X-ray Spectroscopy (EDXS) and Electron Energy Loss Spectrosopy (EELS). This technique could provide a path for Ba-tagging in nEXO. We'll discuss the challenges that remain in developing the entire Ba-tagging chain, from extraction of the single Ba ion out of the 5 tons of LXe to the end-stage identification. We'll also provide a survey of the other promising Ba-tagging techniques being developed for nEXO.   

Current Status and Results of CUORE in the Search for Neutrinoless Double Beta Decay

The Cryogenic Underground Observatory for Rare Events (CUORE) is the largest bolometric experiment searching for neutrinoless double beta (0$\nu\beta\beta$) decay. If observed 0$\nu\beta\beta$ could answer fundamental questions that remain about the nature of the neutrino such as the mass hierarchy, whether they are Majorana fermions, and would present new physics beyond the standard model via lepton number violation. CUORE is comprised of 988 TeO$_2$ crystals (742 kg) arranged into 19 towers, with each crystal operated as a cryogenic bolometer and began taking data in the spring of 2017. This talk will briefly describe the CUORE experiment and summarize the efforts made to improve detector performance, with an emphasis on improving the energy resolution and suppression of backgrounds in the $^{130}$Te 0$\nu\beta\beta$ decay region of interest. Additionally the current status of the ongoing 0$\nu\beta\beta$ search will be discussed as well as the present state of the CUORE experiment.   

The next-phase search for $0\nu\beta\beta$ decay with CUPID

Neutrinoless double beta ($0\nu\beta\beta$) decay is a matter-creating process that violates lepton number conservation. Its discovery would prove the existence of physics beyond the Standard Model. The CUORE Upgrade with Particle IDentification (CUPID) is a proposed next-phase $0\nu\beta\beta$ decay bolometric experiment aiming at a sensitivity that covers the allowed parameter space for the inverted ordering of neutrino masses. CUPID will deploy $\sim250$ kg of $^{100}$Mo embedded in scintillating Li$_2$MoO$_4$ crystals simultaneously acting as source and detector for $0\nu\beta\beta$ decay, and will actively distinguish between $\alpha$ and $\beta$ particles thanks to the readout of both the heat and scintillation light channels. In this talk, the design of CUPID, its active background rejection techniques, and expected background budget are presented.