DUNE's Potential to Search for Neutrinoless Double Beta Decay Elise Chavez University of Wisconsin-Madison - GEM Supervisors: Joseph Zennamo and Fernanda Psihas Fermilab August 07, 2020 Abstract DUNE is projected to be the most capable GeV-scale neutrino experiment the world has seen, which means that it has the ability to search for physics we could not have hoped to observe with our current experiments. One such phenomena is neutrinoless double beta decay, an ultra-rare decay that would indicate new physics. It is a process that violates lepton number and hence is forbidden by the Standard Model, but as stated, it is super rare. The idea then is to take advantage of DUNE's size and capabilities to attempt to search for it by doping the liquid argon with xenon-136, a candidate isotope for the decay. This project, thus, aims to characterize the potential radiologic backgrounds that could come from DUNE itself and the environment to determine if searching for this decay is even feasible. In this paper, the following sources are examined at less than 5 MeV: radiation from the anode, radiation from the cathode, krypton, argon, radon, polonium, and, for the first time, neutrons. Ultimately, it has been determined that these sources do not provide a lot of apparent background that could disguise a neutrinoless double beta decay signal and that DUNE has great potential to search for this process. It must be noted that spallation from cosmic muons and solar neutrino backgrounds were not examined in this study. 1 Contents 1 Introduction 3 2 Background 3 2.1 Neutrinoless Double Beta Decay . 3 2.2 DUNE and Xenon-136 . 4 2.3 The Purpose . 5 3 Methods and Data 6 3.1 DUNE Simulations Part I . 6 3.2 DUNE Simulations Part II . 6 4 Analysis 8 4.1 Results from DUNE Simulation Part I . 8 4.2 Results from DUNE Simulation Part II . 10 5 Conclusion 13 2 1 Introduction radon, and, for the first time, neutrons. The fi- nal states, or particles produced, will be analyzed In the new age of neutrino experiments, the ad- and discussed along with their implications for this vanced capabilities and the sheer sizes of develop- type of study in DUNE. The methods will be dis- ing projects have allowed physicists to explore more cussed in detail as this project worked with real closely the laws of the universe and the things that DUNE simulations to obtain results as comparable make it up. One such process is neutrinoless dou- as possible to DUNE's design. ble beta decay, an ultra rare process forbidden by the standard model. It is a very difficult process to detect due to it rareness, for context the cur- rent best limit on the half-life for xenon-136 to go through the decay is 1027 years, or 1016 ages of the universe. It is very very rare, but observing this de- cay would change completely the way in which we 2 Background understand physics. It would indicate that there is new physics and that our current models need to change. Before diving into all of the specifics of this project, While the decay's rareness is an obstacle, there there is a need for some background to completely are several factors that can be exploited to make understand the choices made and the ideas driving the chances of observation better. The first of this project. First and foremost, it is important to those is amount of material. If there is more decay understand 0νββ and what one needs to make it material, the probability of observation increases. happen. Second is exposure time. If the material is left to decay for a long time, the probability again in- creases. Some of the other factors come from a de- tectors capabilities, such as energy resolution. All of these, help to increase the chance of observation. DUNE (Deep Underground Neutrino Experiment) 2.1 Neutrinoless Double Beta De- is one experiment in development that may give us cay a fighting chance in observing neutrinoless double beta decay (0νββ). The idea is to dope the liquid argon in the far detector with xenon-136, a can- Neutrinoless double beta (0νββ) decay occurs didate isotope for this decay, and to use DUNE's when two neutrons in a nucleus decay into two exposure time and detector capabilities to increase protons and emit two electrons. This is depicted our chances of detecting this decay, which would in Figure 1 and the Feynman diagram is given in be monumental for physics. In addition, this would Figure 2. As can be seen in the figures, this decay lower DUNE's capabilities to less than 5 MeV, as violates lepton number conservation. There are 0 0νββ occurs at 2.459 MeV for xenon-136. leptons at the start but 2 in the final state. This is This paper works to outline the full idea and forbidden by the Standard Model. For this decay potential backgrounds from the detector itself and to occur in nature, the neutrino must be a Ma- from some parts of the environment that could jorana particle meaning the neutrino must be its disguise the 0νββ signal. The sources for such own anti-particle. Here, the electron neutrino and backgrounds are radiation from the anode, radi- anti-electron neutrino annihilate leaving the two ation from the cathode, krypton, polonium, argon, electrons as the final state. 3 stated in the introduction. This makes observing the decay extremely difficult. There have been ex- periments that have tried and hence improved the half life limit over time. In the next section, the choice of DUNE and its potential will be discussed. 2.2 DUNE and Xenon-136 The future of neutrino physics lies in part with the Deep Underground Neutrino Experiment (DUNE) Figure 1: An atomic diagram of neutrinoless dou- which will have both a near and far detector. ble beta decay. [4] The near detector will be located at Fermilab in Batavia, Illinois and the far detector will be lo- cated in South Dakota. The larger one being the far detector. A diagram of the entire experiment is given in Figure 3. Figure 3: A diagram of the DUNE far and near detectors. [6] Figure 2: A Feynmann diagram of neutrinoless double beta decay. [5] The detectors will both contain time projection chambers (TPCs) immersed in liquid argon, called liquid argon time projection chambers (LArTPCs). This means that observing such a decay would There are expected to be two LArTPCs at first, have vast implications for particle physics. It with the plan being 4 total upon completion of the would mean that the Standard Model is wrong project. Each TPC will hold 20 kilotons of liquid and demand change to it or the development of argon. DUNE is also expected to run for a time new theory. In addition, it could be a piece of period of about 5-10 years. Further, DUNE is ex- the matter/anti-matter asymmetry puzzle. While pected to have a better sensitivity to the electron it wouldn't directly be an answer, neutrinoless dou- neutrino and hence electrons. This means a better ble beta decay would lead physicists to a new age sensitivity to the 0νββ signal. of particle physics. Unfortunately, this decay is ex- To combat the rareness of the 0νββ decay, one tremely rare with xenon-136 having a current 0νββ can exploit the mass of decay material, expo- half life limit of 1027, or 1016 ages of the universes as sure time, and detector capabilities to increase the 4 chances of observation. The latter comes with the reason to use it. Putting this all together, the advanced capabilities and sensitivities of DUNE hope is to dope, by mole, the liquid argon with and the second comes from the planned run-length 2% xenon. To put this in context, the world's best of the experiment. It is exploiting the mass of de- 0νββ limits come from experiments that have had cay material that merits a more detailed discussion. about 2 kg of decay material, DUNE would bring Unfortunately, not all isotopes can undergo 0νββ us to 100s of tons of decay material. Thus, DUNE decay. A nucleus would have two neutrons change can exploit its sheer size to exploit the mass of de- into two protons, which is not always a physically cay material. allowed process. The nucleus must lose energy in This may seem like a fairly easy thing to do; order for this new state to be energetically favor- however, doping the liquid argon is still in the re- able, and due to the complicated structure of the search and development (R&D) phase of study. It nucleus that is simply not the case for most nu- is still unclear whether it is currently feasible to clei. In Figure 4 are the few isotopes that can even implement. This uncertainty also stems from how undergo the decay. difficult it is to obtain xenon. In Figure 4, the nat- ural abundance of xenon is 8.9%, but this is all xenon. Xenon-136's abundance is a big fraction of this, but Xenon will be hard to obtain. Thus, it is more than necessary to study the feasibility of this search for 0νββ decay which is the cornerstone of this project. We must know what to expect in DUNE. 2.3 The Purpose Figure 4: A table of isotopes that can undergo neutrinoless double beta decay. [3] The purpose of this project was to study and char- acterize the environmental radioactive decays in The one isotope not on this list is argon, which DUNE's LArTPCs which serve as background for means a different one would have to be used.