Session LB: Mini-Symposium: Neutrinos and Nuclei VIII: Lattice QCD for Double Beta Decay; Double Beta Decay Backgrounds II

Preliminary Lattice QCD Study of nn → ppee Matrix Element for Neutrinoless Double-Beta Decay

Several ongoing experiments are searching for a signal of neutrinoless double-beta decay, which would indicate that the neutrino is a Majorana fermion.  The bounds on decay rate observed in these experiments depend on neutrino properties as well as nuclear structure, and current systematic uncertainties from nuclear shell models are a significant contribution to the uncertainty in the range of excluded parameter space.  As such, additional theoretical understanding of the nuclear matrix elements contributing to double-beta decay rates would be desirable.  We aim to perform a preliminary study of the matrix element of the transition of a dineutron to a diproton.  While this particular transition is unphysical, it serves as a proof of concept of the study of matrix elements relevant for 0νββ on the lattice.   

A path from lattice QCD to the short-distance contribution to 0νββ with a light Majorana neutrino

Neutrinoless double beta (0νββ) decay is a lepton number violating nuclear transition whose observation will have profound consequences on elementary particle physics. To draw reliable conclusions from the current experimental limits and potential future discoveries, the uncertainties in the theoretical predictions of its decay rate are needed to be reduced. A major contribution to these uncertainties comes from the effective field theories (EFT) matched ab initio nuclear many-body calculation of its nuclear matrix element. A recently identified short-distance contribution at leading order in the effective field theory amplitude of the subprocess nn→ pp(ee) remains undetermined, and only lattice quantum chromodynamics (QCD) can directly and reliably determine the associated low-energy constant. We provide here a framework to obtain the physical decay amplitude, and hence the missing contribution, from the lattice QCD calculation of the correlation function for this process. The complications arising from the Euclidean and finite-volume nature of the corresponding correlation functions are fully resolved. This work fills the gap between first-principles studies of the nn→ pp(ee) amplitude from lattice QCD and those from effective field theory and can be readily employed in the ongoing lattice QCD studies of this process.   

Background Control for nEXO – a Neutrinoless Double Beta Decay Experiment

nEXO is a tonne-scale neutrinoless double beta decay (0νββ) search experiment. It searches for 0νββ of ¹³⁶Xe using a liquid Xe time projection chamber (TPC). It is projected to reach a half-life sensitivity of about 10²⁸ years with 10 years of exposure time. To achieve this unprecedented sensitivity, stringent limits on radioactivity are necessary to control backgrounds to a sufficiently low level. Detector construction materials contribute most to the background budget in the form of intrinsic radioactivity and outgassed radon. Material storage and handling also contribute through cosmogenic activation, radon daughter plateout, and dust deposition. This talk will discuss our current plans on the estimation, measurement, and mitigation of backgrounds that affect nEXO.   

Radon daughters plate-out as a background source in nEXO neutrinoless double beta decay experiment

nEXO is a planned next-generation experiment searching for the neutrinoless double beta decay of Xe-136. The experiment will utilize a time projection chamber and 5000 kg of isotopically enriched liquid xenon. The projected half-life sensitivity is > 10²⁸ years after 10 years of exposure. Stringent radioactive background control and careful material selection are necessary to achieve such sensitivity.

One of the considered background sources is radon daughter attachment to detector materials during their exposure to air. The decay of Po-210 can produce neutrons via (α, n) reactions, followed by capture on Xe-136, and subsequent β-decay of Xe-137, in turn creating background events. A literature survey reveals a wide range of measured radon daughter attachment lengths. To better understand the causes for this variability our group at the University of Alabama has started a measurement program using various materials relevant for nEXO, monitoring environmental parameters. In this talk I will present the results of our study.   

Preliminary Background Model for LEGEND-1000

The Large Enriched Germanium Experiment for Neutrinoless double-beta Decay (LEGEND) aims to develop a phased $^{76}Ge$-based neutrinoless double beta decay ($0\nu\beta\beta$) experimental program with a discovery potential at a half-life beyond $10^{28}$ years. The final phase of the experiment, LEGEND-1000, will house a total mass of 1 ton HPGe detectors enriched with $^{76}Ge$ to provide a total exposure of 10 ton.yr. To achieve our discovery sensitivity goal, the detectors need to be operated in a nearly background-free regime enabled by a background index of less than $1 \times 10^{-5}$ cts/keV.kg.yr near the end-point energy $Q_{\beta\beta}$ (2039 keV). Numerous background suppression techniques have been implemented. These techniques include use of large inverted-coaxial point-contact (ICPC) detector with excellent energy resolution and pulse-shape discrimination capability, ultra-radiopure and low-mass components to minimize exposure as well as the active liquid-argon veto to reject backgrounds. In this talk, we will discuss how each of these techniques contribute to meeting our background goals for LEGEND-1000   

The Background Model for the CUPID Experiment

The abundance of matter in the universe is one of the most plainly obvious yet deceptively difficult to explain facts in modern physics. All of our known laws of physics conserve the net abundance of matter and antimatter; yet, it is apparent that at some point in the early universe, that symmetry must have been violated. We can search for vestiges of this symmetry violation by searching for a process called Neutrinoless Double Beta (0$\nu\beta\beta$) decay. The CUORE Upgrade with Particle Identification (CUPID) is a next generation search for 0$\nu\beta\beta$ decay in $^{100}$Mo, with sensitivity to $m_{\beta\beta}$ in the so-called Inverted Hierarchy region. CUPID is the successor to the CUORE experiment and adds the ability to significantly reduce radioactive backgrounds to our signal through the use of a powerful particle identification technique. In order to achieve its sensitivity goal, CUPID must achieve a stringent background level of 10$^{-4}$~counts/(keV$\cdot$kg$\cdot$yr) in the 0$\nu\beta\beta$ decay region of interest — a decrease of 2 orders of magnitude over the backgrounds achieved in CUORE. In this talk, I will describe the projected background model for CUPID, focusing on the major background sources, and the mitigation techniques employed to reduce them.   

Radiogenic and Cosmogenic neutrons at LEGEND-1000

Neutrinoless double beta decay (0νββ) is a rare decay process that is possible only if neutrinos are their own antiparticles. A discovery of 0νββ would also explicitly show that the total lepton number is violated with profound physics implications. The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND) aims to develop a phased ⁷⁶Ge-based 0νββ experimental program with the discovery potential of a half-life beyond 10²⁸ years. The second phase of LEGEND – LEGEND-1000 -- will consist of approximately 400 high purity germanium detectors with a total mass of 1000 kg made from germanium enriched to at least 90% in ⁷⁶Ge. In order to achieve the sensitivity goal, the background level of LEGEND-1000 needs to be less than 1*10^-5 cts/keV kg yr at the Q-value of 0νββ (2039 keV). Background modeling based on Monte Carlo simulations and calculations suggests that this background goal can be achieved. In this talk, we will discuss our investigations of radiogenic and cosmogenic neutrons at LEGEND-1000, which is found to be a subdominant background component for the baseline design of LEGEND-1000.   

Reducing Cosmogenic Backgrounds in CUPID

CUPID, the CUORE Upgrade with Particle IDentification, is a proposed upgrade to CUORE,  a ton-scale bolometric experiment in search of neutrinoless double-beta decay (0??????). CUPID will be located at Gran Sasso National Laboratory (LNGS) in Italy and will search for 0?????? decay with a projected sensitivity corresponding to the full neutrino mass regime in the Inverted Ordering (IO) scenario; this correlates to a ¹⁰⁰Mo 0?????? half-life greater than T_1/2^0?? > 10²⁷ yr. To achieve the expected sensitivity, CUPID will implement several techniques to further mitigate backgrounds including scintillating bolometers for particle identification.  Additionally, CUPID will have an active muon veto system to reduce the background from muons in the region of interest. The muon veto system will have a modular design placed around and below the external shielding to tag cosmic muons and correlate them with bolometer data.  This presentation will discuss the cosmogenic background in CUPID and the use of the muon tagger for the reduction of cosmogenic backgrounds in CUPID.   

Investigation of neutron-induced backgrounds in isotopes of interest for 0νββ decay searches

Even deep underground, neutron-induced reactions can occur in all components of detectors searching for rare events, such as neutrinoless double-beta (0νββ) decay.  The isotopes ⁷⁶Ge, ¹⁰⁰Mo, ¹³⁰Te, and ¹³⁶Xe are all in current or planned 0νββ-decay searches.  Additionally, even enriched detectors made from these isotopes may include other nuclides, such as ^95,97Mo or ¹³⁴Xe.  For ¹³⁶Xe (Q_ββ=2457.8 keV), a recently-discovered level in ¹³⁴Xe by Peters et al. [1] decays with the emission of a 2485.7 keV gamma ray.  For ¹³⁰Te, (Q_ββ=2527.5 keV), a neutron-induced excitation of the 2527.1 keV state was investigated.  In ¹⁰⁰Mo (Q_ββ=3034.4 keV), a nuclear level with energy 3039.4 ± 1.0 keV cascades to the ground state.  The isotopes ⁹⁵Mo and ⁹⁷Mo also have energy levels that lie within the region of interest:  3037 keV and 3035 keV respectively.  Finally, ⁷⁶Ge (Q_ββ=2039.1 keV) has a nuclear level at 3951.9 keV that has been reported to emit a 2040.7 keV gamma ray, but which was not observed in studies by Crider et al. [2].  Here, we will report our results from our cross-section measurements of neutron inelastic scattering (n,n'γ) on ^134,136Xe and ¹³⁰Te, and we will update our initial results from our investigation of gamma-ray cascades resulting from n,n'γ on ^natMo and ⁷⁶Ge.  

[1] E.E. Peters, et al., EPJ Web of Conferences, 93, 01027 (2015).

[2] B.P. Crider, et al., EPJ Web of Conferences, 93, 05001 (2015).